spinels and mg ilmenites from the noril sk 1 and talnakh intrusions and other mafic rocks of the siberian flood basalt province spinels from the mineralized noril sk 1 and talnakh intrusions show a broad and distinctive compositional spectrum from normal chromite to ti enriched aluminous magnetite in the talnakh intrusion in particular the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase spinels enclosed in olivines are less ti enriched than those outside olivine grains compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid the advanced nature of the process is attributed to the open nature of the intrusions continuous prolonged flow through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness the barren chibacheka river intrusion shows some of the same features but differs from talnakh in that spinel olivine pairs record equilibration temperatures higher than those at talnakh temperatures recorded by spinel olivine pairs at talnakh are anomalously low for such a small intrusion again indicating prolonged slow cooling consistent with a flow through model for the sill spinel compositions support a hypothesized comagmatic relationship between the chibacheka river intrusion and the ti cr rich gudchichinsky picritic basalts spinels in the noril sk and talnakh intrusions imply a parent magma with a lower cr al ratio than the gudchichinsky picritic basalts spinels from the siberian intrusions show a similar range in compositions to the feeder intrusions of the karoo flood basalt province jinchuan spinels share the features of high ti contents and anomalously low equilibration temperatures with the noril sk talnakh spinels mg ilmenites are a feature of the siberian and jinchuan intrusions they are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding mg rich cumulus phases their existence implies moderately but not unusually ti rich parent magmas consistent with compositions of the siberian traps lavas

generation and evolution of siliceous high magnesium basaltic magmas in the formation of the permian huangshandong intrusion xinjiang nw china abstract numerous permian orthopyroxene rich mafic ultramafic intrusions in the southern margin of the central asian orogenic belt caob have been emplaced in a post orogenic environment the huangshandong mafic ultramafic intrusion is the largest in the eastern tianshan orogenic belt of the caob and consists of a layered unit intruded by a massive unit the layered unit consists of lherzolite troctolite olivine gabbro hornblende gabbro gabbrodiorite and diorite whereas the massive unit is made of gabbronorite and olivine gabbronorite olivine from the layered unit has fo values of 75 to 84 and contains low nio and mno rocks from this unit have initial 87sr 86sr ratios 0 7031 0 7043 and end values 3 4 7 1 while plagioclase has initial 87sr 86sr ratios ranging from 0 70010 to 0 71569 olivine from the massive unit has relatively low fo values of 59 to 70 and high nio and mno rocks from this unit have relatively low and uniform initial 87sr 86sr ratios 0 7030 and high end values 6 0 7 5 and contain plagioclase with relatively low initial 87sr 86sr ratios 0 70042 to 0 70520 olivines from both units have low cao and cr2o3 both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous depleted mantle source magma forming the layered unit experienced earlier removal of sulfides before emplacement with the earlier sulfide saturation caused by the addition of crustal material the magma that formed the massive unit has undergone fractional crystallization of olivine cr spinel siliceous high magnesium basaltic magmatism produced by melting of a hydrous depleted mantle source may be an important feature of orogenic belts

sims zircon u pb geochronology and sr nd isotopes of ni cu bearing mafic ultramafic intrusions in eastern tianshan and beishan in correlation with flood basalts in tarim basin nw china constraints on a ca 280 ma mantle plume zircon sims u pb dating of the poshi hongshishan bijiashan and huangshan ni cu bearing and xiangshan ni cu ti fe bearing mafic ultramafic intrusions in the eastern tianshan and beishan rift yields a relatively restricted range of 278 6 ma to 284 0 ma the histogram of compiled age data of basalts in the tarim basin and mafic ultramafic intrusions in the eastern tianshan and beishan rift has a peak of 280 ma which probably represents the time of mantle plume activity the basalts have lower end t values in the range of 9 2 1 7 and mg of 2 wt indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting the mafic ultramafic intrusions have higher end t of 1 3 11 2 and mg of 33 90 and lower tio2

modification of continental lithosphere by tectonic processes a tomographic image of central north america the assembly of laurentia by precambrian accretion is also believed to have formed the underlying lithosphere accretionary signatures are detectable by seismic observations but subject to modification by later processes e g orogeny rifting and plumes we examine the archean superior province sp and environs using a set of teleseismic p wave arrivals from canadian and american instruments the resulting tomographic model has high resolution beneath the dakotas and minnesota provides a first look at the lithosphere beneath manitoba and sharpens previously documented features in ontario from the model and previous anisotropy observations we detect i a large high velocity feature beneath the western sp associated with elevated lithospheric anisotropy the high velocity feature does not match crustal boundaries notably its western edge lies approximately 200 km east of the contact with the proterozoic trans hudson orogen tho ii a low velocity channel shaped feature strikes northwest through minnesota and the dakotas associated with weakening anisotropy iii high velocities southwest of ii beneath the minnesota river valley terrane mrvt associated with low anisotropy we interpret i to be accretionary and contemporaneous with superior assembly similar velocity but weaker anisotropy of the mrvt is consistent with vertical tectonic mechanisms the inboard location of the tho contact may indicate modification of the superior root the low velocity channel has no obvious crustal expression but connects to an offset in the proterozoic mid continent rift mcr and may be rift related the mcr is not well imaged but will be examined via the temporary superior province rifting earthscope experiment in progress

precambrian basement geology of the permian basin region of west texas and eastern new mexico a geophysical perspective because most of the permian basin region of west texas and southern new mexico is covered by phanerozoic rocks other means must be found to examine the precambrian upper crustal geology of the region we have combined geologic information on the precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region geophysical anomalies can be related to six precambrian events formation of the early proterozoic outer tectonic belt igneous activity in the southern granite rhyolite province an episode of pre grenville extension the grenville orogeny rifting to form the delaware aulacogen and eocambrian rifting to form the early paleozoic continental mar in two geophysical features were studied in detail the abilene gravity minimum and the central basin platform gravity high the abilene gravity minimum is shown to extend from the delaware basin across north central texas and is interpreted to be caused by a granitic batholith similar in size to the sierra nevada batholith in california and nevada this batholith appears to be related to formation of the southern granite rhyolite province possibly as a continental margin arc batholith because of this interpretation we have located the grenville tectonic front southward from its commonly quoted position closer to the llano uplift middle proterozoic mafic intrusions are found to core the central basin platform and the roosevelt uplift these intrusions formed at about 1 1 ga and are related in time to both the mid continent rift system and the grenville orogeny in texas because these features are likely to be rift related they suggest that the concept of a delaware aulacogen needs to be revised only to the extent that the rifting is proterozoic in age not eocambrian precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying permian basin and thus have potential exploration significance interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre ellenburger basins which may be extensive and of potential exploration interest

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

the mesoproterozoic copperwood sedimentary rock hosted stratiform copper deposit upper peninsula michigan copperwood is located in gogebic county upper peninsula michigan within the porcupine mountains copper district it was discovered in 1956 and in 2008 orvana minerals corp began exploration and development work with production projected in the near future stratiform copper mineralization is hosted by gray to black shales and siltstones of the basal nonesuch formation copper mineralization is overlain by laminated gray siltstones of the nonesuch formation and is underlain by red bed sandstones conglomerates and minor siltstones of the copper harbor formation these rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the mesoproterozoic 1 1 1 0 ga midcontinent rift the copper deposit is a tabular body that averages 2 5 m thick lacks significant basal undulations and dips 8 to 12 northward on the southwest limb of the western syncline an open fold with a shallow plunge to the northwest chalcocite is the only cu bearing mineral in the orebody and occurs as concentrations along stratigraphic laminations and as disseminated grains within the orebody there is no mineral or metal zoning cu ag and s are strongly enriched zn is very slightly enriched and pb and co lack enrichment gangue minerals in the orebody are quartz clinochore muscovite illite plagioclase k feldspar calcite and hematite which are consistent with temperatures of less than 130 c only one significant fault a shallow nw dipping reverse fault with displacement of up to 7 m has disturbed the section the mineralized zone at copperwood is stratigraphically equivalent to that at the white pine mine but copperwood lacks the structural complexity and the hydrothermal overprint manifested by the native cu mineralization at white pine reported canadian national instrument 43 101 compliant measured indicated and inferred resources are 33 million metric tonnes mt of 1 60 cu and 4 1 ppm ag for total contained cu of about 1 165 billion lb and 4 32 moz of ag the characteristics of copperwood are consistent with those of reduced facies or kupferschiefer type sedimentary rock hosted stratiform copper deposits the proposed genetic model is chalcocite replacement of pyrite in unlithified sediments during diagenesis as a result of emanating upward focused compaction driven cu bearing saline basinal waters with cu leached from the underlying red bed paleoquifer the simplicity of the mineralizing event and the undeformed nature of the deposit distinguish copperwood from most other examples of reduced facies or kupferschiefer type sedimentary rock hosted stratiform copper deposits throughout the world

laser ablation icpms study of trace element partitioning between plagioclase and basaltic melts an experimental approach plagioclase melt partition coefficients d for 34 trace elements at natural concentration levels were determined experimentally in a natural morb composition at atmospheric pressure using thin pt wire loops experiments were carried out at three tempera tures 1 220 1 200 and 1 180 c and at three different oxygen fugacities fo2 iw qfm air in order to assess the effect of fo2 on the partitioning of elements with multiple valence fe eu cr run products were analyzed by laser ablation icp ms most trace element ds increase slightly as temperature decreases except for dzr dfe deu and dcr that vary systematically with fo2 applying the lattice strain model to our data suggests the presence of fe 2 entirely in the octahedral

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

ruthenium in chromite as indicator for magmatic sulfide liquid equilibration in mafic ultramafic systems abstract the platinum group element ruthenium ru is an important petrogenetic tracer of earth s accretion history core mantle interaction mantle evolution and the exploration for magmatic sulfide deposits however its geochemical behavior in mafic ultramafic systems is still not fully understood which limits its usefulness in the predictive modelling of geochemical systems to further develop the use of ru as a petrogenetic tracer we analyzed the ru contents of chromites from a global sample set of komatiites komatiitic basalts and ferropicrites by laser ablation icp ms and carius tube isotope dilution icp ms analysis the ru data are combined with full major and minor element microprobe analyses the data show that two groups of chromite can be distinguished on the basis of their ru contents this bimodal distribution occurs across komatiites komatiitic basalts and ferropicrites and corresponds to the sulfide saturation state of the magma during chromite crystallization chromites from sulfide undersaturated magmas contain between 150 and 600 ppb ru whereas chromites that crystallized from sulfide bearing magmas mostly contain less than 150 ppb ru the ru contents are independent of elements that typically document a modification of chromite suggesting that the determined ru concentrations reflect the primary magmatic chromite composition the ru contents are furthermore independent of magma type i e komatiites komatiitic basalts ferropicrites the magma source regions i e different cratons belt and localities the geochemical affinity i e munro type and karasjok type and age i e 2 7 ga and 2 0 ga and neither local fluctuations in fo2 nor emplacement styles i e intrusive vs extrusive can account for the bimodal ru distribution in chromite observed during this study as a consequence it is argued that the ru contents of chromites from mafic and ultramafic systems are indicative of the presence or absence of a sulfide liquid during chromite crystallization in sulfide saturated systems the chalcophile ru will dominantly partition into sulfides whereas in the absence of sulfides ru preferentially partitions into chromite over all other available phases because i high ru contents in chromites are exclusively associated with sulfide undersaturated systems and ii the ru contents of chromites can be measured via fast and cost effective laser ablation icp ms ru variability patterns in chromites allow the identification of magmas that have equilibrated with magmatic sulfide liquids prior to or during chromite crystallization and hence have potential to host metal sulfide orebodies

ruthenium variation in chromite from komatiites and komatiitic basalts a potential mineralogical indicator for nickel sulfide mineralization more than 390 chromite grains from komatiites and komatiitic basalts from the yilgarn craton of western australia and the finnish part of the fennoscandian shield were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry la icp ms to identify ruthenium ru signatures in chromite associated with nickel sulfide bearing rocks results indicate a potential method to discriminate mineralized and barren komatiite and komatiitic basalt units based on ru concentrations in chromite and indicate potential for chromite to be used as a resistate indicator mineral in exploration for komatiite associated nickel sulfide deposits chromites from barren komatiites and komatiitic basalts display ru concentrations mostly between 150 and 600 ppb chromites from mineralized units have distinctly lower ru contents 150 ppb these results can be interpreted in terms of the much higher partition coefficient for ru into sulfide liquid compared to that of ru into chromite resulting in much lower ru concentrations in chromite where both chromite and sulfide liquid are present and competing for ru as a result the ru content of chromite can be used to determine if a komatiite melt equilibrated with a sulfide liquid during crystallization and therefore if a system and or sequence is prospective for nickel sulfide mineralization the strength of this method compared to previous whole rock exploration techniques derives from combining 1 the geochemical properties of a chalcophile element that records an ore forming process while being strongly immobile during postmagmatic processes with 2 the in situ analysis of a mineral that is generally preserved even in highly altered and mildly weathered komatiites and that is a common constituent of detrital heavy mineral samples chromite ru content has potential as a prospectivity indicator applicable to a wide range of media including bedrock laterites and detrital resistates heavy mineral samples

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

genesis of native copper lodes in the keweenaw district northern michigan a hybrid evolved meteoric and metamophogenic model the keweenaw peninsula of northern michigan is well known for early diagenetic sulfide dominant sediment hosted stratiform copper mineralization hosted mainly by basal siltstone shale units of the nonesuch formation in the white pine district and later native copper lodes hosted mainly by flow top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower portage lake volcanics the native copper mineralization is generally attributed to downdip burial metamorphism of the host strata an alternative model is proposed here involving a complementary influx of evolved meteoric water elevated in salinity and progressively equilibrating with its aquifers similar to the fluid which formed the white pine mineralization this model has several advantages over the metamorphogenic model alone it would provide the salinity by dissolution of evaporites and moderately oxidizing conditions as a result of hematitization i e reddening of coarse continental rift sediments necessary for copper transport and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite and chlorite dominant i e reduced hydrothermal mineral assemblages at the same time this influx of meteoric water from highlands adjacent to the keweenaw rift basin would have provided a mechanism to move the deep seated hot dense ore brine upward along the aquifers of the portage lake volcanics on its ascent from deep portions of the rift basin the gravity driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric metamorphogenic oreforming hydrothermal brine

post rift deformation of the midcontinent rift under grenville tectonism abstract using a thin viscous sheet model we investigate the hypothesis that compressive stresses resulting from the grenville orogeny caused uplift along the midcontinent rift the objective of this study is to investigate factors involved in shaping the post rift structure of the midcontinent rift including the size and location of the colliding terranes and the convergence angle between the colliding terranes and the north american craton we find a linear relationship between the size of the terrane and the deformation within the midcontinent rift the calculations predict comparable deformation along both the eastern and western arms of the midcontinent rift our results demonstrate that the geologically indicated ne sw deformation can be produced without a multistage compression event because of the nonlinear interaction of far field compressive stress and the mechanically weak rift therefore it is not possible to determine which compression direction is representative of the compression associated with the grenvillian orogenic event based on the post rift pattern of deformation

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

the santa rita nickel sulfide deposit in the fazenda mirabela intrusion bahia brazil geology sulfide geochemistry and genesis the santa rita ore zone within the fazenda mirabela intrusion is an unusual example of high tonnage magmatic ni cu pge sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion the intrusion as a whole is about 4 2 5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat shaped morphology the ore zone comprises a 50 to 200 m thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites this orthopyroxene dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half sulfide tenor variations and silicate host rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores consistent patterns of sulfide tenor gradually decreasing ni from over 20 to about 10 wt percent near constant cu and gradually decreasing pges with cyclically varying pt pd ratios are found within the santa rita ore zone across the entire intrusion and display relatively little lateral variability the position of the sulfide mineralization envelope within the harzburgite orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene bearing cumulates asymmetrically disposed toward the southern margin of the intrusion nickel tenors of the sulfides range from 15 to 25 wt percent exceptionally high for rocks with relatively low values of mgo mgo feo and are closer to the typical values observed in komatiite hosted deposits although cu tenors typically 5 8 wt are much higher than komatiite values sulfide pge tenors range from less than 100 ppb to greater than 10 ppm of both pt and pd with maximum values in s poor rocks at the base of the sulfide bearing interval pges become significantly more depleted toward the top of the santa rita zone we attribute these features along with the localization of the sulfide bearing interval at the contact between ultramafic and mafic cumulates to mixing of an initially s undersaturated moderately ni enriched resident magma with a relatively low ni pge depleted significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets

sr nd os s isotope and pge geochemistry of the xiarihamu magmatic sulfide deposit in the qinghai tibet plateau china the newly discovered xiarihamu ni cu deposit is located in the eastern kunlun orogenic belt in the northern part of the qinghai tibet plateau western china it is the largest magmatic ni cu sulfide deposit found thus far in an arc setting worldwide and ranks second in china in terms of total ni resources fe ni cu sulfide mineralization occurs in a small ultramafic body that is part of a larger mafic ultramafic complex formed by protracted silurian early devonian basaltic magmatism the mineralized ultramafic body is composed predominantly of lherzolite and olivine websterite with minor dunite websterite and orthopyroxenite here we report new pge platinum group element data and the results of a new integrated sr nd os s isotope study the initial concentrations of rh and pd in the parental magma are estimated to be 0 014 ppb and 0 24 ppb respectively which are more than one order of magnitude lower than those in undepleted mantle derived magmas such as many continental picrites the observed pge depletions in the xiarihamu parental magma are attributed to sulfide retention in the source mantle because the degree of partial melting required to generate the xiarihamu primary magma was not high enough for a magma of that composition to dissolve all sulfides in the source the 87sr 86sr i ratios and end t of the xiarihamu host rocks range from 0 7062 to 0 7105 and from 1 97 to 5 74 respectively indicating 5 30 wt crustal contamination in the xiarihamu magma these data also reveal that the source mantle for the xiarihamu magma is isotopically sr nd more enriched than that for the average cenozoic arc basalt the os t and 34s values of sulfide ores from the xiarihamu deposit range from 78 to 1393 and from 2 to 6 respectively these values clearly indicate addition of crustal os and s to the xiarihamu parental magma metal tenors such as ni and rh are inversely correlated with os t and 34s values this indicates that mixing between different pulses of magma with different isotope compositions took place during magma emplacement at xiarihamu and that external s played a critical role in the formation of the xiarihamu magmatic ni cu sulfide deposit

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

chromite alteration processes within vourinos ophiolite the renewed interest in chromite ore deposits is directly related to the increase in cr price ruled by international market trends chromite an accessory mineral in peridotites is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel bearing rock burkhard in geochim cosmochim acta 57 1297 1306 1993 however the understanding of chromite alteration and metamorphic modification is still controversial e g evans and frost in geochim cosmochim acta 39 959 972 1975 burkhard in geochim cosmochim acta 57 1297 1306 1993 oze et al in am j sci 304 67 101 2004 metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite in this study we investigate the vourinos complex chromitites from the mines of rizo aetoraches xerolivado and potamia with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation the present study has been partially funded by the aliakmon project in collaboration between the public power corporation of greece and institute of geology and mineral exploration of kozani

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

the permian huangshanxi cu ni deposit in western china intrusive extrusive association ore genesis and exploration implications the permian huangshanxi cu ni deposit is the second largest magmatic sulfide deposit discovered to date in a major ni cu province related to protracted basaltic magmatism in eastern xinjiang china it is hosted by a small mafic ultramafic intrusion comprised predominantly of lherzolites olivine websterites gabbronorites and gabbros the huangshanxi intrusion is coeval with permian basalts of tholeiitic and alkaline affinities in the tuha and tarim basins respectively to evaluate a possible genetic relationship between the huangshanxi intrusion and a specific type of coeval basalt in the region as well as ore genesis in the intrusion we have carried out an integrated mineralogical petrological and geochemical study our data reveal that the huangshanxi intrusive rocks are characterized by relatively flat chondrite normalized ree patterns depletion in nb and ta and elevated end values varying between 6 and 10 these features are similar to those of coeval tholeiitic basalts in the nearby tuha basin but are significantly different from those of coeval alkaline basalts in the relatively remote tarim basin the geochemical similarities and differences suggest that the huangshanxi intrusion is genetically related to the tholeiitic basalts in the tuha basin not to the alkaline basalts in the tarim basin as suggested previously by some researchers this implies that regional exploration for the huangshanxi type cu ni deposits should be centered in the tuha basin instead of the tarim basin more specifically the uplifted areas around the tuha basin where similar intrusions may have been brought close to the surface should be carefully examined for mineralization potential intrusive relations and mass balance constraints from incompatible trace elements and sulfide abundances suggest that the huangshanxi intrusion represents a dynamic magma conduit through which multiple pulses of magma ascended to higher levels or to the surface numerical simulation of magma evolution and mixing calculations using sr nd isotopes indicate that selective assimilation of s bearing crustal materials is important for sulfide saturation during the early stages of magma evolution when lherzolites formed fractional crystallization may have also played a role in the attainment of sulfide saturation during the later stages of magma evolution when olivine websterites and gabbronorites formed in both cases immiscible sulfide droplets were retained in the conduit to form disseminated sulfide lenses while the fractionated silicate liquids and buoyant phases such as plagioclase continued to ascend extremely low pge tenors in the sulfide ores of the huangshanxi deposit suggest that the parental magma was highly depleted in chalcophile elements possibly due to previous sulfide segregation at depth

segregation of magmatic fluids and their potential in the mobilization of platinum group elements in the south kawishiwi intrusion duluth complex minnesota evidence from petrography apatite geochemistry and coexisting fluid and melt inclusions abstract pegmatitic and other felsic rock pockets and dike like intrusions are abundant in the south kawishiwi intrusion of the duluth complex including the basal cu ni pge mineralized units these occurrences are found as pockets pods or as veins and contain abundant accessory apatite and quartz quartz hosts primary fluid inclusions as well as silicate melt inclusions combined microthermometry and raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are co 2 rich 95 mol and contain small amounts of h 2 o 4 5 mol ch 4 0 4 mol and trace n 2 respectively this combined technique also made it possible to measure total homogenization temperatures of the inclusions th tot 225 10 c otherwise not detectable during microthermometry silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt composition of the entrapped melt is granitoid peraluminous and is very poor in mafic components we interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the south kawishiwi intrusion the presence of co 2 in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval the composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the south kawishiwi intrusion and the fluid and melt phases coexisted as a heterogeneous melt fluid system until entrapment of the inclusions coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure 1 7 kbar and thus estimate formation depth 5 8 km for the inclusions chlorine is suggested to behave compatibly in the silicate melt phase in the fluid melt system represented by the inclusions indicated by the high up to 0 3 cl concentrations of the silicate melt and co 2 rich nature of the fluid apatite halogen contents provide further details on the behavior of cl apatite in pegmatitic pockets often has elevated cl concentrations compared to troctolitic rocks suggesting enrichment of cl with progressive crystallization systematic trends of cl loss at some differentiated melt pockets suggest that in some places cl exsolved into a fluid phase and migrated away from its source the segregation of cl from the melt is probably inhibited by the presence of co 2 rich fluids until the last stages of crystallization increasing the potential for the development of late stage saline brines platinum group minerals are often present in microcracks in silicate minerals in late stage differentiated sulfide veinlets and in association with chlorapatite indicating the potential role of cl bearing fluids in the final distribution of pges

zoned orthopyroxenes in the ni co sulfide ore bearing xiarihamu mafic ultramafic intrusion in northern tibetan plateau china implications for multiple magma replenishments abstract the xiarihamu mafic ultramafic intrusion is situated in the east kunlun orogenic belt e kob northern tibetan plateau china with ni co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite whole rock and mineral compositions show cyclic variations in drill hole intersections through the xiarihamu mafic ultramafic intrusion microbeam x ray fluorescence xrf mapping shows reverse zoning cr poor core cr rich rim and oscillatory zoning cr poor core cr rich mantle and cr poor rim in orthopyroxene of olivine orthopyroxenite and orthopyroxenite zoned orthopyroxenes are euhedral or subhedral and there are relatively sharp interfaces between zones with distinct cr contents crystal size distribution csd curves of orthopyroxenes in olivine orthopyroxenite show concave upward curvature at size larger than 3 0 mm and larger orthopyroxenes commonly display zoning implying a secondary recrystallization and grain growth of larger orthopyroxenes quantitative analyses show that cr and enstatite contents of cores are relatively lower but incompatible element e g ti zr ga and sr contents of cores are higher than rims in reverse zoned orthopyroxenes and chemical profiles of unzoned orthopyroxenes resemble those of rims but different from those of cores in reverse zoned orthopyroxenes chemical profiles of orthopyroxenes indicate that cr zoning is not due to the change in cr partitioning behavior between orthopyroxene and silicate melt which shows significant temperature and redox dependence textural feature and crystal size distribution of orthopyroxenes suggest the formation of cr rich zones attests to the multi stage growth in the process of magma replenishment rather than a reactive liquid flow model or in situ trapped liquid overgrowth orthopyroxene zoning together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the xiarihamu mafic ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition

comment on paleozoic reactivation structures in the appalachian ouachita marathon foreland far field deformation across pangea by j craddock d malone r porter j compton j luczaj a konstantinou j day and s johnston earth science reviews 169 2017 1 34

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

the susceptibility of oklahoma s basement to seismic reactivation recent widespread seismicity in oklahoma is attributed to the reactivation of pre existing critically stressed and seismically unstable faults due to decades of wastewater injection however the structure and properties of the reactivated faults remain concealed by the sedimentary cover here we explore the major ingredients needed to induce earthquakes in oklahoma by characterizing basement faults in the field in seismic surveys and via rock mechanics experiments outcrop and satellite mapping reveal widespread fault and fracture systems with trends that display a marked similarity to the trends of recent earthquake lineaments our three dimensional seismic analyses show steeply dipping basement rooted faults that penetrate the overlying sedimentary sequences representing pathways for wastewater migration experimental stability analysis indicates that oklahoma s basement rocks become seismically unstable at conditions relevant to the dominant hypocentral depths of the recent earthquakes these analyses demonstrate that the geometry structure and mechanical stability of oklahoma s basement make it critically susceptible to seismic reactivation seismicity induced by wastewater injections is widespread in oklahoma probably because its basement is susceptible to the reactivation of basement rooted faults according to three dimensional seismic analyses rock mechanics experiments and field surveys

roles of xenomelts xenoliths xenocrysts xenovolatiles residues and skarns in the genesis transport and localization of magmatic fe ni cu pge sulfides and chromite abstract most sulfide rich magmatic ni cu pge deposits form in dynamic magmatic systems by partial melting s bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components and generation of barren to weakly mineralized immiscible fe sulfide xenomelts into which ni cu co pge partition from the magma some exceptionally thick magmatic cr deposits may form by partial melting oxide bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components and generation of barren fe ti oxide xenocrysts into which cr mg v ti partition from the magma the products of these processes are variably preserved as skarns residues xenoliths xenocrysts xenomelts and xenovolatiles which play important to critical roles in ore genesis transport localization and or modification incorporation of barren xenoliths autoliths may induce small amounts of sulfide chromite to segregate but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors pge cu ni co and cr v ti respectively is required to make significant ore deposits silicate xenomelts are only rarely preserved but will be variably depleted in chalcophile and ferrous metals less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma but may aid downward sulfide transport by increasing the bulk density of the magma sulfide wets olivine so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets but other silicates and xenoliths may not be wetted by sulfides xenovolatiles may retard settling of or in some cases float dense sulfide droplets reactions of sulfide melts with felsic country rocks may generate fe rich skarns that may allow sulfide melts to fractionate to more extreme cu ni rich compositions xenoliths xenocrysts xenomelts and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas and are more likely to be preserved in less dynamic less turbulent systems domain phases than in more dynamic more turbulent systems domains phases massive to semi massive ni cu pge and cr mineralization and xenoliths are often localized within footwall embayments dilations jogs in dikes throats of magma conduits and the horizontal segments of dike chonolith and dike sill complexes which represent fluid dynamic traps for both ascending and descending sulfides oxides if skarns residues xenoliths xenocrysts xenomelts and or xenovolatiles are present they provide important constraints on ore genesis and they are valuable exploration indicators but they must be included in elemental and isotopic mass balance calculations

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

examining tectonic scenarios using geodynamic numerical modelling halls creek orogen australia abstract paleoproterozoic orogens record the accretion and collision of older tectonic blocks and this record the tectonic development of a region the halls creek orogen hco is a well exposed paleoproterozoic orogenic belts which can provide insight into the assembly of the kimberley craton to the diamantina craton during the nuna supercontinent amalgamation despite the relative abundance of rock exposure there is still controversy as to how the halls creek orogen developed the 1865 ma tickalara metamorphics seem to be a key unit within the central zone of the halls creek orogen that can help solving the controversy the formation of the protoliths to the tickalara metamorphics and other sedimentary and igneous rocks of central zone have been described as either forming in 1 an oceanic island arc setting above an easterly dipping subduction zone outboard of kimberley craton or 2 an ensialic marginal basin located closer to the margin of kimberley craton the two plausible tectonic scenarios of the halls creek orogen are examined through 33 2d thermo mechanical petrological numerical experiments based on i2vis code the initial constraints for model setup aim to best represent the tectonic environment for the protoliths to the tickalara metaomorphics as either intra ocean subduction or ocean continent subduction and collision with this approach we were able to find experiments with specific physical parameters with results that are consistent with the geology observed in the halls creek orogen the results indicate that the geology of the halls creek orogen is best represented by the ensialic marginal basin scenario this scenario is most consistent with the observed geology and reveals processes which led to the generation of key lithological units and major structures the numerical experiments can also explain sources of magmatism and development of metamorphic conditions that occurred during the tectonic evolution of the halls creek orogen

subduction and accretionary tectonics of the east kunlun orogen western segment of the central china orogenic system abstract the kunlun orogen is generally divided into the east kunlun orogenic belt e kob and the west kunlun orogenic belt w kob by the altyn tagh fault the e kob forms part of the western segment of the central china orogenic system ccos and is considered to have formed by the collision between the qaidam block and qiangtang or bayanhar terrane as a consequence of the closure of the kunlun ocean branch of the paleo tethyan ocean based on a compilation recently published high quality data this contribution provides an overview of the composition nature and ages of the principal tectonic elements including ophiolitic melanges and related volcanic rocks intrusive plutons and sedimentary cover sequences in the e kob according to multiple lines of evidence from these tectonic elements we proposed herewith a paleozoic triassic subduction and accretionary tectonic model to interpret the spatiotemporal tectonic framework plate subduction polarity and tectonic processes from accretion to collision of the e kob three main ophiolitic melange zones are identified in the e kob from north to south they are the qimantagh xiangride ophiolitic melange zone qxm the aqikekulehu kunzhong ophiolitic melange zone akm and the muztagh buqingshan anemaqen ophiolitic melange zone mbam according to these ophiolitic melange zones the e kob is divided into four major tectonic units the north qimantagh belt the central kunlun belt the south kunlun belt and the bayanhar terrane based on several lines of evidence from geology geochemistry and geochronology the south kunlun belt is interpreted as a paleozoic to triassic fore arc and accretionary complex related to northward subduction of the kunlun ocean during the ordovician triassic time the akm mabm and the south kunlun belt constitute a wide accretionary complex along the kunlun suture zone that marks final closure of the major paleo tethyan ocean while the qxm represents the best expression of another suture that records final closure of the qimantagh back arc basin the central kunlun belt as a long lived island arc terrane from ordovician to triassic times rifted from the qaidam block due to the spreading of the qimantagh back arc basin during the period of ca 485 425 ma taken into all the geological geochemical and geochronological lines of evidence together a trench arc back arc basin tectonic system in the e kob was built up and evolved into a protracted and long lived northward subduction and accretion along the kunlun suture during paleozoic and triassic time

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

three dimensional electrical resistivity of the north central usa from earthscope long period magnetotelluric data abstract we present initial results from three dimensional inversion of long period earthscope magnetotelluric mt transportable array data from 232 sites covering the north central us the study area covers the 1 1 ga mid continent rift mcr system which cuts across a series of archean and paleoproterozoic lithospheric blocks the western arm of the mcr is clearly evident in shallow depth sections with a narrow resistive core flanked by elongate conductive basins other prominent upper crustal features mapped include the moderately conductive michigan and illinois basins and extremely high conductivities in foreland basin rocks at the southern margin of the superior craton the most prominent conductive anomalies in an otherwise relatively resistive mid lower crust are two elongate east west oriented structures which are closely aligned with previously inferred continental sutures the first underlies the southern margin of the superior craton just north of the niagara fault nf and can be associated with the 1 85 ga penokean orogeny a second further south beneath iowa and western wisconsin lies just south of the spirit lake tectonic zone sltz and can be identified with yavapai accretion at 1 75 ga both of these conductive sutures are cleanly cut by the mcr which is otherwise not clearly evident in the deeper parts of the resistivity model the break in the anomalies is narrow comparable to the surface expression of the mcr indicating that rifting impacts on the entire crustal section were highly localized both suture related anomalies are imaged as extending into and perhaps through the lithosphere as dipping diffuse zones of reduced mantle resistivity sense of dip of these structures southward for the nf anomaly northward for sltz agrees with previously inferred models for subduction and accretion suggesting that a conductive phase most likely carbon has been thrust deep into the lower crust and uppermost mantle providing a marker of the three dimensional boundary between lithospheric blocks resistivities drop below 100 m below 200 km depth in rough agreement with the seismically determined lithosphere asthenosphere boundary lab there are modest lateral variations in this deep low resistivity layer but the reliability and significance of these are not yet clear

zircon dating hf sr nd os isotopes and pge geochemistry of the tianyu sulfide bearing mafic ultramafic intrusion in the central asian orogenic belt nw china abstract the tianyu mafic ultramafic intrusion is one of many important sulfide bearing mafic ultramafic intrusions in the eastern tianshan terrane located in the southern margin of the central asian orogenic belt in north xinjiang nw china the origin of sulfide mineralization in these intrusions their relationship with permian basalts and a type granites and the geodynamic setting of the bimodal magmatism in the region are controversial in this paper we use zircon u pb age hf sr nd os isotope and lithophile chalcophile trace element constraints from the tianyu mafic ultramafic intrusion to address these important issues the u pb age of zircon from the tianyu intrusion determined by sims is 280 2 ma this confirms that the tianyu intrusion is coeval with most sulfide bearing mafic ultramafic intrusions and some basalts and a type granites in the tianshan region similar to coeval basalts in the nearby tuha basin the tianyu intrusive rocks are characterized by negative nb anomaly and elevated end values varying between 1 and 4 6 the negative nb anomaly in the tianyu intrusion is not as pronounced as in the coeval a type granites but the end values for these bimodal rocks are similar the ehf values of zircon from the tianyu intrusion are positive varying between 3 and 8 these values suggest that the parental magma of the tianyu intrusion was dominated by melt derived from a depleted source mantle the os values of pyrrhotite from the tianyu intrusion are high ranging from 725 to 995 which can be modeled by selective assimilation of sulfide from the lower and upper crusts negative pt anomaly is present in relatively sulfide rich samples such as semi massive and massive sulfide ores but not in sulfide poor samples which is more consistent with the effect of post magmatic hydrothermal alteration than the original signature of the parental magma the results of modeling using the compositions of selected disseminated sulfide samples suggest that the parental magma of the tianyu intrusion was depleted in all pge which is consistent with previous sulfide segregation at depth our data and modeling results support the model of decompression melting of upwelling asthenosphere due to slab break off during a transition from oceanic subduction to arc arc or arc continent collision in the permian decompression melting of the upwelling asthenosphere in the region produced mafic magma partial melting in the lower parts of a juvenile arc crust due to mafic magma underplating produced granitic melts mixing of the mafic magma with a small amount of granitic melts at depth followed by contamination with the upper crust at a higher level formed the parental magma of the tianyu intrusion pge depletion in the parental magma of the tianyu intrusion can be attributed to sulfide segregation associated to the first stage of crustal contamination at depth we suggest that the formation of sulfide ores in the intrusion was related to the second stage of crustal contamination in the upper crust

the eagle and east eagle sulfide ore bearing mafic ultramafic intrusions in the midcontinent rift system upper michigan geochronology and petrologic evolution 1 the eagle and east eagle intrusions are small subvertical dike like mafic ultramafic bodies that cut proterozoic sedimentary strata in the baraga basin in northern michigan the eagle intrusion hosts a newly discovered magmatic ni cu pge deposit the nearby east eagle intrusion also contains sulfide mineralization but the extent of this mineralization has yet to be determined by further drilling both intrusions contain olivine bearing rocks such as feldspathic peridotite melatroctolite and olivine melagabbro sulfide accumulations range from disseminated at both eagle and east eagle to semimassive and massive at eagle u pb baddeleyite dating gives a crystallization age of 1107 2 5 7 ma for the eagle intrusion coeval with eruption of picritic basalts at the base of the volcanic succession in the midcontinent rift system mrs the fo contents of olivine cores in the eagle and east eagle intrusions vary between 75 and 85 mol higher than those of olivine in larger layered intrusions in the mrs such as the duluth complex the feo mgo ratios and al2o3 contents of the parental magmas for the eagle and east eagle intrusions inferred from olivine and spinel compositions are similar to those of picritic basalts in the base of the mrs volcanic succession these petrochemical data suggest that the eagle and east eagle intrusions are the intrusive equivalents of high mgo basalts that erupted in the early stages of continental magmatism associated with the development of the rift variations in mineral compositions and incompatible trace element ratios suggest that at least three major pulses of magmas were involved in the formation of low sulfide rocks in the eagle intrusion lower fo contents of olivine associated with semimassive sulfides as compared to that of olivine in low sulfide rocks suggest that the magma associated with the semimassive sulfide was more fractionated than the parental magmas of the low sulfide rocks in the eagle intrusion accumulation of suspended olivine crystals and sulfide droplets from ascending magmas as they passed through wide parts of the conduits at eagle and east eagle played a critical role in the genesis of olivine rich rocks and sulfide ores in the intrusions the eagle ni cu pge deposit typifies the conduit style of magmatic sulfide deposition that is associated with continental basaltic magmatism

sevier laramide deformation of the continental interior from calcite twinning analysis west central north america abstract paleozoic mesozoic carbonates that cover cratonic western north america contain a regional layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the sevier thrust transport direction e w in carbonates of the idaho wyoming portion of the thrust belt and within carbonates as far as 2000 km into the plate interior the inferred calcite twinning differential stress magnitudes generally decrease across the thrust belt and decrease exponentially away from the orogenic front into the craton synorogenic calcite cements and veins preserve a distinct twinning deformation history in the thrust belt twinning strains commonly record local out of transport piggyback strain events with high differential stresses

chromite in komatiites ii modification during greenschist to mid amphibolite facies metamorphism chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes particularly above 500 c metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors as a result of morphism of host rocks this modification is the subject mg fe exchange with silicates and carbonates chromite metaof this paper morphosed to amphibolite facies is enriched in zn and fe and metamorphic modification of chromite has been disdepleted in ni relative to lower metamorphic grades relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions cr al and fe are not ophiolitic or alpine ultramafic complexes onyeagocha greatly modified by metamorphism up to lower amphibolite facies 1974 ulmer 1974 evans frost 1975 hoffman although minor fe depletion occurs during talc carbonate alwalker 1978 loferski lipin 1983 kimball 1990 teration at low temperature significant al is lost from chromite burkhard 1993 and in a few studies of komatiitic rocks cores above 550 c as a result of equilibration with fluids in bliss maclean 1975 donaldson 1983 gole hill equilibrium with chlorite elevated zn content in chromite is 1990 and in a detailed study of the pechenga intrusions restricted to rocks with low metamorphic mg fe ratios and is abzalov 1998 these studies have highlighted two the result of introduction of zn during low temperature alteration important effects first chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or fermetamorphism cobalt and mn also behave similarly except where ritchromit second chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage progressively modified during prograde metamorphism chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite this is the result of incomplete metamorphic reaction silicate minerals evans frost 1975 abzalov 1998 between chromite and chlorite bearing silicate assemblages magthis paper examines the nature and magnitude of these netite compositions at the inner chromite magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade eastern goldfields province of the archaean yilgarn block in western australia fig 1 brief locality descriptions are given in appendix b

geochemical constraints on the petrogenesis of granitoids in the east kunlun orogenic belt northern tibetan plateau implications for continental crust growth through syn collisional felsic magmatism early triassic syn collisional granitoids with mafic magmatic enclaves mmes crop out along the entire east kunlun orogenic belt ekob at the northern margin of the tibetan plateau they are andesitic in composition and enriched in light rare earth elements lrees with a flat heavy ree hree pattern their average composition resembles that of the bulk continental crust the enclosed mmes have the same mineralogy as their host granitoids but contain a greater mode of mafic minerals amphibole and biotite and thus have higher hree abundances zircon u pb dating shows that both the granitoid hosts and mmes have the same crystallization age of similar to 250 ma and indistinguishable bulk rock sr nd pb hf isotope compositions i sr of 0 7080 0 7116 varying pb 206 pb 204 i of 18 53 19 32 essentially constant epsilon nd t of 5 3 to 2 1 and a small range of positive epsilon hf t mostly 1 7 5 2 the complete isotopic overlapping between the granitoid hosts and the mmes is understood to reflect that the mmes are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system the isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted paleo tethyan oceanic crust with terrigenous sediments under amphibolite facies conditions in response to the continental collision c 2014 elsevier b v all rights reserved

origin and emplacement of the aguablanca magmatic ni cu pge sulfide deposit sw iberia a multidisciplinary approach a model is proposed for the origin and emplacement of the ca 341 ma aguablanca magmatic ni cu platinum group element pge sulfi de deposit sw iberia integrating petrological geochemical structural and geophysical data the aguablanca deposit occurs in an unusual geodynamic context for this ore type an active plate margin as an exotic magmatic subvertical breccia located at the northern part of the coeval gabbronorite aguablanca stock 341 1 5 ma structural and gravity data show that mineralized breccia occurs inside the inferred feeder zone for the stock adjacent to the cherneca ductile shear zone a variscan sinistral transpressional structure the orientation of the feeder zone corresponds to that expected for tensional fractures formed within the strain fi eld of the adjacent cherneca ductile shear two distinctive stages are established for the origin and emplacement of the deposit 1 initially the ore forming processes are attributed to magma emplacement in the crust assimilation of crustal s and segregation and gravitational settling of sulfi de melt a scenario similar to most plutonic ni cu sulfi de ores and 2 fi nal emplacement of the ni cu sulfi de bearing rocks by multiple melt injections controlled by successive opening events of tensional fractures related to the cherneca ductile shear zone

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

geochemistry of the j m howland reef of the stillwater complex minneapolis adit area i sulfide chemistry and sulfide olivine equilibrium platinum mineralization in the stillwater complex occurs within a complex sequence of olivine bearing plagioclase rich cumulates about 400 m above the base of the banded zone the reef consists of a 2 m thick layer of disseminated sulfides extending over the entire strike length of the complex in the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine plagioclase cumulate mixed rock to a plagioclase olivine cumulate containing a high proportion of postcumulus bronzite with olivine preserved only as rare resorbed cores on bronzite grains platinum group elements within the reef correlate strongly with abundances of sulfides which show convincing textural evidence for having originated as immiscible sulfide liquid strong correlations are also observed between all six platinum group elements ni and cu despite the gradation in host rock mineralogy the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length containing 9 3 percent ni 6 9 percent cu and approximately 0 7 percent total platinum group elements principally pd these platinum group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt olivines from the reef show a range of compositions from fo 71 to fo 85 and from 2 400 to 4 500 ppm ni a positive correlation is observed between nio and feo in olivines from sulfide bearing samples indicating ni fe exchange between olivine and sulfides on a local scale such that ni contents of olivines were buffered by that of the sulfide phase a value of 9 70 or 1 49 is obtained for the ni fe distribution coefficient between sulfide and olivine in agreement with values obtained at magmatic temperatures by boctor 1982 these observations may be interpreted in the framework of the plume model of campbell et al 1983 a pulse of replenishing magma enters the stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling sulfide liquid attains high platinum group element concentrations during turbulent ascent and spreading of the plume crystallization of olivine from the hybrid layer gives rise to olivine sulfide boulders which settle to the floor of the chamber reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich subsequent reequilibration between olivine and sulfide grains gives rise to the observed ni fe distribution

geochemistry of the kalatongke ni cu pge sulfide deposit nw china implications for the formation of magmatic sulfide mineralization in a postcollisional environment the kalatongke also spelt as karatungk ni cu platinum group element pge sulfide deposit containing 33 mt sulfide ore with a grade of 0 8 wt ni and 1 3 wt cu is located in the eastern junggar terrane northern xinjiang nw china the largest sulfide ore body which occupies more than 50 vol of the intrusion y1 is dominantly comprised of disseminated sulfide with a massive sulfide inner zone economic disseminated sulfides also occur at the base of the intrusions y2 and y3 the main host rock types are norite in the lower part and diorite in the upper part of each intrusion enrichment in large ion lithophile elements and depletion in heavy rare earth elements relative to mid ocean ridge basalt indicate that the mafic intrusions were produced from magmas derived from a metasomatized garnet lherzolite mantle the average grades of the disseminated ores are 0 6 wt ni and 1 1 wt cu whereas those of the massive ores are 2 wt ni and 8 wt cu the pge contents of the disseminated ores 14 69 ppb pt and 78 162 ppb pd are lower than those of the massive ores 120 505 ppb pt and 30 827 ppb pd however on the basis of 100 sulfide pge contents of the massive sulfides are lower than those of the disseminated sulfides very high cu pd ratios 4 5 104 indicate that the kalatongke sulfides segregated from pge depleted magma produced by prior sulfide saturation and separation a negative correlation between the cu pd ratio and the pd content in 100 sulfide indicates that the pge content of the sulfide is controlled by both the pge concentrations in the parental silicate magma and the ratio of the amount of silicate to sulfide magma the negative correlations between ir and pd indicate that the massive sulfides experienced fractionation

up down or sideways emplacement of magmatic fe ni cu pge sulfide melts in large igneous provinces the preferential localization of fe ni cu pge sulfides within the horizontal components of dike sill lava flow complexes in large igneous provinces lips indicates that they were fluid dynamic tra

evidence for grenville age seismicity and thick skinned deformation in northern wisconsin u s a abstract one of the great continental rifts in all of earth s history the ca 1 1 ga midcontinent rift of the lake superior region was closed and inverted less than 100 m yr after its initiation when rift bounding normal faults were reactivated as reverse faults this was a major crustal event involving thick skinned deformation but its tectonic importance has not been fully appreciated owing in part to the lack of good outcrop along the traces of the rift closing faults a rare exposure of one of these faults occurs near the falls of the marengo river in southeastern bayfield county wisconsin the atkins lake marenisco fault is a crustal scale structure that juxtaposes archean granitic gneisses with paleoproterozoic and neoproterozoic metasedimentary and metavolcanic rocks pseudotachylytes cataclasites and microbreccias in the fault zone record large documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont

platinum group element distribution in the main zone and upper zone of the bushveld complex south africa abstract the platinum group element pge contents of the upper portions of the bushveld complex were investigated with three questions in mind a in natural systems does magnetite concentrate os ir ru and rh ipge as has been observed in experimental systems b is there a au pd enriched layer present such as observed in the upper parts of the skaergaard intrusion c can changes in metal ratios be used for prospecting for pge deposits in the sulfide poor main magnetite layer of the eastern bushveld ir and rh are enriched relative to pt pd and au and this could be because magnetite preferentially concentrated ir and rh over pt pd and au however in most other magnetite layers no enrichment was observed this could be because most magnetite layers contain approximately 1 sulfides and the pge budget is dominated by the sulfides these sulfides obscured the effects of magnetite collecting ipge because sulfides collect all the pge and the partition coefficients for the pge into a sulfide liquid are much greater than the partition coefficient for ipge into magnetite the weighted average of the platinum group elements pge and au over the 2000 m of sampled stratigraphy is au 2 1 ppb pd 1 7 ppb pt 1 7 ppb rh 0 18 ppb ru comparison of the composition of the marginal rocks of the bushveld with a weighted average for the complete 6 km of the bushveld cumulates shows that the cumulate pile is much richer in compatible elements ir rh cr and poorer in incompatible elements sm and hf than the marginal rocks two possible solutions to this are a the magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or b fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

chrome spinels from the jinchuan ni cu sulfide deposit gansu province people s republic of china chrome spinels occur in olivine rich cumulates in the jinchuan intrusion host to one of the largest known accumulations of magmatic ni cu in the world the chemistry and mode of occurrence of these spinels is compared with those in two other similar but unmineralized mafic ultramafic intrusions in the same belt the jinchuan intrusion contains mg poor chromites with widely varying tio 2 contents showing many features that are typical of chromites in normal tholeiitic layered intrusions wide compositional variance occurs on the scale of single thin sections and is attributed to reaction between cumulus chromite and trapped intercumulus liquid this process operated extensively even on grains that were armored by cumulus olivine crystals different reaction paths correlate with the abundance of sulfides in the rock the unmineralized intrusions show similar reaction trends but are offset from the jinchuan trends yejili spinels are generally higher in al and ga while zangbutai chromites are systematically higher in fe super 3 jinchuan spinel olivine pairs record equilibration temperatures well below the solidus and are slightly but significantly lower than blocking temperatures recorded at yejili the jinchuan ore zone contains low cr high fe super 3 chromites that are unusual and distinctive a plausible model for these unusual compositions is that these grains originated as normal aluminous spinels crystallizing from the parent silicate melt and subsequently became enlarged and modified during crystallization of the sulfide melt ti enriched chromites enclosed within sulfides record a prehistory of extensive reaction with trapped liquid sulfide associated chromites from jinchuan are strongly nickel depleted relative to expected ni values for their fe super 3 content when compared with the barren intrusions this feature combined with the distinctive chemistry of the sulfide related spinels may have applications in the use of heavy resistate minerals during exploration distinctively high ti chromites are also a feature of intrusions associated with the karoo flood basalts suggesting that jinchuan may have a similar affinity however tio 2 enrichment in chromite can be a product of interaction with differentiated trapped liquid as indicated by similar trends in lava lakes layered intrusions and at sudbury and therefore cannot be taken to prove unusually ti rich parent magmas

compositional effects on the solubility of minor and trace elements in oxide spinel minerals insights from crystal crystal partition coefficients in chromite exsolution chromite from los congos and los guanacos in the eastern pampean ranges of cordoba argentinian central andes shows homogenous and exsolution textures the composition of the exsolved phases in chromite approaches the end members of spinel mgal 2 o 4 spl and magnetite fe 2 fe 2 3 o 4 mag that define the corners of the spinel prism at relatively constant cr 3 r 3 ratio where r 3 is cr al fe 3 the exsolution of these phases from the original chromite is estimated to have accounted at 600 c on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite rich olivine fo 95 the relatively large size of the exsolved phases in chromite up to ca 200 m provided for the first time the ability to conduct in situ analysis with laser ablation inductively coupled plasma mass spectrometry for a suite of minor and trace elements to constrain their crystal crystal partition coefficient between the spinel rich and magnetite rich phases d i spl mag minor and trace elements listed in increasing order of compatibility with the spinel rich phase are ti sc ni v ge mn cu sn co ga and zn d i spl mag values span more than an order of magnitude from d ti spl mag 0 30 0 06 to d zn spl mag 5 48 0 63 our results are in remarkable agreement with data available for exsolutions of spinel rich and magnetite rich phases in other chromite from nature despite their different cr 3 r 3 ratio the estimated crystal crystal partitioning coefficients reflect the effect that crystal chemistry of the exsolved phases from chromite imposes on all investigated elements excepting cu and sc and only slightly for mn the observed preferential partitioning of ti and sc into the magnetite rich phase is consistent with high temperature chromite melt experiments and suggests a significant dependence on fe 3 substitution in the spinel structure a compositional effect of major elements on ga co and zn is observed in the exsolved phases from chromite but not in the experiments this might be due to crystal chemistry differences along the mgfe 1 al 2 fe 2 3 exchange vector which is poorly covered experimentally this inference is supported by the strong covariance of ga co and zn observed only in chromite from layered intrusions where this exchange vector is important a systematic increase of zn and co coupled with a net decrease in ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite which are enriched in the magnetite component the most likely explanation is that the contents of minor and trace elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite

chrome spinels in gabbro wehrlite intrusions of the pechenga area kola peninsula russia emphasis on alteration features abstract chrome spinels are common accessory minerals of the pechenga ultramafic rocks in nw russia varying in metamorphic grade from sub greenschist to amphibolite facies magmatic chrome spinels can be grouped into two main types spinel 1 which crystallized first is represented by al ti chromite it occurs mainly as inclusions in olivine and rather less commonly enclosed within interstitial minerals spinel 2 crystallized after spinel 1 and has composition varying from ti rich chromite to ti rich chrome magnetite spinel 2 occurs in the interstitial spaces between olivine crystals and enclosed within intercumulus silicates zoned crystals with spinel 1 in the core overgrown by spinel 2 are also common cr poor titanomagnetite occurring in pyroxenites and gabbro apparently represents further evolution of spinel 2 phases metamorphic chrome spinels are represented by ferritchromit and cr magnetite which are the dominant chrome spinel phases in the amphibolite facies rocks the continuous trend of the magmatic chrome spinels composition is indicative of their intensive post cumulus alteration by reaction with evolving intercumulus liquid this reaction led to depletion of the earlier crystallized chrome spinels in mg cr and al and their enrichment in ti and fe 3 oxidation exsolution of ilmenite from ti rich chrome spinels is another common process of post crystallization alterations serpentinization of olivine led to formation of magnetite which overgrew and cut chrome spinel grains low temperature hydrothermal processes in particular talc carbonate alterations could cause a local loss of ti and further redistribution of mg mn and zn prograde metamorphic reactions led to formation of ferritchromit and magnetite replacing primary chrome spinels another common process affecting ti rich chrome spinels is a prograde metamorphic recrystallization of exsolved ilmenite which changes from submicron lamellae through a regular grid of coarse ilmenite crystals and further to their irregular blebs and rims metamorphism of epidote amphibolite and amphibolite facies has led to the liberation of most of the elements metamorphically liberated cr was captured by antigorite

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

an experimental study of trace element partitioning between augite and fe rich basalts abstract aluminous titanium rich hedenbergite and fe rich augite were experimentally produced at 1050 1220 c and 0 8 2 2 gpa major element compositions are analogous to clinopyroxene from layered intrusions angrites nakhlites and late stage lunar magma ocean cumulates trace element concentrations in pyroxene and coexisting melt were measured by laser ablation inductively coupled plasma mass spectrometry for rare earth elements ree high field strength elements hfse transition metals and large ion lithophile elements ree hfse and transition metal partition coefficients ds are tightly correlated with the cation abundance of fe on the pyroxene m1 site x fe m1 and also correlated with al on the tetrahedral site x al t parameterized lattice strain models were developed to predict ree and hfse partition coefficients as functions of temperature and pyroxene composition x al t or x fe m1 the parameterized models can be used to calculate ree and hfse partition coefficients for fe rich high ca pyroxene we calculated partition coefficients for clinopyroxene derived from modeled end stage lunar magma ocean cumulate compositions and observe a factor of 3 variation in clinopyroxene melt ree partition coefficients using representative nakhlite clinopyroxene core and rim compositions we calculated partition coefficients and observed a factor of 4 variability in clinopyroxene melt ree partition coefficients hfse partition coefficients are even more sensitive to composition showing variations of a factor of 4 for lunar late cumulates and an order of magnitude for nakhlites the strong dependence of ree and hfse partitioning on composition necessitates careful selection of appropriate partition coefficients for fe rich systems

the geochemistry geochronology and paleomagnetism of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract the midcontinent rift mcr of north america comprises a series of basaltic sheets flows and intrusive rocks emplaced in the lake superior region during the mesoproterozoic the mafic rocks preserved on the northern flank of lake superior represent the older portions of the rift sequence and offer insights into the early development of the rift new geochronological geochemical and paleomagnetic data are presented for the dikes and sills located in and south of thunder bay ontario three sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill the predominant logan sills and nipigon sills in the north of the study area in addition three dike sets are recognized the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike the geochemical data demonstrate that the majority of sills south of thunder bay are of logan affinity and distinct from those of broadly similar age in the nipigon embayment to the north the pigeon river dikes that intrude the sills are geochemically coherent but distinct from the logan sills and could not be feeders to the sills the new age of 1109 2 4 2 ma for the cloud river dike and its r polarity are consistent with published magnetostratigraphy the mt mollie dike age 1109 3 6 3 ma indicates that it is not coeval with the spatially associated crystal lake gabbro as previously thought the complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south the geochemistry and geochronology of the intrusions suggest a long lived and complex magmatic history for the midcontinent rift

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

processes driving estuary infilling by marine sands on an embayed coast abstract detailed measurements of waves currents and sediment transport from an intertidal sandflat at the mouth of a partially filled new zealand estuary in an embayment with low littoral drift are presented with a view to establishing how marine sands are transported into the estuary waves mobilised sediments during episodic events early in events when waves were energetic sediments were in motion throughout the tidal cycle under abating waves sediment motion became intermittent finally becoming restricted to periods around low tide when wave orbital motions were able to penetrate to the bed three components of the total sediment flux were estimated from the data and analysed 1 a wind driven current which presumably has a complicated spatial expression in the vicinity of the estuary mouth controlled the net i e integrated over event direction of flux of suspended sediment entrained by waves and advected by currents there was no evidence of non symmetry over the tidal cycle in the suspension process that might drive a net sediment flux however there is some suggestion that lag in growth and decay of the estuary wave field over the tidal cycle systematically enhances offshore transport 2 net bedload transport by waves and currents was directed onshore which was guaranteed by two mechanisms firstly increasing wave orbital current skewness as the tide ebbed turned bedload transport onshore late in the ebb tide against the current and secondly bedload peaked early in the flood tide when skewed waves were reinforced by the flooding current 3 correlation between seaward directed forced mean flow and enhanced suspension under groups of high waves resulted in persistent offshore flux of suspended sediment at infragravity frequencies suspended sediment flux carried by gravity waves was directed seawards on flood tides and counter to the waves and onshore on ebb tides and with the waves which was explained in terms of a simple vortex ejection model we conclude that sediment transport on the sandflat at the mouth of this partially filled estuary is governed by subtle interactions between waves and currents that vary over the tidal cycle some interactions such as those that drive bedload transport promote estuary infilling by marine sands and others such as those that drive suspended sediment flux at infragravity frequencies do the opposite

variation in trace element content of magnetite crystallized from a fractionating sulfide liquid sudbury canada implications for provenance discrimination abstract laser ablation icp ms analysis has been applied to many accessory minerals in order to understand better the process by which the rock formed and for provenance discrimination we have determined trace element concentrations of fe oxides in massive sulfides that form ni cu pge deposits at the base of the sudbury igneous complex in canada the samples represent the crystallization products of fractionating sulfide liquids and consist of early forming fe rich monosulfide solution mss cumulates and residual cu rich intermediate solid solution iss this study shows that fe oxide geochemistry is a sensitive petrogenetic indicator for the degree of fractionation of the sulfide liquid and provides an insight into the partitioning of elements between sulfide and fe oxide phases in addition it is useful in determining the provenance of detrital fe oxide in a sulfide melt all lithophile elements cr ti v al mn sc nb ga ge ta hf w and zr are compatible into fe oxide the concentrations of these elements are highest in the early forming fe oxide titanomagnetite which crystallized with fe rich mss upon the continual crystallization of fe oxide from the sulfide liquid the lithophile elements gradually decrease so that late forming fe oxide magnetite which crystallized from the residual cu rich liquid is depleted in these elements this behavior is in contrast with fe oxides that crystallized from a fractionating silicate melt whereby the concentration of incompatible elements such as ti increases rather than decreases the behavior of the chalcophile elements in magnetite is largely controlled by the crystallization of the sulfide minerals with only ni co zn mo sn and pb present above detection limit in magnetite nickel mo and co are compatible in fe rich mss and thus the co crystallizing fe oxide is depleted in these elements in contrast magnetite that crystallized later from the fractionated liquid with cu rich iss is enriched in ni mo and co because fe rich mss is absent the concentrations of sn and pb which are incompatible with fe rich mss are highest in magnetite that formed from the fractionated cu rich liquid at subsolidus temperatures ilmenite exsolved from titanomagnetite whereas al spinel exsolved from the cores of some magnetite locally redistributing the trace elements however during laser ablation icp ms analysis of these fe oxides both the magnetite and its exsolution products are ablated so that the analysis represents the original magmatic composition of the fe oxide that crystallized from the sulfide melt

