partial melting of metagreywackes part ii compositions of minerals and melts a series of experiments on the fluid absent melting of a quartz rich aluminous metagreywacke has been carried out in this paper we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe this analytical work includes biotite plagioclase orthopyroxene garnet cordierite hercynite staurolite gedrite oxide and glass over the range 100 1000 mpa 780 1025 c biotites are na and mg rich with ti contents increasing with temperature the compositions of plagioclase range from an17 to an35 with a significant orthoclase component and are always different from the starting minerals at high temperature plagioclase crystals correspond to ternary feldspars with or contents in the range 11 20 mol garnets are almandine pyrope grossular spessartine solid solutions with a regular and significant increase of the grossular content with pressure all glasses are silicic sio2 67 6 74 4 wt peraluminous and leucocratic feo mgo 0 9 2 9 wt with a bulk composition close to that of peraluminous leucogranites even for degrees of melting as high as 60 vol with increasing pressure sio2 contents decrease while k2o increases at any pressure the melt compositions are more potassic than the water saturated granitic minima the h2o contents estimated by mass balance are in the range 2 5 5 6 wt these values are higher than those predicted by thermodynamic models modal compositions were estimated by mass balance calculations and by image processing of the sem photographs the positions of the 20 to 70 isotects curves of equal proportion of melt have been located in the pressure temperature space between 100 mpa and 1000 mpa with increasing pressure the isotects shift toward lower temperature between 100 and 200 mpa then bend back toward higher temperature the melting interval increases with pressure the difference in temperature between the 20 and the 70 isotects is 40 c at 100 mpa and 150 c at 800 mpa the position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process a comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 mpa the difference in fertility between pelites and greywackes decreases with decreasing pressure a review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust greywackes pelites or orthogneisses produces only peraluminous leucogranites more mafic granitic compositions such as the various types of calk alkaline rocks or mafic s type rocks have never been obtained during partial melting experiments thus only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments other types of granites involve other components or processes such as restite unmixing from the source region and or interaction with mafic mantle derived materials

the origin of the upper palaeozoic chanaral melange of n chile the fragmented and mixed strata of the chanaral melange extend for some 220 km from n to s in the coastal ranges of n chile 26 20 to 28 20 s the melange was produced in an accretionary wedge resulting from the ne subduction of the ancestral pacific oceanic plate beneath the gondwana land margin during carboniferous times the melange consists essentially of blocks of sandstone in a pelitic matrix these sediments were deposited as deep sea basin plain turbidites prior to their fragmentation other blocks in the melange include mafic volcanic rocks with the geochemical characteristics of oceanic island basalts and rare shallow marine limestones structures in the melange suggest an origin by two distinct deformation processes involving unlithified sediments initial boudinage and break up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge the second process produced cross cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

evolution from magmatic to hydrothermal activity beneath the cerro escorial volcano nw argentina as sampled by erupted quartz and brines abstract large quartz pebbles erupted with the escorial ignimbrite provide insight into the late magmatic evolution of the shallow cooling magmatic hydrothermal system below the cerro escorial volcano of the southern central volcanic zone in the argentine andes the ignimbrite is of relatively small volume crystal rich dacitic in composition and not particularly water rich as amphibole is absent eruption temperature was estimated to be close to 850 c the quartz pebbles provide insight into the magmatic hydrothermal transition beneath the volcano based on textures trace element composition analyzed by laser ablation inductively coupled plasma mass spectrometry and inclusion content the pebbles can be separated into pegmatite like megacrysts and lower temperature epithermal microcrystalline quartz both types are distinct from magmatic phenocrysts present in porphyritic clasts ejected by the ignimbrite the megacrysts show a wide range of trace element concentrations with elevated al concentrations and al ti and ge ti ratios compared to quartz phenocrysts although some contamination by submicroscopic inclusions and cooling to near solidus temperatures may perturb the signal in some cases the range of trace element concentrations in quartz crystals from this system may reflect a change in crystallization conditions from initial precipitation from typical evolved silicate melt to fast growth from residual melt and or fluid in pegmatitic pockets or coarse grained hydrothermal veins at locally variable precipitation conditions abundant primary and secondary silicate melt inclusions and a variety of secondary fluid inclusions are present within the megacrysts in particular brine inclusions are densely packed with salt crystals sometimes anhydrite and or a silicate crystal but no visible liquid at room temperature and co existing vapor inclusions are of very low density heating experiments of brine inclusions reveal last salt and vapor bubble dissolution temperatures around 600 700 c but an immiscible silicate melt surrounding the homogenized salt globule remains even at unreasonably high temperature the co existence of silicate melt and fluid inclusions reinforces the magmatic nature of the fluids while boiling trails of brine with cu concentrations of several percent and vapor point to relatively low pressures

