listric faulting and continental rifting in western mexico a paleomagnetic and structural study abstract the western trans mexican volcanic belt is characterized by several large scale tectonic depressions which recently have been interpreted in terms of active rifting and independent motion of the jalisco block in this paper we report results for the northwestern part of the jalisco block within the ameca tectonic depression the trend of the central part of the depression changes where it is bounded by the ameca river fault there the fault which is characterized by a steep high scarp and a pronounced surficial curvature bounds the pleistocene amatlan de canas volcanic field which consists of small eroded volcanic structures and mafic lava flows intercalated with conglomeratic talus debris the sinuous pattern of outcrop of the fault indicates that the fault plane dips toward the volcanic field the conglomeratic units are abruptly truncated at the fault contact and generally dip 12 to 20 toward the fault plane paleomagnetic results for the young volcanic units give a mean direction which is interpreted to be the result of 12 to 20 of tilting toward the fault regional listric faulting block sliding of the entire block occupied by the volcanic field is postulated to explain the structural stratigraphic and paleomagnetic patterns observed listric faulting occurs within a transtensional tectonic environment related to the southwesterly motion of the jalisco block as it rifts away from mainland mexico

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

volcan las navajas a pliocene pleistocene trachyte peralkaline rhyolite volcano in the northwestern mexican volcanic belt volcan las navajas a pliocene pleistocene volcano located in the northwestern portion of the mexican volcanic belt erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite and is the only volcano in mainland mexico known to have erupted pantellerites las navajas is located near the northwestern end of the tepic zacoalco rift and covers a 200 m thick pile of alkaline basaltic lavas one of which has been dated at 4 3 ma the eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation during the first stage a broad shield volcano made up of alkali basalts mugearites benmoreites trachytes and peralkaline rhyolites was constructed eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera the second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls this stage ended about 200 000 years ago with the eruption of pumice falls and ash flows which led to the collapse of the southern portion of the volcano to form the second caldera during the third stage two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano finally the calc alkaline volcano sanganguey was built on the southern flank of las lavajas alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along nw trending fractures through the area although other mildly peralkaline rhyolites are found in the rift zones of western mexico only las navajas produced pantellerites greater volumes of basic alkaline magma have erupted in the las navajas region than in the other areas of peralkaline volcanism in mexico a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

the la brena el jaguey maar complex durango mexico ii petrology and geochemistry a suite of 16 basanitic volcanic rocks representing all stages in the evolution of the la brena el jaguey lbej maar complex has been studied petrographically and analyzed for mineral compositions and whole rock major element trace element and sr nd pb isotopic compositions two feldspathic granulite xenoliths were also studied as possible lower crustal contaminants to the lbej magmas the volcanic rocks contain the stable minerals olivine plagioclase augite and titanomagnetite ilmenite plus a diverse suite of xenocrusts derived from disaggregation of mantle xenoliths of spinel lherzolite olivine orthopyroxene spinel and lower crustal granulite xenoliths plagioclase quartz augite ilmenite late stage interstitial melts rich in fe and ti migrated into vesicles in several samples forming coarse grained segregation vesicles that are dominated by ilmenite blades up to 2 mm long the whole rock elemental data are typical of intra plate basanitic rocks with strong enrichments in large ion lithophile elements i e k th u as well as high field strength elements i e nb ta relative to mid ocean ridge basalts morb and estimates of primordial mantle abundances mg increased systematically with time during the evolution of the lbej maar complex from 57 0 58 2 in the pre maar lavas to 59 1 63 8 in the post maar lavas compatible elements ca sc cr co ni correlate positively with mg whereas a large group of incompatible elements al na k p rb sr zr nb ba la ce sm hf ta th u correlate negatively with mg these trends can be closely reproduced by simple models of fractional crystallization provided that the incompatible element abundances of the parental high mg magmas are allowed minor variability all successful fractionation models demand an important role for augite despite its presence in the lbej volcanic rocks as only a late stage microphenocrystic and groundmass mineral minor garnet fractionation is necessary to produce depletion of heavy rare earth element ree abundances in the pre maar lavas whose ree patterns cross those for the rest of the suite the importance of augite and garnet fractionation indicate that the differentiation of the lbej magmas took place within the upper mantle a conclusion that is supported by the presence of spinel lherzolite xenoliths in magmas from all stages in the evolution of the maar complex isotopic data for seven lbej volcanic rocks show the following ranges 87sr 86sr 0 70327 0 70347 nd 4 2 5 0 206pb 204pb 18 60 18 81 207pb 204pb 15 58 15 65 208pb 204pb 38 19 38 58 sr nd values are negatively correlated and form a trend parallel to the mantle array overlapping the field for ocean island basalts oib the lbej rocks have similar 87sr 86sr values but lower nd compared to basanitic rocks from the us basin and range province brp pb isotopic ratios are positively correlated and overlap the braod fields for morb and oib and the small fields for mexican ore deposits and volcanic rocks from the active subduction related mexican volcanic belt the lbej rocks have slightly more radiogenic pb than basanitic rocks from the us brp despite correlations among the isotopic ratios of the lbej suite none of these ratios correlate with position in the eruption sequence mg or any other compositional parameter the two lower crustal xenoliths have high 87sr 86sr values 0 707 0 710 and low nd 1 5 8 0 compared to the lbej volcanic rocks but their pb isotopic compositions are only slightly more radiogenic than the volcanic rocks these data do not support the widely held view that the lower crust is a major reservoir of unradiogenic pb in order to further constrain the role played by crustal contamination in generating the isotopic diversity in the lbej suite we conducted an extensive investigation of sr nd pb isotopic ratios for scoria clasts from different levels of a single scoria fall horizon in the pyroclastic sequence related to the formation of la brena maar our results do not support an important role for crustal contamination in the lbej magmas rather we conclude that minor isotopic variability exists in the mantle source regions beneath the maar complex

