u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

formation of intra arc volcanosedimentary basins in the western flank of the central peruvian andes during late cretaceous oblique subduction field evidence and constraints from u pb ages and hf isotopes during late early to late cretaceous the peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism the peruvian coastal batholith and formation of volcanosedimentary basins known as the western peruvian trough wpt we present high precision u pb ages and initial hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the wpt hosting the perubar massive sulfide deposit zircons extracted from both the volcanic and intrusive rocks yield concordant u pb ages ranging from 67 89 0 18 ma to 69 71 0 18 ma indicating that basin subsidence submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the andean magmatic arc during the late cretaceous field observations satellite image interpretation and plate reconstructions suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction wrench tectonics is therefore considered to be the trigger for the formation of the wpt as a series of pull apart basins and for the emplacement of the coastal batholith the zircon initial ehf values of the dated magmatic rocks fall between 5 5 and 7 4 and indicate only very subordinate influence of a sedimentary or continental component the absence of inherited cores in the zircons suggest a complete lack of old basement below the wpt in agreement with previous u pb and sr isotopic data for batholithic rocks emplaced in the wpt area this is supported by the presence of a most likely continuous block of dense 3 0 g cm3 material observed beneath the wpt area on gravimetric crustal cross sections we suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and or oceanic crust inherited from a possible late permian triassic rifting such young and mafic crust was the most probable source for arc magmatism in the wpt area

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

the late archean history of the wyoming province as recorded by granitic magmatism in the wind river range wyoming abstract the wyoming province a small ca 500 000 km 2 archean craton is the most southwestern of the archean provinces in north america it is composed primarily of late archean potassium rich granitic rocks in contrast to many other archean provinces rocks of tonalite trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2 8 ga field petrologic geochemical and isotopic study of the late archean granites exposed in the wind river range have allowed us to identify at least four periods of potassic calc alkalic magmatism at 2 8 2 67 2 63 and 2 55 ga granitic rocks of these ages appear to be widespread across the wyoming province the oldest calc alkalic granites of the wind river range emplaced at ca 2 8 ga appear to be derived predominantly from pre existing crust however nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses during at least two other periods of plutonism at 2 67 and 2 63 ga generation of the wind river range batholiths involved the incorporation of substantial amounts of isotopically juvenile material either from depleted mantle or young continental crust the information presented below as well as data available from elsewhere in the wyoming province is interpreted to suggest that the wyoming province unlike other archean cratons is not composed of a tectonic amalgamation of smaller exotic terranes although the wyoming province did experience crustal addition in archean time it was not by lateral accretion but by incorporation of mantle derived melts into large granitic batholiths

lead isotope and trace element geochemistry of paleoproterozoic metasedimentary rocks in the lead and rochford basins black hills south dakota usa implications for genetic models mineralization ages and sources of leads in the homestake gold deposit abstract mixed silicate carbonate sulfide facies banded iron formations with elevated detrital components were deposited in the northern black hills during middle paleoproterozoic time between 2 012 and 1 974 ga homestake iron formation and equivalents and at 1 887 ga rochford iron formation these iron formations deposited atop shales and basalts in intracontinental rifted basins separated in time by 80 130 myr are the hosts of extensive late paleoproterozoic gold mineralization gold sulfide ore bodies occur within dilated segments of late stage ductile brittle shears that formed after isoclinal folding related to wyoming superior continental collision lead stepwise leaching pbsl data for monazite bearing garnet separated from a sample of homestake iron formation has yielded an isochron age of 1746 10 ma 2 mswd 0 42 which represents a maximum age for both the isoclinal folding and subsequent gold mineralization monazite bearing garnet from a nearby harney peak type granitic pegmatite believed to be coeval with latest stage semibrittle shears and quartz veins that crosscut homestake ore bodies has yielded a pbsl isochron age of 1713 10 ma 2 mswd 4 1 which probably represents a minimum age for the gold event pb isotopes of sulfides from replacement ore and shear ore within the homestake iron formation cannot distinguish between these distinct ore types however both pb isotopes and rey rare earth and yttrium abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation thus revealing an epigenetic sensu stricto component to the gold mineralization the pb isotope characteristics of the ore further reflect an admixed component with low 232th 204pb and variable th u ratios in addition to compositions directly derived from bif typical leads this added component corresponds to pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite the apparent pb isotopic match between ore and granite is further consistent with the pbsl data for shear ore type arsenopyrite which define an isochron age of 1719 45 38 ma 2 mswd 0 2 and by a striking overlap of the pb isotopic data fields defined by the homestake galenas and pegmatitic feldspars the precambrian ore leads are therefore best explained as mixtures of magmatic hydrothermal and host rock mobilized leads the lead area also contains significant gold bearing deposits of tertiary age which are hosted in both paleoproterozoic and phanerozoic rocks tertiary mineralization was temporally and spatially related to coeval plutons emplaced during laramide uplift of the black hills and represents the final mineralization event recognized in the homestake mine area like the precambrian galena leads the leads of tertiary galena hosted by middle paleoproterozoic rocks including the homestake iron formation define a trend best explained by mixtures of pb derived from magmatic fluids exsolved from tertiary intrusives and pb leached from the same country rocks that contaminated the granite derived hydrothermal fluids ascending through them during the much earlier shearing and mineralization event inferred at 1730 1715 ma

concentric slow cooling of a low p high t terrane evidence from 1600 1300 ma mica dates in the 1780 1700 ma black hills orogen south dakota u s a the crystalline core of the southern black hills south dakota exposes an extensive low p high t aureole of garnet to second sillimanite zone schists centered on the plutonic core of the 1715 ma harney peak granite hpg this paper demonstrates regional patterns of apparent ages observed for 52 40 ar 39 ar dates of muscovite and biotite in diverse rocks from across the 1000 km 2 metamorphic aureole and its plutonic center about 20 biotite dates sampled mostly near faults are influenced by excess 40 ar and obscure the regional trends the remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite with previously unrecognized elliptical age zones centered on the main hpg pluton and its outliers the regional pattern of 40 ar 39 ar cooling ages indicates non uniform slow cooling of the mid crust between 1600 1250 ma this scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite rb sr and apatite u pb which range from 1690 to 1550 ma and from 1700 to 1500 ma respectively to explain these results it is likely that ambient pre granite temperatures of the country rocks were 350 c at the 10 14 km depth of granite emplacement as previously proposed and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years alternatively or in addition the hpg and inner aureole were not uplifted until 1480 1330 ma whereupon they finally cooled through 300 350 c

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

step leach pb pb dating of inclusion bearing garnet and staurolite with implications for early proterozoic tectonism in the black hills collisional orogen south dakota united states we report 207 pb 206 pb stepwise leaching pbsl ages of 1762 15 ma 1759 8 ma and 1760 7 ma for almandine garnet staurolite and garnet staurolite combined in an early proterozoic metapelite from the black hills collisional orogen south dakota pbsl derived 208 pb 206 pb th u trends reveal that staurolite contains inclusions of both cogenetic monazite ca 1760 7 ma and older detrital zircon 2040 ma whereas coexisting garnet contains only the zircon these results and petrologic data indicate prograde sequential growth of garnet monazite and staurolite during tectonic burial at temperatures between 400 c and 550 c i e well below pb closure temperatures we interpret the 1760 7 ma date as a maximum age for wyoming superior continental collision in the black hills which apparently postdated the 1800 1900 ma trans hudson orogeny in canada but was nearly synchronous with north directed accretion of the central plains orogen in southeastern wyoming

high closure temperatures of the u pb system in large apatites from the tin mountain pegmatite black hills south dakota usa abstract the cores of several large 1 2 cm apatites from the early proterozoic tin mountain pegmatite black hills south dakota have concordant u pb ages the age uncertainties of these samples are less than 5 ma the lead isotopic compositions of muscovite and k feldspar coexisting with the apatites were used for initial lead isotopic corrections the resulting u pb concordia ages agree well with u pb and pb pb isochron ages based solely on multiple fragments from single apatites this agreement justifies the use of coexisting muscovite and k feldspar for initial lead corrections the precision of these u pb ages is comparable to those of the u pb systems of more commonly analyzed minerals such as zircon monazite and sphene the estimated u pb closure temperature t c of these apatites is about 620 c this t c is in close agreement with the t c calculated for 0 5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite apatite can therefore yield high precision u pb ages that are close to its crystallization age in igneous rocks

nd o and sr isotopic constraints on the origin of precambrian rocks southern black hills south dakota abstract the nd o and sr isotopic characteristics of precambrian metasedimentary metavolcanic and granitic rocks from the black hills of south dakota are examined two late archean granites 2 5 2 6 ga have t dm ages of 3 05 and 3 30 ga suggesting that at least one of the granites was derived through the melting of significantly older crust early proterozoic metasedimentary rocks have t dm ages that range from 2 32 to 2 45 ga these model ages in conjunction with probable stratigraphic ages ranging from 1 9 to 2 2 ga indicate that mantle derived material was added to the continental crust of this region during the early proterozoic previous studies of the harney peak granite complex have reported u pb and rb sr ages of about 1 71 ga and most granite samples examined in this study have sr isotopic compositions consistent with that age two granite samples taken from the same sill however give two point rb sr and sm nd ages of 2 08 0 08 and 2 20 0 20ga 2200 nd 15 5 respectively in addition whole rock and apatite samples of the spatially associated tin mountain pegmatite give a sm nd isochron age of 2000 100 ma 2200 nd 5 8 1 8 the sm nd o and rb sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post crystallization processes and the age of the pegmatite and parts of the harney peak granite complex remain uncertain processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material assimilation mixing of ree between granite and country rock during crystallization via a fluid phase and post crystallization mobility of sr the nd isotopic compositions of the pegmatite and the harney peak granite indicate that they were not derived primarily from the exposed metasedimentary rocks

a gravity traverse across the coastal batholith peru a 80 km long gravity traverse with 43 stations was made across the lima segment of the peruvian coastal batholith eastwards from the coast at huacho lat 11 s through sayan to churin a second traverse 30 km long with 14 stations was made from near sayan southwards to the coast at rio seco laboratory measurements of specific gravity were made on 74 samples taken from all the major units of the batholith and its envelope a bouguer anomaly gradient of 5 mgal km strikes parallel to the axis of the batholith this gradient fits a model with an upper crust of 2650 2750 kg m 3 in density which thickens from 2 5 km at the coast to 25 km at 125 km inland this wedge of light rock includes the coastal batholith and the miogeosynclinal sediments in addition the model requires either a buried granitoid beneath churin at the eastern end of the traverse or a lateral change of density in the lower crust after the regional gradient is removed the residual anomalies can be correlated well with the distributions and densities of acid and basic rocks in the batholith positive residuals over the acid central complex of the rio huaura are interpreted quantitatively in terms of bell jar subsidence of gabbro diorite blocks

petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the black hills south dakota abstract four suites of mafic to intermediate intrusions and flows that occur in the precambrian core of the black hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late archean and early proterozoic history of the region most of the mafic rocks were later metamorphosed to amphibolites during the black hills orogeny the collision of the wyoming and superior cratons which began at 1760 ma compositions of the mafic suites define at least two distinct tectonic settings of magmatism between 2480 and 1880 ma the 2 48 ga blue draw intrusion located on the eastern edge of the precambrian core is differentiated with compositions ranging from gabbro to granodiorite it is a calcalkaline intrusion with relatively high sio2 from 51 8 to 60 5 wt and k2o mostly 1 wt ree patterns are slightly concave upward and show lree enrichment the intrusion is also enriched in ba rb th and zr these and other geochemical discriminants are typical of modern calcalkaline basalts the intrusion s calcalkaline character is similar to the neighboring slightly older 2 56 ga little elk granite gneiss these two intrusions support the presence of late archean early proterozoic arc magmatism on the eastern margin of the wyoming craton the three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments including black shales limestones iron formations and cherts u pb zircon ages suggest their emplacement between 2 01 and 1 88 ga mafic rocks in the eastern half of the precambrian core in the mt rushmore and pactola dam quadrangles and near bear mountain on the western margin of the precambrian core have slightly depleted to enriched tholeiitic compositions with sio2 50 wt and k2o generally 2 5 wt and the lowest sio2 concentrations typically

three dimensional p and s wave velocity structures of southern peru and their tectonic implications in 1981 an 18 station regional array was operated in southern peru above the area where the dip of the subducted nazca plate changes from virtually horizontal in the northwest to about 30 in the southeast arrival times of compressional p and shear s waves from microearthquakes recorded by this array are used in this paper to investigate the three dimensional velocity structure of the crust and upper mantle in the region between the coast and the cordillera occidental the results suggest a crustal thickness of about 40 km beneath the coast increasing to about 70 km beneath the cordillera occidental this change in thickness occurs abruptly in the northwest but is gradual in the southeast the inverse correlation between the dip of the moho and the dip of the slab suggests a broad scale causal relation between the two at the same time free air gravity anomalies suggest that the steepening of the moho in the northwest may occur only in the region under the array because the crust is thicker in the southeast than originally surmised a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust the association of these earthquakes with a shallow dipping moho is suggestive of ongoing crustal deformation to the west of the andes however other seismological and geological observations make the role of these earthquakes in crustal deformation unclear s wave velocities in the mantle between 70 and 130 km depth above the 30 dipping slab are low suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern peru

neoarchean and paleoproterozoic events in the minnesota river valley subprovince with implications for southern superior craton evolution and correlation abstract the lithologic diversity and protracted geologic history of the minnesota river valley mrv gneiss terrane at the southern margin of the archean superior province provide key perspectives on the growth and modification of this craton innovations in geochronology specifically the application of the chemical abrasion technique to remove pb loss domains in zircon and the tandem use of in situ geochemistry and precise isotope dilution age analysis for establishing the petrologic context of radioisotopic ages are yielding new insights into these old rocks first u pb geochronology on a variety of plutonic to mesoscale intrusive rocks from the morton and montevideo blocks of the mrv provide precise constraints on the neoarchean 2603 2584 ma sacred heart orogeny which sutured the mesoarchean gneisses of the mrv to the southern margin of the superior craton in an episode of contractional deformation high grade metamorphism and voluminous plutonism second we report the first radioisotopic age constraints on the st leo and taunton belts of greenstone and supracrustal rocks within the yellow medicine shear zone an inferred lithosphere scale structure juxtaposed between the morton and montevideo gneisses titanite from a metadiorite intrusion into the st leo belt yields igneous 2539 ma and metamorphic 2510 ma ages confirming the archean provenance of the host greenstones and revealing a latest neoarchean episode of magmatism and deformation focused along and within the shear zone third a new 1781 ma u pb zircon age for one of a number of small granitic plugs in the southern montevideo block confirms its correlation to the more voluminous east central minnesota batholith and its origin in the yavapai orogeny again later magmatism was focused along the yellow medicine shear zone suggesting the lithosphere scale significance of that structure this more resolved history for the neoarchean to proterozoic evolution of the southern superior craton provides new opportunities to test archean craton correlations and reconstructions including links to the wyoming and slave cratons

extension related to a high topography results from a microearthquake survey in the andes of peru and tectonic implications summary a microearthquake study performed in 1985 october november in the cordillera blanca western cordillera of northern peru allowed us to make a precise mapping of its shallow seismicity 160 local events are accurately located in the vicinity of an active normal fault system showing a vertical displacement of 4500 m over the last 7 myr part of the seismic activity can be related to this west dipping system but a great number of events indicate a new seismic zone situated under the highest peaks of the chain by using an inversion algorithm of p wave polarities we show a regional deviatoric stress tensor which defines an extensional axis in a direction approximately orthogonal to the andean structures this result is in good agreement with the theoretical distribution of stresses expected in an elongated high topography bordered by tectonic convergent zones moreover we suggest that an absolute uplift of this young batholith occurs on a normal fault system and that this phenomenon is partly due to buoyancy forces originating in its crustal root

the isotopic ecology of late pleistocene mammals in north america part 1 florida abstract mammoths and mastodons are common in pleistocene deposits yet these proboscideans and many other animals disappeared suddenly 10 000 years ago in this study we reconstruct the diets of proboscideans and associated mammals through isotopic analysis of carbonate in tooth enamel apatite in order to test nutritional hypotheses for late pleistocene extinction we analyzed specimens from six sites in florida ranging from full glacial 21 000 bp to late glacial 14 750 to 10 000 bp age the oxygen isotope composition of mammalian apatite covaries with meteoric water composition which in turn varies with climate consequently oxygen isotope analysis can be used to assess the potential for time averaging or mixing of specimens from different geographic regions within fossil assemblages the carbon isotope composition of an herbivore is controlled by the isotopic composition of the plants that it ingests carbon isotope analysis reveals that mastodons ate chiefly c3 plants presumably trees shrubs and herbs whereas mammoths consumed chiefly c4 grass several nutritional hypotheses for late pleistocene extinction entail the assumption that extinct taxa had specialized diets the resource partitioning and focused feeding preferences of florida s proboscideans corroborate this assumption but they do not in themselves prove that nutritional stress was the cause of the late pleistocene extinction

40ar 39ar evidence for ca 1800 ma tectonothermal activity along the great falls tectonic zone central montana late paleoproterozoic 1900 1600 ma tectonothermal activity on all borders of the archean wyoming province has long been established by low precision k ar and rb sr studies however recent tectonic models advanced for supercontinent aggregation require improved constraints on the timing of tectonothermal activity along major boundaries on its northwestern boundary the great falls tectonic zone separates the archean wyoming and hearne provinces recently published u pb ages and geochemical data reveal the presence of a paleoproterozoic ca 1860 ma marginal arc magmatic complex along a portion of the great falls tectonic zone in central montana we present nine new 40ar 39ar mineral ages on hornblende and biotite from these same arc rocks which indicate ca 1800 ma thermal activity 500 c and subsequent rapid cooling to below 300 c by 1775 ma this new data set constrains the timing of wyoming hearne collision to between 1860 and 1800 ma and the timing of last significant tectonothermal activity

block and shear zone architecture of the minnesota river valley subprovince implications for late archean accretionary tectonics the minnesota river valley subprovince of the superior province is an archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east northeast trending linear geophysical anomalies two of the block bounding zones are verified regional scale shears the geological nature of the third boundary has not been established potential field geophysical models portray the boundary zones as moderately north dipping surfaces or thin slabs similar in strike and dip to the morris fault segment of the great lakes tectonic zone at the north margin of the subprovince the central two blocks of the subprovince morton and montevideo are predominantly high grade quartzofeldspathic gneiss some as old as 3 6 ga and late tectonic granite the northern and southern blocks benson and jeffers respectively are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late tectonic plutons a belt of moderately metamorphosed mafic and ultramafic rocks h

ground motions from subduction zone earthquakes a total of 258 horizontal components of accelerogram data recorded at soil sites during thrust normal and strike slip earthquakes occurring in seven subduction zones around the pacific ocean were analyzed the results of statistical analyses of 5 per cent damped pseudo velocities psv computed from these accelerograms at 10 periods t between 0 1 and 4 sec indicated no significant differences in the average psv levels of the thrust normal and strike slip data from the northern honshu zone each of these groups of northern honshu data which together comprised one half of the total data base were fit equally well by the same attenuation model the analyses also revealed that at intermediate periods 0 8 t 3 sec the psv observed at stiff soil sites within the northern honshu nankai kuril mexico and alaska zones were on the average significantly greater than the psv observed at similar sites in the peru northern chile and new britain bougainville zones this observation indicates that differences in the source and or travel path characteristics between the two groups accounted for the differences in psv independent evidence supporting source differences is the correlation noted between psv in this period band and the characteristic source complexities for these zones which hartzell and heaton inferred from the teleseismic data of the larger magnitude earthquakes certain anomalous tectonic characteristics of the new britain bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff soil sites within this zone some of the differences in the psv among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics geology greatly influenced the mexican psv data from soft soil sites to a lesser extent geology probably affected the alaskan data which were recorded mostly on deep stiff soils and the new britain bougainville data recorded at yonki which is underlain by softer soil deposits

neogene forests from the appalachians of tennessee usa geochemical evidence from fossil mammal teeth abstract neogene land mammal localities are very rare in the northeastern u s therefore the late miocene early pliocene gray fossil site in eastern tennessee can clarify paleoecological dynamics during a time of dramatic global change in particular the identification of ancient forests and past climate regimes will better our understanding of the environmental context of mammalian evolution during the late cenozoic stable isotope analyses of bulk and serial samples of fossil tooth enamel from all ungulates present at the gray site elucidate paleoecological reconstructions the herbivorous megafauna include taxa of likely north american and eurasian ancestry including the tapir tapirus polkensis rhino teleoceras cf t hicksi camel cf megatylopus sp peccary tayassuidae and proboscidean gomphotheriidae the tapir rhino camel and peccary yield mean stable carbon isotope 13c tooth enamel values of 13 0 13 3 13 8 and 13 1 respectively suggesting forest dwelling browsers this range of 13c values indicates the presence of a c3 dominated ancient local flora because 13c values decline with increasing canopy density the ancient temperate forests from the gray site were moderately dense the lack of significant c4 plant consumption i e tooth enamel 13c values

zircon u pb ages of super units in the coastal batholith peru implications for magmatic and tectonic processes zircon u pb ages on 50 plutonic samples from super units in the lima arequipa and toquepala segments of the peruvian coastal batholith range from 188 to 37 m y revealing previously unrecognized jurassic elements the jurassic plutons represent a distinct time defined continental arc separated from the cretaceous arc by a quiescent period of 50 m y only 4 of the 50 samples have discordant zircons and these are all from the arequipa and toquepala segments this regionally limited discordance interpreted to be due to inherited radiogenic pb from the precam brian basement suggests that there are fundamental differences in the nature of the crust beneath the lima and the arequipa toquepala segments on a grand scale magmatism in the coastal batholith generally evolved through time from mafic to siliceous melts but zircon u pb ages show that the occurrence of small bodies of old siliceous and young mafic rocks was also important close age association between all rock types raises the possibility that mantle derived mafic magmas provided the heat that produced siliceous magmas in either the underplated wedge of the subduction complex or the lower crust throughout the period of batholith emplacement in general age results confirm that super units have short time spans for the santa rosa paccho and linga super units however mapping generalizations were made in the absence of thorough isotopic age and trace element data remapping of these super units with supporting geochemical and geochronological studies will be required before the emplacement history of the batholith is fully understood dated plutons in the paccho super unit yield ages between 64 0 and 39 0 m y they are thus considerably younger than has been previously recognized and cannot belong to one super unit their young ages also verify the eastward migration of the locus of magmatism through time a migration rate of 1 3 mm yr computed for the rocks along latitude 11 08 1 s on the basis of the 105 m y old patap super unit and a 61 m y old pluton in the paccho super unit suggests that the geometry of the andean subduction zone in peru was very stable during batholith emplacement

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

structure of the frontal part of the andean convergent margin a multichannel seismic reflection record across the central peru margin and trench was improved by processing 24 rather than the 12 channels previously processed and by thorough migrating to reject the strong diffractions that obscured weaker primary reflections the increased resolution clarifies the structure of the 15 km wide frontal accretionary complex and the adjacent truncated continental framework against which the trench sediment was imbricated resolved are individual thrust slices and packets adjacent to the trench axis subducting sediment filled graben in the ocean crust beneath the lower slope and a tertiary stratigraphic section of the upper slope yaquina basin which is cut by normal faults the resolution in this multichannel record provides criteria for reinterpreting single channel data off chile with increased confidence the previously proposed truncation of the south american continent along much of the peru chile trench is confirmed and the development of an accretionary complex in front of the truncated continental crust appears to vary with the amount of sediment seen in the trench axis the andean margin frontal structure is similar to that off central america the aleutian trench and the japan trench which suggests common truncation as well as accretion at the front of convergent margins

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

velocity structure of the andes of central peru from locally recorded earthquakes arrival times of local events recorded during two short term seismic fied investigations in central peru are used to determine the p and s velocity structures above the slab in a segment of the andes where the subduction is horizontal the velocities in the upper crust can be related to the sedimentary cover the moho depth is estimated to be about 50 km below the western part of the sub andean zone and deepens gently to the west reaching 60 km under the western cordillera an uplift of high velocity material is observed under the coastal zone associated with a cluster of earthquakes in and above the slab the asymmetrical structure of the crust below the eastern cordillera reflects the convergence process in this part of the andes the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera the compression then decreases toward the east

interpretation of reconnaissance gravity and aeromagnetic surveys of the antarctic peninsula two dimensional models are presented for the crustal structure of the antarctic peninsula a mesozoic cenozoic magmatic arc on the southeast pacific margin the models are constrained by measured rock properties the west coast magnetic anomaly wcma is caused by a zone of mafic plutons which is 70 150 km wide and over 1500 km long in modelling of bouguer anomalies isostatic compensation is maintained by crustal thickness variations the model moho is deepest 35 km beneath the mid cretaceous axis of the arc and shallowest 20 km beneath a late cenozoic marginal basin positive isostatic residual gravity anomalies are associated with the wcma and a mafic terrane in the mesozoic accretionary complex three broad areas of consistent crustal structure are identified the junctions of these areas are sinistral fault zones which displace the curvilinear magnetic physiographic and gravity patterns by several tens of kilometers

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

dietary variability of extinct tayassuids and modern white lipped peccaries tayassu pecari as inferred from dental microwear and stable isotope analysis abstract prior geochemical work documents dietary shifts coincident with climate change in extinct peccaries with taxa typically shifting between browsing and mixed feeding this study employs dental microwear texture analysis dmta and stable carbon isotopes to interpret the diet of extinct peccaries in florida specifically platygonus mylohyus and protherohyus from the late miocene throughout the pleistocene to evaluate how dietary behavior has fluctuated over time when mylohyus and protherohyus coexisted in the hemphillian mylohyus consumed c3 food sources with harder textures consistent with the consumption of woody browse while protherohyus consumed a greater proportion of c4 vegetation likely grass during the blancan both platygonus and mylohyus can be considered primarily c3 browsers based on both dmta and stable isotope data however during the irvingtonian both mylohyus and platygonus consumed a larger proportion of c4 vegetation than during previous north american land mammal ages nalmas platygonus consumed predominantly c3 vegetation during the rancholabrean while mylohyus consumed a mixture of c3 and c4 vegetation in more open environments independent of nalma platygonus consumed predominantly tough vegetation e g tough leaves and or grass while mylohyus consumed harder food items consistent with morphological differences in their teeth in contrast extant tayassu peccaries are obligate c3 feeders and show no evidence of c4 consumption further tayassu are indistinguishable in all dental microwear textural properties from both of their extinct relatives suggesting that while extant peccaries eat primarily foods from forested environments those foods are highly variable in textural properties collectively these data suggest that extinct peccaries mylohyus and platygonus altered their diet over time and possibly in response to changing climates

chemical and isotopic evolution of the coastal batholith of southern peru southeast of arequipa the coastal batholith of southern peru is composed of two segments arequipa and toquepala including five superunits which were emplaced in discrete magmatic pulses from the jurassic to the paleocene eras 190 61 ma most superunits intruded a precambrian basement dominated by granulitic and amphibolitic rocks showing a strong enrichment in large ion lithophile elements low end p 21 to 29 and 206pb 204pbp 16 11 17 03 tilton and barreiro 1980 and high esr p 396 to 999 values major and trace element analyses reveal that each superunit is formed by distinct suites of calc alkaline plutons i e i type that range in composition from quartz gabbro to monzogranite for the whole plutonic suite located in southern peru the evolution toward negative end i and positive esr i values is followed by a significant decrease in 206pb 204pbi ratios but is also related to the density of precambrian outcrops this led us to classify the intrusives into three groups group 1 consists of intrusives carrying positive end i 2 4 to 0 4 and generally negative esr i values 7 4 to 0 7 they are located in the ilo moquegua transect 17 22 17 80 s an area where precambrian exposure is scarce group 2 consists of plutons with intermediate e values i e end i 0 5 to 2 2 and esr i 7 1 to 55 7 which are found in the vicinity of arequipa and tarata where numerous precambrian outcrops are present finally group 3 is composed of intrusives showing negative end i 4 4 to 8 0 and positive esr i values 27 1 to 56 1 including one anomalous granodiorite exposed near tarata and two samples collected in the arequipa quadrangle near the contact with the charcani gneiss there are several petrogenetic models which can explain the trace element isotopic and geographic correlations observed within the coastal batholith of southern peru one simple model advocates that the parental mafic magma s of the plutonic suites of each superunit must be ultimately derived from an isotopically depleted mantle wedge above the subduction zone the gabbro diorite granodiorite monzogranite association is formed via fractional crystallization and crustal assimilation i e afc during ascent in the andean crust if the exposed precambrian crust represents a good compositional average of the upper crustal material assimilated then the isotopic and some of the trace element variations observed in the three groups can be related to various amounts of crustal assimilation i e small for group 1 but substantial for group 3 although surprisingly little assimilation is required when considering all southern peruvian intrusives more complex models advocate different parental mafic magmas formed by various mixtures of lower crustal and mantle derived melts both having heterogenous but depleted nd and sr isotopic compositions the parental magmas would then ascend differentiate and assimilate upper crustal materials of different compositions i e a crust similar to the exposed precambrian basement for groups 2 and 3 and a less enriched crust not exposed in the area for group 1 if the first model is correct then the relatively small degree of crustal assimilation experienced during the formation of the coastal batholith of southern peru is consistent with the hypothesis that the thick andean crust was thinned during the prevailing extensional tectonic regime that led to the formation of a marginal basin in central peru during the late mesozoic this allowed the basic magmas to encounter the base of the precambrian section at shallower depth and spend less time ascending and differentiating in this enriched crust effectively limiting the amount of assimilation

common pb isotopic compositions of the lima arequipa and toquepala segments in the coastal batholith peru implications for magmagenesis abstract common pb isotopic compositions for 65 feldspar samples from the 188 37 ma old gabbroic to monzogranitic plutons of the peruvian coastal batholith show changes along strike that can be related to variable contamination of mantle derived magmas by the local precambrian basement results have the following isotopic ranges 206 pb 204 pb 17 580 20 803 207 pb 204 pb 15 555 15 709 and 208 pb 204 pb 38 104 41 177 with averages of 206 pb 204 pb 18 630 207 pb 204 pb 15 610 and 208 pb 204 pb 38 500 the gabbroic dioritic and tonalitic plutons north and east of lima the lima segment have ratios similar to the isotopically homogeneous reservoir identified by tilton 1979 tilton and barreiro 1980 and barreiro and stern 1982 for rocks in central and southern chile the homogeneous reservoir has been suggested to be enriched subcontinental mantle more siliceous rocks granodiorites and monzogranites in the lima segment have considerably higher ratios 206 pb 204 pb 18 680 20 803 207 pb 204 pb 15 610 15 709 and 208 pb 204 pb 38 500 41 177 nine of ten monzogranites from the ring dike complexes in the lima segment define a linear array with a slope of 0 1143 correlation coefficient 0 9398 and a pb isochron age of 1 9 ga this isochron age has no simple chronological meaning although it may represent a component of sedimentary debris derived from 1 9 ga crustal materials feldspars from plutons in the arequipa and toquepala segments south of lima tend to have low pb isotopic ratios 206 pb 204 pb 17 580 18 603 207 pb 204 pb 15 555 15 633 and 208 pb 204 pb 38 104 38 749 the ratios plot between the isotopic signatures of local precambrian granulites and gneisses and the chile type enriched subcontinental magmatic reservoir indicating that these end members were the principal magma sources the oldest plutonic rocks in the segments early to mid jurassic are closest to the pb isotopic ratios of the precambrian basement contamination of plutons in the arequipa and toquepala segments by unradiogenic pb from the precambrian basement and lack of it in the lima segment supports crustal models by couch et al 1981 and jones 1981 which show a thick precambrian basement layer throughout southern peru and an extremely thin layer beneath the lima segment

pressure temperature time path related to the thermotectonic evolution of an early proterozoic metamorphic terrane black hills south dakota a pressure temperature time path determined from zoned garnet and associated minerals in a metapelite from a dome in the western part of the black hills south dakota precambrian core shows two clockwise loops that link the previously postulated thermal and tectonic events affecting early proterozoic age rocks between ca 1 850 and 1 715 ga the shape of the loops indicates that two distinctly different styles of tectonic processes occurred during this time period the first loop established from the garnet core is correlated with crustal thickening by thrusting and nappe development followed by a period of erosional uplift the second loop from the garnet rim resulted from homogeneous thickening represented by north northwest trending isoclinal folds the peak conditions of this event are interpreted to be associated with anatexis doming and intrusion of harney peak s type granite 1 715 ga

inland california during the pleistocene megafaunal stable isotope records reveal new paleoecological and paleoenvironmental insights abstract we measured oxygen and carbon isotope compositions from teeth from three fossil localities in california the mckittrick tar seeps rancholabrean fairmead landfill and irvington both irvingtonian these sites have produced a variety of megafauna fossils and to date are relatively understudied previous ecological studies of pleistocene california megafauna have focused on the coastal rancho la brea tar seeps rlb neglecting inland faunas both fairmead landfill and mckittrick are located in california s san joaquin valley and provide the first isotopic data from the region we sampled a wide range of taxa equus camelops mammuthus mammut hemiauchenia odocoileus tetrameryx capromeryx platygonus canis dirus canis latrans arctodus simus smilodon homotherium miracinonyx panthera onca and panthera atrox stable carbon values from both middle pleistocene localities are consistent with a c3 dominated environment mean annual precipitation map estimates for the middle pleistocene localities are 340 mm year for fairmead landfill and 900 mm year for irvington while the inland map estimate is similar to modern levels irvington map estimates are significantly higher than the modern average in contrast the mckittrick tar seeps show clear evidence of c4 consumption among equus bison and camelops suggesting a mixture c4 grasses and halophytes similarly comparing between mckittrick and published data for rlb indicate a higher level of c4 resources inland serially sampled teeth from both inland localities reveal diet and resource partitioning between browsers and grazers

dating of archean basement in northeastern wyoming and southern montana rb sr whole rock and u pb zircon ages of granite and gneiss cores from three deep drill holes extend known occurrences of archean rocks in the subsurface of northeastern wyoming and southern montana rb sr and k ar mineral ages are discordant and reflect early or middle proterozoic disturbance highly altered rocks occur in a thin zone immediately below the sub cambrian unconformity samples from a few metres deeper in the basement are much fresher but show the effects of this alteration in filled fractures and thin adjacent alteration haloes whole rock rb sr systems have retained a fair degree of integrity in spite of increased susceptibility to modification because of the disturbed mineral systems interaction of the rocks with water a few metres below the sub cambrian unconformity probably occurred for only a relatively short time fractures filled rapidly with secondary minerals such as chlorite anhydrite and carbonate to maintain a relatively impermeable crystalline basement in which the silicates and their contained isotopic systems were preserved ages of cores from two wells extend the archean basement to the nacp anomaly a possible suture related to early proterozoic tectonism these and published ages limit a possible ensimatic mobile belt to a zone 300 km wide beneath the williston basin however the possibility remains that archean crust was continuous between the wyoming age province and the superior province

tooth enamel biomineralization in extant horses implications for isotopic microsampling abstract isotopic analyses of tooth enamel from fossil equids are increasingly being used to reconstruct paleoclimatic and paleoenvironmental conditions however the accuracy of these reconstructions is currently limited partly because the precise timing and spatial patterns of enamel mineralization in equids have not been documented we used radiographic analyses of mandibles collected from modern juvenile and adult domestic horses equus caballus to document the timing of enamel mineralization in equid cheek teeth premolars and molars optical and radiographic analyses of thin sections of mature and developing teeth were used to document the spatial pattern of enamel mineralization in each tooth we found that the enamel layers in equine cheek teeth took longer to mineralize than had been previously assumed largely because the enamel continued to mineralize for approximately 6 to 12 months after each tooth had begun to erupt total enamel mineralization times for individual teeth ranged from 1 5 to 2 8 years examination of thin sections revealed that retzius striae extend outward from the enamel dentin junction at an angle of 5 to 10 and run near parallel to the nonocclusal surface of the tooth daily cross striations average 5 m in width suggesting that during the initial phase of enamel mineralization matrix formation new material is added at a rate of 5 m day radiographic analyses demonstrate that the secondary mineralization front enamel maturation front is orientated approximately parallel to the retzius striae but complete maturation i e full mineralization lags behind matrix formation by several weeks to months

a bimodal archean volcanic series owl creek mountains wyoming wind river canyon in the owl creek mountains of central wyoming contains an approximately 1 5 km long section of isoclinally folded amphibolite grade metavolcanic rocks of dominantly basaltic and dacitic composition few metasedimentary rocks appear to be present the rocks have experienced three separate deformational events including isoclinal folding an amphibolite facies metamorphic event is denoted by garnet biotite temperatures of approximately 570 c the dominant rock in the section is a hornblende amphibolite this lithology has two compositional variants a morb like tholeiitic basalt and interlayered with it a slightly more sio 2 rich mafic rock that spans the compositional range from high silica basalt to basaltic andesite approximately 51 54 sio 2 the lree enriched pattern la 40 x of the basaltic andesites suggests they cannot be derived from the tholeiitic basalts by closed system fractionation in the northern portion of the section the tholeiitic basalts are interlay

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

archean geochronological framework of the bighorn mountains wyoming the bighorn mountains of the central wyoming province expose a large tract of archean crust that has been tectonically inactive and at relatively high crustal levels since 2 7 ga seven sensitive high resolution ion microprobe shrimp u pb zircon and titanite age determinations on samples of the main lithologic units provide a geochronological framework for the evolution of this area the oldest precisely dated magmatic event occurred at 2950 5 ma when diorite to granite dykes and sills intruded an older gneiss complex exposed in the central and southern bighorn mountains rocks as old as 3 25 ga may be present in this gneissic basement as indicated by the oldest dates obtained on areas of zircon grains that are interpreted as inherited cores a tonalitic gneiss was intruded into the gneiss complex at 2886 5 ma deformation of the central and southern gneisses preceded the intrusion of the bighorn batholith a tonalitic to granitic intrusion that occupies the northern portion of the uplift this