sedimentology and stable isotopes from a lacustrine to palustrine limestone deposited in an arid setting climatic and tectonic factors miocene pliocene opache formation atacama desert chile abstract field relations petrography gis and geochemistry of a late miocene pliocene limestone the opache formation calama basin atacama desert chile shows that depositional facies diagenetic histories and isotopes differ from eastern to western parts of the basin we recognize two facies lacustrine characterized by ostracod diatom mudstone and wackestone and palustrine characterized by both shallow water and pedogenic features gis analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west with the calama constriction restricting flow between east and west petrography and geochemical results confirm that the calama constriction influenced depositional parameters ponding groundwater to the east forming a shallow lake and creating palustrine conditions to the west our analyses indicate that lake waters were less enriched in 13c or 18o than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history derived from groundwater rich in dissolved minerals this seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the pacific ocean and was less evolved whereas eastern limestones precipitated from water which had become depleted in 18o during transport from the east today moisture from the pacific very rarely if ever reaches inland as far as calama indicating that during the late miocene mid pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal pacific current allowed moisture from the pacific to reach inland this changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the andes allowing it to reach farther westward into the atacama region during the late miocene pliocene together these moisture sources were sufficient to produce carbonate rich palustrine and lacustrine environments in the calama basin

plutonic and metasedimentary rocks from the coastal range of northern chile rbsr and upb isotopic systematics more than 50 of the surface in the coastal range of northern chile between the towns of taltal and chan aral consists of granitoid rocks age determinations by means of upb isotopic analyses on sized zircons and by means of rbsr isotopic measurements in conjuction with other geochemical studies reveal a complex pattern for the development of the plutonites though closely associated two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data in a west east transect southeast of chan aral several calc alkaline plutonites with basic to acid members intruded mainly into paleozoic metasedimentary rocks the upb data of the zircons from the plutonites yielded concordant ages ranging from lower jurassic to lower cretaceous upb ages are identical with rbsr biotite ages within the error limits and almost coincide with the kar mineral ages both the ages and the initial87sr86sr ratios decrease continuously from west to east the low initial sr isotopic ratios 0 703 0 705 and other geochemical parameters indicate i type characteristics in the west east segment northeast of chan aral mainly alkaline plutonic rocks intruded in an interval from upper permian to upper triassic the upb data of zircons are slightly discordant and indicate a small portion of inherited crustal material 1 ga old rbsr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks the initial 87sr86sr ratios of 0 710 point to an origin of these rocks by anatexis of crustal rocks s type the oldest rocks in the area of chan aral consist of thick sequences of metasediments the material for these rocks was derived from a source 2 5 ga old as indicated by the upb data of the zircons the lower intercept age of the zircons points to metamorphism of the source area at about 600 ma due to uplift in the early paleozoic the source area supplied the material for the sediments which were deposited in the devonian contact metamorphism in the triassic transformed the sediments into metasedimentary rocks as demonstrated by rbsr whole rock data this event was caused by the emplacement of the s type granitoid rocks which probably originated however from a deeper level of the crust

the north chilean coast range an example for the development of an active continental margin analysis of extended plutons in the coast range of north chile between 25 30 and 26 35 led to the recognition of a complex magmatic and structural evolution from the upper paleozoic to the tertiary