on b be ratios in the mexican volcanic belt boron and beryllium concentrations were measured in a diverse suite of well characterized rocks from the mexican volcanic belt mvb low b and high be result in relatively low b be ratios in the mvb compared to other arcs nevertheless b systematics resemble those of other arcs and provide insights into mantle processes in the mvb b enrichment depends first on magma type and second on edifice type and location b be values are highest 5 15 in andesites and dacites erupted from calcalkaline strato volcanoes located along the volcanic front such as volcan colima and v san juan rocks from strato volcanoes located behind the volcanic front generally have lower values 1 5 b be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation in the westernmost mvb west of the michoacan guanajuato volcanic field mgvf cindercone ejecta including basalts lamprophyres and basanites contain low b be values 5 the lamprophyres and basanites have very low b be despite high ba ce and other common measures of subduction signature in the mgvf where cinder cones occur exclusively b be values in primitive calc alkaline basalts are distinctly higher than those from alkaline basalts 3 8 vs 1 3 indicating that high b be is a mantle derived feature and not an artifact of crustal assimilation comparison among various elemental ratios indicates that cs and u show enrichment patterns similar to b all are enriched in calc alkaline rocks but not in lamprophyres or basanites in contrast ba k and sr are enriched in both calc alkaline rocks and the lamprophyres and basanites multi stage processes and differing melting mechanisms are inferred to explain the variable characteristics of mvb volcanic rocks first slab derived fluids rich in fluid mobile elements including b and ba infiltrate the mantle wedge these fluids cause fluid fluxed melting that produces calc alkaline magmas enriched in all fluid mobile elements these lavas erupt from large volcanic front strato volcanoes the slab derived fluids also metasomatize portions of the mantle wedge producing phlogopite and or amphibole these phases have high partition coefficients for ba sr k but may have low partition coefficients for b cs u accordingly subduction zone metasomatism produces a mantle wedge enriched in ba sr k but not necessarily in b cs u decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high ba ce sr la and k la but low b be cs la and u la this interpretation implies two types of subduction zone signatures one involving enriched ba sr k elements that have longer residence times in the mantle wedge and another involving enriched b cs u which all partition so strongly into fluids or melts that they have short residence times in the mantle and are only enriched in magmas generated by fluid fluxed melting or that have assimilated crustal material assimilation of granites and crustal rocks can also enrich differentiated lavas in b

pleistocene to recent alkalic volcanism in the region of sanganguey volcano nayarit mexico forty five cinder cones and associated lava flows have erupted within the last 300 000 years along five parallel lines through the calc alkaline volcano sanganguey in the northwestern segment of the mexican volcanic belt lavas erupted from these cinder cones include ne and hynormative alkali basalts hawaiites mugearites and benmoreites it is unusual that this suite has erupted in a calc aikaline volcanic belt where volcanoes in the vicinity have been erupting calc alkaline andesites dacites and rhyodacites

basaltic volcanism and extension near the intersection of the sierra madre volcanic province and the mexican volcanic belt three groups miocene pliocene and pleistocene off oceanic type basalts associated with normal faulting have been identified in an area surrounding the city of guadalajara mexico although most basalts within these groups have compositional characteristics of an asthenospheric source each group is also associated with lavas that have subduction related traits the first group the san cristobal plateau basalts has a minimum volume of 1 800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre existing extensional basin of unknown configuration 10 m y ago the relatively rapid eruption of these basalts along with their volume structural association and chemistry suggest an upwelling mantle plume beneath the guadalajara region this region may be the earliest indication of the separation of the jalisco block from north america the second basalt group the guadalajara basalts consists of small volumes of porphyritic basalts and basaltic andesites 3 3 5 0 ma in age that outcrop near the northern outskirts of guadalajara and near the town of hostotip aquillo to the northwest the youngest basalts in the guadalajara area are the 0 4 1 4 ma santa rosa suite of basalts hawaiites mugearites close to the contemporary 0 2 ma andesitic central volcano v tequila these either flowed from nearby cinder cones toward the rio santiago canyon or erupted in the canyon to dam the river the flat lying tertiary 23 27 ma ash flows of the sierra madre volcanic province that lie to the north of guadalajara are faulted north northeast into horsts and grabens that in one case can be dated as being older than 21 8 ma this fault trend has been reactivated closer to guadalajara as normal faults 200 to 300 m displacement cut the miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city also hidden by the young volcanic succession there is the north west southeast fault zone that extends to the gulf of california as the tepic zacoalco graben system faulting of this zone is well displayed in the region of the santa rosa dam in the santiago canyon northwest of guadalajara two styles of faulting are found there an older 1 ma 3 has been approximately north northeast for at least 4 m y characteristic of the mexican basin and range province

neotectonics in central mexico regional neotectonic activity in central mexico was mapped through analysis of eleven landsat thematic mapper images the study area stretches from mexico city on the east to the gulf of california on the west between 17 and 21 n latitude fault scarps of plio quaternary age define several large scale tectonic features within the mexican volcanic belt dominated by rifting transtension and shear with probable sinistral components three large crustal blocks south of the mexican volcanic belt are separated from the north american plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks a simple vector diagram for plates and crustal blocks in central mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study the relative motion between the blocks varies from about 2 5 to 8 mm year 1 which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world in the mexican volcanic belt there is a clear relationship between volcanism and neotectonic activity fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt this may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the mexican volcanic belt relative to the trench the ultimate factor controlling the location of plio quaternary volcanism and tectonism in central mexico may be large zones of crustal weakness formed during major mesozoic and cenozoic tectonic events

geoacoustic and source tracking using particle filtering experimental results a particle filtering pf approach is presented for performing sequential geoacoustic inversion of a complex ocean acoustic environment using a moving acoustic source this approach treats both the environmental parameters e g water column sound speed profile ssp water depth sediment and bottom parameters at the source location and the source parameters e g source depth range and speed as unknown random variables that evolve as the source moves this allows real time updating of the environment and accurate tracking of the moving source as a sequential monte carlo technique that operates on nonlinear systems with non gaussian probability densities the pf is an ideal algorithm to perform tracking of environmental and source parameters and their uncertainties via the evolving posterior probability densities the approach is demonstrated on both simulated data in a shallow water environment with a sloping bottom and experimental data collected during the swellex 96 experiment

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

present day kinematics of the rivera plate and implications for tectonics in southwestern mexico we model the present day motion of the rivera plate relative to the pacific north america and cocos plates and combine the results with the nuvel i model for cocos north america motion to examine present day deformation in southwestern mexico the pacific rivera data which include 25 three m y average spreading rates determined from original surface ship magnetic data and 22 azimuthal data along the rivera transform are systematically misfit by the nuvel i pacific north america and pacifie cocos euler vectors indicating that the rivera plate is kinematically distinct from north america and cocos an f test shows that the improvement in fit to the pacific rivera data from a model with a distinct rivera plate exceeds that expected solely from adding three model parameters the rivera north america euler vector derived from closure of the rivera pacific north america plate circuit predicts slower and more trenchnormal convergence along the acapulco trench than prior models in addition the model predicts convergence normal to the eastern tamayo fracture zone at a rate 60 slower than the right lateral strike slip predicted by prior models an observed systematic misfit of the pacific rivera euler vector to azimuths from the western rivera transform fault may have several causes there may be a time averaging problem between the 3 0 m y average rates and the shorter time average transform fault azimuths and earthquake slip vectors alternatively rivera transform trends may be biased by deformation within lithosphere adjacent to the transform valley or future detailed mapping of the transform may reveal the misfit to be an artifact of the presently available bathymetry along the entire pacific rivera rise spreading rates averaged over the past 0 7 m y are systematically faster than 3 0 m y average rates with the difference increasing southward along the rise the post 3 ma change in the rate gradient has caused a southward migration of the pacific rivera euler pole since 3 ma possibly in response to the 3 ma completion of a spreading reorganization that doubled the length of the rivera transform fault and halved the area of the rivera plate we hypothesize that deformation in southwestern mexico including opening along the colima rift is related to oblique subduction of the cocos plate along the northern middle america trench three unes of evidence support this hypothesis first the sense of oblique subduction predicted by nuvel i along the middle america trench is consistent with a model in which part of western mexico located southeast of the colima rift moves to the southeast relative to north america second field geologic data and landsat imagery suggest that in the past few million years the chapala oaxaco fault zone has accommodated several kilometers of sinistral motion and sinistral transtension has occurred along the trans mexican volcanic belt either or both of these fault zones could accommodate any southeastward coastal block motion that might result from oblique subduction third the observation that earthquake slip vectors from the northern middle america trench trend systematically counterclockwise from the predicted cocos north america convergence direction is also consistent with southeastward motion the discrepancy between the observed and predicted slip directions along the middle america trench suggests sinistral slip is 0 10 mm yr 1 if oblique subduction is driving southeastward transport of a coastal sliver the colima rift is a passive pull apart zone at the northwestern end of this sliver rather than an incipient spreading ridge that will replace the pacific rivera rise

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

the great jalisco mexico earthquakes of 1932 subduction of the rivera plate we have analyzed seismograms of the 3 june 1932 ms 8 2 and the 18 june 1932 ms 7 8 jalisco earthquakes and their aftershocks at manzanillo mnz guadalajara gum and tacubaya tac the aftershocks locations the first motions at mnz and the isoseismic maps of the two main shocks strongly suggest that a the 3 june 1932 earthquake initiated nw of but close to mnz and propagated nw for an estimated length of rupture of 220 km b the 18 june 1932 earthquake nucleated sw of mnz offshore and perhaps ruptured a length of about 60 km and c the width of rupture was approximately 80 km there is no evidence of events occurring se of mnz even up to 1 1 2 yr after the first main shock a gap of about 60 km remains between the aftershock areas of the 1932 jalisco and the 1973 colima earthquakes whose seismic potential is unknown although the boundary between the rivera and cocos plates is uncertain there is little doubt that the 1932 earthquakes broke the shallow part of the rivera subduction zone our results may help in quantifying seismic potential of tectonically similar areas such as the juan de fuca subduction zone in the nw united states

contemporaneous calc alkaline and alkaline volcanism at sanganguey volcano nayarit mexico sanganguey volcano located in the northwestern segment of the mexican volcanic belt is a calc alkaline stratovolcano that has erupted a chemically continuous suite of andesites and dacites during the past 200 000 yr alkalic volcanism however appears to have been predominant in the area since the pliocene alkalic basalts and their derivatives erupted along extensional fractures trending northwest through sanganguey and continued to erupt through the flanks of the stratocone following cessation of calc alkaline activity the calc alkaline andesite dacite volcanism was focused beneath sanganguey and appears to have been controlled by crystal fractionation in a shallow magma chamber no basalts that erupted in the area have chemical compositions that could be parental to the sanganguey calc alkaline suite the most basic calc alkaline rocks basaltic andesites have incompatible trace element concentrations nearly identical to the most basic alkaline rocks implying that the two suites were derived from distinct parental magmas alternatively it is suggested that stagnation of mantle derived alkalic basalts provided heat input to mobilize lower crustal rocks resulting in the generation of calc alkaline magmas

active rifting in southwestern mexico manifestations of an incipient eastward spreading ridge jump a major ongoing episode of continental rifting has formed a system of three intersecting grabens in the western mexican volcanic belt these structures may mark the initial stage of an eastward spreading ridge jump beneath continental crust eastward ridge segment jumps have been propagating northward along this part of the east pacific rise for the past 12 m y but this is the first plate boundary reorganization event to involve continental crust since the separation of baja california in the miocene since pliocene time small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant subduction related calc alkaline magmas erupted from andesitic stratovolcanoes the alkaline magmas are varieties found in zones of active rifting elsewhere in the world

seismicity and tectonics of the rivera plate and implications for the 1932 jalisco mexico earthquake seismicity on and near the boundaries of the rivera plate is examined in order to determine the subduction regime of the rivera plate and its influence on the earthquake behavior of the northernmost section of the mid america trench in particular we address the question of a rivera cocos boundary there have been several large historic earthquakes in the coastal areas of the mexican states colima and jalisco but the last large event was in june 1932 the 1932 jalisco earthquake ms 8 1 the present quiescence has lasted longer than the average recurrence interval for the mexican subduction zone and the jalisco area is now categorized as a seismic gap however if subduction in this area is dominated by the motion of the rivera plate which is subducting at half the rate of the cocos plate the deficiency of earthquake activity may not be unusual in an attempt to better delineate the rivera cocos boundary intermediate sized earthquakes along and near the rivera fracture zone and the mid america trench were relocated with the joint epicenter determination method we found that in general the catalog locations are fairly accurate with the average change in epicenter upon relocation being 12 km seismicity along the rivera fracture zone follows the physiographic trend of the fracture zone but east of its intersection with the east pacific rise seismicity is more diffuse two intermediate sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent rivera cocos boundary motion reexamination of arrival time data for the 1932 jalisco earthquake yields an epicenter at 19 57 n 104 42 w close to the boundary zone inferred from these two earthquakes we conclude that the possibility that the 1932 jalisco earthquake broke the northernmost section of the cocos north american plate interface as opposed to the rivera north american plate interface cannot be ruled out

a potassic volcanic front in western mexico the lamprophyric and related lavas of san sebastian the superposition of two tectonic regimes rifting and subduction in western mexico has led to the development of a lamprophyric volcanic front 70 km closer to the middle america trench than is the main axis of andesitic volcanism the san sebastian cindercone field represents the northernmost extent of lamprophyric activity in western mexico and lies 40 km trenchward of volcan ceboruco a historically active andesitic stratovolcano the predominant country rocks are cretaceous silicic ash flows although marine sedimentary rocks emplaced between 45 and 115 ma also occur minette absarokite and kersantite lavas were erupted between 0 48 and 0 26 ma in the san sebastian field in close association with flows of basaltic andesite and hornblende andesite there is a gradual transition observed between the two intermediate lava types basaltic andesite and kersantite as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition this transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions examination of the primary lamprophyric lava types absarokite and minette reveals a remarkable range in their oxidation states if these lavas reflect their mantle source conditions then the redox state of western mexico9s sub arc mantle spans the interval between the ni nio and magnetite hematite buffers a variation in fo 2 of five log units the oxidation and hydrous enrichment of mexico9s sub arc mantle probably formed in response to subduction the eruption of lamprophyric lavas in front of the andesitic suite of the main axis however may have depended upon rifting 1 to trigger melting of the modified mantle and 2 to provide an easy passage to the surface through crustal thinning and block faulting

minette lavas and associated leucitites from the western front of the mexican volcanic belt petrology chemistry and origin during the late pliocene k rich minette and leucitite lavas erupted in the western mexican volcanic belt near the town of los volcanes a region which is located much closer to the middle america trench than the main line of currently active andesite stratovolcanoes during this period the tectonic regime in western mexico was highly complex due to the simultaneous occurrence of active subduction of the young rivera plate and rifting caused by crustal extension most of these basic lavas contain phenocrysts of phlogopite augite and apatite along with microphenocrysts of leucite and fe ti oxide olivine is absent from all but two of the flows one an olivine leucitite and the other a felsic minette the phlogopite phenocrysts and high whole rock fe2o3 feo ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities all of these rock types are highly enriched in the large ion lithophile and light rare earth elements with sr 5100 ppm ba 4800 ppm and ce 330 ppm they are also mildly enriched in the high field strength elements e g zr 260 700 ppm and display the strong relative enrichment of the lile over the hfse that is characteristic of magmas erupted in convergent margins consideration of high pressure phase equilibria in the mg2sio4 camgsi2o6 kalsio4 sio2 system shows that the minettes from this region are not related through fractional crystallization to the more mgo rich olivinebearing minettes which have erupted in other parts of the western mexican volcanic belt during the quaternary this conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models instead the data suggests that these high k magmas were derived from a source region which consists predominantly of phlogopite clinopyroxene and apatite and which has formed through hydrous enrichment of the subarc mantle in response to subduction

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

cummingtonite and the evolution of the mount st helens washington magma system an experimental study hydrothermal experiments have been conducted on the 3 5 ka cummingtonite bearing yn tephra from mount st helens washington to determine the stability field of cummingtonite in volcanic rocks and to gather additional data on the evolutionary history of the mount st helens magmatic system cummingtonite is not stable in experiments conducted at or above 810 c at pressures 2 o analyses of melt inclusions that yield preeruption melt h 2 o contents of 6 4 wt the yn tephra belongs to a series of cummingtonite bearing dacites that erupted early in the history of mount st helens these early dacites have relatively homogeneous compositions and are characterized by low temperature 800 c high water pressure and high oxygen fugacity nno 1 phenocryst crystallization silicic magmas that erupted after eruption of basaltic andesite at mount st helens 2 3 ka are more heterogeneous in composition and last equilibrated under higher temperatures 900 c lower water pressures and lower oxygen fugacities than the early dacites these changes in the magma chamber conditions appear to have been related to the intrusion of mafic magma into an upper crustal dacitic magma chamber

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

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

geochemistry of exposed granulite facies terrains and lower crustal xenoliths in mexico the trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma mass spectrometer to constrain the composition and evolution of the mexican lower crust most xenoliths found are pyroxeneplagioclase orthogneisses although quartzo feldspathic paragneisses form up to 50 of the xenolith population in some areas lithologies in the exposed terrains consist of pelitic and carbonate rich paragneisses charnockites and less abundant basic intermediate and felsic orthogneisses both xenoliths and exposed granulites generally have low abundances of incompatible elements such as k rb th and u the xenoliths however often have lower abundances of these elements than the exposed granulites high k rb ratios coupled with an inverse correlation between k2o and k rb indicate that most samples from the mexican lower crust are depleted in both k and rb furthermore th u ratios similar to upper crustal materials and high la th ratios suggest depletion in both th and u relative to average crustal material removal of a melt phase either after the crystallization of cumulates or following a partial melting event can explain many of the chemical features observed and may be responsible for the elemental depletions some samples however appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths available geobarometric data suggest that xenoliths equilibrated near the crust mantle boundary 10 kbar and represent the lowermost crust while the exposed granulites equilibrated at shallower crustal levels 7 kbar thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the mexican lower crust the lowermost crust in mexico consists of crystallized basaltic liquids cumulates and or residue after the removal of a partial melt as well as garnet rich metasediments

pliocene quaternary volcanism and faulting at the intersection of the gulf of california and the mexican volcanic belt the junction of the western mexican volcanic belt and the gulf of california represents a superposition of subduction and continental rifting tectonic regimes in the late cenozoic subduction of the oceanic rivera plate has caused late cenozoic uplift of western mexico forming the jalisco block this paper addresses three questions where is the northern boundary of the jalisco block how is the continental fracture system related to that offshore and what is the spatial distribution and composition of subduction related lavas 40 ar 39 ar dates on cretaceous to paleogene silicic ash flows show that the northern boundary of the jalisco block may be defined by the abrupt change in basement age from cretaceous to miocene 40 ar 39 ar and k ar dates on faulted lavas from the nayarit region indicate that extension at the edge of the jalisco block has occurred since ca 4 2 ma the least principal stress 3 direction associated with these faults has had two different orientations from 4 2 ma to the present several pliocene to holocene 4 20 1 05 and 0 65 ma lava flows and aligned cinder cones have a n45 w associated least principal stress direction whereas three pliocene 3 36 3 38 and 3 11 ma lava flows are cut by faults indicating a north to north northeast least principal stress direction the two different stress directions may arise either from structural features in the basement of the arc or from changes in offshore plate boundaries e g 2 5 ma when the rivera and mathematicians plates were locked together whole rock major and trace element analyses of lavas from the coastal nayarit region reveal three different lava types an alkali basalt series basaltic andesite and andesite the alkali basalts show feo and tio 2 enrichment and have low ba zr and ba la ratios consistent with derivation from an oceanic island type mantle the basaltic andesites and andesites are both alkaline and calc alkaline contain hydrous phenocrysts and have high sr zr and la nb ratios consistent with a subduction related source among these lavas there is a correlation between sr zr oxygen fugacity and h 2 o contents suggesting that an oxidized hydrous fluid is involved in the genesis of the subduction related lavas the distribution of subduction related volcanic centers in the coastal nayarit region and areas southeast within the jalisco block defines front that parallels the middle america trench and is consistent with a 45 dip on the subducted rivera plate

peralkaline and metaluminous mixed liquid ignimbrites of the guadalajara region mexico abstract two widespread ignimbrites the 4 8 ma san gaspar ignimbrite and the 3 3 ma guadalajara ignimbrite are distinctive units in the guadalajara region mexico both ignimbrites contain fiamme of two distinct compositions and in some fiamme two glasses are intricately intermixed indicating that two magmas were erupted simultaneously the metaluminous san gaspar ignimbrite is characterized by high k dacite fiamme containing abundant phenocrysts of andesine augite hypersthene hornblende and biotite and greatly subordinate shards and small aphyric fiamme of colorless rhyodacitic glass geothermometry based on coexisting mafic phenocrysts indicates pre eruptive magma temperatures of about 1000 c conversion of common hornblende to oxyhornblende at the top of the ignimbrite revesiculation of larger fiamme and dense welding throughout the ignimbrite are indicative of high emplacement temperature the guadalajara ignimbrite contains in nearly equal proportions aphyric peralkaline rhyolite and sparsely porphyritic peralkaline low silica rhyolite several plio pleistocene occurrences of peralkaline volcanism in the western portion of the dominantly calc alkaline mexican neovolcanic belt suggest that locally this zone accommodates extension which may be related to opening of the gulf of california

the intensity and magnitude of holocene plinian eruptions from mount st helens volcano dispersal characteristics of the t we wn pu ps ye yn and yb plinian fall deposits of mount st helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities magma discharge based on a theoretical model of tephra dispersal new proximal thickness measurements are combined with an empirical distal extrapolation based on studies of 53 plinian deposits to calculate the magnitude erupted mass of each eruption layer yn 3510 y r b p represents the highest intensity and largest magnitude eruption at mount st helens in post glacial times modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase 26 hours several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of mount st helens peak intensity of the plinian phase was 108 kg s and the total erupted volume was 4 km3 dre of magma small plinian style eruptions are represented by layers such as ps and pu of the pine creek eruptive period 3000 2500 yr b p and have intensities of only 106 kg s when compared with plinian eruptions from other volcanoes the holocene eruptions of mount st helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate sized stratovolcanoes there is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle this relationship may be related to a time dependent process for the accumulation of differentiated and volatile rich magma within the chamber beneath mount st helens

the geodetic signature of the m8 0 oct 9 1995 jalisco subduction earthquake the october 1995 mw 8 0 jalisco subduction earthquake has provided a thorough geodetic observation of the coseismic subduction process an 11 station regional gps network located directly onshore of the rupture demonstrates consistent vertical subsidence verified by tide gauge data and southwest directed extension with measured displacements reaching 1 meter unusually shallow and non uniform faulting is required to explain the displacements we determine that up to 5 meters of slip occurred within the upper 15 km of the thrust fault zone and 2 meters possibly as shallow as 8 km and that slip was likely distributed in two main patches the paucity of continental sediments in this subduction zone could be responsible for the anomalously shallow faulting

steep subduction geometry of the rivera plate beneath the jalisco block in western mexico the morphology of the rivera plate subducted beneath the jalisco block in western mexico is determined from accurately located hypocenters of locally recorded microearthquakes and from earthquakes with magnitude mb 4 5 recorded at teleseismic distances the hypocenters of these latter earthquakes are relocated and for five of them the focal depth is constrained by the inversion of long period body waves the wadati benioff zone inferred from these data indicates a steep subduction of the rivera plate that resembles the geometry of subduction of the cocos plate beneath central america it is however very different from the shallower and almost subhorizontal subduction of the cocos plate observed in southern mexico southeast of this region the rivera plate is comparable to the juan de fuca plate in terms of the small areal extent young seafloor age low relative velocity and low teleseismic activity in the subduction zone this study shows that the dip of both the juan de fuca and rivera plates are similar once they are decoupled from the overriding continental crust the downgoing rivera plate initially starts with a dip of 10 down to a depth of 20 km and then increases gradually to a constant dip of 50 below a depth of 40 km intermediate depth seismicity is low in this zone associated with the subduction of the slow 2 cm yr and young 9 m y rivera plate the maximum depth extent of earthquakes observed in the rivera subduction zone is about 130 km the andesitic calc alkaline colima volcano appears to be directly related to the subduction of the rivera plate to the nw of this volcano the observed quaternary volcanism in the jalisco block which is parallel to the trench may also be explained by the subduction of the rivera plate

miocene shearing along the northern boundary of the jalisco block and the opening of the southern gulf of california new geologic and structural mapping in western mexico shows that the tectonic boundary between the eocene early miocene sierra madre occidental volcanic province and the cretaceous paleocene jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late miocene time transpressional folding left lateral strike slip faulting and minor thrusting occurred between 14 5 and 11 5 ma during initial rifting at the mouth of the gulf of california inferred right lateral transtension between 12 and 9 ma took place concurrently with widespread alkaline volcanism and east west extension at the gulf mouth and east of the jalisco block these events can be explained in light of the long extensional history that led to the separation of baja california from mainland mexico since the birth of the magdalena microplate about 14 5 ma left lateral shearing at the sierra madre occidental jalisco block boundary indicates that initial 14 12 ma rifting at the mouth of the gulf was accomplished mainly by east southeast motion of the block this was caused by differential stresses applied to the north america plate at the two sides of the subducting magdalena cocos plate boundary as a consequence of the waning of subduction of the magdalena plate subsequent right lateral transtension at the sierra madre occidental jalisco block boundary and extension to the east imply a limited west northwest motion of the block which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of baja california

the colima volcanic complex mexico iii a 34 meter section of ash and scoria fall units has been studied on the upper ne flank of the active volcan colima charcoal and soil horizons are restricted to the topmost 12 m nine 14c dates show a smooth progression from 235 years near the surface to 8 300 years at 10 meters depth indicating a post pleistocene accumulation rate of 1 3 m 1 000 years at 6 km from the vent this figure allows an estimate of the magmatic eruption rate for fallmaterial of 0 31 km3 1 000 years less than 15 of colima s lava eruption rate an unusually thick and coarse grained scoria fall horizon at about 4 m depth in the section appears to have been produced by the caldera forming eruption of colima some 4 300 years ago

beach and shoreface ooid deposition on shallow interior banks turks and caicos islands british west indies abstract modern ooids are forming and accumulating as beach dune complexes which face into and prograde across the very shallow broad low energy caicos bank this contrasts with other modern oolite occurrences that are located at shelf margins or at the heads of steep ramps facing deeper water where tidal energy plays an important role in ooid generation ooid quality and distribution coupled with radiocarbon dating indicates that the caicos ooids are forming in the beach and shoreface environment in response to wave and current agitation generated by prevailing southeasterly trade winds the caicos model is a useful analog for many ancient sheetlike oolite accumulations deposited on broad epeiric shelves or gently sloping ramps

water co2 cl and f in melt inclusions in phenocrysts from three holocene explosive eruptions crater lake oregon rare melt inclusions 100 m in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from plinian pumice of three holocene eruptions were analyzed by ir spectroscopy for molecular h 2o oh groups and co 2 and by electron microprobe for cl and f the three rhyodacitic eruptions each of which began with aplinian phase occurred over 200 yr the llao rock and cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite groundmass glass and melt inclusion compositions anhydrous are similar inclusions with total h 2o concentrations of 1 2 3 4 wt are adjacent to fractures or are hourglass inclusions suggesting partial degassing caused by depressurization melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason intact inclusions in plinian pumices have total h 2o concentrations believed to represent magmatic h 2o contents wt llao rock 5 3 5 3 cleetwood 3 8 4 7 climactic 3 9 0 2 l n 6 ratios of or molecular h 2o indicate closure temperatures of 200 500oc that reflect syn or posteruptive cooling co 2 concentrations are 25 ppm h 2o and co 2 concentrations indicate saturation pressures of 1 0 1 8 kbar or depths 5 km six inclusions from climactic pumice average 400 40 l ppm f 1880 70 ppm cl f and cl concentrations of the other samples are similar and not well correlated with each other or with total h 2o location of melt inclusions near boundaries of patchy zones which are mantled by oscillatory zoned overgrowths suggests that their h 2o concentrations represent magmatic values significantly before eruption although mean h 2o concentrations of analyzed melt inclusions decrease for the three successive eruptions it is not certain that this indicates a corresponding change in magmatic h 2o content during the interval between eruptions

the geology and petrology of volcan san juan nayarit mexico and the compositionally zoned tepic pumice abstract volcan san juan is the westernmost quaternary composite volcano in the mexican volcanic belt its activity is divided into three stages of decreasing eruptive volume during stage 1 activity the main cone of san juan and the adjacent cerro alto grew to a combined volume of 60 km3 through eruption of hornblende bearing andesites dacites and rhyodacites stage 1 included explosive eruptions of hornblende dacite and an andesite dated at 33 750 a years ago and 23 010 a stage 2 activity involved a major plinian eruption 14 770 a that produced the hypersthene hornblende bearing compositionally zoned rhyodacitic andesitic tepic pumice and led to formation of an oval shaped caldera 4 1 km in diameter tepic the capital city of nayarit state with a population of 250 000 lies at the ne foot of volcan san juan this location was buried under 2 9 m of rhyodacitic pumice fall deposits from the tepic pumice eruption erosion has since removed virtually all exposures of tepic pumice 15 km from the caldera the estimated volume of erupted tephra is 5 6 km3 equivalent to 1 2 km3 of magma modeling of the eruption dynamics indicates that as the eruption evolved the column height increased from 15 km to 24 km and maximum mass discharge rate increased from 3 106 kg s to 6 107 kg s during stage 3 activity a hornblende andesitic lava dome partially filled the caldera and sent block lava flows down the north slope of the cone total erupted volume was 0 55 km3 compared to other composite volcanoes of the western mexican volcanic belt the products from san juan have the highest sr contents and the lowest rb y yb and lu contents resulting in high la yb and sr y with low rb sr these observations indicate a strong slab melt component at san juan consistent with its proximity to the middle america trench where young hot lithosphere is subducting the san juan andesites dacites and rhyodacites also have relatively high 87sr 86sr 206pb 204pb 207pb 204pb and 208pb 204pb and low end values compared to these other volcanoes which appear to reflect significant crustal contamination of san juan s magmas

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

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock