possible crustal contamination of midcontinent rift igneous rocks examples from the mineral lake intrusions wisconsin interlayered mafic telsic intrusions from the mineral lake intrusive complex in northwest wisconsin reflect the typical bimodal basalt rhyolite compositional pattern of the midcontinent rift flood

u th pb zircon ages of some keweenawan supergroup rocks from the south shore of lake superior new single crystal zircon u th pb ages for plutonic and rhyolitic keweenawan supergroup rocks from the south shore of lake superior provide geochronological constraints on magmatic evolution associated with the 1 1 ga midcontinent rift analyses of a granophyric phase of the mineral lake intrusion and the mellen granite both parts of the mellen intrusive complex and a laterally extensive rhyolite from the top of the kallander creek volcanics have weighted average 207pb 206pb ages of 1102 0 2 8 ma n 2 1100 9 1 4 ma n 5 and 1098 8 1 9 ma n 4 respectively analyses of a pyroclastic rhyolite flow at the top of the porcupine volcanics result in variable 207pb 206pb ages that range from 1080 to 1137 ma this rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals the latter yielding concordant analyses having a weighted average 207pb 206pb age of 1093 6 1 8 ma n 2 four prismatic zircons from an aphyric rhyolite of the chengwatana volcani

1 8 ga svecofennian post collisional shoshonitic magmatism in the fennoscandian shield abstract at least 14 small 1 11 km across 1 8 ga svecofennian post collisional bimodal intrusions occur in southern finland and russian karelia in a 600 km long belt from the aland islands to the nw lake ladoga region the rocks range from ultramafic calc alkaline apatite rich potassium lamprophyres to peraluminous hibasr granites and form a shoshonitic series with k 2 o na 2 o 5 k 2 o na 2 o 0 5 al 2 o 3 9 over a wide spectrum of sio 2 32 78 although strongly enriched in all rocks the lile ba and sr and the lree generally define a decreasing trend with increasing sio 2 depletion is noted for hfse ti nb and ta available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded initial magmas mg 65 in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post collisional uplift some 30 ma after the regional svecofennian metamorphic peak however prior to the melting episode the lithospheric mantle was affected by carbonatite metasomatism more extensively in the east than in the west the melts generated in the more carbonate rich mantle are extremely enriched in p 2 o 5 4 f 12 000 ppm lile ba 9000 ppm sr 7000 ppm lree la 600 ppm and ce 1000 ppm the parental magma underwent 55 60 fractionation of biotite clinopyroxene apatite magnetite sphene whereupon intermediate varieties were produced after further fractionation 60 80 of k feldspar amphibole plagioclase minor magnetite sphene and apatite leucosyenites and quartz monzonites were formed in the west where the source was less affected by carbonatite metasomatism calc alkaline lamprophyres vogesites minettes and spessartites and equivalent plutonic rocks monzonites were formed removal of about 50 of biotite amphibole plagioclase magnetite apatite and sphene produced peraluminous hibasr granites the impact of crustal assimilation is considered to be low at about 1 8 ga the post collisional shoshonitic magmatism brought juvenile material particularly enriched in alkalis lile lree and f into the crust although areally restricted the regional distribution of the post collisional intrusions may indicate that larger volumes of 1 8 ga juvenile material resides in unexposed parts of the crust

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

os nd o and s isotope constraints on country rock contamination in the conduit related eagle cu ni pge deposit midcontinent rift system upper michigan abstract the eagle cu ni pge deposit is hosted by mafic to ultramafic intrusive rocks associated with the marquette baraga dike swarm in northern michigan sulfide mineralization formed in a conduit system during early stages in the development of the 1 1 ga midcontinent rift system the conduit environment represents a prime location for melt rock interaction in order to better assess the extent of country rock contamination in the eagle system a combined trace element nd os o and s isotope study of country rocks sulfide bearing igneous rocks and massive sulfide was undertaken both the eagle and the weakly mineralized east eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development the trace element os nd and o isotopic values of the igneous rocks are consistent with os 1100 and e nd 1100 values for the magmas are 34 and 2 18 o values of pyroxene in feldspathic pyroxenite range from 6 5 to 6 6 and provide the only indication that bulk contamination may locally have exceeded 20 sulfur isotopic values of disseminated and massive sulfide in the eagle intrusion range from 0 3 to 4 6 the 34 s values are much lower than those that characterize most of the country rocks but could still be indicative of a contribution of s from country rocks of up to 50 33 s values of the disseminated and massive sulfides range from 0 10 to 0 09 indicating a source in paleoproterozoic country rocks semi massive sulfide in the eagle deposit has 34 s values between 2 2 and 5 3 and 33 s values show a broad range between 0 86 and 0 86 indicating a major contribution from an archean source isotopic data from the eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the ni cu pge mineralization and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the eagle deposit our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end member values for mixing calculations sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios in addition sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and 34 s values near those of mantle sulfur