metamorphism isotopic ages and composition of lower crustal granulite xenoliths from the cretaceous salta rift argentina crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the cretaceous salta rift at quebrada de las conchas provincia salta argentina were investigated to get information about the age and the chemical composition of the lower crust most of the xenoliths have a granitoid composition with quartz plagioclase garnet rutile k feldspar as major minerals the exceedingly rare mafic xenoliths consist of plagioclase clinopyroxene garnet hornblende all xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca 900 c and pressures of ca 10 kbar the sm nd mineral isochron ages are 95 1 10 4 ma 91 5 13 0 ma 89 0 4 2 ma granitoid xenoliths and 110 7 23 6 ma mafic xenolith these ages are in agreement with the age of basanitic volcanism ca 130 100 and 80 75 ma and are interpreted as minimum ages of metamorphism lower crustal temperature at the time given by the isochrons was above the closure temperature of the sm nd system 600 700 c the sm nd and rb sr isotopic signatures 147sm 144nd 0 1225 0 1608 143nd 144ndt0 0 512000 0 512324 87rb 86sr 0 099 0 172 87sr 86srt0 0 708188 0 7143161 and common lead isotopic signatures 206pb 204pb 18 43 18 48 207pb 204pb 15 62 15 70 208pb 204pb 38 22 38 97 of the granitoid xenoliths are indistinguishable from the isotopic composition of the early paleozoic metamorphic basement from nw argentina apart from the lower 208pb 204pb ratio of the basement the sm nd depleted mantle model ages of ca 1 8 ga from granitoid xenoliths and early paleozoic basement point to a similar proterozoic protolith time constraints the well equilibrated granulite fabric p t conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca 90 ma and to a prominent thermal anomaly in the lithosphere the composition of the xenoliths is similar to the composition of the early paleozoic basement in the andes of nw argentina and northern chile a thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths

cyclostratigraphy orbital tuning and inferred productivity for the type kimmeridge clay late jurassic southern england three independently measured variables magnetic susceptibility photoelectric factor and total gamma ray obtained from throughout the type kimmeridge clay fm in dorset southern england were used to identify regular metre scale sedimentary cycles spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large amplitude cycles of 1 87 4 05 m wavelength and smaller amplitude cycles of around half that wavelength these cycles are interpreted to record orbital obliquity and precession respectively the much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high latitude climatic forcing transferred to lower latitudes via sea level variations orbital tuning indicates that the early kimmeridgian sensu anglico lasted at least 3 6 ma 95 longer wavelength cycles and the late kimmeridgian at least 3 9 ma 103 longer wavelength cycles the first detailed productivity estimates for the kimmeridge clay fm on a cycle by cycle calculation indicate that average productivity of the type kimmeridge clay 220 g m 2 a 1 was less than the average productivity on modern continental shelves the high average organic carbon content of the type kimmeridge clay 3 8 total organic carbon cannot be attributed to high average productivity however the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and or elevated preservation linked to reduced bottom water oxygenation

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

0 3 byr of drainage stability along the palaeozoic palaeo pacific gondwana margin a detrital zircon study the palaeo pacific margin of gondwana in the present day south central andes is marked by tectonic activity related to subduction and terrane accretion we present detrital zircon u pb data encompassing the palaeozoic era in northern chile and northwestern argentina cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase the main age clusters for these zircon grains are ediacaran to palaeozoic with an additional peak at 1 3 0 9 ga and they can be correlated with grenvillian age and the brasiliano pampean and famatinian orogenies the zircon data reveal main transport from the nearby ordovician famatinian arc and related rocks the silurian sandstone units are more comparable with cambrian units with brasiliano and transamazonian ages 2 2 1 9 ga being more common because the silurian deposits were situated within or east of the extinct famatinian arc hence the arc acted as a transport barrier throughout palaeozoic time the complete suite of zircon ages does not record the accretions of exotic terranes or the palaeozoic glacial periods we conclude that the transport system along the palaeo pacific margin of gondwana remained stable for c 0 3 byr and that provenance data do not necessarily reflect the interior of a continent hence inherited geomorphological features must be taken into account when detrital mineral ages are interpreted supplementary material u pb data cl images and detailed geological maps are available at www geolsoc org uk sup18796

sm nd ages of mafic rocks from the coastal cordillera at 24 s northern chile we investigated the sm nd systematics of mafic granulite and undeformed layered gabbro which form a midcrustal section of the jurassic magmatic belt in the north chilean coast range south of antofagasta mineral isochrons indicate ages between ca 171 and 150 ma for the granulite and an age of ca 161 ma for the gabbro these ages are interpreted as closure time of the sm nd system in the area the age of metamorphism is late jurassic the minimum intrusion age of the protolith of the granulite is likely early jurassic ca 200 ma but an exact intrusion age could not be derived from the data the intrusion age of the gabbro is ca 185 ma granulite generated from mantle derived gabbroic magmas has nd200 between 6 28 and 7 05 and the gabbro that intruded the granulite has nd185 between 5 89 and 6 09

pb isotopes define basement domains of the altiplano central andes detailed pb isotopic maps of the central andes based on 345 163 previously published 182 new analyses of ores volcanic rocks and their host rocks elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region the data clearly show that most of the pb in central andean igneous and ore samples is derived from the local basement including pb in ore deposits of the bolivian tin belt some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern as well as to metallogenic boundaries such as the western edge of the bolivian tin belt

crustal recycling of metamorphic basement late palaeozoic granitoids of northern chile 22 s implications for the composition of the andean crust upper palaeozoic silicic magmatism is widespread in the central introduction andes but its origin is poorly constrained we investigated wholethe magmatic arc in the puna altiplano plateau of the rock chemical and isotopic composition of upper palaeozoic grancentral andes rests on a continental crust of a maximum itoids and their early palaeozoic high grade country rocks in thickness of 70 km crustal thickening is mainly atthe chilean coastal cordillera and precordillera at 22 s in tributed to shortening of the crust and not to juvenile comparison with an upper cretaceous granitoid the age of the magmatic additions from the subduction zone see review late palaeozoic granitoids from a rb sr isochron of 300 ma by allmendinger et al 1997 this implies a major is consistent with k ar cooling ages of hornblende and biotite contribution of pre mesozoic material to the present similar major and trace element patterns as well as nd and pb thick crust because mesozoic juvenile additions to the isotopic composition of upper palaeozoic granitoids and gneisses crust are focused in the mesozoic magmatic arcs mainly point to a source of the granitoids that is similar to the gneisses at in the chilean coastal cordillera fig 1 e g scheuber outcrop sr isotope ratios of the upper palaeozoic granitoids are et al 1994 lucassen et al 1996a which is not involved less radiogenic than those in many of the gneisses we propose a in the crustal thickening the last major reorganization of stratification of the early palaeozoic crust with a rb deficient the crust before the cenozoic was in the early palaeozoic granulitic mid lower crust resulting in less radiogenic sr compared orogeny between 500 ma the age of peak metawith the upper crust based on the interpretation of the p t t morphism and 400 ma the age of the final exhumation history and isotopic composition of the lower palaeozoic meta damm et al 1990 1994 lucassen et al 1996b unmorphic basement and of the isotopic composition of the late published data 1998 becchio et al 1997 the widepalaeozoic granitoids and younger magmatic rocks nd isotopic spread upper palaeozoic granitoid magmatism starting composition is identical in lower and upper crust and in crustal at 300 ma was assigned to arc magmatism e g brown melts from the late palaeozoic to recent the cretaceous granitoid 1991 bahlburg herve 1997 or to melting of the evolved from partial melts of a mantle derived source with concrust at a passive margin breitkreuz zeil 1984 and siderable contamination by the old crustal component the crust references therein the metamorphic history of the prethat formed in the early palaeozoic is the major source of material mesozoic crust was studied in detail before lucassen et for the cenozoic tectonic thickening of the andean crust al 1999a unpublished data 1998 summary of previous

mesozoic magmatic and tectonic events within the andean plate boundary zone 26 27 30 s north chile constraints from 40 ar 39 ar mineral ages in north chile jurassic cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences new 40 ar 39 ar mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes c 202 188 ma c 160 153 ma c 142 138 ma c 130 127 ma and c 106 103 ma conjugate dike sets were emplaced immediately before periods of plutonism and each distinct episode of magmatism prior to c 127 ma was located successively eastward inboard from the subduction boundary features interpreted to result from regional extension within the magmatic arc steeply dipping ductile mylonitic shear zones were developed in wall rocks along western contacts of late jurassic and early cretaceous plutons steeply plunging mineral stretching lineations and a variety of kinematic indicators record an east side down extension along eastern faults of the atacama fault system steeply dipping ductile mylonitic shear zones display subhorizontal mineral stretching lin

