tectonic implications of metamorphism and gravity field in the penokean orogen of northern michigan published geologic data together with new metamorphic pressure and temperature data and gravity data provide strong evidence for northward directed thrusting of lower proterozoic rocks in northern michigan and wisconsin metamorphic studies indicate pressures of 7 kbar at watersmeet node and 4 kbar at the peavy node the republic metamorphic node located 30 km to the north of peavy attained pressures of 2 3 kbar all three metamorphic highs reached temperatures in the range of 600 650 c and are correlated with areas of negative gravity anomalies relative to surrounding regions in the peavy high the center of the gravity anomaly lies on the sillimanite isograd and we suggest that the source of this negative anomaly may be remobilized underlying archean gneiss the highest grade rocks of the republic node lie well within a residual gravity anomaly which is interpreted to have a very shallow source and the watersmeet high lies above gneiss domes which show negative gravity anomalies a north south geologic cross section constructed from a two dimensional gravity model which extends northward from the dunbar dome in the magmatic arc terrane of wisconsin across a terrane boundary interpreted as a suture and marked by the niagara fault and across the peavy metamorphic node suggests that the dunbar dome was transported northward onto the continental foreland during the penokean orogenic event these north verging fold thrust sheets or nappes produced prominent tectonic thickening at the peavy and watersmeet as indicated by the relatively high metamorphic pressures there the thrust sheets evidently thinned to the north as suggested by the relatively low pressures at republic we suggest that the archean continental edge defined by the steep gradient in the regional northeast trending gravity high lies beneath the magmatic rocks of wisconsin and thus extends south of the niagara fault suture

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

evidence for widespread approximately 1760 ma metamorphism and rapid crustal stabilization of the early proterozoic 1870 1820 ma penokean orogen minnesota

geochemical and nd pb isotopic evidence for the provenance of the early proterozoic virginia formation minnesota implications for the tectonic setting of the animikie basin the early proterozoic animikie basin is located at a major crustal boundary between archean crust of the superior craton and penecontemporaneous early proterozoic crust of the wisconsin magmatic terranes the provenance of sedimentary rocks within the animikie basin constrains the tectonic evolution of the early proterozoic margin of the superior craton published geological petrographic and nd pb isotopic data indicate that the pokegama quartzite lower passive margin unit of the animikie group has a late archean provenance in contrast the virginia formation upper deep water turbidite shale unit of the animikie group was derived from dominantly early proterozoic sources with isotopic compositions similar to igneous rocks of the wisconsin magmatic terranes shales of the virginia formation have nd depleted mantle model ages 2 14 to 2 35 ga and mu 1 of 7 81 to 8 09 ashes within the virginia are more evolved geochemically than shales and have younger model ages of 1 86 and 1 99 ga and simila

on the evolution of salt lakes episodic convection beneath an evaporating salt lake 1 the transient evolution of salt lake brines and the accompanying underlying groundwater salinity distribution with depth is discussed a convective diffusive equation with a novel surface source term modeling the evaporative process is introduced and an approximate analytical solution of this equation is obtained using a perturbation approach a linear stability analysis of the resulting solution is carried out for typical field scale parameters the model predicts that instability may initially occur within timescales on the order of weeks or decades and that a near surface layer of salty fluid whose thickness is on the order of decimeters is responsible for the initial onset of convection furthermore the analysis indicates that when convection starts the decimeter layer mixes and this results in the loss of the destabilizing force in this period the salty fluid diffuses downward this results in a return to a hydrologic system where the salt profile is then well mixed throughout but now with a background concentration with a higher average salinity the process can then repeat for an extensive but finite period as the surface layer and subsurface groundwater concentration eventually become hypersaline and where salt precipitation occurs we have called this phenomenon episodic convection variable density groundwater flow and solute transport modeling experiments of a field based salt lake system confirm the existence of episodic convection and semiquantitatively validate the findings of the analytical approach

was the deposition of large precambrian iron formations linked to major marine transgressions geochemical evidence suggests the iron and silica rich precipitates that compose large neoarchean and paleoproterozoic iron formations formed along a chemocline between iron rich deep water and iron poor surface water sedimento logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions we suggest this occurred because the chemocline could only impinge on continental shelves during times of sea level rise or highstand on the other hand the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others to explain this we propose that iron concentrations in the precambrian ocean were greatest at mid water depths in a zone on the order of 100 m thick radiometric age dates from the hamersley basin of western australia suggest major iron formation deposition had a periodicity of ca 20 to 125 m y close to second order cycles in phanerozoic sea level driven by long te

geochemistry and petrogenesis of the early proterozoic hemlock volcanic rocks and the kiernan sills southern lake superior region during the early proterozoic the 2 km thick differentiated gabbroic kiernan sills were emplaced into a thick accumulation of pillow basalt and associated deep water strata the hemlock formation in the southern lake superior region on the basis of major elements and trace elements including rare earth element data the kiernan sills and the hosting volcanic rocks of the hemlock formation were determined to be comagmatic in origin and both evolved from assimilation crystal fractionation processes the major assimilated components in these igneous rocks are identified as terrigenous sedimentary rocks assimilation affected the abundance of nb ta light rare earth elements and most likely p rb th and k in the magma the effect of chemical contamination from wall rock assimilation accumulates with increasing differentiation with wall rock contamination carefully evaluated a series of tectonic discriminating methods utilizing immobile trace elements indicates that the source magma was a high ti t

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

geology and petrology of proterozoic mafic dikes north central minnesota and western ontario proterozoic diabase and gabbro dikes 2120 ma old form a major northwest oriented swarm extending about 300 km from the mesabi iron range in minnesota to the vicinity of kenora ontario the dikes were emplaced into archean crust at about the time that the early proterozoic basin of the lake superior region was opening by rifting and the swarm and the basin may therefore be tectonically related the dikes are overlain unconformably by the animikie group the upper sedimentary sequence in the proterozoic basin in minnesota but may be approximately coeval with mafic volcanic rocks in the pre animikie mille lacs group a two stage tectonic model involving 1 regional right lateral crustal shear in the late archean and 2 hot spot rifting in the early proterozoic is proposed to account for the swarm the dikes are iron rich quartz tholeiites that are differentiated toward dioritic compositions late alteration to hydrous phases including blue green amphibole chlorite and sericite together with lesser

sedimentology of the lower huronian supergroup early proterozoic elliot lake area ontario canada abstract the early proterozoic huronian supergroup is a slightly metamorphosed sedimentary succession locally 10 km thick exposed to the north of lake huron ontario canada the lower part of the supergroup comprising the matinenda mckim ramsay lake and pecors formations was studied in the quirke syncline area elliot lake and regions immediately to the west and south thirteen lithofacies are present trough and planar cross stratified sandstone quartz pebble conglomerate clay rich sandstone lenticular bedded sandstone siltstone mudstone laminated sandstone siltstone mudstone massive sandstone massive pebbly sandstone massive diamictite stratified matrix supported diamictite stratified clast supported diamictite graded diamictite and massive siltstone the lithofacies may be grouped into associations which correspond approximately to the mapped formations the crossbedded sandstones and conglomerates comprise a fluvial association matinenda formation which was formed mainly in shallow braided channels clay rich sandstone and the lenticular and laminated lithofacies constituting portions of the mckim and pecors formations were deposited on the strand and offshore areas of braid deltas interfingering laterally and vertically with the matinenda fluvial deposits four glacigenic subassociations are present in the ramsay lake formation an ice proximal subassociation i consists of massive pebbly sandstone and diamictite which progressively truncates the matinenda and mckim formations to the north the massive sandstone may be an ice override deposit formed by erosion and reworking of the underlying matinenda formation or a sub ice shelf rain out accumulation of locally eroded glacial debris subassociations ii and iii consist mainly of subaqueous sediment gravity flow and ice rain out deposits subassociation iv is interpreted as an ice proximal fluvial outwash deposit containing resedimented blocks of moraine material stratigraphic and paleocurrent data indicate a south dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units a trough of relatively deep water extended nw se across the region transverse to the e w divergent continental margin hinge line that is assumed to have existed to the south of the project area analogies are suggested to mesozoic recent basins on the atlantic margin of canada where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones or reactivation of earlier structural lineaments

early proterozoic volcanogenic massive sulfide deposits in wisconsin an overview the early proterozoic greenstone belt of northern wisconsin possesses some of the best volcanogenie volcanic hosted massive sulfide potential in north america a 100 millionton resource of base and precious metal bearing mineralization distributed in 13 or more deposits and occurrences and clustered in three districts has been identified in the belt host rocks for the volcanogenie massive sulfide mineralization are part of the 144 mi long east west trending ladysmith rhinelander metavolcanic complex which consists of various greenschists amphibolites cherty iron formations and sericite to quartz sericite schists these 1880 to 1860 ma metamorphic rocks are concealed beneath pleistocene glacial cover development of the flambeau deposit initiation of mine permitting for the lynne deposit and reactivation of the crandon project indicate that the belt will receive a higher level of activity than in the past geologic and geophysical data compiled since the late 1960s define three depositional environments each containing volcanogenie massive sulfide mineralization in the 1880 to 1860 ma ladysmith rhinelander metavolcanic complex 1 a main volcanic arc sequence the structural core of the complex 2 a laterally equivalent and or younger back are basin volcanic volcaniclastic succession that includes a series of marie volcanic piles and 3 major felsic volcanic centers in the back arc basin and along the flanks of the main volcanic arc volcanogenie massive sulfide mineralization in all three depositional environments includes 1 syngenetic and epigenetic strata bound to stratiform massive sulfide mineralization within along the flanks of or near the top of the felsic volcanic centers 2 syngenetic strata bound to stratiform massive sulfide mineralization associated with cherty magnetic iron formation within the main volcanic arc sequence and 3 epigenetic stringer sulfide mineralization and syngenetic stratiform massive sulfide mineralization associated with marie volcanic piles developed within the back arc basin identified volcanogenie massive sulfide deposits and occurrences are classified by metal content into three groups cu zn cu and zn pb cu each group exhibits various styles of mineralization which include sheets mounds stacked lenses and replacements potentially economic deposits are associated with felsic volcanic centers and sulfide bearing meta argillite formations that are favorable stratigraphic units deposited before after or during the ore forming event s stratigraphic correlations supported by lead isotope data suggest that most volcanogenie massive sulfide deposits in the greenschist succession formed in a narrow time interval

paleomagnetism of the paleoproterozoic gowganda and lorrain formations ontario low paleolatitude for huronian glaciation abstract to further our understanding of the paleolatitudes of precambrian glaciations a paleomagnetic study has been conducted on the glaciomarine coleman member of the gowganda formation and the conformably overlying deltaic firstbrook member and lorrain formation huronian supergroup 2 4 2 3 ga in ontario canada many of the rocks notably the gray and grayish red argillic facies display unstable magnetization or overprints carried by magnetite and or hematite that give negative fold tests some red sandstones however have stable high temperature 690 c components marked by very shallow northerly or southwesterly directions five sites in pale red sandstone from the coleman member near elliot lake give a dip corrected a direction of d 216 3 i 5 5 95 6 7 n 30 and 17 sites in lorrain red sandstone near desbarats give a dip corrected a direction of d 5 4 i 5 5 95 5 9 n 75 unfavorable structural attitudes rendered fold tests inconclusive these very shallow directions may be ascribed to early chemical remanent magnetization crm because a some specimens display polarity reversals b such directions are unknown either as original or as overprint components in post huronian rocks of the region and c a prior site in coleman red beds gave a high temperature shallow nnw direction that is not significantly different from lorrain a and which a positive conglomerate test suggested is not an overprint the very shallow directions are near primary directions for the slightly older 2 45 ga matachewan dyke swarm in the region lorrain argillic rocks at five sites give a normal component b t 670 c d 18 6 i 59 6 95 6 1 n 16 dip corrected and a reverse component e t 500 c d 190 9 i 60 9 95 7 3 n 14 in situ which are close to a prefolding direction carried by magnetite previously identified in coleman gray argillites morris a however a primary origin for morris a is dubious because of the widespread occurrence of overprints carried by magnetite in coleman gray argillites and because the fold test employed folds formed during the penokean orogeny at 1900 1800 ma our results suggest that morris a lorrain b and lorrain e may data from immediately before and just after penokean folding respectively hence coleman sandstone a and lorrain a may be better estimates of original directions those components and matachewan paleomagnetic data constrain huronian paleolatitudes and suggest that huronian glaciation occurred within 11 and possibly within 4 of the paleoequator

evidence for multiple deformation during the penokean orogeny in the middle precambrian thomson formation minnesota the penokean orogeny a major event that occurred at 1850 1900 ma in the great lakes region involved deformation metamorphism and igneous activity in minnesota the effects of the penokean defo

properties and behaviour of hong kong marine deposits with different clay contents large scale marine reclamation has taken place on soft hong kong marine deposits hkmd more and more reclamation is to be undertaken due to the rapid development of infrastructures and housing pr

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

the chocolay group lake superior region u s a sedimentologic evidence for deposition in basinal and platform settings on an early proterozoic craton the early proterozoic chocolay group in the lake superior region is a cratonal sequence consisting of a basal unit overlain successively by quartzite dolomite and slate formations two regional depositional environments are described 1 present day structural troughs marquette menominee and felch michigan show evidence of having sedimentary basin precursors and 2 other areas gogebic range michigan show evidence of sedimentation in platform settings the basal unit in the marquette and menominee troughs is as much as 160 m thick and was probably deposited by alluvial processes in the gogebic range the basal unit is less than 10 m thick and resembles lag deposits at the base of transgressive sequences the overlying quartzite contains cross beds that show two regional sedimentary vector paleocurrent distributions 1 unimodal low variance distributions with sedimentary vectors oriented subparallel to local structural trough axes marquette menominee and felch troughs and 2 polymodal distributions with sedimentary vectors showing greater dispersion about the mean gogebic range as indicated by many varieties of sedimentary structures silicified evaporites cryptalgal structures intraclast breccias and so on the carbonates and interstratified siliciclastics of the chocolay group were deposited in subtidal to supratidal conditions taylor noted in 1972 that certain members of the chocolay group carbonates thicken toward the center of the marquette trough with concomitant facies changes the uppermost pelite unit of the chocolay group was probably deposited below normal wave base it is concluded that the basins and platforms were formed on a craton undergoing rift type tectonism

evidence for nappe development during the early proterozoic penokean orogeny minnesota two major phases of early proterozoic penokean folding are now recognized in the thomson formation of east central minnesota effects of the first deformation are found only in the southern two thirds of the exposed thomson formation the second deformation affected the entire region of outcrop several lines of evidence indicate that northward directed nappes developed during the early phase of folding this evidence includes lithologic differences between the areas of one and two deformations the pervasive nature of an s 1 foliation in the area of its occurrence facing directions of f 2 folds and the refraction pattern of the s 1 foliation in graded metagraywacke slate beds taken together the data show that an area of several hundred square kilometres is on the upper limb or limbs of a large northward directed recumbent fold or folds nappes of the early phase of penokean deformation

the reappearance of the huronian in wyoming rifting and drifting of ancient continents striking stratigraphic and sedimentological similarities between the early proterozoic huronian supergroup of the canadian shield and the snowy pass supergroup of wyoming suggest that they were deposited in a single broad epicratonic basin developed atop a large archean continent that included the superior and wyoming geological provinces breakup of the continent after the 2 2 ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the archean superior and wyoming cratons and their early proterozoic covers these crustal fragments were subsequently reassembled during early proterozoic 1 85 ga orogenesis the end result being the present 2000 km separation of the huronian and snowy pass supergroups and their archean basements

two early proterozoic successions in central wisconsin and their tectonic significance geological mapping of 7 500 km 2 of previously virtually unmapped precambrian rocks in central wisconsin suggests the presence of two early proterozoic sedimentary volcanic plutonic successions evidence for the two successions includes structural stratigraphic metamorphic and lithologic relations that are consistent throughout the area the older succession unconformably overlies archean rocks it is characterized by amphibolite facies metamorphism folding about west to northwest plunging isoclinal f 1 fold axes with more open coaxial f 2 folding basal quartzite and pelitic sediments of the older succession are overlain by volcanic rocks and intruded by syntectonic tonalite granodiorite and trondhjemite plutons the younger succession consists primarily of a greenschist facies calc alkaline basalt rhyolite sequence that unconformably overlies the older amphibolite facies rocks structures in the greenschist sequence trend northeasterly with fold axes that plunge easterly the plutonic rocks are dominantly syntectonic to late tectonic granites and quartz monzonites with only minor tonalite in most areas the two successions have been juxtaposed along major faults the older amphibolitic sequence may correlate with the chocolay group farther north in the lake superior region and the younger green schist rocks with the menominee and or baraga groups if this correlation is correct the deformation metamorphism and intrusion of the green schist succession would have occurred during the penokean orogeny metamorphism to amphibole rank isoclinal folding and tonalite intrusion of the older succession would be a major pre penokean erogenic event within the early proterozoic that may correlate with the post chocolay pre menominee break in northern michigan wisconsin and minnesota we suggest that this earlier event has been unrecognized and that it may be a significant part of the early proterozoic history of the lake superior region

book review iron formation facts and problems developments in precambrian geology 6 edited by a f trendall and r c morris elsevier sciences publishers b v amsterdam oxford new york tokyo 1983 xiv 588 pp price us 106 00 u s a and canada dfl 275 00 rest of world isbn 0 444 42144 0 hardback

paleomagnetism structure and longitudinal correlation of middle precambrian dykes from northwestern ontario and minnesota a primary magnetization about 2 1 ga old is well preserved in the kenora kabetogama dyke swarm in minnesota and ontario its direction the mean for 12 dykes incorporating results from 30 paleomagnetic sites is 115 9 55 6 yielding a paleomagnetic pole position of latitude 42 7 s longitude 4 2 e dm 7 3 dp 5 2 the primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km whereas the remanence direction varies between dykes it remains constant along a dyke sufficient proof that it was formed during initial cooling of the intrusion a weak longitudinal increase in hydrous alteration mgo content and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a precambrian continental margin that underwent rifting sedimentation and deformation during the time interval 2 2 1 6 ga bp although the cause of the longitudinal changes in terms of ma