crustal structure beneath the florida to edmonton broadband seismometer array 1 ps receiver functions were stacked to determine crustal thickness and vp vs ratios for the 28 stations of the florida to edmonton broadband seismometer array and 6 adjacent permanent stations roots of thickened crust exist beneath the paleozoic appalachian orogen and the proterozoic trans hudson orogen the ratios of surface elevation to root thickness are lower than observed in young active orogens as are average crustal root buoyancies determined from gravity data these results are consistent with global trends and may be explained by increased crustal root density due to metamorphism crustal thickening is also observed beneath the mid continent rift in iowa consistent with compressional inversion of the rift during the grenville orogeny

geochemical discrimination between barren and mineralized intrusions in the svecofennian 1 9 ga kotalahti nickel belt finland abstract the geochemistry of 11 svecofennian 1 9 ga mafic ultramafic intrusions in the kotalahti nickel belt in finland was studied the intrusions were classified into three groups 1 mineralized 2 intermediate and 3 barren the classification was made on the basis of the amount and composition of the sulfides found in each intrusion the whole rock geochemical results show evidence of country rock contamination in mineralized intrusions contamination is best seen in elevated al 2 o 3 cao ratio and zr p 2 o 5 th and lree contents in peridotites olivines in mineralized intrusions show nickel depletion which in some intrusions is interpreted as in situ depletion barren intrusions have nickel undepleted olivines or olivines with low fo and low ni

the 1 1 ga midcontinent rift system central north america sedimentology of two deep boreholes lake superior region abstract the midcontinent rift system mrs of central north america is a 1 1 ga 2500 km long structural feature that has been interpreted as a triple junction rift developed over a mantle plume as much as 20 km of subaerial lava flows mainly flood basalts are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds this rock sequence known as the keweenawan supergroup has been penetrated by a few deep boreholes in the search for petroleum in this paper two deep boreholes in the upper peninsula of michigan are described in detail for the first time both the amoco production 1 29r test herein referred to as the st amour well and the nearby hickey creek well drilled by cleveland cliffs mining services were 100 cored the former is 7238 ft 2410 m deep and the latter is 5345 ft 1780 m deep the entirety of the stratigraphic succession of the hickey creek core correlates very well with the upper portion of the st amour core as determined by core description and point counting of 43 thin sections selected out of 100 studied thin sections two lower paleozoic units and two keweenawan red bed units the jacobsville sandstone and the underlying freda sandstone are described the jacobsville is largely a feldspatholithic sandstone and the freda is largely a lithofeldspathic sandstone below the freda the remaining footage of the st amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate sandstone siltstone and shale black shale individual basalt flows and a basal ignimbritic rhyolite this lower portion of the st amour core presents an enigma as it correlates very poorly with other key boreholes located to the west and southwest while a black shale sequence is similar to the petroleum bearing nonesuch formation farther west there is no conglomerate unit to correlate with the copper harbor conglomerate other key boreholes are distributed over a 1300 km distance along the better known southwest arm of the triple junction mrs and can be correlated rather well with the units that are exposed in the lake superior region however a definitive explanation of the anomalous deeper st amour stratigraphy is elusive and any explanation is tenuous a possible explanation for this anomalous stratigraphy may be the geographic proximity of the st amour borehole to the keweenawan hot spot mantle plume the suggested thermal force behind the development of the mrs similarly a drastic change in structural architecture may be explained by this geographic relationship thus within the locale of this rifting center complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm

p wave teleseismic traveltime tomography of the north american midcontinent the remains of the 1 1 ga midcontinent rift mcr lie in the middle of the tectonically stable portion of north america previous and ongoing studies have imaged strong heterogeneity associated with the mcr in the crust but have not imaged such within the mantle it is unclear whether this is due to the absence of rift related mantle structures or these studies had insufficient resolution to image them to address this issue we measured 46 374 teleseismic p wave delay times from seismograms recorded by the usarray transportable array superior province rifting earthscope experiment and surrounding permanent stations we included these and 54 866 delay times from prior studies in our tomographic inversion we find that high velocity anomalies are widespread in our study area but there are also prominent low velocity anomalies two of these are coincident with high bouguer gravity anomalies associated with the mcr in iowa and the minnesota wisconsin border at 50 to 150 km depth extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift related material that underplated the crust although we cannot exclude that the subcrustal mantle lithosphere beneath the mcr is anomalously enriched hydrated or warm other anomalies occur at syntaxes of the penokean orogen one with the superior province and marshfield terrane in southern minnesota and another with the yavapai and mazatzal terranes both at 100 to 250 km depth in the midmantle we image two linear high velocity anomalies interpreted as subducted fragments of the farallon and kula plates

geochronology petrology and hf s isotope geochemistry of the newly discovered xiarihamu magmatic ni cu sulfide deposit in the qinghai tibet plateau western china this paper reports the first set of data for the newly discovered xiarihamu magmatic ni cu sulfide deposit in the eastern kunlun paleozoic arc terrane which is located in the northern part of the qinghai tibet plateau an on going drilling campaign reveals 100 million tons of sulfide mineralization with the average grade of 0 8 wt ni and 0 1 wt cu for the deposit this makes the xiarihamu deposit one of the 20 largest magmatic ni cu sulfide deposits in the world and the largest ever found in arc settings the deposit is hosted in a small ultramafic body intruding older gabbroic and metamorphic rocks new zircon u pb isotope age data reveal that the ultramafic body 411 6 2 4 ma is 20 ma younger than the host gabbroic intrusion 431 3 2 1 ma the ultramafic body is composed predominantly of lherzolite and olivine websterite with minor dunite websterite and orthopyroxenite mineralization mainly occurs as sub horizontal to gently dipping 30 disseminated sulfide zones that are generally concordant with the lithological structure of the ultramafic intrusion the lateral extension and thickness of individual mineralized zones are up to 200 m and 100 m respectively sulfide mineral assemblages are composed of pyrrhotite pentlandite and chalcopyrite the xiarihamu ultramafic rocks show light ree enrichments and pronounced negative nb anomalies plus significant ca depletion in olivine 700 ppm ca which are characteristic of many arc basalts in the world olivine crystals in the xiarihamu ultramafic rocks have relatively primitive compositions with fo contents up to 90 mol close to the mantle value the contrasting ni contents of olivine crystals with similar fo contents from different sulfide mineralized zones in a single drill core indicate that at least two pulses of sulfide laden magma with different ni compositions were involved in the development of the deposit estimated parental magma for the xiarihamu lherzolites contains 52 4 wt sio2 and 9 8 wt mgo which are within the ranges of boninites worldwide supporting the interpretation that the xiarihamu sulfide ore bearing ultramafic intrusion is the product of subduction related boninitic magmatism the ehf t values of zircon crystals from the xiarihamu ultramafic rocks vary from 1 to 5 indicating minor crustal contamination of the parent magma the involvement of external sulfur in the genesis of the xiarihamu magmatic sulfide deposit is evident from higher 34s values of sulfide ores 3 5 to 6 8 than typical mantle value 0 2 although the actual contaminants are yet to be identified

geochemistry of north shore hypabyssal dikes and sills in the midcontinent rift of minnesota an example the 47th avenue sill this study presents geochemical data for several of the numerous small to large dikes and sills including the 47th avenue sill exposed along the shore of lake superior in and north of duluth minnesota these intrusions are late magmatic features of the proterozoic midcontinent rift system and together form the north shore hypabyssal group the dikes are geochemically distinct from the sills and when the two are exposed together the younger dike intrudes the older sill dikes are primitive with mg up to 68 have positive end values and are oriented approximately north south with steep westerly or near vertical dips the older sills are more evolved usually have end values near or below 0 and have the same gentle easterly dip as the thick sequence of north shore volcanic group flows they intrude dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5 with primitive geochemistry and positive end values whereas sill compositions are similar to widespread ba