geodynamic and magmatic stages on a traverse through the andes between 20 and 24 s n chile s bolivia nw argentina metamorphic precambrian basement is exposed in the chilean pre cordillera in the form of a narrow horst and also in the argentinian puna miogeosynclinal pelites and psammites of devonian age dominate in the western part of the andes while early palaeozoic clastics occur in the e insignificant triassic sediments were followed by wide spread shallow marine jurassic deposition cretaceous and tertiary strata are predominantly terrestrial deposits whereas the precambrian and palaeozoic series were intensely deformed by folding block faulting dominates in the volcano sedimentary cover magmatic activity has been significant since the precambrian while s type intrusives and extrusives occurred mainly in the palaeozoic i type magmatic rocks predominated in the mesozoic the cenozoic volcanism in the cordillera occidental originated in the lower and upper crust magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the chilean pre cordillera and the cordillera occidental this phenomenon can best be explained by the existence of partial melts at depths of more than 10 km the bouguer values decrease by about 430 mgal from the coast to the cordillera occidental in the precambrian and palaeozoic the continent apparently extended farther to the w the change from a passive to an active continental margin took place at the palaeozoic mesozoic boundary with the break up of pangea in order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the central andes a combined geological geophysical investigation has been made along a traverse through the andes between 20 and 24 s fig 1 the following pages present a review of the geological development together with the first results of work from the chilean section of the traverse

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

formation of magmatic crust at the andean continental margin during early mesozoic a geological and thermal model of the north chilean coast range the pacific margin of south america has existed since at least the early paleozoic developing a diversified continental crust in the triassic and jurassic mantle derived plutonic and volcanic rocks formed a ca 100 km wide belt of new magmatic crust in the coast range of northern chile the tectonic regime was extensional surface geology seismic velocities and the gravity field indicate that the magmatic rocks in the upper 20 km of the crust have a mainly basic composition the pre andean continental crust is only preserved in isolated small areas but the rifting had no proveable influence on the surface elevation of the area which was always above or close to sea level the coastal magmatic belt marks the onset of the andean cycle and was its most intensive phase of magmatic additons to the crust numerical experiments were performed using a two dimensional finite difference grid with spatial relations derived from the geological model the duration of two phases of extension and contemporaneous compensation of the crustal thinning by magmatic accretion and the location of the extension in the lithosphere were varied in order to evaluate the duration of the tectonic magmatic phases and to distinguish between the effects of tectonic movements and magmatic additions on temperature gradient and elevation the effect of these two first order heat advection processes on the thermal structure of the lithosphere depends on their duration it is possible to set reasonable constraints on the duration of extension by comparing calculated temperature distributions with geological data the temperature distribution in the crust is dominated by processes in the crust and only to a minor extent by deep seated thermal anomalies surface elevation is largely controlled by deep seated thermal anomalies in the mantle if the thickness of the crust is held constant by magmatic accretion during extension the time scale of the tectonic magmatic processes has minor influence on the calculated elevation

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen abstract lead isotopic data on leached k feldspars plagioclases and muscovite provide constraints on the premetamorphic evolution of the grenville orogen in ontario and new york a boundary between reworked archean crust to the northwest and new mantle derived crust generated during mid proterozoic times to the southeast is identified within the northern portion of the gneiss belt of ontario the identification of juvenile mid proterozoic crust at the southeastern margin of the orogen is consistent with previous investigations based on neodymium model ages however near the grenville front lead isotopes indicate the presence of reworked archean crust rather than primary early proterozoic crust postulated on the basis of neodymium model ages this discrepancy between the lead and neodymium isotopic terranes underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle derived material the combined neodymium and lead data imply that the grenville orogen in ontario consists essentially of reworked archean crust and juvenile mantle derived material generated around 1 5 ga the grenville orogen involved archean crust that was depleted in thorium and uranium several hundred ma before high grade metamorphism at ca 1 1 ga such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the gneiss belt

