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Groundwater of northern Mexico contains high concentrations of geogenic fluoride (F−), a contaminant known to affect human health. The origin of F− in groundwater in this region has been related to the weathering of rhyolite and other volcanic rocks present in the alluvium. However, the relationship of F− concentration to water depth has not been established. F− concentrations, pH, and total dissolved solids (TSD) were determined for 18 wells within the Meoqui-Delicias aquifer in 2021. The F concentrations varied between 0.62 mg L−1 and 4.84 mg L−1, and 61% of the wells exceeded the 1.5 mg L−1 guideline. F− concentrations did not correlate to TDS but correlated to well depth (r = −0.52, p < 0.05). Because of the less-than-strong correlation coefficient value obtained, a diagram of F− concentrations vs. well depth was constructed. The diagram showed a distinct enrichment of F− in shallow wells, suggesting that groundwater residence time and evaporation may be important factors in explaining the F− content within the aquifer. This pattern was confirmed after plotting 2003 and 2006 data for the same wells. These findings are important to better understand the distribution of F− in neighboring alluvial aquifers as well as in alluvial aquifers elsewhere. [ABSTRACT FROM AUTHOR]

Crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae. Spherulites typically range from microscopic to a few centimeters in diameter. Larger spherulites, termed "megaspherulites," are rare but have been reported in the USA and Mexico. The uncommon nature of such structures supports the need for a study to explore the factors and special conditions that allow them to reach such a large size. In the Southern Puna of Argentina, El Viejo Coulee includes megaspherulites up to 4 m in diameter. We present observations from field work, petrography, scanning electron microscopy, bulk geochemistry, and Fourier transform infrared spectroscopy. The megaspherulites occur in obsidian lenses that differ in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee. The obsidian lacks vesicles and microlites and is unaltered. The megaspherulites comprise growth cones consisting of micropoikilitic texture where quartz encloses potassium feldspar. The growth cones are separated by interconal areas composed of lithophysae. We propose that the megaspherulites were formed above the glass transition temperature (Tg) and are the product of primary crystallization of rhyolitic melt. The exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperatures above Tg during crystallization. The large size also suggests scarcity of nucleation sites, which is consistent with the megaspherulites being hosted by unaltered microlite- and vesicle-free glass. The position of the obsidian lens at the base of the coulee may have played a critical role in maintaining the temperature above the Tg long enough to allow the crystallization of the megaspherulites. These conditions also favored crystallization in the most advanced stage where micropoikilitic texture replaced the fans of crystal fibers typical of spherulites. Crystallization of anhydrous quartz and feldspar in the growth cones led to the concentration of volatiles in the melt in the interconal areas, resulting in volatile exsolution and formation of vesicles that became nucleation sites for lithophysae. The study advances our understanding of some of the special processes that are involved in the cooling and solidification of rhyolitic magmas. Fundamentally, we find that the position of the obsidian at the base of the coulee was critically important because this position favored maintenance of the temperature above the Tg which, in turn, favored high diffusion rates. Also, the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulites to nucleate; those that nucleated therefore grew very large. The meter-scale megaspherulites may have taken ~ 55 years to create. [ABSTRACT FROM AUTHOR]

Abstract Precise timing and constraints for magma source(s) are critical to understand the post-subduction tectonomagmatic evolution of the world's largest Cenozoic Silicic Large Igneous Province in western Mexico. We provide in-situ zircon U–Pb ages and Hf isotope compositions of rhyolitic ignimbrites exposed in the Palmarejo and Guazapares at the northwestern extremity of Sierra Madre Occidental. Four rhyolite ignimbrite samples from Palmarejo yielded zircon weighted mean U–Pb ages between 23 and 24 Ma. However, five individual analyses from one sample yielded the oldest age of 25.09 ± 0.41 Ma (MSWD = 1.7, n = 5). A rhyolitic tuff from the Guazapares district yielded older individual dates varying from 25.7 to 28.6 Ma with a main population yielding a weighted mean age of 27.31 ± 0.36 Ma. Zircon U–Pb ages suggest that the ignimbrite flare-up in southwestern Chihuahua remained restricted to the Early Middle Oligocene to Early Miocene. We suggest that the zircon U–Pb dates >24 Ma (particularly from Guazapares) are the antecrystic zircons included from the underlying older ignimbrites. The zircon U–Pb dates of 25–28 Ma are related to the culmination of syn -extensional first ignimbrite flare-up and bimodal volcanism started as early as 35 Ma. The first ignimbrite flare-up is possibly resulted from the detachment of the leading edge of the Farallon slab around 35–28 Ma. The younger age of 24–23 Ma from Palmarejo can be correlated to the second episode of ignimbrite flare-up erupted during the pervasive regional extension, normal faulting and onset of formation of metamorphic core complexes. This second flare-up is resulted by the subsequent events of break off/rollback of the Farallon plate. The positive Hf isotope composition (εHf (t) = +1.0 to +5.8) and concomitant mafic volcanism suggest partial melting of subcontinental lithospheric mantle and the overlying Laramide crust by upwelling of decompressing asthenosphere during post-Laramide regional extension. This interpretation is consistent with the successive detachments, fragmental removal and/or rollback of the leading edges of the Farallon slab beneath the western continental margin of North America. We did not find any xenocrystic zircon inheritance neither from the Laramide plutonic basement/roots of the Jurassic Guerrero terrane nor from the Proterozoic North American crust. However, zircon inheritance of Laramide age has been reported from the Guazapares district. Partial melting of Laramide basement was possibly facilitated by the heat advection and ponding of subcontinental lithospheric mantle-related magmas in the crust during the Cenozoic regional extension. Highlights • Zircon U–Pb ages and Hf isotope compositions of ignimbrites from southwestern Chihuahua, Mexico. • Rhyolite ignimbrite eruption has been restricted between 28 and 23 Ma. • Partial melting of lithospheric mantle and Laramide crust by upwelling asthenospheric magmas. • Cenozoic ignimbrites and Late Cretaceous -Tertiary granodiorites share similar magma sources. [ABSTRACT FROM AUTHOR]

Rhyolitic magmas in occasions can be related to explosive eruptions in active volcanoes that may not exhibit precursory signals, making them very dangerous for the surrounding communities. To understand the pre-eruptive conditions of such magmas it is important to decipher the mechanisms capable to produce these explosive events. In this study we analyzed the pre-eruptive conditions of Guangoche stratovolcano, located to the southwest of the Los Azufres Volcanic Field, in the central part of the Trans-Mexican Volcanic Belt. During the late Pleistocene Guangoche generated three Plinian-subPlinian eruptions and some effusive eruptions, all of them involving high-silica rhyolitic magmas. One of these Plinian events (VEI 5) occurred ~29 ka and deposited the White Pyroclastic Sequence (WPS). This eruption ejected ~0.7 km 3 (DRE volume) of rhyolitic magma and deposited a pumice fallout followed by three pumice-rich pyroclastic density currents. Petrography, combined with mineral composition, melt inclusions, and water-saturated phase equilibria experiments were used to determine the evolution of the magma storage of the WPS event. Prior to the eruption, the magma was a crystal-poor (~14 vol%) rhyolite, containing the following mineral assemblage: biotite, Fe-Ti oxides, amphibole, sanidine and quartz, set in a rhyolitic matrix (77.3 wt% SiO 2 ), with low-Ba (121 ± 16.7 ppm), Eu (0.15 ± 0.02 ppm) and Sr (37 ± 7 ppm). Based on Fe-Ti oxythermometers and phase equilibria experiments, we determine that the magma was stored between ~75 and 100 MPa (3–4 km depth), at a temperature of 769 ± 8 °C. Moreover, H 2 O-CO 2 dissolved in quartz melt inclusions and the textural features of the mineral assemblage of the WPS products, provided insights of the presence of a quartzo-feldspathic crystal-mush, located at minimum depths between 4.3 and 9.5 km (98–218 MPa). Large sanidine, quartz, plagioclase, and amphibole phenocrysts and mineral clots in the WPS rocks represent assimilated parts from this mush by a melt extraction process probably triggered by the arrival of a relatively hotter magma at the base of the crystal-mush, which in turn caused reheating and partial melting of the quartzo-feldspathic crystal-mush, generating the 29 ka rhyolitic WPS magma. [ABSTRACT FROM AUTHOR]

