evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

introduction to calderas special issue large pyroclastic eruptions and associated caldera collapse structures represent the most catastrophic geologic events that have occurred on the earth s surface in phanerozoic time other than large impacts of asteroids or meteorites calderas are large collapse depressions more or less circular in form the diameters of which are many times greater than the diameter of any included vents williams and mcbirney 1979 the largest known calderas associated with silicic pyroclastic eruptions that produce ash flow tuffs or ignimbrites are as much as 60 km in diameter related pyroclastic deposits have magmatic volumes exceeding 103 km3 in addition to potential problems of volcanic hazards calderas and associated volcanic activity are important controls on many of the earth s ore deposits and geothermal energy resources as well as providing key insights into the generation and evolution of large volume silicic magma bodies

geochemistry of evolved magmas and their relationship to subduction unrelated mafic volcanism at the volcanic front of the central mexican volcanic belt abstract this study reports new geochemical and radiogenic isotope data on hy normative pleistocene to recent 87 sr 86 sr 0 70370 0 70469 143 nd 144 nd 0 51278 0 51289 206 pb 204 pb 18 64 18 72 207 pb 204 pb 15 59 15 62 and 208 pb 204 pb 38 39 38 52 these isotopic ratios are generally similar to those for the associated mafic rocks although some differences do exist between them particularly for the 87 sr 86 sr ratios that are somewhat higher and 143 nd 144 nd that are slightly lower for the evolved rocks the available geochemical evidence does not support the generation of the scn evolved magmas by simple fractional crystallization fc or even by assimilation coupled with fractional crystallization afc of such mafic magmas nor by direct slab melting or indirect fluid transport to the mantle participation of the subducted cocos plate the viable mechanism seems to be that the most evolved andesitic and dacitic magmas in the scn were produced by partial melting of a heterogeneous mafic granulite source in the lower crust the magmas of intermediate compositions may reflect magma mixing processes between the most evolved andesitic and dacitic magmas generated in the lower crust and the mantle derived mafic magmas basalt and basaltic trachy andesite at the volcanic front of the central part of the mvb these results have direct bearing to other volcanic areas with complex tectonic setting

geology and geochemistry of amealco caldera qro mexico amealco caldera located in the central part of the mexican volcanic belt mvb about 130 km nw of mexico city is characterized by a quasi circular 11 km diameter structure whose southern portion is partly truncated by a regional fault the caldera was formed 5 m y ago due to a voluminous eruption of 65 km3 eq magma volume of ignimbrites which covered an area of 1800 km2 this was followed by eruption of 2 km3 of andesitic and dacitic lava flows from the border of the caldera and emplacement of a total of 8 silicic rhyolitic domes 8 km3 inside the caldera finally postcaldera volcanism 4 km3 consisted of extrusion of andesitic and basaltic andesite lavas and scoriae we have analyzed 17 representative rock samples for their major and trace element contents by x ray fluorescence spectrometry and for rare earth elements by high performance liquid chromatography all samples were also analyzed for their sr and nd isotopic compositions which range as follows 87sr 86sr 0 70379 0 70451 143nd 144nd 0 51267 0 51278 the chemical and isotopic data are used to constrain the origin and evolution of magmas in the caldera no combination of altered morb and overlying sediments from the subducted slab cocos plate can produce the sr and nd isotopic compositions observed in rocks from amealco caldera further modelling reveals that the mantle derived primary magmas in amealco caldera may have assimilated up to 10 of a crustal component during their evolution from basaltic andesites to dacites but no further assimilation appears to have taken place during the evolution of the rhyolitic magmas the structure and genesis of the amealco caldera are comparable to some other calderas of the mvb

geochemical effects of decoupled fractional crystallization and crustal assimilation most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization afc in some magmatic systems however assimilation and crystallization are not strictly related and the mass assimilated is decoupled from and therefore varies independently of the mass crystallized fca the geochemical consequences of fca are examined and compared to those of afc the behavior of incompatible elements is identical during afc and fca and ratios of these elements do not allow discrimination between the two processes major oxide least squares mass balance models do not discriminate between afc and fca at f 0 7 f fraction of melt remaining however fca yields magmas richer in compatible elements and with higher sr isotopic ratios than afc at a given value of f repeated cycles of fca and afc combined with magma mixing fam may result in unusual geochemical trends such as the evolution of a calc alkaline basaltic parent to a tholeiitic daughter magma or the evolution of low and medium k calc alkaline basalts to high k andesites dacites trachyandesites or trachydacites lavas erupted by the volcano micro profitis ilias on santorini hellenic arc greece provide an example of magmas which evolved by combined fractionation assimilation by fca and mixing

geochemical evidence for a lithospheric source for magmas from los humeros caldera puebla mexico abstract this study reports new geochemical and sr nd pb radiogenic isotope data on hy normative quaternary basaltic to rhyolitic volcanic rocks as well as two samples of mesozoic to palaeozoic intrusive igneous rocks from los humeros caldera lhc located in the eastern part of the mexican volcanic belt mvb the isotopic ratios of mafic basaltic rocks range as follows 87 sr 86 sr 0 703350 0 703387 143 nd 144 nd 0 512867 0 512888 206 pb 204 pb 18 849 18 866 207 pb 204 pb 15 593 15 598 and 208 pb 204 pb 38 490 38 491 these values are similar to the rift related mafic rocks from the volcanic front of the central mvb for comparison the entire suite from basalt b to rhyolite r in the lhc shows the following ranges 87 sr 86 sr 0 703350 0 704678 143 nd 144 nd 0 512702 0 512888 206 pb 204 pb 18 710 18 866 207 pb 204 pb 15 593 15 631 and 208 pb 204 pb 38 444 38 594 the basaltic rocks from los humeros do not show the characteristic negative nb anomaly with respect to ba and ce as is typical of such magmas in volcanic arcs the available geochemical and isotopic evidence does not support the generation of the lhc mafic magmas by direct slab melting nor by indirect fluid transport to the mantle participation of the subducted cocos plate instead they could be generated in the upper mantle from a lithospheric source the evolved andesitic to rhyolitic magmas could have been originated from such mafic magmas through assimilation coupled with extensive fractional crystallisation two different compositions representing lower lc and upper crusts uc as well as a hypothetical crust ha were tested as plausible assimilants for the assimilation coupled with fractional crystallisation process

paricutin re examined a classic example of crustal assimilation in calc alkaline magma following its birth on the 20th of february 1943 the mexican volcano paricutin discharged a total of 1 38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952 until 1947 when 75 of the volume had been erupted the lavas varied little in chemical or isotopic composition all were basaltic andesites with 55 to 56 sio2 18o of 6 9 to 7 0 and 87sr 86sr ratios close to 0 7038 subsequent lavas were hypersthene andesites with silica contents reaching 60 18o values up to 7 6 and 87sr 86sr of 0 7040 to 0 7043 the later lavas were enriched in ba rb li and k2o and depleted in mgo cu zn cr ni sr and co the isotopic and other chemical changes which appeared abruptly over a few months in 1947 are interpreted as the result of tapping a sharply zoned and density stratified magma chamber xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70 18o of 5 6 to 9 9 and 87sr 86sr between 0 7043 and 0 7101 in many respects they resemble samples of basement rocks collected from nearby outcrops three analysed samples of the latter have silica contents of 65 to 67 18o of 7 7 to 8 6 and 87sr 86sr between 0 7047 and 0 7056 these new data provide strong support for the original interpretations of wilcox 1954 who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight continental crust except for a few trace elements particularly ba sr and zr the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late stage lavas some of the discrepancies may be explained by postulating a contaminant that was older and richer in ba sr and zr than those represented by the analysed xenoliths others can be attributed to chemical changes accompanying disequilibrium partial melting contact metamorphism and meteoric hydrothermal alteration of the country rock many of the xenoliths show evidence of having been affected by such processes the lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption the order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities

geochemical and sr nd pb isotopic evidence for a combined assimilation and fractional crystallisation process for volcanic rocks from the huichapan caldera hidalgo mexico abstract this study reports new geochemical and sr nd pb isotopic data for miocene to quaternary basaltic to andesitic dacitic and rhyolitic volcanic rocks from the huichapan caldera located in the central part of the mexican volcanic belt mvb the initial sr and nd isotopic ratios except for one rhyolite range as follows 87sr 86sr 0 70357 0 70498 and 143nd 144nd 0 51265 0 51282 the sr nd pb isotopic ratios are generally similar to those for volcanic rocks from other areas of the central and eastern parts of the mvb the isotopic ratios of one older pre caldera rhyolite hp30 from the huichapan area particularly its high 87sr 86sr are significantly different from rhyolitic rocks from this and other areas of the mvb but are isotopically similar to some felsic rocks from the neighbouring geological province of sierra madre occidental smo implying an origin as a partial melt of the underlying crust the evolved andesitic to rhyolitic magmas could have originated from a basaltic magma through a combined assimilation and fractional crystallisation afc process different compositions representing lower crust lc and upper crust uc as well as a hypothetical crust similar to the source of high 87sr 86sr rhyolite hp30 were tested as plausible assimilants for the afc process the results show that the uc represented by granitic rocks from a nearby los humeros area or by cretaceous limestone l rocks outcropping in the northern part of the study area and the lc represented by granulitic xenoliths from a nearby san luis potosi slp area are not possible assimilants for huichapan magmas whereas a hypothetical crust ha similar in isotopic compositions to rhyolite hp30 could be considered a possible assimilant for the afc process chemical composition of assimilant ha although not well constrained at present was inferred under the assumption that hp30 type partial melts could be generated from its partial melting these data were then used to evaluate the proposed afc process to explain the evolution of the huichapan magmas

molluscs as evidence for a late pleistocene and early holocene humid period in the southern coastal desert of peru 14 5 s the southern peruvian coastal desert around palpa southern peru 14 5 s is currently characterized by hyper arid conditions however the presence of two species of molluscs scutalus pupoides and desert loess deposits indicates the past development of semi desert and grassland ecosystems caused by a displacement of the eastern desert margin due to hydrological changes radiocarbon dating shows that the transition to a semi arid climate in the southern peruvian coastal desert took place during the greenland interstadial 1 13 5 cal ka bp at the beginning of the holocene the mollusc fauna vanished due to increasing humidity and the development of grasslands dust particles were fixed by the grasses as indicated by abundant poaceae phytoliths and desert loess was formed the humid period we observe here is out of phase with the palaeoenvironmental records from the titicaca region which indicates dry conditions at that time this paper offers a new idea for this contradiction an orbitally driven meridional shift of the bolivian high might have altered the moisture supply across the andes

lead isotope studies of the guerrero composite terrane west central mexico implications for ore genesis new thermal ionization mass spectrometry and multi collector inductively coupled plasma mass spectrometry pb isotope analyses of three cenozoic ores from the la verde porphyry copper deposit located in the zihuatanejo huetamo subterrane of the guerrero composite terrane are presented and the metal sources are evaluated lead isotope ratios of 3 cenozoic ores from the el malacate and la esmeralda porphyry copper deposits located in the zihuatanejo huetamo subterrane and of 14 ores from the zimapan and la negra skarn deposits from the adjoining sierra madre terrane are also presented to look for systematic differences in the lead isotope trends and ore metal sources among the proposed exotic tectonostratigraphic terranes of southern mexico comparison among the isotopic signatures of ores from the sierra madre terrane and distinct subterranes of the guerrero terrane supports the idea that there is no direct correlation between the distinct suspect terranes of mexico and the isotopic signatures of the associated cenozoic ores rather these pb isotope patterns are interpreted to reflect increasing crustal contribution to mantle derived magmas as the arc advanced eastward onto a progressively thicker continental crust the lead isotope trend observed in cenozoic ores is not recognized in the ores from mesozoic volcanogenic massive sulfide and sedimentary exhalative deposits the mesozoic ores formed prior to the amalgamation of the guerrero composite terrane to the continental margin which took place during the late cretaceous in intraoceanic island arc and intracontinental marginal basin settings while the tertiary deposits formed after this event in a continental arc setting lead isotope ratios of the mesozoic and cenozoic ores appear to reflect these differences in tectonic setting of ore formation most pb isotope values of ores from the la verde deposit 206pb 204pb 18 674 18 719 are less radiogenic than those of the host igneous rocks but plot within the field defined by the huetamo sequence suggesting that these ores may also contain metals from the sedimentary rocks the pb isotope ratios of ore samples from the zimapan deposit 206pb 204pb 18 771 18 848 are substantially higher than the whole rock pb isotope compositions of the basement rocks the similarity of ore pb to igneous rock pb in the zimapan district 206pb 204pb 18 800 18 968 may indicate that the proximal source of ore metals in the hydrothermal system was the igneous activity

composition and smnd isotopic data of the lower crust beneath san luis potosi central mexico evidence from a granulite facies xenolith suite quaternary volcanics from san luis potosi slp in central mexico sampled a xenolith suite comprising uppermantle nodules spinell lherzolites hornblende pyroxenites spinel pyroxenites and lower crustal xenoliths the latter consist of granulite facies mafic to intermediate metaigneous rocks and high grade metasediments precursors of the metaigneous suite range from gabbroic to tonalitic compositions whole rock ree patterns permit classification of metaigneous rocks into cumulates and fractionated melts mafic xenoliths consist of plagioclase clinopyroxene orthopyroxene garnet amphibole apatite rutile scapolite garnet has reacted extensively with clinopyroxene forming corona textures of secondary plagioclase orthopyroxene and amphibole intermediate xenoliths are garnet free and quartz bearing all samples display similar ca al zoning patterns documented in orthopyroxene clinopyroxene and plagioclase which are along with the presence of coronas around garnet interpreted as representing partial re equilibration stages from 940 60 c and 7 11 kbar down to lower temperatures and pressures there is no indication of a secondary heating event in these xenoliths sm nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath slp similar sr and nd isotopic signatures reveal that the hornblende pyroxenites are high pressure cumulates linked to the host basanites four mainly intermediate metaigneous granulites from a single vent which are free of secondary amphibole and two spinel pyroxenites regarded as the most primitive member of the metaigneous suite define a regression line in a sm nd isochron diagram yielding an age value of 1248 69 ma and an initial nd value of 2 3 metaigneous xenoliths containing secondary amphibole plot significantly above the regression line which is probably due to the introduction of externally derived nd during amphibole formation under retrograde p t conditions discussing the geological significance of the calculated age value there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors this gives evidence for formation of lower crust of basic to intermediate compositions during the mid proterozoic now situated beneath slp

epithermal au ag and related deposits of the hauraki goldfield coromandel volcanic zone new zealand epithermal au ag vein deposits of the hauraki goldfield in the coromandel volcanic zone new zealand produced 320 000 kg au and 1 5 million kg ag between 1862 and 2006 from a total land area of approximately 2 900 km2 approximately 69 percent of the production was from the world class martha deposit at waihi some 97 percent of total past gold production was from deposits hosted in miocene andesite and dacite of the coromandel group although this unit represents only 61 percent of the rocks exposed in the goldfield the deposits consist of quartz calcite vein systems 0 5 to 4 5 km long within alteration zones up to 20 km2 in size they were mined over vertical intervals of 170 to 330 m but up to 575 m at martha and 700 m at karangahake electrum and acanthite are the main ore minerals although pyrite sphalerite galena chalcopyrite sulfosalt minerals and te and se bearing minerals are present in some deposits stibnite and cinnabar occur locally as late minerals and some deposits are overprinted by late massive calcite fluid inclusion ice melting temperature tm measurements indicate that fluids associated with base metal and precious metal deposition had apparent salinities of between 0 5 and 2 wt percent nacl equiv with a maximum of 6 1 wt percent nacl equiv when fitted to hydrostatic boiling point with depth curves the homogenization temperature th data define paleodepths spanning a vertical interval from near the water table to a depth of approximately 1 300 m the presence of vapor rich fluid inclusions and occurrences of lattice textures and vein adularia indicate that periods of boiling coincided with deposition of some epithermal ores sulfur and carbon isotope values of vein minerals are consistent with a magmatic origin for sulfur and carbon oxygen isotope values of vein quartz and calcite are consistent with derivation from evolved meteoric water with possibly a small magmatic water component in some samples the epithermal deposits can be broadly subdivided into northern southern and eastern groups based on their host rocks age variability of vein strike au ag ratios occurrence or relative abundance of key minerals such as adularia calcite and arsenopyrite the grain size of vein quartz and differences in vein textures early ca 14 10 ma steeply dipping planar veins occur in the oldest andesite and dacite in the northern portion of the goldfield but some of these are bonanza grade veins that contained significant gold resources larger veins typically with well developed crustiform textures formed in andesite and dacite in the southern gold field at 7 to 6 ma and these contributed most of the au and ag that has been recovered from the goldfield in contrast rhyolite in the eastern goldfield hosts smaller less well developed veins plus stockworks that have produced the least amount of au and ag epithermal mineral deposits in the hauraki goldfield evolved through time reflecting changes in volcanic activity and more speculatively tectonic and structural history and setting

enso tropical extratropical climate teleconnections and mechanisms for holocene debris flows along the hyperarid coast of western south america 17 24 s abstract el nino the warm phase of the enso cycle involves ocean climate anomalies in the tropical pacific ocean and in the extratropics which frequently result in heavy rainfall episodes along the equatorial and subtropical regions of western south america here we investigate meteorological mechanisms producing heavy rains floods and debris flows along the less known hyperarid coasts of southern peru and northern chile to evaluate the paleoclimate significance of holocene debris flow deposits our results reveal that heavy rainfall over the coast of southernmost peru occurs either during austral summer at the mature stage of el nino in connection with warmer sea surface temperatures and anomalous jet streams off northern chile or during the previous austral winter spring associated with equatorward shifted pacific south america psa atmospheric teleconnection patterns at antofagasta in northern chile such events occur almost exclusively in the latter season when deeper psa related anomaly poles extend their influence equatorwards beyond central chile during non el nino conditions short lived heavy rainfall episodes in southernmost peru can be associated with similar albeit weaker or less persistent circulation anomalies in addition to that a seasonally enhanced low level southerly flow provides orographic uplift rainfall favourable conditions by the andes at the 18 s coastal bend ultimately the trigger for rainfall events in all seasons and phases of the enso cycle was invariably connected with mid troposphere wave disturbances from mid latitudes the chronostratigraphy of debris flow deposits from both areas and its comparison with other paleoclimate records at the western side of the andes suggests that the enso related teleconnection patterns operated only during the second half of the holocene supporting an onset of modern el nino manifestations at 5300 5500 cal bp and increased frequency of major events during recent times we suggest that several debris flows dated between 12 900 and 8400 cal bp in southernmost peru previously interpreted as an indication of strong el nino events were associated with short and intense heavy rainfall episodes similar to those described here during the late winter spring season in non el nino conditions concomitant with stronger low level southerlies strengthened south eastern pacific subtropical anticyclone and intensified coastal upwelling

multiple episodes of aridity in southern africa since the last interglacial period there is generally a dearth of evidence of the nature of quaternary climate change within desert systems which has limited previous interpretations of past environmental change at low latitudes the last glacial maximum has previously been identified as the peak of late quaternary aridity when desert systems expanded to five times their present extent1 2 3 and low latitude aridity has been described for previous glaciations4 but little evidence has been derived directly for large desert basins particularly southern africa here we report new chronological optical dating evidence of arid episodes recorded in aeolian sediments from the mega kalahari sand sea episodic aeolian activity is recorded at the northeastern desert margin whereas more sustained activity is evident from the southwestern desert core several significant arid events are apparent since the last interglacial period with dune building arid phases at 95 115 41 46 20 26 and 9 16 kyr before present existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast southwest summer rainfall gradient which are in turn related to sea surface temperatures in the southeastern atlantic ocean

volcanic stratigraphy of the los azufres geothermal area mexico abstract the los azufres volcanic center located 200 km northwest of mexico city is one of a number of pleistocene silicic volcanic centers with active geothermal systems that lie north of the axis of the mexican neovolcanic belt silicic volcanism began approximately 1 m y ago with eruption of the agua fria rhyodacite to high silica rhyolite lava domes and flows they are covered by rhyodacite and dacite lava domes and flows of the san andres volcano dated at 0 3 m y a north to northeast trending set of high angle normal faults is offset by a younger and more prominent east trending set that parallels the principal regional structural trend these faults cut all but the youngest volcanic rocks of the center the yerbabuena rhyodacites to high silica rhyolites these morphologically well preserved lava domes are approximately 0 15 m y old and are probably associated with air fall tuffs that blanket the western portion of the volcanic center two episodes of high heat flow suggested by studies of drill core samples and fluids from the los azufres geothermal field may correspond to two different episodes of magma injection to high levels the most recent one resulting in eruption of the yerbabuena lavas

isotopic and trace element constraints on the origin and evolution of alkaline and calc alkaline magmas in the northwestern mexican volcanic belt a wide variety of rock types has been produced by pliocene to recent volcanism in the northwestern portion of the mexican volcanic belt composite cones erupted rocks typical of calc alkaline volcanic belts associated with subduction monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks one larger center las navajas produced basalts trachytes and peralkaline rhyolites such alkaline rocks are typically associated with crustal extension related to rifting both subduction and rifting appear to be taking place in western mexico the most basic calc alkaline rocks although somewhat fractionated show depletion in high field strength elements hfse and enrichment in alkaline earth elements relative to other incompatible elements a characteristic typical of magmas in subduction related volcanic arcs the basic alkaline rocks show no depletion in hfse and show higher concentrations of incompatible elements than the basic calc alkaline rocks the calc alkaline rocks show a tight cluster of 87sr 86sr and 143nd 144nd ratios both of which correlate weakly with sio2 basic alkaline rocks have lower sr and higher nd isotopic ratios whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc alkaline rocks trace element data indicate that calc alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas the evolution of both calc alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation afc of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite assuming a contaminant located in the lower or middle crust afc modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc alkaline magmas calc alkaline and alkaline rocks appear to have resulted from melting of two distinct sources alkaline from an oib type source and calc alkaline from a mixed mantle and slab derived source

sr and nd isotopic evidence for petrogenesis of mid tertiary felsic volcanism in the mineral district of zacatecas zac sierra madre occidental mexico abstract twelve samples of mid tertiary felsic volcanic rocks from zacatecas and san luis potosi both belonging to the sierra madre occidental and one sample of lower tertiary porphyritic andesite from zacatecas are analyzed for 87 sr 86 sr k rb and sr eight selected samples are also analyzed for 143 nd 144 nd a linear regression of the present day 87 sr 86 sr and 87 rb 86 sr of the felsic volcanic rocks in zacatecas gives an approximate date of 30 8 ma the initial 87 sr 86 sr ratios are high and widely distributed from 0 705 to 0 712 or higher whereas the initial 143 nd 144 nd ratios are somewhat low and show a narrow range 0 5125 0 5127 the available isotopic and trace element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted mantle fractionate and mix with varying proportions of the overlying middle upper continental crust and undergo further shallow level fractional crystallization before eruption this model is also compatible with the trace element and sr isotopic data published from other areas of the sierra madre occidental for which a purely mantle origin has been proposed

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

deep geothermal wells in the los azufres mexico caldera volcanic basement stratigraphy based on major element analysis abstract the los azufres caldera michoacan mexico is located in the trans mexican volcanic belt mvb the pre caldera volcanic basement consists of a 3 km thick pile of andesitic flows intruded and or overlain by rhyolitic and dacitic domes and flows the geochemistry of the volcanic layers has been investigated in three deep wells a9 a25 and a48 down to 3 0 km such deep wells have given the opportunity to investigate the geochemical changes related to the hydrothermal metamorphism as well as the stratigraphy of the thick volcanic pile systematic analysis of major elements in cores and cuttings shows that the partial to total hydrothermal metamorphism of the series from the greenschist to the amphibolite facies produces significant geochemical changes on the whole rock geochemistry gains in co 2 h 2 o and depletions in alkalis are especially observed and may be related with the crystallization of carbonates and phyllosilicates however the depth concentration profiles have shown that most other elements such as al fe ca mg p and ti display moderate changes due to hydrothermal alteration in comparison with their initial variations of magmatic origin rythmic compositional changes are observed all along the volcanic pile with progressive changes in rock composition from basalts to dacites within four megasequences of a few hundred meters thickness elements rather immobile under hydrothermal conditions such as ti have been especially useful for the reconstruction of magmatic sequence stratigraphy variations of element concentrations seem mostly due to magmatic differentiation of parental basalts which produces compositions from basaltic andesites to dacites however the four magmatic suites distinguished display calc alkaline features coherent with other suites of the mvb volcanoes

magma genesis and chamber processes at los humeros caldera mexico nd and sr isotope data the mexican volcanic belt mvb a roughly east west structure consists of many late tertiary and quaternary cindercones domes calderas and stratovolcanoes1 2 los humeros caldera approximately 19 40 n latitude 97 25 w longitude lies on the northeastern part of the mvb where the belt overlaps with another major volcanic province the eastern cordillera3 fig 1 a compilation6 of the bulk chemical analyses of the two major volcanic provinces indicates that the mvb is characterized largely by calc alkaline series whereas rocks of the alkaline series dominate the eastern cordiliera ec pleistocene to recent basaltic to rhyolitic volcanism in los humeros caldera one of the best known examples of a well developed caldera in mexico7 9 presumably associated with the subduction of cocos plate along the middle america trench shows that the initial 87sr 86sr ratios range from 0 7039 to 0 7048 and the initial 143nd 144nd ratios from 0 5126 to 0 5129 we show here that these isotope ratios are negatively correlated and lie on the mantle array defined by morb and oceanic island rocks implying that los humeros magmas were generated in the upper mantle with very little if any contribution from the subducted oceanic crust sediments or continental crust

episodic eolian deposition in the past ca 50 000years in the alto ilo dune field southern peru abstract the alto ilo dune field in the coastal desert of southern peru is located at the edge of the atacama desert and is one of the driest landscapes on earth a stratigraphic sequence of eolian sands paleosols and debris flow deposits are identified in this dune field spanning the past 55 ka we infer that eolian deposition is preceded by an increase in sediment supply from local alluvial fans that are upwind of the alto ilo dune field and thus reflects periods of relative hydrologic excess eolian sediment supply may increase also with the concomitant fall in sea level providing an additional foreland source area for eolian particles the presence of paleosols indicate millennial scale periods of locally wetter conditions possibly associated with spread of lomas vegetation fed from coastal fog sources debris flows reflect significant rainfall events possibly associated with enso variability the chronology is provided by optically stimulated luminescence using multiple aliquot regeneration protocols which reveal four major eolian depositional episodes at ca 55 to 45 ka 38 to 27 ka 22 to 16 ka and 12 ka eolian deposition in the late pleistocene dominates the record and is generally coincident with metrics of hydrologic excess from the peruvian andes and the altiplano which appears to translate into enhanced sediment availability on the coast a significant period of soil development between 18 and 13 ka is consistent with timing of central andean pluvial event and indicates widespread wet conditions the holocene record of eolian deposition is sparse reflecting limited alluvial fan sources and decreasing particle availability upwind with the rise in postglacial sea level

application of the gauss theorem to the study of silicic calderas the calderas of la primavera los azufres and los humeros mexico abstract we explored applications including limitations of gauss s theorem to the study of silicic calderas first it enables us to determine the mass deficiency from calderas mass deficiency itself has also other potential applications it enables to make qualitative comparisons between calderas we can use the mass deficiency to test in a quick way and as a preliminary step to a formal gravity inversion for the feasibility of caldera types of simple geometry i e piston subsidence and funnel models this application can be done in a straightforward way once the mass deficiency has been determined for this purpose the mass deficiency is converted to the volume of material missing at the caldera subsequently for example this volume and the respective caldera diameter enable us to estimate the height of the cylinder fitting the piston subsidence model if the obtained parameters are congruent with the known geology and geophysical information then the model may be considered further in the inversion of the gravity data for the detailed structure other simple models i e the funnel model can also be analyzed in this way in particular when working with a piston subsidence caldera type the gauss theorem enables us to estimate the caldera collapse very difficult to obtain based on geologic information alone these possible uses of gauss s theorem are illustrated with the calderas of la primavera los azufres and los humeros caldera mexico the obtained mass deficiency from these calderas follow the linear mass deficiency diameter trend observed for other calderas in particular because of their diameters and mass deficiencies la primavera and krakatau calderas can be considered equiparable this comparison is of the most importance considering that la primavera is located in the neighbourhood of a metropolis guadalajara city since geophysical studies have already established a piston subsidence model for these calderas we assessed gauss s theorem used to estimate their collapses the respective estimated subsidences for la primavera 600 m los humeros 500 m and los azufres calderas 800 and 1200 m respectively are supported by borehole data these relatively small subsidences in relation to many calderas in western us can be explained by the presence of cylindrical to sub spherical magma chambers of smaller size than those underlying the western us calderas of yellowstone valles and long valley for example in an epicontinental setting of young quaternary rocks against precambrian cratonic basement in western us a reassessment of all the available geological and geophysical information was also done in particular the caldera limits were checked and corrected when necessary based on available 2 d gravity modeling gravity gradients and the location of the volcanic vents regional tectonic lineaments might explain many of the particular details of their structures la primavera caldera might have formed through a piecemeal subsidence controlled by the pre caldera tectonics los humeros caldera is an example of a caldera that evolved by incremental growth as the vulsiniani calderas in italy the los azufres caldera comprises two contiguous calderas emplaced in a regional tectonic depression

geometry of the benioff zone and state of stress in the overriding plate in central mexico analysis of depths and focal mechanisms of 16 small earthquakes in central mexico along with previous data shows that a the subducted cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km and b the bottom part of the overriding continental plate is in tensional stress regime neotectonic structures in central mexico and stress orientations estimated from borehole elongations cinder cone alignments and fault slip analysis in the trans mexican volcanic belt which lies in the northeastern portion of the area under study also indicate the same stress regime absolute motion of the north american plate in the region has a component normal to the trench of 20 mm yr which if true would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference if we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the benioff zone can be explained this however implies that seaward retreat of a trench is possible even for a young 15 m y old subducting slab alternatively tectonic erosion of the leading edge of the continent which has been proposed to explain truncated igneous and metamorphic continental terrane could give rise to the tensional stress but would not explain the geometry of the benioff zone

the great atacama flood of 2001 and its implications for andean hydrology in february 2001 widespread flooding occurred throughout the atacama desert of northern chile and southern peru it was particularly severe in the rio loa basin where roads and bridges were disrupted and the town of calama inundated the instantaneous peak flow in the rio salado a tributary of the rio loa reached 310 m3 s 1 an order of magnitude higher than any previously recorded event the flood is estimated to have a return period of 100 200 years and is shown to have been caused by intense long duration rainfall in the western cordillera associated with la nina the surface water response is typical of arid areas and highly dependent on antecedent conditions but is quite different in perennial and ephemeral catchments ephemeral flood flows suffer high transmission losses recharging phreatic aquifers perennial rivers have lower runoff coefficients but baseflow levels remained high after the event for several months due to bank storage rebound and interflow extremely high energies of 3000 w m 2 were generated by the floods in the cordillera becoming less in the precordillera and downstream erosion and sediment transport were consequently highest in the upper and middle reaches of the rivers with mixed erosion deposition in the lowest reach the new insights gained from the interpretation and quantification of this event have important implications for palaeoenvironmental analysis hazard management water resource evaluation and the palaeohydrological evolution of the andes copyright 2005 john wiley sons ltd

generation of high silica rhyolite a nd sr and o isotopic study of sierra la primavera mexican neovolcanic belt high silica rhyolites of the sierra la primavera a late pleistocene center near guadalajara are extremely sr poor 0 3 1 3 ppm yet with one exception values of 87sr 86sri are relatively low at 0 7041 0 7048 values of 143nd 144nd for all the rhyolites are within errors identical to a basalt at 0 5129 these surprisingly primitive values along with feldspar 18o of 6 6 are consistent with an origin by fractional crystallization of mantle derived basalt however absence of the large volume of associated intermediate rocks that would be expected if the 40 km3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of mesozoic or tertiary mafic intrusive rocks or lower crustal metamorphic equivalents the latter interpretation is preferred especially in light of comparative data for other north american cenozoic high silica rhyolites isotopic compositions correlate with basement age but generally lie between values for associated basalts and the underlying crust nearly all can be interpreted as containing both a young mantle derived component and a crustal component probably derived by partial melting at intermediate to deep levels of the crust no matter what the proportions of mantle and crust derived material in parental magmas the extremely low concentrations of sr and ba in the high silica rhyolites require extensive fractional crystallization of feldspar rich assemblages after parental liquids attain rhyolitic compositions

fault controlled geothermal fluids of the northern trans mexican volcanic belt a geochemical and isotopic study of the los geysers field valley of queretaro mexico abstract this study focuses on the geochemical and isotopic composition of natural spring waters and gases discharging at los geysers thermal area northern trans mexican volcanic belt tmvb and its relations to the regional tectonic framework water is of na hco3 type and samples were collected at temperatures ranging from 48 c to 98 c and ph from 6 8 to 9 3 stable isotope compositions 10 0 to 8 0 for 18o and 75 6 to 69 7 for d deviate from the global and local meteoric lines suggesting fractionation by steam loss the 87sr 86sr values of the thermal waters range from 0 70664 to 0 70698 indicating that most of the strontium comes from the dissolution of carbonate belonging to the late berriasian to early valanginian in the guanajuato conglomerate group and a minor component of less radiogenic sr from paleogene neogene volcano sedimentary and volcanic rocks solute geothermometers reveal a reservoir temperature around 164 c a mixture between air and hydrothermal co2 composes the bubbling gases helium isotopes data r ra 0 96 1 71 and 4he 20ne ratios 0 53 9 41 reveal that helium is dominated by a radiogenic component with a considerable mantle he contribution up to 20 8 since there is no active volcanism at the northern tmvb mantle he is best explained by transport of volatiles along active faults reaching the crust mantle boundary the conceptual hydrogeochemical model involves a thermal aquifer at 1000 m depth which is recharged by regional meteoric water flow a high thermal gradient 153 c km and fast fluid upflow to the surface along discontinuities related to regional horst and graben tectonics

partition coefficients for rare earth elements in mafic minerals of high silica rhyolites the importance of accessory mineral inclusions abstract ree concentrations of mafic mineral separates from high silica rhyolites measured by inaa are high and variable compared to electron microprobe analyses of the minerals themselves the mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element ree rich minerals optical and electron microscopic observation revealed discrete grains of chevkinite rare earth titano silicate included within clinopyroxenes from the sierra la primavera mexico rhyolite and monazite grains adhering to ortho and clinopyroxenes from the bishop tuff california during hand picking of mineral separates inclusions are only partly removed as a result the magnitude and variability of true mineral melt partition coefficients for light ree have been overestimated the true ree partition coefficients of la primavera and bishop tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower silica rhyolites and are not as variable as previously thought the partitioning slope is positive as in less silicic systems the relative partitioning behavior of ree in high silica rhyolites is dominated by crystal chemical controls and not by liquid structural effects when a partition coefficient is used for crystallization calculations adhering phases and inclusions should be retained or the minor phases must be accounted for separately alternatively a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode

do major oxide tectonic discrimination diagrams work evaluating new log ratio and discriminant analysis based diagrams with indian ocean mafic volcanics and asian ophiolites many geochemical diagrams exist that classify old volcanic terranes of ambiguous provenance into various modern plate tectonic settings with variable success recently proposed diagrams based on log ratios and linear discriminant analysis with large datasets of major oxides were tested here with data for ocean island arc and mid ocean ridge lavas from the indian ocean success rates are 45 100 with misclassifications potentially caused by alteration although alteration demonstrably need not cause misclassification the diagrams were further applied to some asian ophiolites representing tethyan and indian ocean crusts to see if the diagrams confirm their tectonic setting inferred from trace and isotopic data lower success rates 30 60 but 75 100 for specific suites are not surprising in view of the ubiquitous and complex alteration in ophiolites log ratio transformation and linear discriminant analysis appear to be powerful methods when discrimination diagrams are based on major oxide data alone

adakitic like magmatism in western ossa morena zone portugal geochemical and isotopic constraints of the pavia pluton abstract granitic rocks are a major component of the earth s continental crust and occur in a wide variety of tectonic settings their chemical and isotopic characterization is crucial to the recognition of the potential sources and mechanisms involved in their generation in this study we present the first whole rock chemical and isotopic sr nd o data for the pavia pluton 328 317 ma located near the western border of the ossa morena zone evora massif portugal major and trace element geochemistry suggests that the different granitic phases composing this intrusive body enclaves granites s l and crosscutting dikes represent independent magma pulses and the majority is similar to ttgs and adakites the little sr nd o isotopic variation with 87 sr 86 sr 328 0 70428 0 70560 end 328 ranging between 3 4 and 0 4 and 18 o varying from 5 6 to 8 4 implies an isotopically similar protolith for all phases the most viable mechanism for the generation of the pavia pluton adakitic like magmatism is assimilation fractional crystallization of a mantle derived magma this mechanism was also invoked to explain the genesis of other plutons within the evora massif but they have a distinct chemistry typical arc calc alkaline rocks the chemical differences between them and the pavia pluton granitic rocks are interpreted as the result of lower degrees of crustal assimilation and higher degrees of contamination of mantle derived magmas by the sinking slab after subduction blocking and subsequent slab break off