hypothetical southeast pacific continent revisited new evidence from the middle paleozoic basins of northern chile the truncation of orogenic trends at the modern chilean coast paleocurrent indicators and facies distributions indicating the derivation of sediment from sialic sources in the present day pacific have suggested to students of the paleozoic foundation of the andes that a south east pacific continent may have been located oceanward of the present margin in paleozoic time these features are well represented in the middle paleozoic siliciclastic platform deeper basin system in northern chile furthermore the middle devonian platform sections contain brachiopods both of malvinokaffric and eastern americas realm affinity sedimentological and tectonic features are consistent with the evolution of the basin in a devonian and early carboniferous extensional regime which was succeeded by the inception of subduction by oblique convergence in the late carboniferous the extensional regime was possibly governed by the clockwise motion of laurentia along the proto andean margin as recently proposed by ian dalziel subduction of pacific oceanic crust then began in the wake of laurentia moving toward its permian position in the pangea supercontinent

the crustal evolution of south america from a zircon hf isotope perspective hf isotope data of 1100 detrital zircon grains from the palaeozoic south central andean gondwana margin record the complete crustal evolution of south america which was the predominant source the oldest grains with crustal residence ages of 3 8 4 0ga are consistent with complete recycling of existing continental crust around 4ga we confirm three major archaean palaeoproterozoic transamazonian and late mesoproterozoic to early neoproterozoic crust addition phases as well as six igneous phases during proterozoic to palaeozoic time involving mixing of juvenile and crustally reworked material a late mesoproterozoic to early neoproterozoic grenville age igneous belt can be postulated along the palaeo margin of south america this belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages crustal reworking likely dominated over juvenile addition during the palaeozoic era and proterozoic and archaean zircon was mainly crustally reworked from the eroding thickened ordovician famatinian arc

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

the early palaeozoic high grade metamorphism at the active continental margin of west gondwana in the andes nw argentina n chile the evolution of the early palaeozoic orogen of west gondwana in the cambrian to ordovician basement of the andes between 18 and 32 s is investigated for pressure and temperature conditions and age of metamorphism it is characterized by mid crust temperatures commonly above the wet granite solidus 650 c widespread felsic migmatite and rare granulite formed at pressures of ca 0 5 0 7 gpa locally 1 0 gpa these rocks represent the deepest exhumed sections of the early palaeozoic crust high pressure low temperature rocks are absent the crystallization ages compiled from the literature in combination with new data for near peak metamorphic conditions of these high grade metamorphic rocks in nw argentina and n chile are 530 500 ma and 470 420 ma both age groups are spatially overlapping radiogenic isotope signatures sr nd pb are used to characterize the early palaeozoic basement the pb and sr isotope compositions of the early palaeozoic basement indicate mixing arrays between pre palaeozoic unradiogenic and radiogenic crust crustal residence ages sm nd tdm indicate a prominent event of crust formation around 2 ga which is known continent wide this material was recycled during midproterozoic and early palaeozoic orogenies without prominent additions of new crust present in the isotope record i e accretion of compositional exotic material is absent

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

orbitally driven fertility cycles in the palaeocene pelagic sequences of the southern alps northern italy abstract palaeocene pelagic sequences cropping out in the southern alps are often characterized by a cyclic alternation of marly and calcareous lithologies in particular two sets of limestone marlstone couplets are well exposed in the ponte nelle alpi and tabiago sections both danian to thanetian in age located in the belluno region venetian prealps and in the brianza region central prealps respectively a multidisciplinary approach was adopted to determine the nature of these alternations by analyzing for each semicouplet the abundance and composition of planktonic foraminiferal faunas the degree of bioturbation and the carbonate content both sections show similar general features the marly semicouplets of ponte nelle alpi and tabiago sections are poor in planktonic foraminifera but enriched in more tolerant for nutrients and temperature foraminiferal forms and they record more oxygenated conditions a possible interpretation of the lithological rhythms suggests that the marly semicouplets were deposited in periods of high seasonality a marked seasonality leading to an increased circulation of water masses induced a nutrient enrichment in surface waters and oxygenated conditions at the seafloor the data prove that the marly semicouplets represents fertile surface waters whereas the calcareous ones record the opposite conditions low seasonality sluggish circulation and fewer nutrients at the sea surface spectral analysis was applied to evaluate the possible periodicities that could have driven the observed cyclicity the periodicities recorded agree with those predicted by the palaeoclimatic milankovitch theory for the precession obliquity and eccentricity cycles the stronger signal associated to obliquity has also been recorded during the mid cretaceous confined to some intervals this inconstancy is probably attributable to the response of the earth to a signal that may only be detected in the low latitudes when climatic or oceanic conditions are particularly favorable to its transmission

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

chemical and isotopic evolution of groundwater through the active andean arc of northern chile abstract the role of rock water reactions leakage of sedimentary brine evaporite dissolution and variations in lithology on the chemical and isotopic evolution of groundwater draining the active arc and preandean basins in the antofagasta region of northern chile has been investigated using li b and sr isotopes the studied basin is that of the upper loa river and its sub basins which is adjacent to the basin occupied by the salar de atacama where li rich brines are commercially exploited due to the arid climate water draining this part of the andes does so primarily through the subsurface water from a thermal source is a common component of water sampled but its geochemical signature evolves through multiple processes including dilution cold rock water reactions influxes of sedimentary brine and evaporite mineral dissolution these processes lead to a range of li and b concentrations in groundwater 0 01 14 mg l and 0 18 20 mg l and isotope compositions 1 to 12 and 6 to 14 the effect of rock dissolution on 7li and 11b within aquifers is apparent from their low values but even in streams these values only undergo small increases relative to other rivers worldwide this may be due to the weathering of ignimbrites producing water with less al and limited clay formation the large reservoir of thermally sourced li and b of which only a small component is removed and because the streams acquire groundwater through their beds the wider range in 11b is a result of variable lithology and dissolution of evaporite minerals which have lesser effects on 7li our multi isotope approach highlights the role of lithology and different processes in defining groundwater chemistry isotopic signatures derived from marine sediments are recognizable in some springs even though this type of rock has nearly no surface exposure due to extensive volcanic rock cover and are acquired directly or via incorporation of these sediments into volcanic systems these high cl rocks can weather to produce water with a chemical composition like that of sedimentary brine but is distinguishable through 11b composition combination of 87sr 86sr 7li and 11b also indicate that mixed salts which contribute to groundwater draining the ignimbrite covered areas of the basin did not form following cold temperature weathering but from water where thermal contributions dominated solute budgets keeping 7li low and li cl and b cl high compared to other fluid reservoirs such as seawater we propose that these saline systems lay within the confines of one of the multiple late miocene pliocene calderas following the active stage of volcanism and while heat gradients remained high

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

lithospheric evolution of the pre and early andean convergent margin chile abstract the proto andean and early andean evolution of the southwestern gondwana margin comprises three stages that differ in their magmatic evolution and deformational style the gondwana cycle 330 280 ma the pre andean stage 280 210 ma and the early andean cycle 210 100 ma these stages have been traditionally interpreted as the upper crustal response to changes in the tectonic setting which include cordilleran style continental arc gondwana cycle orogenic collapse and possibly slab break off that led to continental rifting and extensive crustal melting pre andean stage and subsequent subduction re initiation in oceanic arc style context early andean cycle the petrological and geochemical characteristics of carboniferous to jurassic igneous rocks from this region however do not support the described model elemental and sr nd pb isotopic data of 86 samples along with a compilation of 1230 samples from the literature suggest that subduction was the most likely process by which the magmatic record was generated sub alkaline affinities lile enrichment over hfse nb ta troughs porphyritic textures and hornblende and biotite bearing lithologies are present in all studied units whereas isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust even though the aforementioned features are common to all igneous rocks some changes point to a decline in the contribution of crustal lithospheric sources to the magmatism with time thus sio2 lan ybn and 87sr 86srinitial exhibit a systematic decrease from 285 to 150 ma whereas the endinitial parameter increases in the same period these changes were accompanied by the shift from dominant compressional carboniferous early permian to transtentional middle permian jurassic stresses in the upper crust suggesting that the margin went from advancing to retreating due to pangea reorganization and break up following a potential flat slab event slab roll back may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion