age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

mapping the piute mountains california with thermal infrared multispectral scanner tims images thermal infrared multispectral scanner tims data were acquired in 1990 over the piute mountains california to evaluate their usefulness for lithologic mapping in an area of metamorphosed structurally complex igneous and sedimentary rocks the data were calibrated and atmospherically corrected and emissivity variations in the form of alpha residuals were extracted from which color composite images were made there was an excellent visual correlation between the units revealed in the color composite image and the lithologic units mapped in the eastern side of the area it was also possible to correct improve and extend the recent map for example several areas mapped as granodioritic gneiss had tims alpha residual spectra consistent with mafic rocks and were subsequently mapped as amphibolite the presence of a swarm of mafic dikes of which only a few had previously been identified was also revealed the images also showed color variations in granitoid plutons that correlated with compositional variations previously determined by extensive field and geochemical work the western piute mountains is an area that has proven to be especially difficult to map with field and air photographic methods due to heterogeneous nature of certain units and various levels of desert varnish the images permitted the extremely heterogeneous proterozoic schists gneisses and granites to be easily mapped because the small scale compositional variability was averaged to the tims pixel size 12 m 12 m square areal measurements from the images show that the proterozoic rocks in the piute mountains consist of granitoids 50 biotite gneiss 15 pelitic gneiss 15 quartzite 10 and amphibolite 10 tims data can dramatically increase the efficiency and the quality of geologic mapping in well exposed heterogeneous areas with minimal vegetation cover where detailed mapping of lithologic contacts by traditional methods is unusually difficult

age inheritance and uplift history of the old woman piute batholith california and implications for k feldspar age spectra ion probe u pb zircon analyses yield a crystallization age of 74 pm 3 ma for both peraluminous and metaluminous plutons within the old woman piute batholith these analyses reveal that about 50 of the zircons from the peraluminous sweetwater wash granite are restitic inherited zircons from this batholith yield ages ranging from older than 1800 ma to 1100 ma and most are similar in age to proterozoic igneous rocks in the eastern mojave desert 40 ar 39 ar hornblende ages from the metaluminous granodiorite are 73 pm 2 ma and mica ages from both plutonic series are 70 pm 2 ma k feldspar age spectra yield age gradients with initial steps between 60 and 67 ma and rise to plateau ages of 70 pm 2 ma several k feldspars from the granodiorite yield elevated plateau ages after 50 39 ar released that result from excess argon uplift rates as high as 4 mm yr between 74 and 70 ma are required by the narrow age range for all minerals

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

ascent of a granitoid diapir in a temperature varying medium a relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth dependent temperature to predict emplacement histories ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it ascent velocity is described by the expression u 2 gd3z 0r where is the density contrast between diapir and country rock g is gravitational acceleration d is the deformation zone width z is a parameter which describes the increase in viscosity across the zone 0 is the country rock viscosity at the contact and r is the diapir radius the width of the deformation zone as dictated by the temperature distribution and rheological function is the point at which the viscosity increases by a factor f from the contact value although ascent rates vary with choice of this parameter ascent distances are not strongly dependent on f results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock but only weakly dependent on the initial diapir temperature ascent velocity varies with the diapir size with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons as a result of the proportionately larger amount of thermal energy available for ascent larger bodies will rise greater distances than smaller diapirs however the additional ascent appears nominal relative to the contrast in available energy modeling diapirs using a no slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress free boundary condition the time scale of granitoid diapirism over crustal distances cannot be less than 104 years due to the rate limiting effect of thermal conduction above the body and not greater than 105 years reflecting solidification times of kilometer sized plutons a consequence of ascent by a deformation zone mechanism is the generation following emplacement of a mechanically weakened crust which extends from beneath the diapir to near the source perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust

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

the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

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

mid crustal cretaceous roots of cordilleran metamorphic core complexes thermobarometry for cretaceous to mid tertiary plutonism and deformation in the lower plate of whipple and santa catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust moreover they are characterized by a striking acceleration of tectonic decompression coincident with middle tertiary low angle detachment faulting leading to erosional and tectonic unroofing by mid miocene time depth estimates for emplacement of five intrusive suites within the whipple complex include 1 33 4 km for the peraluminous 89 ma whipple wash plutonic suite 2 29 1 km for the 73 ma axtel quartz diorite 3 16 5 km for mylonitization and synkinematic plutonism at 26 ma 4 6 2 1 9 km for the postkinematic 19 ma war eagle gabbro quartz diorite complex and 5 5 2 2 3 km for a 17 ma postkinematic granodiorite decompression initially occurred at a low rate of 0 3 mm yr from 89 to 26 ma and increased to approximately 2 mm yr during the late oligocene to middle miocene estimated depths for four pluton emplacement or deformational events in the santa catalina mountains include 1 21 1 km for the magmatic epidote bearing 68 ma leatherwood quartz diorite 2 15 3 km for the garnet two mica 47 ma wilderness granite 3 9 3 1 9 km for post wilderness mylonitzation and 4 6 3 2 6 km for the 27 ma catalina monzogranite post laramide decompression estimated at 0 3 mm yr accelerated to 1 3 mm yr prior to the cessation of detachment faulting whereas most batholithic terranes of the north american cordillera are representative of an upper crustal setting core complexes provide as a consequence of their tectonic evolution a petrological and structural view into middle crustal processes

two phase cement grout propagation in homogeneous water saturated rock fractures abstract modeling of cement grout flow in rock fractures is important for the design monitoring and execution of rock grouting that is widely used in a variety of rock engineering applications this study presents a mathematical model based on the reynolds flow equation for cement grout flow in a homogeneous water saturated rock fracture the model is based on two phase flow i e grout as a bingham fluid and groundwater as a newtonian fluid and is used for investigating the importance of the water phase in rock grouting the modeling results for the two phase flow generally show the importance of the water phase that can significantly affect the pressure distribution and grout penetration in the fracture especially under the condition of grout hardening such effects depend on the viscosity ratio between the grout and groundwater which becomes increasingly important for cases with smaller values of the viscosity ratio the grout density also affects the grout penetration length applying an analytical solution based on single phase flow i e neglecting the impact of groundwater flow for modeling grout injection will generally overestimate the penetration length

40ar 39ar age spectrum analysis of detrital microclines from the southern san joaquin basin california an approach to determining the thermal evolution of sedimentary basins abstract detrital microcline grains from sedimentary strata preserve a record of thermal evolution in the temperature range 100 to 200 c which can be revealed by40ar 39ar age spectrum analysis microcline separates from deep drill hole intersections with eocene to miocene sediments in the basin and tejon blocks of the southern san joaquin valley california analysed by the age spectrum approach show radiogenic40ar 40ar gradients that record both the slow cooling of the uplifting sediment source 65 ma ago and a recent thermal event this information in conjunction with the observation of fission track annealing in the coexisting apatites allows estimation of the temperature time conditions of this thermal event at about 140 c for 200 ka present and paleotemperature data is in accord with heating related to several kilometers of pleistocene sediment deposition heat flow calculations suggest that this recent subsidence has depressed the thermal gradient from about 30 c km 1 to the present apparent gradient of 24 c km 1 40ar 39ar analysis of detrital microcline crystals yields thermochronological information in the temperature time range of petroleum maturation and provides this technique with potential as both a useful exploration tool and as a means of probing the fundamental geodynamic processes of basin evolution

controls on source and depth of emplacement of granitic magma large granitic batholiths emplaced in high grade metamorphic rocks at mid crustal levels were relatively hydrous compared with major batholiths emplaced at very shallow levels those emplaced at mid crustal levels were probably generated by partial melting of crustal rocks at only slightly deeper levels those emplaced at shallow levels are associated with penecontemporaneous volcanic ejecta and were probably generated at deep crustal or possibly mantle depths

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

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

the role of manganese in the paragenesis of magmatic garnet an example from the old woman piute range california a reply strongly peraluminous plutons of the old woman piute range southeastern california display the differentiation sequence muscovite biotite granite rightarrow muscovite garnet biotite granite rightarrow garnet muscovite granite rightarrow garnet muscovite aplite and pegmatite the garnets show no evidence of reaction but biotites in garnet bearing rocks are rimmed by muscovite metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4 5 kb the plutons become relatively enriched in manganese with differentiation whole rock ratios of molar mn fe mg increase from 0 01 in least felsic garnet free rocks to 0 3 in most felsic garnet rich rocks mno concentrations show a corresponding rise in biotites 0 4 to 1 8 wt and less clearly in garnets 9 to 25 the differentiation sequence textural relations and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction biotite mn rich liquid rightarrow garnet

shrimp u pb in zircon geochronology of the chor granitoid evidence for neoproterozoic magmatism in the lesser himalayan granite belt of nw india abstract porphyritic and peraluminous granitoids of the lesser himalayan granite belt fringe the high himalaya from nepal to pakistan magmatic zircons in the chor granitoid in himchal pradesh define a shrimp u pb age of 823 5 ma this neoproterozoic age contrasts with cambro ordovician ages for other dated lesser himalayan granites but is coeval with granite magmatism in the south china craton neoproterozoic magmatism within the northern margin of gondwanaland may provide a more local source for detrital zircons in high himalayan metasediments during the neoproterozoic and early paleozoic than the recently postulated east african orogen

constraints on the tectonics of the mule mountains thrust system southeast california and southwest arizona the mule mountains thrust system crops out discontinuously over a 100 km strike length in the blythe quartzsite region of southeast california and southwest arizona along the thrust system middle and upper crustal metamorphic and plutonic rocks of proterozoic and mesozoic age are thrust north northeastward 015 degree to 035 degree over a lower plate metamorphic terrane that formed part of the proterozoic north american craton its paleozoic sedimentary rock cover overlying mesozoic volcanic and sedimentary rocks and the intruding jurassic and cretaceous granitic rocks stratigraphic petrologic and pb isotopic ties for jurassic granitoids and for jurassic and cretaceous sedimentary rocks across the various parts of the thrust system indicate that related crustal blocks are superposed and preclude it from having large displacements the thick skinned thrust system is structurally symmetrical along its length with a central domain of synmetamorphic thrust faults that are flanked by western and eastern domains where lower plate domains where lower plate synclines underlie the thrusts deformation occurred under low greenschist facies metamorphic conditions in the upper crust movement along the thrust system was probably limited to no more than a few tens of kilometers and occurred between 79 plus minus 2 ma and 70 plus minus 4 ma the superposition of related rocks andmore the geometry of the thrust system preclude it from being a major tectonic boundary of post middle jurassic age as has been previously proposed rather the thrust system forms the southern boundary of the narrow zone of cretaceous intracratonic deformation and it is one of the last tectonic events in the zone prior to regional cooling less

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

correlation of metamorphosed paleozoic strata of the southeastern mojave desert region california and arizona isolated outcrops of deformed regionally metamorphosed paleozoic strata are scattered within the southeastern mojave desert region of california and western arizona these strata unconformably overlie a basement of proterozoic crystalline rocks and are overlain in turn by metamorphosed mesozoic sedimentary rocks the strata can be correlated lithostratigraphically with the classic cratonal paleozoic section of the western grand canyon arizona and with nonmetamorphosed paleozoic sections transitional between cratonal and miogeoclinal in the ship marble and providence mountains california the strata evidently were once continuous with paleozoic epicontinental strata exposed throughout the southern great basin and colorado plateau outcrops of paleozoic strata and of the underlying proterozoic basement in the southeastern mojave desert region define a terrane that has been disrupted by mesozoic thrust faults and by tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence this coherent terrane extends at least as far west and southwest as the big maria palen and calumet mountains and possibly beyond to include paleozoic exposures in the san bernardino mountains and near victorville poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western mojave desert and the eastern transverse ranges

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

late cretaceous ductile deformation metamorphism and plutonism in the piute mountains eastern mojave desert late cretaceous deformation in the piute mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones the shear zones often separate proterozoic lithologies with strong competency contrasts suggesting that proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network shear zones now divide the crust into wedge and lozenge shaped blocks the piute mountains can be separated into two kinematic domains each characterized by a dominant sense of shear thrusting in the southwest directed kinematic domain ended at 85 7 ma the age of the late synkinematic east piute pluton thrusting in the northeast directed kinematic domain could have lasted until 74 3 ma the age of the lazy daisy pluton postthrusting northwest southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage f2 strain increases toward the south and is concentrated around the margins of the lazy daisy pluton this deformation was synchronous with pluton emplacement late cretaceous peak metamorphism outlasted all ductile deformation peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the lazy daisy pluton is approached temperature and pressure of approximately 450 c and 2 5 4 0 kbar were achieved during ductile thrusting temperatures of 500 540 c existed at the onset of upright open folding and peak temperatures as high as 620 c near the lazy daisy pluton occurred after f2 upright folding all penetrative deformation ended before 72 71 ma by which time the area had cooled to below 300 c foster et al 1989

combining nd isotopes in monazite and hf isotopes in zircon to understand complex open system processes in granitic magmas mapping the age and trace element and sm nd isotope compositions of monazite grains from a peraluminous cretaceous granite using laser ablation split stream analysis reveals a wide range in nd isotope and rare earth element ree compositions within and between single grains these data corroborate isotopic variability indicated by hf isotope analysis of zircon in the same granite sample the ree variations indicate that monazite grew during fractional crystallization hf and nd isotopes indicate that the granitic magma was generated from at least two distinct proterozoic sources of approximately the same age one component that had highly radiogenic initial 176hf 177hf and 143nd 144nd and a second component that was notably less radiogenic this study highlights the utility of in situ ree and sm nd isotope data in monazite in magmatic systems further it refines the zircon based constraints on magmatic processes because of sensitivity of light rees to fractional crystallization lower probability of complications owing to inheritance and smaller analytical volumes required