origin of the copper cobalt deposits of the zambian copperbelt an epigenetic view from nchanga the zambian copperbelt is arguably the most significantly mineralized neoproterozoic basin on earth preserving a truly spectacular scale of mineralization in excess of 1 109 t of ore at 2 7 copper has been extracted to date and there are also major cobalt accumulations the origin of these deposits has been hotly debated for more than six decades yet the driving forces that generated this system are poorly understood in particular the relationships between tectonics paleo fluid circulation and ore deposition we present new field and isotopic data for the nchanga deposits in which the bulk of the mineralization is hosted by shale capped feldspathic arenites and arkoses that have undergone recrystallization and hydrothermal alteration within a host rock package controlled by low angle thrust faults by using in situ laser combustion we show for the first time that the range of 34s for copper cobalt ore sulfides 1 to 18 cannot have the same source as diagenetic pyrite 1 to 17 we suggest a new epigenetic model for the formation of these spectacular nchanga orebodies that involves the introduction of metal and sulfate bearing hydrothermal fluids into quartzofeldspathic units during basin inversion with sulfide derived from thermochemical reduction of the sulfate near the site of deposition

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

synmagmatic deformation patterns in the old woman mountains se california abstract synmagmatic structures within granitic plutons may provide insight into the interplay between plutonism and regional deformation at mid crustal levels within the late cretaceous old woman pluton mojave desert se california synmagmatic structures include magmatic fabrics parallel to wall rock foliations melt filled shear zones folded gneissic xenoliths and schlieren with axial planar magmatic fabric deformation continued below granite solidus temperatures in shear zones on the pluton margins these structures permit reconstruction of a sheet like pluton that was emplaced under regional extension taken together with published thermochronometric data the synmagmatic structures constrain the role played by magmatic events in the collapse of a thickened crustal section the post plutonic cooling history suggests rapid uplift and denudation ending by 65 ma we argue that extensional deformation overlapped and post dated pluton emplacement and may have contributed to tectonic unroofing we suggest that magma emplacement can trigger subsequent extensional collapse in overthickened crustal blocks

construction of a pluton evidence from an exposed cross section of the searchlight pluton eldorado mountains nevada a top to bottom cross section of the searchlight pluton is exposed in a large tilt block in the colorado river extensional corridor of southern nevada hornblende barometry and geologic relations indicate that the pluton was about 10 km thick extending from approximately 3 to 13 km depth the pluton is stratified with about 2 km of fine grained quartz monzonite below the roof 2 km of granite in the center and 6 km of coarser more mafic quartz monzonite at the bottom the lower unit has a pronounced magmatic foliation that was subhorizontal prior to tilting contacts between the units are gradational over a few centimeters to about 20 m geometry field relations and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber the upper unit formed as a solidification front that migrated downward from the roof and the middle unit granite and the lower unit represent complementary segregated melt and crystal rich cumulate thus the dominant part of the pluton appears to have solidified in monotonic fashion from more or less uniform magma that was compositionally similar to the upper unit the basal quartz monzonite and the lower part of the granite enclose widely scattered synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale these rocks are commonly fine grained and formed as quenched mafic melts they are isotopically distinct from the main sequence and represent discrete injections into the magma chamber the mafic magmas were contaminated by but did not strongly affect the main sequence magma the nd and sr isotopic compositions of the main sequence magma are intermediate between those of regional crust and enriched mantle derived basalts like those represented by the mafic pods this suggests that the principal magma was a hybrid with about 60 mantle component and thus the pluton represents both addition to and internal reorganization of the crust

ireteba pluton eldorado mountains nevada late deep source peraluminous magmatism in the cordilleran interior abstract the ireteba pluton is a 66 ma documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm biotite pm mathrm muscovite pm mathrm garnet end document granite emplaced at the eastern edge of the cordilleran plutonic belt in southeastern nevada in common with other cordilleran peraluminous granites its mineralogy major element chemistry isotopic composition and abundant proterozoic zircon inheritance document crustal origin distinctive trace element chemistry field relations and inherited components furth

a petrologic comparison of triassic plutonism in the san gabriel and mule mountains southern california triassic magmatism in the southwest u s cordillera forms a semicontinuous magmatic arc extending from northwestern nevada to southeastern california quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern california and we suggest that these rocks represent exposure of a structurally deeper part of the triassic arc where it was emplaced into comparatively thick proterozoic crust elemental and isotopic data suggest that triassic quartz monzodiorites and quartz monzonites in the mule and san gabriel mountains were derived from a relatively undepleted nonradiogenic mafic lithospheric source with virtually no upper crustal interaction very limited data for associated triassic diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature younger triassic granites record a strong geochemical signature of interaction with continental crust including inherited zircon and high initial sr ratios but comparatively less radiogenic pb isotopic compositions the major and trace element geochemistry of late triassic plutonic rocks in southeastern california is similar in many respects to alkalic components of the triassic arc in the mojave desert however contemporaneous rocks farther north have a calc alkalic signature perhaps reflecting the variation in age and composition of lithosphere across which the triassic arc was constructed

metal sources for the nkana and konkola stratiform cu co deposits zambian copperbelt insights from sr and nd isotope ratios abstract the central african copperbelt lies within the lufilian orogenic belt in the border region between the democratic republic of congo drc and zambia a sr and nd isotope study was performed on gangue carbonates associated with multiphase mineralisation at the zambian konkola and nkana deposits comparison with isotopic signatures of basement rocks provides new insights into the likely metal source s for the cu co mineralisation at least three mineralisation phases can be identified with respect to the lufilian orogeny gangue carbonates of the first pre to syn kinematic mineralisation phase in both deposits have sr and nd isotopic compositions that correspond to felsic rocks of the domes region a tectonic zone in the zambian part of the lufilian belt where basement rocks crop out however the isotopic signatures from both deposits differ this can be attributed to local variation in isotopic composition of the basement below the deposits radiogenic isotope ratios suggest that subsequent syn kinematic mineralisation at both sites occurred due to the remobilisation of precursor ore petrographic evidence indicates that the third late kinematic mineralisation phase at nkana resulted from a renewed input of metals with a mafic affinity e g co ni however sr and nd isotope ratios resemble those of the earlier mineralisation phases and do not reflect a change in source composition nonetheless comparison with the isotopic signatures of the co poor konkola deposit and co rich stratiform deposits in the drc might indicate a mafic component in the nkana metal source calculated mixed isotopic compositions support such a mafic component

synkinematic contact metasomatism implications for the timing of pluton emplacement and regional deformation in the scanlon shear zone south eastern california structures in rocks altered by the infiltration of magmatic fluids provide key constraints on the relative timing of plutonism and regional deformation in the central old woman mountains south eastern california in this well exposed area the scanlon shear zone a belt of attenuated shallowly dipping amphibolite facies palaeozoic strata is in contact with two tabular late cretaceous 74 ma granitoid plutons the shear zone contains massive wollastonite bearing skarns 40 m above the contact with the old woman granodiorite field evidence petrological data and stable isotope compositions indicate that the mineral assemblages in these skarns formed when the granodiorite crystallized and exsolved water rich magmatic fluids structural features in the skarns which include transposed wollastonite foliations syntectonic vesuvianite and garnet porphyroblasts and quartz wollastonite veins afford an opportunity to monitor the regional strain at the time of pluton crystallization these structures yield a broad range of timing relationships that indicate synkinematic mineral growth with deformation ending first at c 74 ma the metasomatic structures are kinematically compatible with the regional deformation observed in other lithologies and no post tectonic features overprint the fabrics in the skarns observations of isoclinally folded and boudinaged dykes and pegmatites granodiorite saddle reefs in isoclinal fold hinges and undeformed locally pegmatitic dykes also yield a broad range of timing relationships indicative of synkinematic plutonism with deformation ending first our results show that coupling petrological and stable isotope data with structural analysis is effective for unravelling the relative timing of heat and mass transfer processes in pluton wallrock systems

gravity and aeromagnetic constraints on the structure of the woods mountains volcanic center southeastern california the woods mountain volcanic center is a well exposed mildly alkaline volcanic center that formed during the miocene in southeastern california detailed geologic mapping and geochemical studies have distinguished three major volcanic phases precaldera caldera forming and postcaldera geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava flow eruptions rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude circular gravity and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern california analysis of a bouguer gravity anomaly map reduced to the pole magnetic intensity map and three dimensional gravity and magnetic models indicates that there is a single funnel to bowl shaped caldera approximately 4 km thick and approximately 10 km wide at the surface this model is consistent with other siliceous pyroclastic filled calderas on continental crust except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

genesis of mg fe carbonates from the sierra menera magnesite siderite deposits northeast spain evidence from fluid inclusions trace elements rare earth elements and stable isotope data the mg fe carbonate deposits of sierra menera underneath exploited goethite gossan are located in the iberian range of northeasten spain within the late ordovician dolomitic formation of the sierra menera unit mgo and feo contents of the carbonates range between 13 74 and 44 02 and 5 52 and 41 73 wt percent respectively corresponding to the breunnerite and pistomesite members the morphology petrographic mineralogical and geochemical characteristics of the mg fe carbonates are consistent with a metasomatic hydrothermal origin by replacement of a dolomitic protolith as the result of the influx of hot fe mg rich fluids under acid and reducing conditions these deposits have similarities to alpine type magnesite deposits austria slovakia western carpathian mountains pyrenees and mississippi valley type fe deposits basque cantabrian basin northern spain the mg fe carbonates have normalized rare earth element ree patterns characterized by low light ree heavy ree ratios and positive eu anomalies dolomite rocks that host mg fe occurrences have been recrystallized to saddle dolomite crystals and have less fractionated normalized ree patterns but are also characterized by positive eu anomalies these patterns are distinct from the pristine host dolomite which has ree distribution typical of marine carbonates based on ree fluid inclusion and stable isotope data the mg fe carbonates are interpreted to have been formed by interaction between hot 100 150 c 18o enriched and saline 20 wt nacl equiv iron rich hydrothermal fluids and marine carbonates recrystallization of the host dolomite resulted from moderate to low fluid rock ratios 10 the mg fe carbonates were partially replaced by late dolomite redolomitization due to the action of more dilute solutions although this event resulted in 18o depletion ree patterns and 13c values of the precursors were preserved

geochemistry and tectonic setting of lower proterozoic supracrustal rocks of the pinal schist southeastern arizona two lithologic assemblages are identified in the early proterozoic 1700 ma supracrustal rocks of the pinal schist in southeastern arizona a western assemblage is composed largely of quartz wacke turbidites and an eastern assemblage includes bimodal volcanics and variable amounts of sediment chiefly feldspathic quartzite and conglomerate basalts from the eastern assemblage have incompatible element distributions similar to those found in basalts from continental margin arcs or evolved oceanic arcs whereas those from the western assemblage have distributions similar to within plate or back arc basin basalts felsic volcanics range in composition from rhyodacite to rhyolite and exhibit incompatible element distributions similar to those found in felsic volcanics from continental rifts and back arc basins near continents provenance studies of sediments from both assemblages indicate sources rich in granite and felsic volcanics the quartz wackes are similar in composition to sediments from continental margin arcs and associated back arc basins an increase in mafic components in these rocks from north to south in the pinal basin may reflect compositional polarity in an arc system the eastern assemblage may represent remnants of a major northeast trending continental margin arc system and the western assemblage may have been deposited in an intra arc basin or a northwest trending aulacogen which opened on the southwest

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

chronologic and isotopic framework for early proterozoic crustal evolution in the eastern mojave desert region se california the early proterozoic geologic evolution of the eastern mojave desert region as defined by characteristics of its supracrustal rocks granitoids metamorphism structural history and pb and nd isotopic signature contrasts sharply with other proterozoic provinces of the southwestern united states the oldest supracrustal rocks of the mojave desert region contain zircons over 2 0 ga corroborating nd isotopic evidence for a much older crust here than elsewhere in the southwestern united states granitoids widely emplaced within these supracrustal rocks range from 1 76 to 1 64 ga the earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 ma in a migmatite producing event that we term informally the ivanpah orogeny subsequent granitoids emplaced from 1 69 to 1 67 ga were voluminous along a north trending belt in the middle of the mojave province younger plutons were emplaced at about 1 66 ga in several places and at about 1 64 ga along the extreme southern part of the province commonalities between the proterozoic evolutions of the mojave and arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed the data only suggest that the juxtaposition occurred between about 1 76 and 1 64 ga

detrital zircon as a proxy for tracking the magmatic arc system the california arc example coupled age and trace element geochemical analyses of detrital zircons provide criteria that may be applied to describe average melt compositions through virtually the entire history of a long lived magmatic arc detrital zircon data suggest that the california western united states cordilleran arc was characterized by five mean magmatic states three geochemically distinct pulses were characterized by high th u and progressively heavy rare earth element hree depleted melts the first and last pulses were also characterized by higher than average u yb suggesting that progressive crustal thickening coupled with variable fluid inputs from the subducting slab modulated pulse volumes lulls between pulses were states generally characterized by low magmatic volumes and low th u and u yb suggesting fluid poor conditions with minimized crustal involvement in magmatism

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

petrogenesis of the highly potassic 1 42 ga barrel spring pluton southeastern california with implications for mid proterozoic magma genesis in the southwestern usa syenites from the barrel spring pluton were emplaced in the early proterozoic mojave crustal provine of southeastern california at 1 42 ga all rocks even the most mafic are highly enriched in incompatible elements e g k2o 4 12 wt rb 170 370 ppm th 12 120 ppm la 350 1500xchondrite la ybn 35 100 elemental compositions require an incompatible element rich but mafic or ultramafic source trace element models establish two plausible sources for barrel spring magmas 1 lree enriched garnet websterite with accessory apatite rutile enriched lithospheric mantle and 2 garnet amphibolite or garnet hornblende granulite with enriched alkali basalt composition also with accessory apatite rutile mafic lower crust nd and pb isotopic ratios do not distinguish a crust vs mantle source but eliminate local mojave province crust as the principal one and indicate that generation of the enriched source occurred several hundred million years before emplacement of the barrel spring pluton 1 40 1 44 ga potassic granites are common in southeastern california suggesting a genetic link between the barrel spring pluton and the granites however although the same thermal regime was probably responsible for producing both the granitic and syentic magmas elemental and isotopic compositions preclude a close relationship isotopic similarity of the barrel spring pluton to 1 40 1 44 ga granites emplaced in the central arizona crustal province to the east may imply that a common component was present in the lithosphere of these generally distinct regions

origin of the nchanga copper cobalt deposits of the zambian copperbelt the zambian copperbelt forms the southeastern part of the 900 km long neoproterozoic lufilian arc and contains one of the world s largest accumulations of sediment hosted stratiform copper mineralization the nchanga deposit is one of the most significant ore systems in the zambian copperbelt and contains two major economic concentrations of copper and cobalt hosted within the lower roan group of the katangan supergroup a lower orebody copper only and upper orebody copper and cobalt occur towards the top of arkosic units and within the base of overlying shales the sulfide mineralogy includes pyrite bornite chalcopyrite and chalcocite although in the lower orebody sulfide phases are partially or completely replaced by malachite and copper oxides carrollite is the major cobalt bearing phase and is restricted to fault propagation fold zones within a feldspathic arenite hydrothermal alteration minerals include dolomite phlogophite sericite rutile quartz tourmaline and chlorite quartz veins from the mine sequence show halite saturated fluid inclusions ranging from 31 to 38 wt equivalent nacl with homogenisation temperatures thtot ranging between 140 and 180 c diagenetic pyrites in the lower orebody show distinct relatively low 34s ranging from 1 to 17 whereas arenite and shale hosted copper and cobalt sulfides reveal distinctly different 34s from 1 to 12 for the lower orebody and 5 to 18 for the upper orebody there is also a clear distinction between the 34s mean of 12 1 3 3 n 65 for the upper orebody compared with 5 2 3 6 n 23 for the lower orebody the 13c of dolomites from units above the upper orebody give 13c values of 1 4 to 2 5 consistent with marine carbon however dolomite from the shear zones and the alteration assemblages within the upper orebody show more negative 13c values 2 9 to 4 0 and 5 6 to 8 3 respectively similarly shear zone and upper orebody dolomites give a 18o of 11 7 to 16 9 compared to lower roan dolomites which show 18o of 22 4 to 23 0 two distinct structural regimes are recognized in the nchanga area a weakly deformed zone consisting of basement and overlying footwall siliciclastics and a moderate to tightly folded zone of meta sediments of the katangan succession the fold geometry of the lower roan package is controlled by internal thrust fault propagation folds which detach at the top of the lowermost arkose or within the base of the overlying stratigraphy and show vergence towards the ne faulting and folding are considered to be synchronous as folding predominantly occurred at the tips of propagating thrust faults with local thrust breakthrough the data from nchanga suggests a strong link between ore formation and the development of structures during basin inversion as part of the lufilian orogeny sulfides tend to be concentrated within arenites or coarser grained layers within shale units suggesting that host rock porosity and possibly permeability played a role in ore formation however sulfides are also commonly orientated along but not deformed by a tectonic fabric or hosted within small fractures that suggest a significant role for deformation in the development of the mineralization the ore mineralogy hydrothermal alteration and stable isotope data lend support to models consistent with the thermochemical reduction of a sulfate and metal enriched hydrothermal fluid at the site of mineralization there is no evidence at nchanga for a contribution of bacteriogenic sulfide produced during sedimentation or early diagenesis to the ores

isotopic geochemical and temporal characterization of proterozoic basement rocks in the quitovac region northwestern sonora mexico implications for the reconstruction of the southwestern margin of laurentia a detailed geochemical characterization of 19 representative proterozoic basement rocks in the quitovac region in northwestern sonora mexico has identified two distinct paleoproterozoic basement blocks that coincide spatially with the previously proposed caborca and north america blocks new u pb zircon geochronology revises their age ranges the caborca 1 78 1 69 ga and north america 1 71 1 66 ga blocks at quitovac and precludes a simple age differentiation between them in addition grenvillian age granitoids ca 1 1 ga spatially associated with the caborca block have been identified at quitovac nd isotopes and major and trace element geochemistry support the distinction of these paleoproterozoic blocks granitoids of the north america block are characterized by depleted end values 3 4 3 9 and younger nd model ages 1800 1740 ma and have lower k 2 o y rb ba th ree and fe mg values than coeval rocks of the caborca block the caborca block granitoids are likewise characterized by slightly less depleted end 0 6 2 6 and older nd model ages 2070 1880 ma despite the subtle differences granitoids from both the caborca and north america blocks exhibit island arc like affinities we propose that the proterozoic basement rocks from the quitovac region are an extension of the proterozoic crustal provinces in the southwestern united states specifically rocks of the caborca block exhibit an affinity to rocks of either the yavapai province or the mojave yavapai transition zone whereas rocks of the north america block have signatures similar to those of the mazatzal province or possibly the yavapai province of arizona the new isotopic ages and geochemical data do not support the existence of the late jurassic mojave sonora megashear at quitovac as originally proposed however the quitovac region accounts only for a small fraction of the proterozoic basement in sonora so these findings do not eliminate the possibility of a megashear elsewhere in northern sonora our new data create the possibility of alternative hypotheses for the distribution of paleoproterozoic crustal provinces in southwestern north america that affect reconstructions of the original southwestern margin of laurentia and reduce uncertainties in the configuration timing and existence of the proterozoic supercontinent rodinia

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

the geology and geochemistry of the lumwana cu co u deposits nw zambia the lumwana cu co u deposits of nw zambia are large tabular disseminated ore bodies hosted within the mwombezhi dome of the lufilian arc the host rocks to the lumwana deposits are two mineralogically similar but texturally distinct gneisses a granitic to pegmatitic gneiss and a banded to augen gneiss which both comprise quartz feldspar biotite muscovite haematite amphibole and intervening quartz feldspar biotite schist the sulphide ore horizons are typically developed within a biotite muscovite quartz kyanite schist although mineralization locally occurs within internal gneiss units contacts between the ore and host rocks are transitional and characterized by a loss of feldspar kinematic indicators such as s c fabrics and pressure shadows on porphyroblasts suggest a top to the north shear sense the sulphides are deformed by a strong shear fabric enclosed within kyanite or concentrated into low strain zones and pressure shadows around kyanite porphyroblasts this suggests that the copper mineralization was introduced either syn or pre peak metamorphism in addition to cu and co the ores are also characterized by enrichments in u v ni ba and s and small discrete zones of uranium mineralization occur adjacent to the hanging wall and footwall of the copper ore bodies or in the immediate footwall to the copper mineralization unlike typical copperbelt mineralization unmineralized units show very low background copper values whole rock geochemical analyses of the interlayered schist and ore schist compared to the gneiss show depletions in ca na and sr and enrichments in mg and k consistent with replacement of feldspar by biotite the mineral chemistry of muscovite biotite and chlorite reflect changes in the bulk rock chemistry and show consistent increases in x mg as the schists develop 34s for copper sulphides range from 2 3 to 18 5 with pyrite typically restricted to values between 3 9 and 6 2 these values are atypical of sulphides precipitated by bacteriogenic sulphate reduction 34s data for chimiwungo cu co show a broader range and increased 34s values compared to the malundwe cu mineralization the lumwana deposits show many characteristics which distinguish them from classical copperbelt mineralization and which suggests that they are formed by metasomatic alteration mineralization and shearing of pre katangan basement although this style of mineralization is reported elsewhere in the copperbelt sometimes associated with the more widely reported stratiform ores of the lower roan none of the previously reported occurrences have so far developed the tonnages of ore reported at lumwana

petrology of biotite cordierite garnet gneiss of the mccullough range nevada i evidence for proterozoic low pressure fluid absent granulite grade metamorphism in the southern cordillera proterozoic migmatitic paragneisses exposed in the mccullough range southern nevada consist of cordierite almanditic garnet biotite sillimanite plagioclase k feldspar quartz ilmenite hercynite this assemblage is indicative of a low pressure facies series at hornblende gr anulite grade textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite thermobarometry utilizing cation exchange between garnet biotite cordierite hercynite and plagioclase yields a preferred temperature range of 590 750 c and a pressure range of 3 4 kb equilibrium among biotite sillimanite quartz garnet and k feldspar records ahlo between 0 03 and 0 26 the low ah o together with low fol qfm and optical properties of cordierite indicate metamorphism under fluid absent conditions preserved mineral compositions are not consistent with equilibrium with a melt phase earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage the relatively low pressures attending high grade metamorphism of the mccullough range paragneisses allies this terrane with biotite cordierite garnet granulites in other orogenic belts closure pressures and temperatures require a transient apparent thermal gradient of at least 50 c km during part of this proterozoic event in the southern cordillera

miocene rapakivi granites in the southern death valley region california usa abstract rapakivi granites in the southern death valley region california include the 12 4 ma granite of kingston peak the ca 10 6 ma little chief stock and the 9 8 ma shoshone pluton all of these granitic rocks are texturally zoned from a porphyritic rim facies characterized by rapakivi textures and miarolitic cavities to an equigranular aplite core these granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern finland chemical and isotope nd sr pb data suggest that rapakivi granites of the southern death valley region were derived by partial melting of lower crustal rocks possibly including mesozoic plutonic component with some mantle input as well they were emplaced at shallow crustal levels 4 km in an actively extending orogen

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

gold butte crustal section south virgin mountains nevada the gold butte block consists of a complex of proterozoic medium to high grade metamorphic and plutonic rocks unconformably overlain by steeply east dipping paleozoic sedimentary rocks on its east side this complex is one of the largest continuous exposures of crystalline rocks in the southwestern united states that lie structurally beneath a steeply tilted section of phanerozoic strata and it may represent part of an intact proterozoic miocene crustal section reaching paleodepths of approximately 15 km the section was exposed during tectonic denudation in miocene time at about 15 ma and lies directly adjacent to the colorado plateau to the east proterozoic and tertiary geological features support the hypothesis that the block represents a relatively intact crustal section a large exposure of proterozoic rapakivi granite the gold butte granite circa 1 45 ga crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance which increases toward the top of a 10 to 12 km thick magma chamber the wallrocks of the granite are predominantly garnet bearing gneisses circa 1 7 ga which display increasing pressures of final equilibration westward away from the basal phanerozoic unconformity thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2 3 kbar whereas those 12 13 km west of the unconformity yield pressures of 5 6 kbar suggesting the crustal section was established by 1 7 ga the style of deformation displayed by denudation structures also varies from east to west with brittle faulting only in eastern portions of the block and retrograde metamorphism mylonitization and chlorite brecciation of the garnet gneiss observed in western areas

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

lead isotopic compositions of diverse igneous rocks and ore deposits from southwestern new mexico and their implications for early proterozoic crustal evolution in the western united states u pb zircon measurements from some of the basement rocks in southwestern new mexico have ages that range from 1 650 to 1 450 m y the largest ore deposits occur near silver city in the southwest part of the region studied they are associated with laramide alkali to calc alkaline plutons and their lead isotopic compositions are the least radiogenic in the region this lead exhibits lower crustal upper mantle characteristics and for the major producing porphyry copper deposits the 206 pb 204 pb ratios are less than 18 0 lead from more silicic mid tertiary volcanic rocks and associated ore deposits has somewhat higher 208 pb 204 pb ratios that reflect a greater crustal involvement in their origin in a regional trend to the northeast ore lead becomes more radiogenic and at hansonburg 200 km from silver city lead in the mississippi type deposits is clearly derived from upper crustal sources on the 207 pb 204 pb 206 pb 204 pb plot data from all of the rocks and ores that we have analyzed form an array that lies below the average orogene curve of doe and zartman 1979 a common source is implied for the lead the isotopic composition of which was similar to that found in 1 750 m y old stratiform deposits as far apart as pecos new mexico and jerome arizona such a composition indicates that over a large region of the southwestern united states continental crust developed between 1 750 and 1 450 m y ago possibly in an island arc environment basement rocks that are 1 750 m y old extend northward through colorado to utah galena data obtained in previous studies show that the fraction of older sialic lead in those rocks increases toward the archean craton in wyoming the crust apparently developed southward from wyoming in stages at 2 400 m y ago or before 2 100 m y ago and 1 750 m y ago with incorporation of older sialic material in each stage as far south as milford utah

ree and sr isotope characteristics of carbonate within the cu co mineralized sedimentary sequence of the nchanga mine zambian copperbelt carbonate largely in the form of dolomite is found throughout the host rocks and ores of the nchanga mine of the zambian copperbelt dolomite samples from the hanging wall of the mineralization show low concentrations of rare earth elements ree and roof shaped upward convex shale normalized ree patterns with positive eu sn anomalies 1 54 and 1 39 and marginally negative ce anomalies ce sn 0 98 0 93 in contrast dolomite samples associated with copper and cobalt mineralization show a significant rotation of the ree profile with hree enrichment and la lusn ratios 1 0 06 0 42 these samples also tend to show variable but predominantly negative eu sn and positive cerium anomalies and an upwardly concave mree distribution gd er malachite samples from the lower orebody show roof tile normalized ree patterns with negative europium anomalies eu sn 0 65 0 80 and negative cerium anomalies ce sn 0 86 0 9 the carbonate 87sr 86sr signature correlates with the associated ree values the uppermost dolomite samples show neoproterozoic seawater like 87sr 86sr ratios ranging from 0 7111 to 0 7116 whereas carbonate from cu co mineralized samples show relatively low concentrations of strontium and more radiogenic 87sr 86sr ranging between 0 7136 0 7469 the malachite samples show low concentrations of strontium but give a highly radiogenic 87sr 86sr of 0 7735 the most radiogenic 87sr 86sr ratio these new data suggest that the origin and timing of carbonate precipitation at nchanga is reflected in the ree and sr isotope chemistry the upper dolomite samples show a modified but essentially seawater like signature whereas the rotation of the ree profile the mree enrichment the development of a negative eu sn anomaly and more radiogenic 87sr 86sr suggests the dolomite in the cu co mineralized samples precipitated from basinal brines which had undergone significant fluid rock interaction petrographic ree and 87sr 86sr data for malachite are consistent with the original sulfide lower orebody being subject to a later oxidizing event

assessing the origin of sr and nd isotopes and ree y in middle upper pleistocene travertines from the acquasanta terme area marche central italy and implications for neotectonics abstract pleistocene bedded travertines from acquasanta terme display ranges of sr and ree y contents 462 1576 mg kg and 250 13 789 ng g respectively which are consistent with an evaporitic rock source paas normalized ree y patterns may support the origin of travertine bearing fluids from marine rocks travertines exhibit a very narrow range of 87sr values from 1 9 to 1 84 or 87sr 86sr ratios from 0 70782 6 to 0 70786 6 indicating an unchanged rock source throughout the travertine depositional time from about 350 to 26 ka likely represented by burano evaporites upper triassic as suggested by previous authors no sr contribution from magmatic sources has been detected travertines also display a very narrow range of nd values from 9 1 to 7 8 or 143nd 144nd ratios from 0 51217 to 0 51224 indicating an unchanged source also for ree y through time likely the same one providing sr lastly chemical and isotopic data of acquasanta travertines have been compared with those of bedded travertines from semproniano tuscany of similar age in particular 87sr values clearly distinguish between the two groups in agreement with their different rock sources as a whole these new geochemical and isotopic data on acquasanta travertines have provided a deeper insight into the origin of their parent hydrothermal fluids which upwelling from significant depth circulated through an area still affected by major seismic activity in particular the application of nd isotopes has resulted useful as unlike sr isotopes of fluid which tend to re equilibrate rather quickly with rocks along upwelling pathways nd isotopes sluggishly do so thus better reflecting the fluid source

upward flow of magmatic fluids from the old woman granodiorite old woman mountains southeastern california isotopic compositions mineral equilibrium and field relations at the contact between the midcrustal cretaceous old woman granodiorite and paleozoic carbonates indicate that water rich silica saturated magmatic fluids were transported upward away from the pluton across an impermeable 30 to 40 m thick marble which caps the granodiorite to higher structural levels along a complex network of hydrologically induced fractures within the fractures fluids reacted to form symmetrical radiating splays of wollastonite with minor amounts of diopside vesuvianite and quartz in many cases pegmatites are found in the center of these calc silicate skarns cross cutting pegmatites and wollastonite veins in the aureole indicate that during late stages of crystallization of the granodiorite there were multiple episodes of fluid expulsion above the marble layer at higher structural levels magmatic fluids flowed both laterally and vertically interacting with lithologies in a more pervasive manner values of 18o for calcite in the vein skarns average 11 8 and pegmatite whole rock silicate 18o values average 9 4 thus oxygen isotopic compositions are consistent with a magmatic origin for the skarn forming fluids away from the vein skarns values of 18o for the capping marble range from 18 7 to 22 1 avg 21 and values of 18c range from 3 8 to 3 0 avg 3 4 the high 18o values provide evidence that the marble largely retained its premetamorphic isotopic composition indicating that fluids from the granodiorite did not flow pervasively across the unit lithologies at higher structural levels show evidence of more pervasive interaction with magmatic fluids forsterite bearing calc silicates have 18o values down to 11 8 and coarse grained vesuvianite and wollastonite bearing skarns have 18o values of 13

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

anhydrite pseudomorphs and the origin of stratiform cu co ores in the katangan copperbelt democratic republic of congo the stratiform cu co ore mineralisation in the katangan copperbelt consists of dispersed sulphides and sulphides in nodules and lenses which are often pseudomorphs after evaporites two types of pseudomorphs can be distinguished in the nodules and lenses in type 1 examples dolomite precipitated first and was subsequently replaced by cu co sulphides and authigenic quartz whereas in type 2 examples authigenic quartz and cu co sulphides precipitated prior to dolomite and are coarse grained the sulphur isotopic composition of the copper cobalt sulphides in the type 1 pseudomorphs is between 10 3 and 3 1 relative to the vienna canyon diablo troilite indicating that the sulphide component was derived from bacterial sulphate reduction bsr the generation of text hco 3 during this process caused the precipitation and replacement of anhydrite by dolomite a second product of bsr is the generation of h2s resulting in the precipitation of cu co sulphides from the mineralising fluids initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core precipitation of authigenic quartz was most likely induced by a ph decrease during sulphide precipitation fluid inclusion data from quartz indicate the presence of a high salinity 8 18 eq wt nacl fluid possibly derived from evaporated seawater which migrated through the deep subsurface 87sr 86sr ratios of dolomite in type 1 nodules range between 0 71012 and 0 73576 significantly more radiogenic than the strontium isotopic composition of neoproterozoic marine carbonates 87sr 86sr 0 7056 0 7087 this suggests intense interaction with siliciclastic sedimentary rocks and or the granitic basement the low carbon isotopic composition of the dolomite in the pseudomorphs 7 02 and 9 93 relative to the vienna pee dee belemnite v pdb compared to the host rock dolomite 4 90 and 1 31 v pdb resulted from the oxidation of organic matter during bsr

ree and their relevance to the development of the kupferschiefer copper deposit in poland abstract over one hundred samples representing mainly clayey organic and carbonate rich shales kupferschiefer but also other members of different ore sections including hangingwall dolomites z1 werra and footwall weissliegend sandstone lower permian have been collected in different mines of the lubin glogow mining district mainly near the contact transitional zone between the copper mineralized zone and secondarily oxidized rote faule rf zone in general the polish kupferschiefer shales are enriched in mree in comparison to nasc in a typical copper rich ore section the ree amounts and patterns depend on lithologies being generally similar in shales and dolomite ree vary among sandstones shales and dolomites average 73 143 and 85 ppm respectively probably reflecting mainly their clay contents sandstones have strongly convex ree patterns with positive eu and negative gd anomalies and depletion in lree and enrichment in mree relative to hree the ree patterns of shale and dolomite are similar to one another and rather flat with strong negative eu anomalies and a small positive gd anomaly in the case of shales the ree patterns of shales from the mineralized cu zone are generally convex mree enriched and have negative eu anomalies however in a section with cu zn and pb shales the ree pattern of pb bearing shales shows a positive eu anomaly in contrast to other shales and overlying dolomite more oxidizing conditions of deposition can be assumed for pb shales no significant differences between ree distributions in transitional and oxidized zones have been observed their ree patterns are more convex and are much higher av 247 ppm than those in the mineralized zone and they do not show eu anomalies the strongly convex pattern may suggest either enrichment in mree or relative depletion in lree due to localized precipitation of light ree minerals both in shales and in the uppermost part of the sandstones two unique sections one cu rich and one cu lean partly oxidized comprising three shale beds interbedded with dolomites have been compared generally ree contents are similar in these two sections also similar are changes in contents of ree between beds in both sections which decrease significantly upwards from 157 171 ppm to 54 60 ppm the ree patterns of the lowermost beds directly overlying sandstones are ramp like with lree enrichments the upper beds have concave ree patterns comparison between sections shows generally stronger negative eu and positive gd anomalies in the highly mineralized section there is a highly significant positive relationship between cu and ree contents in cu rich shales and slightly less significant negative relationship for their concentration in oxidized and transitional shales there is a moderate significant positive correlation between p 2 o 5 and ree contents in cu rich shales the observed differences in ree contents cannot be provenance dependent but have been caused by diagenetic processes possibly related to mineralization and oxidation processes europium anomalies generally reflecting different eh conditions in the deposit can be eliminated by the prolonged oxidation strong enrichment of the rf zones in ree may result from their desorption from large volumes of oxidizing including mineralizing solutions which probably emerged from the underlying molasse lithologies into the rote faule areas higher contents of ree in the lowermost shales suggest upward movement of solutions from the underlying sandstones also far away from the rf areas

isotopic evidence for the provenance of turquoise in the southwestern united states the archaeological record shows that turquoise was widely mined and highly valued by pre hispanic societies in the southwestern united states and it has long been assumed that much of the turquoise noted in ancient mesoamerica was traded from this region however little is understood about the acquisition and exchange of turquoise by native american societies because the geological sources of most turquoise artifacts from archaeological sites in the southwestern u s and mexico are not known we evaluate the potential for pb and sr isotopic ratios to indicate the geological provenance of turquoise artifacts recovered from these regions pb and sr isotopic measurements were made on 137 geological samples of turquoise from 19 mining districts across the southwestern united states and northern mexico these data reveal that isotopic signatures of turquoise deposits vary geographically according to regional and local differences in the geologic settings of turquoise mineralization as an archaeological case study we also report pb and sr isotopic data on 10 turquoise artifacts recovered from three late pre hispanic ca a d 1250 1400 ancestral zuni sites located in the el morro valley of western new mexico these have isotopic signatures uniquely consistent with the turquoise deposits of the cerrillos hills a location identified in zuni traditional history as an ancient source of turquoise these data thus establish pb and sr isotopic measurements as powerful tools for determining the sources of turquoise artifacts and provide a new framework for evaluating the role of turquoise in pre hispanic exchange networks across north america

assembling and disassembling california a zircon and monazite geochronologic framework for proterozoic crustal evolution in southern california abstract the mojave province in southern california preserves a comparatively complete record of assembly postorogenic sedimentation and rifting along the southwestern north american continental margin the oldest exposed rocks are metasedimentary gneisses and amphibolite enclosing intrusive suites that range from tonalite and quartz monzodiorite to granite with minor trondhjemite discrete magmatic episodes occurred at approximately 1790 1730 and 1690 1640 ma evidence from detrital and premagmatic zircons indicates that recycling of 1900 1790 ma paleoproterozoic crust formed the unique isotopic character of the mojave province peak metamorphic conditions in the mojave province reached middle amphibolite to granulite facies metamorphism occurred locally from 1795 to 1640 ma with widespread evidence for metamorphism at 1711 1689 and 1670 1650 ma structures record early tight to isoclinal folding and penetrative west vergent shear during the final metamorphic event in the west mojave province prot

ree composition of primary and altered feldspar from the mineralized alteration zone of alkaline intrusive rocks western yunnan province china abstract the yao an gold deposit is situated northwest of kunming in yunnan province china is associated spatially with yao an alkaline intrusion the yao an gold mineralization may be divided into two ore stages the stage i sulfide stage fine grained pyrite chalcopyrite galena stringer veins and the stage ii oxide plus sulfide tectonic breccias and banded ore the stage i ores occur in the zones of pervasive potassic alteration the laser ablation icp ms technique was used to analyse ree in primary perthites and altered k feldspars of potassic alteration zone related to mineralization in the yao an gold deposit the primary perthites from yao an syenite porphyry all have similar ree features with light ree enrichment large positive eu anomalies and small but significant negative ce anomalies the ree patterns of altered k feldspars have broadly similar features but compared to the primary perthite have much higher ree concentrations and smaller positive eu anomalies these characteristics show that the hydrothermal fluids responsible for alteration were ree rich due to high levels of complex forming anions but had large negative eu anomalies and other features best ascribed to magmatic crystal fractionation these fluids were derived essentially from the magmatic system

hafnium oxygen neodymium strontium and lead isotopic constraints on magmatic evolution of the supereruptive southern black mountains volcanic center arizona u s a a combined lass zircon whole rock study the 700 km 3 peach spring tuff pst erupted at 18 8 ma from the silver creek caldera in the southern black mountains volcanic center sbmvc of western arizona is the only supereruption scale ignimbrite in the northern colorado river extensional corridor the sbmvc contains pre and post caldera volcanic rocks and caldera related intrusions 19 17 ma that provide a detailed petrologic record of ignimbrite antecedence and aftermath whole rock sr nd pb hf isotopic data combined with complementary zircon o and hf isotopic data from a suite of pre through post pst samples provide robust constraints on 1 how the sbmvc evolved with respect to magmatic sources and processes throughout its 2 ma history and 2 the petrogenetic relationships between the pst and slightly younger intracaldera plutons both pre and post pst units have isotopic ranges nd 8 3 to 11 6 hf 8 2 to 14 0 87 sr 86 sr i 0 709 0 712 206 pb 204 pb 18 19 18 49 207 pb 204 pb 15 60 15 62 208 pb 204 pb 38 95 39 29 that fall within the spectrum of miocene colorado river extensional corridor rocks and are consistent with mixing of substantial fractions of proterozoic mojave crust and juvenile material derived from regional enriched mantle compared to the pst which has relatively uniform isotopic ratios nd 11 4 to 11 7 hf 13 8 to 14 3 87 sr 86 sr i 0 709 0 712 206 pb 204 pb 18 20 18 29 207 pb 204 pb 15 60 15 62 208 pb 204 pb 39 02 39 33 individual pre and post pst units are isotopically more variable and generally more primitive consistent with whole rock isotopes zircon hf 8 to 14 and oxygen 18 o 4 5 to 7 2 for most pre and post pst units also have wider ranges and more mantle like values than those of the pst 12 to 15 6 1 to 7 1 moreover zircon isotopic compositions decrease in post pst samples a few zircons from post pst intrusions have 18 o values lower than mantle values whole rock and zircon elemental and isotopic analyses indicate that 1 most pre and post pst units are less evolved and less homogenized than the pst itself 2 intrusions in the silver creek caldera are petrogenetically distinct from the pst and therefore represent discrete magmatic pulses not unerupted pst mush 3 enriched mantle input increased in the sbmvc following the paroxysmal pst eruption 4 post pst history of the sbmvc was characterized by periodic influx of magmas with varying juvenile fractions into pre existing mushy or solidified intrusions resulting in variable and incomplete hybridization and 5 melting and assimilation of hydrothermally altered crust played a relatively minor role in the generation and evolution of magmas in the sbmvc

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

proterozoic geochronologic and isotopic boundary in nw arizona u pb ages determined from zircon sphene and apatite in conjunction with pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north trending proterozoic crustal boundary in northwestern arizona two regions west of the boundary hualapai mountains and lost basin range garnet mountain are characterized by complex u pb zircon systematics evidence for inheritance of an older zircon component 1 8 2 3 ga and elevated sup 207 pb sup 204 pb from feldspars compared to the east although the discordia patterns are complex supracrustal rocks are interpreted to be ca 1 73 ga and are intruded by plutonic rocks ca 1 70 ga deformation is younger than ca 1 70 ga foliated granites and older than the 1 682 plus minus 4 ma garnet mt monzogranite the rocks in one area east of the boundary cottonwood cliffs are characterized by relatively simple u pb zircon systematics no evidence for inheritance of any older component and feldspar sup 207 sup 204 pb near model mantle values supracrustal rocks are older than 1 73 ga as they are intruded by a 1 730 plus minus 9 ma foliated granodiorite timing of deformation is constrained by the late syn kinematic valentine granite dated at 1 713 plus minus 12 ma mineral ages indicate thatmore the rocks on either side of the boundary had different cooling histories and inferentially different uplift histories west of the boundary the cooling history is inferred from minerals separated from an amphibolite metamorphic zircon is 1 687 13 minus 8 ma sphene is 1 660 plus minus 5 ma hornblende has a sup 40 ar sup 39 ar age of 1 552 plus minus 5 ma and apatite has a u pb age of 1 520 plus minus 45 ma east of the boundary an amphibolite has sphene with an age of 1 670 plus minus 11 ma and apatite with an age of 1 630 plus minus 8 ma less

origin of gneisses in the aureole of the san gabriel anorthosite complex and implications for the proterozoic crustal evolution of southern california granulite facies gneisses along the southwestern margin of the san gabriel anorthosite complex include compositionally banded locally migmatitic felsic gneisses and mafic granulites with rare interlayered augen gneiss and garnet hypersthene spinel aluminous gneiss previous workers accepted a 1 4 ga age for these granulites and a 1 2 ga age for the adjacent anorthosite complex and hence that this proterozoic basement is allochthonous with respect to the southwestern united states part of the north american craton solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900 950 c and new u pb zircon data yield a minimum metamorphic age of 1186 ma coincident with that of the adjacent anorthosite we reinterpret these rocks as a 1190 ma anorthosite complex and its granulite grade thermal aureole geochemical data suggest that the anorthosite complex intruded early proterozoic rocks correlative with the mojave crustal province representing the north american craton exposed to the northeast we conclude that these proterozoic basement rocks represent the southwestern margin of the north american craton and not an exotic terrane accreted in mesozoic or tertiary time