paleomagnetism of the 1 1 ga coldwell complex ontario canada implications for proterozoic geomagnetic field morphology and plate velocities we report new paleomagnetic data from the 1108 ma intrusive coldwell complex ontario canada to investigate the apparent reversal asymmetry observed in some midcontinent rift mcr rocks the rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of d 114 8 i 63 7 95 3 6 n 30 the corresponding paleomagnetic pole at plat 47 2 n plong 206 5 e a95 4 8 is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the mcr system including the lower lava flows at mamainse point the rocks of western part of the complex are normally magnetized with a group mean direction d 298 0 i 56 9 95 5 8 n 10 that passes the reversal test with respect to the reversed group mean direction our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals instead our new data indicate that the coldwell complex records only two polarity intervals separated by a symmetrical reversal at 1102 1105 ma this reversal is likely equivalent to the lowermost reversal recorded at mamainse point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry together with a high quality data from the 1098 ma north shore volcanics our data indicate a rapid velocity of laurentia at 25 4 cm yr the fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at 1 1 ga

the midcontinent rift in the lake superior region with emphasis on its geodynamic evolution abstract the midcontinent rift is a middle proterozoic continental rift which records about 15 m y of extension subsidence and voluminous volcanism in the period 1109 1094 ma in the central part of north america during that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression following extension and volcanism a longer period of subsidence resulted in development of a post rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited partial inversion of the central depression occurred about 30 50 m y after extension to produce the current configuration of a central horst composed mostly of thick volcanic accumulations between shallower flanking basins

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

mineral deposits of the mesoproterozoic midcontinent rift system in the lake superior region a space and time classification abstract the mesoproterozoic midcontinent rift system mrs of north america hosts a diverse suite of magmatic and hydrothermal mineral deposits in the lake superior region where rift rocks are exposed at or near the surface historically hydrothermal deposits such as michigan s native copper deposits and the white pine sediment hosted stratiform copper deposit were major mrs metal producers on going exploration for and potential development of copper nickel sulfide deposits hosted by the duluth complex of minnesota and the opening of the eagle nickel mine in michigan indicate an expanding interest in mrs magmatic deposits mrs hydrothermal and magmatic mineral deposits many of which are significant past present and likely future providers of critical minerals here are placed into a space and time metallogenic framework to construct this framework regional mrs mineral deposits extracted from the u s geological survey mineral resources data system mrds and the ontario ministry of energy northern development and mines mineral deposit inventory mdi were supplemented by other known and recently recognized mineral deposits described in the literature all mineral deposits were classified by deposit type host rock age and type and estimated timing of mineralization deposits were then put into a tectonic evolutionary framework stages for the mrs which shows that deposits formed within discrete spatial and temporal stages of rift evolution each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization examples include mrs nickel rich conduit type magmatic sulfide deposits which are restricted to an early magmatic stage mrs history when magma compositions were characterized by ni rich picrites and high mg basalts in contrast contact type magmatic sulfide deposits with cu ni were derived from more evolved al rich tholeiitic magmas that dominated a later time of voluminous magmatic activity hydrothermal sediment hosted stratiform chalcocite mineralization along the margins of western lake superior in michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post volcanic mrs sedimentary basin widespread native cu and native ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks these descriptions of the diverse mineral deposits in the lake superior region cataloged within the space of the local and regional geology and over the more than 60 million year mineralizing history of the mrs are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province

paleomagnetism of the powder mill group michigan and wisconsin a reassessment of the logan loop the apex and the western arm of the 1 1 ga logan loop a prominent feature of the north american precambrian apparent polar wander path are defined by a large body of data from keweenawan rocks of the lake superior basin until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the powder mill group of michigan a reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the logan loop data from the mellen complex in wisconsin together with our data from the powder mill group suggest that for steeply dipping volcanic units the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike reversely magnetized sites from localities where the powder mill volcanics have shallow dips yield a pole 146 w 46 n a95 9 2 on the apex of the loop a well defined stratigraphically continuous normal to reversed polarity change near the base of the powder mill group is not confirmed normally magnetized flows have been found at two localities only one of which is at the base of the lava sequence the higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the powder mill group are secondary and should not be used in definition of the eastern arm of the logan loop

multiple mantle metasomatism in central southern italy geochemical effects timing and geodynamic implications the regional petrological and geochemical variations observed in the recent magmatism along the italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle these are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the alpine subduction by introduction of a composition similar to the deeply subducted dora maira metagranites into a residual lithospheric mantle of upper crustal material metasomatic events beneath the roman and neapolitan areas are younger these are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west directed apennine subduction of the adria plate and during the current northwest subduction of the ionian sea floor

lithospheric structure beneath the alboran basin results from 3d gravity modeling and tectonic relevance a three dimensional gravity modeling combined with integrated heat flow and elevation modeling is conducted to map out the crustal and lithospheric mantle thickness in the alboran basin in the westernmost mediterranean a sediment corrected bouguer anomaly has been derived using a depth to the basement map and densities determined from well logs and seismic data the gravity effect of the base of the lithosphere has been removed from the sediment corrected bouguer anomaly to obtain a crustal bouguer anomaly which has been inverted for crustal thickness the resulting lithospheric structure is further constrained by elevation data under the assumption of local isostasy the low residual elevation anomalies obtained 100 m in average suggest that the area is in local isostasy particularly the medium and long wavelength topography features variations in crustal thickness range from 36 km underneath the betic and rif chains to 12 km beneath the easternmost part of the alboran sea basin in the transition to the south balearic basin in western alboran the moho lies at a rather constant depth of 18 km deepening sharply toward the gibraltar strait down to 30 32 km the base of the lithosphere shallows from 140 km depth in the gibraltar strait to 45 km depth in the easternmost alboran sea lithospheric thinning penetrates to the southeastern side of the iberian margin crosscutting topographic highs of the central and eastern betic chain our results favor mantle delamination produced by detachment and subsequent peeling away of the lithospheric mantle rather than convective removal of the lithospheric mantle either by orogenic collapse or detachment and sinking of a lithospheric slab

the role of slab detachment processes in the opening of the western central mediterranean basins some geological and geophysical evidence abstract a review of the geological and geophysical data from the central and western mediterranean region and the present day upper mantle structure derived from tomographic studies are utilized in order to define the oligocene recent geodynamic evolution for the area in line with previous work we suggest that the miocene quaternary opening of the western and central mediterranean basins is the result of back arc extension due to the roll back toward the southeast of a northwestward subducting african slab in a geodynamic setting pinned between the alpine and betic collisional zones we find however that this general pattern is complicated by four different detachment events which occurred beneath the alps early oligocene the betic chain aquitanian northern africa langhian and the apennines late miocene pliocene we show that each of these events determines a major tectonic reorganization within the european plate

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

paleomagnetism of the native copper mineralization keweenaw peninsula michigan the age and genesis of michigan s world class native copper deposits are poorly constrained the copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 2 ma keweenawan por

depleted arc volcanism in the alboran sea and shoshonitic volcanism in morocco geochemical and isotopic constraints on neogene tectonic processes abstract samples of volcanic rocks from alboran island the alboran sea floor and from the gourougou volcanic centre in northern morocco have been analyzed for major and trace elements and sr nd isotopes to test current theories on the tectonic geodynamic evolution of the alboran sea the alboran island samples are low k tholeiitic basaltic andesites whose depleted contents of hfs elements 0 5 n morb especially nb 0 2 n morb show marked geochemical parallels with volcanics from immature intra oceanic arcs and back arc basins several of the submarine samples have similar compositions one showing low ca boninite affinity 143nd 144nd ratios fall in the same range as many island arc and back arc basin samples whereas 87sr 86sr ratios on leached samples are somewhat more radiogenic our data point to active subduction taking place beneath the alboran region in miocene times and imply the presence of an associated back arc spreading centre our sea floor suite includes a few more evolved dacite and rhyolite samples with 87sr 86sr 0 up to 0 717 that probably represent varying degrees of crustal melting the shoshonite and high k basaltic andesite lavas from gourougou have comparable normalized incompatible element enrichment diagrams and ce y ratios to shoshonitic volcanics from oceanic island arcs though they have less pronounced nb deficits they are much less lil and lree enriched than continental arc analogues and post collisional shoshonites from tibet the magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input sr nd isotope compositions point to significant crustal contamination which appears to account for the small nb anomalies the unmistakable supra subduction zone ssz signature shown by our alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all neogene volcanism in the alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over thickened lithosphere our data support recent models in which subsidence is caused by westward rollback of an eastward dipping subduction zone beneath the westernmost mediterranean moreover severance of the lithosphere at the edges of the rolling back slab provides opportunities for locally melting lithospheric mantle providing a possible explanation for the shoshonitic volcanism seen in northern morocco and more sporadically in se spain

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy abstract pb nd and sr isotope data are reported on mineral separates and whole rock samples of mg rich metapelites and associated rocks that have been subducted to depths of ca 100 km in the mantle within the last 100 ma and returned to the surface and for which metamorphic conditions were estimated to be 700 800 c at 28 kilobars pressure three rock types fine and coarse grained pyrope quartzites and nafe rich jadeite kyanite quartz layers were collected from a single outcrop in the coesite bearing portion of the dora maira massif samples analyzed include pyrope pseudomorphs after pyrope phengite zircon the new mineral ellenbergerite and whole rocks isotope data from all three decay systems indicate minimal if any isotopic exchange with mantle rocks isotope ratios from minerals with low parent daughter ratios cluster around 143 nd 144 nd 0 51225 nd 7 6 87 sr 87 sr 0 7 15 75 15 80 with 206 pb 204 pb 19 4 20 7 these compare closely with ratios from precambrian granitic rocks or their derived sediments from the pb isotope data and the sm nd whole rock model ages we estimate a mean age of 2000 500 ma for the sediment protoliths pyrope mineral separates record approximate ages for alpine metamorphism of 34 38 ma by the sm nd method the u pb method applied to zircon and ellenbergerite yields possible additional metamorphic ages of 50 55 ma these data illustrate a possible mechanism for creating isotopically enriched mantle had these rocks yielded their isotopes to the mantle enriched mantle could have resulted the magnitude of the enrichment depending upon the total volume attained by the mixing process similarly the metasediments could also provide the high potassium abundances and appropriate pb sr and nd isotope ratios that are postulated to accompany mantle metasomatic processes

reversal asymmetry in mesoproterozoic overprinting of the 1 88 ga gunflint formation ontario canada non dipole effects or apparent polar wander abstract eighty two palaeomagnetic samples of calcareous and jaspilitic grainstones iron formation or taconite and chert carbonate were collected from the 1 88 ga gunflint formation at 22 sites in the thunder bay area ontario twenty clasts of gunflint taconite also were sampled from the basal conglomerate of the overlying mesoproterozoic sibley group anisotropy of magnetic susceptibility measurements indicate the gunflint formation in the sampling area has not experienced regional dynamic metamorphism analyses by variable field translation balance and x ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples altogether 213 gunflint specimens and 59 sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 gunflint specimens to stepwise alternating frequency demagnetisation the following components were isolated for the taconites gunflint magnetite normal declination d 293 4 inclination i 30 8 95 7 2 n 21 reverse d 86 7 i 54 6 95 5 8 n 29 gunflint hematite normal d 243 6 i 23 6 95 6 0 n 11 reverse d 70 3 i 51 4 95 3 2 n 79 sibley clasts magnetite normal d 282 7 i 33 4 95 7 6 n 20 sibley clasts hematite normal d 254 5 i 56 2 95 8 4 n 13 reverse d 110 6 i 55 7 95 8 3 n 11 none of these sets passed the reversal test with the normal component generally being the shallower fold tests were negative or inconclusive and the conglomerate test also was negative chert carbonate at one other site appears to have acquired a remanence carried by magnetite d 97 3 i 78 2 95 6 3 n 18 prior to folding related to keweenawan 1 1 ga logan diabase sill emplacement most of the components we identified match components for keweenawan sills volcanic rocks intrusions and baked contact rocks in the thunder bay area indicating that keweenawan magmatism caused widespread chemical remagnetisation of the proterozoic country rock in our sampling area although others have argued that asymmetry was a feature of the keweenawan geomagnetic field the declinations of our gunflint and sibley hematite and magnetite components are different suggesting that the components were acquired at significantly different times we conclude that the reversal asymmetry shown by our gunflint and sibley data may best be ascribed to apparent polar wander during keweenawan times

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

palaeomagnetic geochronological and geochemical study of mesoproterozoic lakhna dykes in the bastar craton india implications for the mesoproterozoic supercontinent abstract palaeomagnetic analysis of the lakhna dykes bastar craton india yields a palaeopole at 36 6 n 132 8 e dp 12 4 dm 15 9 and the u pb zircon age obtained from one of the rhyolitic dykes is 1466 4 2 6 ma mswd 0 21 concordia age based on two analyses with identical pb u ages similar to previously published u pb ages major and trace element analyses of the lakhna dykes show shoshonitic and high k calc alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting this is in keeping with the palaeo to mesoproterozoic tectonic environments in the eastern indian margin the new 1460 ma indian palaeopole was used to test possible palaeopositions of india within the mesoproterozoic supercontinent columbia of the four palaeomagnetically permissible reconstructions juxtaposing western india against south west baltica is geologically the most reliably constrained and best fitting model our preferred reconstruction implies a long palaeo to mesoproterozoic accretionary orogen stretching from south eastern laurentia through south western baltica to south eastern india breakup of india and baltica probably occurred in the late mesoproterozoic but additional constraints are needed

synthetic saline aqueous and hydrocarbon fluid inclusions trapped in calcite at temperatures and pressures relevant to hydrocarbon basins a reconnaissance study abstract we conducted reconnaissance experiments to synthesize aqueous and hydrocarbon inclusions trapped in calcite at conditions relevant to petroleum basins and characterize the microthermometric properties of such inclusions fluid inclusions fis were synthesized in a system of saline aqueous solution 5 or 20 wt nacl coexisting with either heavy crude oil or gasoline under gas undersaturated conditions from 90 to 210 c and 200 550 bar the synthetic inclusions are not representative of gas bearing systems and methane ch 4 was not detected in any aqueous inclusions the fis are mainly distributed along planar healed cracks indicating that the inclusions formed by fracture healing in the calcite crystal microthermometric measurements were conducted on coeval aqueous and hydrocarbon inclusions and raman spectroscopic analyses were done on aqueous inclusions to determine the properties of fis trapped at these conditions homogenization temperatures of synthetic fis are mostly lower than the experimental trapping temperature although the fis show high variability in measured homogenization temperature results allow comparison of t h values for each sample with the expected t h isochores and pressure corrections calculated for the system h 2 o nacl the latter parameters are broadly consistent with the known pvtx properties of h 2 o nacl fluids suggesting little effect of hydrocarbons on the homogenization behavior although the low precision of the t h data limits this assessment nevertheless this result is not unexpected considering that light hydrocarbons gas is not present in the experiments as corroborated by raman spectroscopy a consequence of using dead oil in the experiments simulation of gas bearing petroleum basins will require additional protocols for producing gas either by in situ cracking of the starting hydrocarbon material or by other means the reconnaissance experiments documented here provide a basis for such future experiments

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

constraining the thermal history of the north american midcontinent rift system using carbonate clumped isotopes and organic thermal maturity indices abstract the midcontinent rift system mrs is a late mesoproterozoic 1 1 ga sequence of volcanic and sedimentary rocks exposed in the lake superior region of north america the mrs continues to be the focus of much research due to its economic mineral deposits as well as its archive of precambrian life and tectonic processes in order to constrain the post depositional thermal history of the mrs samples were analyzed for carbonate clumped isotope composition and organic thermal maturity clumped isotope values from sedimentary early diagenetic samples were partially reset during burial to temperatures between 68 and 75 c solid state reordering models indicate that maximum burial temperatures of 125 155 c would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift clumped isotope results from late stage veins in the white pine mine encompass a greater temperature range 49 116 c indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100 c during regional cooling and uplift clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the mrs observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early degradation processes these results demonstrate that the mrs experienced a spatially consistent relatively mild thermal history 125 155 c and is therefore a valuable archive for understanding the late mesoproterozoic environment

gravity and magnetic models of the midcontinent rift in eastern lake superior gravity and magnetic models of the midcontinent rift mcr in eastern lake superior supplement recent structural and stratigraphic interpretations based on the seismic reflection method an algorithm developed to accommodate spatially varying direction and magnitude of magnetization within a magnetic source is used in both forward and inverse modeling procedures structural attitudes of rift filling basalts derived from seismic reflection sections are used to rotate the keweenawan remanent magnetization vectors in the direction of deformation an iterative linear inversion routine calculates magnitudes of induced and remanent magnetizations as well as normal and reversed polarity basalt flow distributions the results indicate that the koenigsberger ratios of these basalts generally range from 1 to 3 which is in agreement with values obtained from rock property measurements the models also suggest that the greater volume of the keweenawan basalt section in eastern lake superior is reversely polarized a

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

discussion of origins of hinterland basins to the rif betic cordillera and carpathians abstract the hinterland basins of the rif betic cordillera western alboran sea and the carpathians pannonian basin are in part composed of a thick in excess of 3 4 km thermal sag basin sequence the underlying crust has been thinned to 25 km in parts of the pannonian basin and to 15 km in parts of the alboran sea modern tectonic models for these areas envision tectonic collision and thickening of the crust followed by crustal thinning either by gravitational collapse back arc extension or a combination of the two contemporaneous with thrusting basin and range type extension has been envisioned for parts of these extensional collapse systems extensional features are common and they interact in a complex way with strike slip systems flexural cantilever modelling agrees with earlier mckenzie type pure shear modelling of the extensional systems in suggesting that extension alone cannot generate sufficient thermal decay to explain the great thickness of the sag basins associated with the alboran sea and pannonian basin consequently it is suggested that uplift of the asthenosphere accompanying subduction roll back is an important factor in the evolution of these basins roll back of the mantle lithosphere permits hot asthenospheric mantle to upwell and lie close to the base of the crust initially uplift and erosion of the crust overlying the asthenosphere occurs as the thermal perturbation cools the crust thermally subsides to produce a large sag basin

magmatic activity and plate motion during the latent stage of midcontinent rift development the keweenawan midcontinent rift of north america records signifi cant continental rifting between ca 1110 and 1085 ma and preserves the most detailed paleomagnetic record of plate motion of any continent in precambrian time u pb dates from extrusive and intrusive rocks of the western lake superior basin suggest a latent stage of reduced magmatic activity from ca 1106 to 1100 ma that places constraints on the dynamics of rift development and the record of plate motion however it has remained unclear whether this stage is a feature of the entire 2500 km long rift the succession of picritic and basaltic lava fl ows at mamainse point in the eastern lake superior basin may be the most continuous and best exposed record of rift related volcanism and magnetic reversals but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates we present a weighted mean 206 pb 238 u date of 1100 36 0 25 ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the great conglomerate this date indicates that eruptive activity at mamainse point continued during the interval of diminished magmatic activity in the western lake superior basin this result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at mamainse point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

post collisional volcanism in a sinking slab setting crustal anatectic origin of pyroxene andesite magma caldear volcanic group neogene alboran volcanic province southeastern spain abstract caldear volcanic group cvg a stratigraphically well defined calc alkaline rock complex within sa de gata in the eastern part of the alpine betic mountain chain s spain consists of three distinct formations hernandez pyroxene andesites bujo hornblende bearing pyroxene andesites and viuda hornblende bearing pyroxene dacites rhyolites the letter rock formation may have developed through crystal fractionation of mainly plagioclase and pyroxenes however there is no direct relation between two formations cvg has a domainal structure with a northeastern domain where hernandez formation is overlain by bujo formation while viuda formation is absent and a southwestern domain where viuda formation forms the only fractionate after hernandez formation hernandez parent magma is thought generated through crustal anatexis by dehydration melting of a predominantly amphibolitic source rock complex which was formed by metamorphism from c 500 ma volcano sedimentary parent material the domainal structure of cvg is explained by compositional variation within this protogenetic complex single crystal u pb ages of c 500 ma to 1800 ma for inherited zircon support the presence of clastic material of proterozoic derivation within the original volcano sedimentary complex regional study of syn collisional rock formations alpine nappe complexes indicate that the collisional tectonic stage in the betic rif orogenic belt took place rather early 25 30 ma and was followed by a stage of rapid regional rock uplift fast cooling c 500 c my and extensional tectonics in the period 22 17 ma this later tectonic stage was set into motion by slab break off which set the stage for a high temperature regime in the overlying lithosphere providing the framework for the crustal melting and magma production responsible for the calc alkaline rocks of alboran volcanic province miocene zircon with ages ranging from c 17 to 11 ma indicate a rather protracted magmatic development prior to eruption at c 11 ma post collisional character of caldear volcanic group thus seems well established

paleomagnetism of mesoproterozoic margins of the anabar shield a hypothesized billion year partnership of siberia and northern laurentia abstract siberia and laurentia have been suggested as near neighbors in proterozoic supercontinents nuna and rodinia but paleomagnetic evidence has been sparse and ambiguous here we present four new paleomagnetic poles from undeformed paleo mesoproterozoic lower riphean sedimentary rocks and mafic intrusions of the northwestern anabar uplift in northern siberia combining these results with other proterozoic data from siberia and laurentia we propose a tight juxtaposition of those two blocks euler parameters 77 098 137 for anabar to north america spanning the interval 1 7 0 7 ga constituting a long lived connection that outlasted both the nuna and rodinia supercontinental assemblages

high δ18o and possible pre eruptional rb sr isochrons in cordierite bearing neogene volcanics from se spain o and sr isotope data are reported for cordieritebearing dacites and andesites forming part of the neogene volcanic province of se spain the almandine bearing biotite cordierite labradorite dacite from the cerro del hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail a syngenetic derivation of the hoyazo dacite magma and part of its inclusions interpreted as restite by means of anatexis of semi pelitic rocks has previously been proposed

latent thermal effects from porcupine volcanics calderas underlying the white pine presque isle stratiform copper mineralization northern michigan economic stratiform copper mineralization in the fine grained carbonaceous basal nonesuch formation of the 50 km wide white pine presque isle mining district northern michigan has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal nonesuch strata from the underlying copper harbor conglomerate aquifer followed by copper precipitation mainly by reactions with in situ iron sulfide based on that model why do lesser amounts of total copper measured in vertical sections occur in coarser grained presumably more permeable basal nonesuch strata situated laterally to the east and west of the mining district the answer may lie in increased infiltration rates for copper harbor hosted ore forming brine warmed locally by latent volcanic heat from a resurgent caldera or calderas located within the large porcupine volcanics dome underlying the mine district not only would caldera heat have warmed the brine and lowered its density but more importantly it would have produced a remarkable decrease in the brine viscosity an increase in temperature from 15 to 100 c could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous because progressive compaction would have opposed the temperature effect a very early diagenetic timing is suggested for main stage copper mineralization over the porcupine volcanics caldera textural evidence has also long supported an early mineralization event at the rift basin scale the exceptionally rapid upward flux of warmed brine into fine grained nonesuch over the caldera would have required equivalent volumes of replacement brine which would in turn have required a lateral convergence of brine within the footwall copper harbor conglomerate aquifer toward the white pine presque isle district this lateral focus of brine toward the warm caldera area would have created a more efficient basin scale ore forming system than that resulting from a linear hydrogeologic flow most concepts described here deserve further evaluation through quantitative hydrogeologic modeling thermal effects on ore brine viscosities and densities developed here for the sediment hosted stratiform copper mineralization of the white pine presque isle district could be considered in genetic models for other deposits and deposit types the effects are especially strong for solutions warmed from low temperatures e g 20 c or lower to 100 c and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

early history of the midcontinent rift inferred from geochemistry and sedimentology of the mesoproterozoic osler group northwestern ontario the mesoproterozoic 1108 1105 ma osler group a3k m thick succession of basaltic flows and sedimentary units on the north shore of lake superior is among the oldest expressions of the midcontinent rift basal sediments of the simpson island formation new name deposited by braided fluvial systems record westward transport of debris eroded from local archean and proterozoic rock units strata deposited by this fluvial system are intercalated with and overlain by ocean island basalt oib like basalts which become increasingly contaminated up section end 1100ma 0 3 to 5 3 the light rare earth element lree enriched la smn 1 5 3 9 and heavy ree hree fractionated gd ybn 1 5 3 7 subaerial flows are divisible into two units that correlate with other sections of the osler group to the east but simple correlations with more distant sequences are difficult the volcanic rock dominated portion of the succession is overlain by a thin 25 m thick conglomerate sandstone assemblage representing southeast progradation of an alluvial fan in a semi arid climatic setting clast lithologies and geochemistry indicate no extra rift detritus was delivered from the hinterland of the fan various lines of evidence in both volcanic and sedimentary rocks support a scenario where early pre 1108 ma subsidence along a north south axis from the western arm of the rift to the nipigon embayment was replaced by subsidence along the east west rift axis between 1108 and 1105 ma resume le groupe d osler mesoproterozoique 1108 1105 ma une succession de coulees basaltiques et d unites sedimentaires d une epaisseur de 3 km sur la rive nord du lac superieur est l une des plus anciennes expressions du rift mi continental les sediments a la base de la formation simpson island nouveau nom deposes par un systeme fluvial anastomose enregistrent le transport vers l ouest de debris erodes des unites rocheuses locales archeennes et proterozoiques des basaltes de points chauds recouvrent et sont intercales avec les strates deposees par ce systeme fluvial ces basaltes sont de plus en plus contamines en remontant la section end 1100ma 0 3 to 5 3 les ecoule ments sub aeriens de terres rares legeres enrichies la smn 1 5 3 9 et de terres rares lourdes enrichies fractionnees gd ybn 1 5 3 7 peuvent etre divises en deux unites qui correspondent a d autres sections du groupe d osler vers lst mais de simples correlations avec des sequences plus eloignees sont difficiles la portion de la succession dominee par des roches volcaniques est recouverte d un mince 25 m assemblage conglomerat gres qui represente la prograda tion vers le sud est d un delta alluvionnaire dans un environnement climatique semi aride la lithologie des clastes et la geochimie indiquent qu aucun detritus hors du rift ne provient de l arriere pays du delta diverses preuves a la fois dans les roches sedimentaires et les roches volcaniques soutiennent un scenario selon lequel une subsidence precoce avant 1108 ma le long d un axe nord sud a partir du bras ouest du rift vers la baie nipigon a ete remplacee par de la subsidence le long de l axe est ouest du rift entre 1108 et 1105 ma

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

further refinement to the timing of mesoproterozoic magmatism lake nipigon region ontario the midcontinent rift in the lake superior region of north america is one of the best preserved examples of an aborted precambrian intercontinental rift one that hosts a diverse suite of rock types in addition to the well studied and voluminous rift fill flood basalts although there is a growing database of high precision age information for the main volcanic packages and the largest mafic intrusions there is relatively little information available on the absolute timing of mafic ultramafic intrusions dyke swarms and alkaline complexes especially in the ontario portion of the rift we report new high precision u pb ages for 29 samples primarily collected in the lake nipigon area ontario from these new age results it is now possible to expand the known distribution of geon 15 magmatism in the region confirm an early stage of midcontinent rift mafic magmatism between 1150 and 1130 ma provide evidence that significant mafic ultramafic magmatism occurred in the lake nipigon region slightly earlier

element fluxes associated with subduction related magmatism destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element ree ratios most island arc suites have low ce yb and remarkably restricted isotope ratios of 87 sr 86 sr 0 7033 143 nd 144 nd 0 51302 206 pb 204 pb 18 76 207 pb 204 pb 15 57 and 208 pb 204 pb 38 4 however they also have rb sr 0 03 th u 2 2 and ce yb 8 5 ratios which are significantly less than accepted estimates for the bulk continental crust the high ce yb suites have higher incompatible element contents more restricted heavy ree and much more variable isotope ratios such rocks are found in the aeolian islands grenada indonesia and philippines and their isotope and trace element features have been attributed both to contributions from subducted sediment and or old trace element enriched material in the mantle wedge it is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid ocean ridge basalts since these may contain ca 80 of the water in the subducted slab and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids the isotope data indicate that not more than 15 of the sr and th in an average arc magma were derived from subducted material and that the rest were derived from the mantle wedge the fluxes of elements which cannot be characterized isotopically are more difficult to constrain but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab

an early miocene arc tholeiitic magmatic dike event from the alboran sea evidence for precollisional subduction and back arc crustal extension in the westernmost mediterranean a discussion is made on the general significance of an early miocene 22 23 ma k ar dating basaltic dike swarm that outcrops within large areas of the central and western betic zone of the betic cordilleras southern spain this dike swarm is thought to represent the remains of one of the earliest magmatic episodes within the the neogene volcanic province that is associated with the late orogenic evolution of the alboran region and undoubtedly offers the best preserved igneous material related to this early stage that is known at present the dike rocks hypabyssal equivalents of andesitic basalts and basaltic andesites have both major and trace element abundance patterns that are akin to those of arc tholeiitic suites the field relationships and areal distribution of the dike swarm suggest on the other hand that a major e w trending rifting belt could have been developing within the alboran block in oligo miocene times previous to its collision with adjacent continental borderlands in the early middle miocene

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region