Abstract: Tlaloc is a late Pleistocene stratovolcano located NE of México City. It is the northernmost volcano of the N–S Sierra Nevada Volcanic Range, which consists from north to south of Tlaloc, Telapón, Iztaccíhuatl, and Popocatépetl volcanoes. Tlaloc has always been considered the oldest (and extinct) volcano of the Sierra Nevada Volcanic Range. Recent field data revealed that Tlaloc was very active during late Pleistocene through a series of explosive eruptions. One of these eruptions produced the Multilayered White Pumice (MWP) a rhyolitic pyroclastic sequence. The eruption began with a 24-km high Plinian column MWP-F1 that was dispersed to the NE by prevailing winds. It was interrupted by fountaining of the column with the generation of a pyroclastic density current that emplaced MWP-S1 layer. Then, followed five unstable sub-Plinian columns (MWP-F2 to F6) that reached altitudes between 16 and 19km. Fall deposits as a whole are 1m thick at 12km from the vent, cover a minimum area of 577km2 for a total volume of 4.68km3 (DRE 1.58km3). The eruption ejected a total mass of 3.45×1012 kg at different mass discharges. The last sub-Plinian column (MWP-F6) collapsed and produced dense pyroclastic density currents that deposited pumiceous pyroclastic flows (MWP-PF) following main ravines to the north and east of the vent. These density currents filled gullies with 23m-thick deposits at a distance of 12km from the vent totaling a minimum DRE volume of 0.2km3. Pyroclastic flow deposits charred tree trunks that yielded an age of 31,490+1995/−1595yrB.P. that closely date the age of the eruption. Rain during this phase of the eruption generated syn-eruptive lahars (MWP-DF). Post-eruptive lahars (MWP-ED) finally swept the volcano flanks. The MWP deposits consist of abundant white pumice (up to 96vol.%), rare gray pumice, cognate lithics, accidental altered lithics, xenocrysts. White and gray pumice clasts contain phenocrysts of quartz, plagioclase, sanidine, biotite, rare Fe–Ti oxides, monazite, zircon and apatite. Xenocrysts are represented by plagioclase, microcline, orthoclase and quartz likely coming from a deeper plutonic body. Both pumices have a rhyolitic composition (74.98±1wt.% SiO2 in water free basis) representing one of the most acidic products of Tlaloc and the entire Sierra Nevada Volcanic Range. The rhyolitic liquid was likely extracted from a mush-like plutonic body reaching the minimum predicted vesicularity values for Plinian fragmentation for which the triggering mechanism of the MWP eruption is attributed to the high viscosity of the magma that provoked overpressure of the magmatic system. The parameters studied suggest that eruption behavior evolved from Plinian to sub-Plinian eruptive columns over time due to conduit dynamics rather than variations in magma properties of the MWP products. [Copyright &y& Elsevier]

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials. [ABSTRACT FROM AUTHOR]

Abstract: Oligocene dome complexes of trachydacitic to rhyolitic composition are common in the southern portion of the Mesa Central physiographic province, which forms part of the southern Basin and Range extensional province as well as of the southern Sierra Madre Occidental volcanic province. Generally, dome complexes occur aligned with regional fault systems, mostly associated with the southern Basin and Range province, and thus suggesting that faults controlled the felsic magmas that formed these domes. Two distribution patterns are evident, one aligned NE–SW and another aligned NNE. The set of domes were emplaced at 33–28 Ma. Emplacement of domes occurred in three continuous phases starting with those of trachydacite affinity at 33–32 Ma, to trachydacite–rhyolitic at 32–31 Ma, and finally to those with rhyolitic composition at 31–28 Ma. Felsic magmas that originated the domes were apparently generated by partial melting at the base of the continental crust. Contrary to previous hypothesis, our evidence suggest that these magmas in these particular areas of the Mesa Central were not accumulated in large magma reservoirs emplaced at shallow levels in the crust, but crossed the continental crust directly. Since continental crust in this region is relatively thin (30–33 km), we propose that an intense extensional episode favored the direct ascension of these magmas through the brittle crust, with little interaction with the country rock during ascent to the surface, to end up forming aligned dome chains or complexes. Geochemical data favors this model, as the felsic rocks show no depletions in Nb and Th but instead relatively enrichment in these elements. REE show flat or concave up patterns, suggesting that the magmas involved enriched (fertile), metasomatized lithospheric fluids that generated partial melting at the base of the continental crust. Based upon these data, we infer an intra-plate tectonic setting for these rocks. [Copyright &y& Elsevier]

Abstract: The Mascota volcanic field is located in the Jalisco Block of western Mexico, where the Rivera Plate subducts beneath the North American Plate. It spans an area of ∼2000 km2 and contains ∼87 small cones and lava flows of minette, absarokite, basic hornblende lamprophyre, basaltic andesite, and andesite. There are no contemporary dacite or rhyolite lavas. New 40Ar/39Ar ages are presented for 35 samples, which are combined with nine dates from the literature to document the eruptive history of this volcanic field. The oldest lavas (2.4 to 0.5 Ma) are found in the southern part of the field area, whereas the youngest lavas (predominantly <0.5 Ma) are found in the northern portion. On the basis of these ages, field mapping, and the use of ortho aerial photographs and digital elevation models, it is estimated that a combined volume of 6.8±3.1 km3 erupted in the last 2.4 Myr, which leads to an average eruption rate of ∼0.003 km3/kyr, and an average volume per eruptive unit of <0.1 km3. The dominant lava type is andesite (2.1±0.9 km3), followed by absarokite (1.6±0.8 km3), basaltic andesite (1.2±0.5 km3), basic hornblende lamprophyre (1.0±0.4 km3), and minette (0.9±0.5 km3). Thus, the medium-K andesite and basaltic andesite comprise approximately half (49%) of the erupted magma, with twice as much andesite as basaltic andesite, and they occur in close spatial and temporal association with the highly potassic, lamprophyric lavas. There is no time progression to the type of magma erupted. A wide variety of evidence indicate that the high-MgO (8–9 wt.% ) basaltic andesites (52–53% wt.% SiO2) were formed by H2O flux melting of the asthenopheric arc mantle wedge, whereas the mafic minettes and absarokites were formed by partial melting (induced by thermal erosion) of depleted lithospheric mantle containing phlogopite-bearing veins. There is only limited differentiation of the potassic magmas, with none more evolved than 55.4 wt.% SiO2 and 4.4 wt.% MgO. This may be attributable to rapid crystallization of the mantle-derived melts in the deep crust, owing to their low volumes. Thus, the andesites (58–63 wt.% SiO2) are notable for being both the most voluminous and the most evolved of all lava types in the Mascota volcanic field, which is not consistent with their extraction from extensively crystallized (60–70%), low-volume intrusions. Instead, the evidence supports the origin of the andesites by partial melting of amphibolitized, mafic lower crust, driven by the emplacement of the minettes, absarokites, and the high-Mg basaltic andesites. [Copyright &y& Elsevier]

Abstract: The effects of hydrothermal alteration on major, rare-earth, and other trace-element concentrations in rhyolitic rocks of the Los Azufres geothermal field, Mexico, were investigated by statistically comparing the chemical compositions of altered drill cuttings (taken above 450m depth) with those of fresh rock outcrop samples. Altered rhyolitic rocks show predominantly vitreous and fluidal textures, with alteration products (mainly clay minerals, chlorites and, less commonly, sericite) comprising up to 40% of the rock mass; cryptocrystalline quartz, chlorite, and zeolites fill fractures and vesicles. In altered rocks the greater statistical variances of several elements (Y, Ce, Pr, Nd, Sm, Lu, and Pb) are probably due to alteration effects, whereas smaller variances for CaO, Sr, Rb/Sr, and Rb/Ba suggest that alteration processes have resulted in more uniform chemical compositions. Only MnO, P2O5, Ta, Zr, and Nb have significantly different concentrations in hydrothermal altered rocks as compared to fresh rocks. MnO, P2O5, Ta, Rb/Zr, and Rb/Nb decrease, whereas Zr, Nb, and Nb/Y increase in the altered rocks. The present study stresses that caution should be taken when using these chemical parameters for petrogenetic studies of old hydrothermally altered areas, particularly with rhyolitic rocks. Rare-earth element (REE) concentrations were not significantly different between fresh and altered rhyolitic rocks. This may indicate that these elements were relatively immobile during the hydrothermal alteration processes affecting the rhyolites at Los Azufres, or more likely that they were reincorporated into hydrothermal minerals after being mobilized from the primary phases. [Copyright &y& Elsevier]

We assess the damage on ignimbrite blocks used to build major architectonic monuments in Morelia, Mexico. Morelia belongs to the UNESCO Cultural Heritage and is located at the crossing of the Mexican Volcanic Belt, the Sierra Madre Mountains, the Coconuts and the Rivera plaques. The Cathedral and a beautiful Aqueduct are the main monuments built with blocks of ignimbrites. This study was performed on blocks of ignimbrites removed from the cathedral during a restoration campaign. We characterized the mechanical, chemical and physical properties of ignimbrite blocks from the four facades and the balustrade of the cathedral and compared them with the corresponding properties of “healthy” blocks from recently excavated nearby quarries. Techniques include mechanical testing with a sclerometer, Scanning Electron Microscopy and Energy Dispersive Spectroscopy, Petrography, sulfur and carbon analyses by combustion technique, X Ray fluorescence and X Ray diffraction. The blocks from nearby quarries have lower values of mechanical strengths than those used to built the cathedral, therefore, the former blocks cannot be used to replace particularly damaged blocks. We found a clear correlation between the contents of environmental sulfur and the degradation of the mechanical strength of the ignimbrites. Since the main sources of environmental sulfur in the city are motor vehicles and a paper factory in the southwest from where the main winds come from, we conclude that the damage is from anthropogenic origin. [Copyright &y& Elsevier]

The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano-sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene–early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVF is located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting. [Copyright &y& Elsevier]

The Paradise Mountain Caldera (~20 × 14 km), located west of Fort Davis, Texas, produced two major ignimbrite/lava complexes at ~36.2 and 35.9 Ma. The older unit (Fort Davis Tuff) is composed of rheomorphic tuff and overlying chemically similar lava. The younger unit (Wild Cherry Tuff) forms thick intracaldera deposits and an outflow sheet that spread over an estimated 2000 km2. Near the caldera, the two ignimbrites are separated by trachytic lava of the Mount Locke Formation. The 5-km diameter Point of Rocks intrusion, which crops out within the eastern margin of the caldera, may have fed the Mount Locke flows. Wild Cherry Tuff is overlain by silicic lava (Casket Mountain Formation, ~35.8 Ma), with which is shares chemical similarities. Intracaldera Wild Cherry Tuff was intruded by silicic intrusions and locally intensively silicified and kaolinized. In a revision of stratigraphic nomenclature, we assign all of the above volcanic units to a Barrel Springs Group. Collectively, the above units form a series ranging from trachyte to alkali rhyolite. Glomeroporphyritic trachyte of Mount Locke lavas represents silicic melt incompletely segregated from more mafic material within a source magma chamber. Variation between Mount Locke Formation trachyte and Fort Davis Tuff rhyolite can be modelled by fractionation of a mineral assemblage of plagioclase, clinopyroxene, FeTi oxides, and apatite; variation within the two major tuff units can be modelled almost exclusively by alkali feldspar. Central eruption within the caldera produced intracaldera tuff more evolved than most of its outflow. Alkali rhyolite of the Wild Cherry Tuff and Casket Mountain lavas, although similar in major element chemistry and mineralogy, show wide variation in incompatible trace elements, suggesting that evolution of silicic magmas in terms of major element chemistry had reached an impasse due to fractionation of a mineral assemblage whose bulk composition was similar to that of parental rhyolite. Paradise Mountain units represent a transition from alkalic magmatism in northeastern Trans-Pecos Texas to metaluminous to peraluminous magmatism in southwestern Trans-Pecos Texas and adjacent Mexico. Over a span of 10 Ma, silicic magmatism terminated southwestward over this region at a rate of about 24 km/Ma. This termination may reflect continued foundering of a sinking Farallon slab and associated asthenospheric mantle upwelling. Mafic magma derived by melting of lithospheric mantle formed plutons that, through filter pressing of silicic magma from crystalline mush, generated silicic melts. The transition across the Trans Pecos region from alkalic to metaluminous magmatism may have resulted through preferential modification of lithospheric mantle along the easternmost extent of Laramide subduction. • Geochemical evolution of a transitional alkalic-metaluminous magma system. • Migration of silicic Cenozoic silicic magmatism from southwestern Texas into Mexico. • Transition from alkalic to metaluminous magmatism due to Farallon slab foundering. [ABSTRACT FROM AUTHOR]

Postcaldera (≤ 180 ka) peralkaline trachytes and rhyolites from Socorro Island, Mexico, have relatively restricted ranges of (230Th)/(232Th)o (0.99−1.14) and (238U)/(232Th) (0.91−0.98). Most of the samples exhibit 238U−230Th disequilibria, with initial (230Th) enrichments of up to 21%. In general, (230Th)/(232Th)o values are lower in the rhyolites than in the trachytes. (230Th)/(232Th)o of postcaldera alkalic basalts may also have a relatively restricted range (0.94−0.96), although ratios as low as 0.84 and as high as 1.04 are permitted by uncertainties in some of the ages. These values are also lower than those of the trachytes. Sr and Nd isotopic ratios of the basaltic and silicic suites have fairly restricted ranges that collectively overlap. Previous work suggests that trachytic magmas form by partial melting of basaltic basement, but closed-system partial melting of a source with Th isotope ratios like those of the postcaldera basalts would probably produce trachytic melts with lower rather than higher (230Th)/(232Th)o. Open-system processes affecting melts from such basement are also ruled out because of the lack of a reasonable contaminant that could increase the (230Th)/(232Th) of the trachytic magmas. Postcaldera trachytic magmas probably derive from basaltic basement characterized by Th isotope ratios similar to those of the silicic rocks, and moderate degrees of melting of such material can account for the Th/U of the trachytes. Differences in (230Th)/(232Th)o between the trachytes and rhyolites delimit maximum residence times for rhyolitic magmas of 40−50 kyr, but it is likely that residence times are shorter. This coupled with correlations between Sr and Nd isotopes and indices of differentiation within the silicic suite suggest that rhyolites are related to trachytes by assimilation–fractional crystallization. The most likely assimilant is basement that formed during the silicic phases of magmatism on Socorro. Our results indicate that volcanic basement may be an important component in the genesis of evolved ocean island magmas. [ABSTRACT FROM AUTHOR]

This work advances research on the role of hurricanes in degrading the rocky coastline within Mexico's Gulf of California, most commonly formed by widespread igneous rocks. Under evaluation is a distinct coastal boulder bed (CBB) derived from banded rhyolite with boulders arrayed in a partial-ring configuration against one side of the headland on Ensenada Almeja (Clam Bay) north of Loreto. Preconditions related to the thickness of rhyolite flows and vertical fissures that intersect the flows at right angles along with the specific gravity of banded rhyolite delimit the size, shape and weight of boulders in the Almeja CBB. Mathematical formulae are applied to calculate the wave height generated by storm surge impacting the headland. The average weight of the 25 largest boulders from a transect nearest the bedrock source amounts to 1200 kg but only 30% of the sample is estimated to exceed a full metric ton in weight. The wave height calculated to move those boulders is close to 8 m. Additional localities with CBBs composed of layered rock types such as basalt and andesite are proposed for future studies within the Gulf of California. Comparisons with selected CBBs in other parts of the world are made. [ABSTRACT FROM AUTHOR]

San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km2 bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin's faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma. [ABSTRACT FROM AUTHOR]

Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72 °C, composed of sodic and non-sodic glasses of surface activity caused by IVAl substituting in Q4(1Al) and Q4(2Al) positions, minor VAl, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H2Om in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H2Om molecules. Sodic glasses contain 71.80-77.77 wt% SiO2, are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 μm size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO2, show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 μm size). [ABSTRACT FROM AUTHOR]