Michoacan-Guanajuato Research Articles

Case study of a cluster of simple and complex monogenetic volcanoes in the north‐east part of the Michoacan–Guanajuato Volcanic Field, Central Mexico: Nomenclature implications

With the advent of new terminologies to categorize and characterize the simple and complex monogenetic volcanoes, also came the semantic issues, which caused a predicament for the usage of terms like simple, complex, polycyclic, polymagmatic, complex monogenetic volcanoes with polygenetic inheritance. To analyse and validate this nomenclature, we studied an overlapping volcanic structure located south of the present‐day town of Irapuato, Central Mexico, that appears to be a monogenetic complex at first sight. Field observations, tephra stratigraphy, petrography and geochemistry of the tephra deposits confirms that the structure is in fact a cluster of three simple (San Joaquin tuff ring and two scoria mounds) and one complex, polycyclic, polymagmatic (La Sanabria‐San Roque tuff ring) monogenetic volcanoes formed by independent events, governed by distinct conduits and magma bodies of different origin (subduction‐related, OIB and E‐MORB origin) and separated by different tephra sequences of dissimilar components and depositional characteristics. We estimate the magma volumes (using the juvenile content and their vesicularity percentage) to be at 0.40–1.31 × 108, 0.25 × 108 and 0.42–0.90 × 108 m3 for San Joaquin, La Sanabria and San Roque, reckoning an eruption duration of 77 and 48 and 81 days, respectively (considering an average eruption rate of 6 m3/s from the well‐documented shallow crater (<30 m), Ukinrek maars in Alaska) occurring within the age range of 40–70 k years (crater diameter and depth ratio). This study not only aided to validate the above‐mentioned terms for monogenetic volcanoes, but also reconsider a few of them and avoid confusion with the polygenetic counterparts.

The 2020 and 2021 Seismic Swarms in the Tancítaro-Paricutín Area (Uruapan- Michoacán, México) Evidence Magma Intrusion in an Area with High Density of Monogenetic Cones

The Michoacán-Guanajuato Volcanic Field (MGVF) in central-western Mexico, with more than 1,000 monogenetic volcanoes, has been well known since 1943 when an eruption formed the Paricutín volcano, 11 km to the NW of the summit of Tancítaro stratovolcano. In the highly fractured zone around Tancítaro, referred to as the Paricutín-Tancítaro region (PTR), two major seismic swarms have been recorded, the first between January 5 and February 22, 2020, and the second between May 30 and July 22, 2021. The Mexican National Seismological Service (SSN) reported a total of 4,956 earthquakes with coda magnitudes between 2.6 and 4.2. With the aim to determine the causes of the swarms and the potential hazard they may pose on the region, we first analyze their spatial distributions by relocating all of the events using the Double Difference method and testing different 1-D velocity models to select the one with the lowest residual errors. Secondly, we analyze the temporal distributions finding that their temporal occurrence fits a Mogi’s type 3 volcanic seismic swarm. We conclude that each swarm is due to stresses induced by dike-fed intrusions of moderate volumes of magma in a pre-existing temporal magma reservoir, probably a sill.

Holocene volcanic eruptions of the Malpaís de Zacapu and its pre-Hispanic settlement history

AbstractThe Michoacán-Guanajuato volcanic field (MGVF) hosts >1,400 monogenetic structures younger than 5 Ma. Here we focus on the Malpaís de Zacapu Late Holocene cluster located in the western part of the Zacapu lacustrine basin, situated in the heart of native Purepecha province. The Malpaís de Zacapu comprises four distinct eruptions: the Infiernillo lava flow emitted at ~1450 b.c.; Malpaís Las Víboras, a purely effusive eruption at ~1000 b.c.; the Capaxtiro compound lava flow at ~150 b.c.; and the most recent eruption, the Malpaís Prieto lava flow at ~a.d. 900. Although these lava flows are not inhabited today, they were densely populated in pre-Hispanic times (before a.d. 1521), especially during the Milpillas phase (a.d. 1200–1450). Volcanological studies (geochemical studies and detailed mapping using high-resolution DEM from LiDAR) allowed us to characterize these eruptions in terms of their magma source (rock chemical composition, mineral assemblage), age (radiocarbon and paleomagnetic dating), magnitude and dynamics (volume, morphology of the deposits), as well as lava flow emplacement duration. The findings allow us to infer the potential impact that these eruptions had on the pre-Hispanic settlement history of the area.

Petrogenesis of mafic-intermediate magmatism of the Michoacán–Guanajuato volcanic field in Western Mexico. A geochemical review

The Michoacán–Guanajuato volcanic field (MGVF) in the western Trans-Mexican Volcanic Belt is one of the largest and most diverse monogenetic volcanic fields in the world holding more than 1200 volcanic vents. Its eruptive activity goes back to 7 Ma, it is considered an active volcanic field, and the composition of its rocks varies from mafic to silicic. It is essential to understand the geochemical evolution of its products, the complex petrogenetic processes, and the origin of magmas in central Mexico. Although these processes are linked to the subduction of the Cocos plate beneath the North American plate, the magmatic plumbing system of the MGVF remains not completely understood. The MGVF has been studied for decades, focusing in its dominant intermediate magmas. Nevertheless, the origin and evolution of the mafic components and their relation with the intermediate rocks have been poorly discussed. Here, we compile geological and geochemical data of the MGVF to discuss the petrogenesis of mafic magmas along the volcanic field and the role they play in the generation of intermediate melts. We used data published for 429 samples of mafic and intermediate volcanic rocks. Conventional procedures and statistical techniques were used to process the dataset. We propose that MGVF mafic magmas are derived from low degrees (∼1–15%) of partial melting of a spinel-bearing lherzolite source/mantle related to the rapid ascent of the asthenosphere caused by an extensional regime that is present in the area. In contrast, intermediate magmas where divided into two main groups based on the Mg content: high-Mg intermediate rocks, which seems to be derived from different rates of assimilation and fractional crystallization process, and low-Mg intermediate rocks, which can be related to fractional crystallization of mafic magmas. In addition, mafic and intermediate magmas display a chemical diversity which is related to mantle heterogeneity domains in the mantle wedge.

Silicic magmas in the Michoacán-Guanajuato Volcanic Field: an overview about plumbing systems, crustal storage and genesis processes

The origin of silicic rocks in the Michoacán-Guanajuato volcanic field (MGVF) has been understudied since the volcanic field attracted the attention of researchers. Using geochemical, petrological and structural data from the literature, here we propose a model for the origin of silicic magmas. We found that all volcanic rocks known to date in the MGVF can be divided in 40 % andesite, 33 % basaltic andesite, 15 % basalt, 2 % trachybasalt to trachyandesite, and 10 % dacite-rhyolite. The structural systems that deformed the crust in the MGVF are NNW-SSE-oriented normal faults of the Taxco-San Miguel de Allende fault system, developed during the Oligocene, and the Morelia-Acambay fault system consisting of ENE-SSW to E-W sinistral strike-slip faults developed during the Oligocene-Miocene. In addition to bibliographic data, we present a gravimetric-magnetometric model to investigate the characteristics of the local basement where magmas acquire their final silicic composition, and a seismic tomography model to investigate the deep plumbing system that contribute to form the silicic rocks emplaced on the surface. The only report of assimilation experiments we found in the MGVF literature suggest that plagioclase and pyroxene are more easily digested than quartz by hotter magmas. The digestion of these mineral phases has a direct consequence on the generation of dacites and rhyolites. We propose that regardless of the genesis of andesitic melts, such intermediate magmas arrive to the upper-crust and are forced to evolve within local compression zones where they melt the local granitic basement and form crystal mushes. The compositional variability of silicic rocks in the MGVF is a consequence of the variable mixing between the intermediate magmas and the granitic partial melts.

Rancho Seco monogenetic volcano (Michoacán, Mexico): Petrogenesis and lava flow emplacement based on LiDAR images

Given the high eruption recurrence in the Michoacán-Guanajuato volcanic field (MGVF) in central Mexico, the birth of a new monogenetic volcano can be expected in the future. It is important, therefore, to reconstruct the past eruptions of its many different volcanoes, including estimates of lava flow emplacement times and their rheological properties. These studies define the range of possible future eruptive scenarios and are necessary to evaluate potential risk. The Rancho Seco monogenetic volcano, located in the central part of the MGVF (19°37′03”N, 101°28′21”W), was radiocarbon-dated at ~27,845 years BP. Its eruption initiated with a violent-Strombolian phase that produced a scoria cone and was followed by the effusion of at least seven associated andesitic lava flows, reflecting drastic changes in the eruptive dynamics. Effusive activity probably involved decreases in the magma ascent and discharge rates linked to efficient degassing in an open system. Lava chemical composition suggest an origin of partial melting of a subduction-modified hydrated heterogenous mantle wedge and textural and mineralogical analysis indicates significant crystal fractionation and minor assimilation of granodioritic basement rocks. High-resolution LiDAR imagery was used to estimate lava flow viscosities and emplacement times by following a morphology-based methodology. Results indicate that lava flow viscosities ranged from 105 to 109 Pa·s and emplacement durations between 32 and 465 days for the flow units considered (F5 and F6). The entire eruption may have lasted from 2 to 6 years with a mean effusion rate of 4 to 15 m3/s. Our results are also pertinent to archaeologists studying the architectural remains of Angamuco, a large urban pre-Hispanic site built on Rancho Seco's lava flows.

Petrographic, Geochemical and Isotopic (Sr–Nd–Pb–Os) Study of Plio-Quaternary Volcanics and the Tertiary Basement in the Jorullo-Tacámbaro Area, Michoacán-Guanajuato Volcanic Field, Mexico

AbstractThe origin of the large diversity of rock types erupted along the subduction-related Trans-Mexican Volcanic Belt (TMVB) remains highly debated. In particular, several hypotheses have been proposed to explain the contemporary eruption of calc-alkaline and alkaline magmas along the belt. The Michoacán-Guanajuato Volcanic Field (MGVF) is an atypical, vast region of monogenetic activity located in the western-central part of the TMVB. Here we present new petrographic, geochemical, and isotopic (Sr–Nd–Pb–Os) data on recent volcanics in the Jorullo-Tacámbaro area that is the closest to the oceanic trench. TMVB-related volcanics in this area are Plio-Quaternary (<5 Ma) and mainly form a calc-alkaline series from basalts to dacites, with rare (<5 vol. %) alkaline rocks that range from trachybasalts to trachydacites, and transitional samples. Crystal textures are consistent with rapid crystallization at shallow depth and processes of mixing of similar magma batches (magma recharge). All of the samples exhibit an arc-type trace element pattern. Alkaline and transitional magmas have higher Na2O and K2O, lower Al2O3, and higher concentrations in incompatible elements (e.g. Sr, K, Ba, Th, Ce, P) compared to calc-alkaline rocks. Calc-alkaline rocks are similar isotopically to transitional and alkaline samples, except for a few low 87Sr/86Sr samples. Sr, Nd and Pb isotopes do not correlate with MgO or 187Os/188Os, indicating that they were not significantly influenced by crustal contamination. Isotopic and trace-element systematics suggest that the Tacámbaro magmas are produced by melting of a mantle wedge fluxed by fluids derived from a mixture of subducted sediments and altered oceanic crust. Alkaline and transitional magmas can be derived from a lower degree of partial melting of a similar source to that of the calc-alkaline rocks, whereas the few low 87Sr/86Sr calc-alkaline rocks require a lower proportion of fluid derived from oceanic sediments and crust. Volcanism at the trenchward edge of the MGVF was thus driven purely by subduction during the last 5 Ma, hence discarding slab rollback in this sector of the TMVB.

The ~ AD 500–700 (Late Classic) El Astillero and El Pedregal volcanoes (Michoacán, Mexico): a new monogenetic cluster in the making?

The recent identification of Holocene volcanic clusters in small areas within the Michoacan Guanajuato Volcanic Field (MGVF) opens several questions regarding future volcanic hazard assessments in this region. Documenting vent alignments and eruption recurrence intervals within clusters will provide parameters necessary for making temporal and spatial hazard evaluations. Here, we present a possible new case of a small cluster consisting of only two monogenetic volcanoes, El Astillero and El Pedregal located in the ~ 4400-km2 Tancitaro-Nueva Italia region in the southwestern part of the MGVF, only 25 km to the south of Paricutin volcano. We determined from paleomagnetic and radiocarbon dating that El Astillero and El Pedregal most likely erupted one after the other between AD 500 and 700 (within the Late Classic period of Mesoamerican archeology). While the eruptions were likely separated by a short period of time, the exact length is difficult to ascertain. After the ~ 6 years of total estimated eruption duration of the two volcanoes, both together occupied an area of 14.7 km2 and emitted a dense rock equivalent (DRE) volume of magma of ~ 0.5 km3. Notable characteristics of the eruptions include a switch from the explosive activity exclusive of El Astillero (Strombolian) to effusive activity early after the initiation of the El Astillero eruption, a shift in the active vents, and a progressive change in the bulk magma composition from basaltic andesite to andesite throughout the duration of the eruption. This activity first formed the El Astillero scoria cone and tephra deposits followed by its lava field and ended with the emplacement of the El Pedregal viscous lavas. The discovery of pre-Hispanic pottery sherds and obsidian artifacts underneath the El Astillero tephra fallout unambiguously attests to human activities in the area before the eruption. Judging by their eruptive style, the eruptions probably had a limited impact on the small area affected and the surrounding human activities, but the hazard for this area remains since El Astillero and El Pedregal could represent the initial stages of a new cluster that is still in the making. If so, another eruption should be expected in this area again.

Geological Heritage of the North Region of the State of Michoacan, Mexico

The inventory of the geological heritage of a region is a necessary action to promote its conservation and its diffusion. We selected the Northern region of Michoacan to advance the inventory of its geological heritage, particularly in the field of volcanology, hydrology and geothermal. The term “scientific geoheritage” was used to refer to those geological features that may be susceptible to be considered as having a potential geological heritage of north Michoacan. The aim was to focus on the complex context and significance of the geological heritage, so that the protection schemes take into account its use as a natural, scientific, and cultural heritage. The Michoacan Guanajuato Volcanic Field is a highlighted geo-area with the Volcan Paricutin as an important geosite. Important hydrological sites are Lago Camecuaro and Los Chorros del Varal, which are also protected natural areas. Finally, the Azufres is a geothermal area with high economic and scientific value.

The eruptive history of the Pátzcuaro Lake area in the Michoacán Guanajuato Volcanic Field, central México: Field mapping, C-14 and 40Ar/39Ar geochronology

Monogenetic volcanic fields occur in many tectonic settings representing an unpredictable and uncertain volcanic hazard. The Pátzcuaro Lake is located in the central part of the Michoacán-Guanajuato Volcanic Field (MGVF). Field mapping of an area of 560 km2, stratigraphy and new radiometric ages (40Ar/39Ar and 14C) aided in the identification of 45 individual volcanic landforms formed during the past 3.9 Ma. These landforms comprise five medium-size shield volcanoes, 25 scoria cones, 10 lava domes, two lavas associated to scoria cones, one fissural lava flow, and two volcanic complexes. These volcanic edifices erupted a minimum volume of 20 km3 since Pliocene with an output rate of 0.02 km3/ka. Chemical composition of magmas ranges from 47.6 to 65.3 wt% silica, being dominantly andesites (~76 vol%), followed by basaltic-andesites (~12 vol%), dacites (~9 vol%) and basalts (~3 vol%). Volcanic products vary from mainly andesitic during the Pliocene, basaltic to dacitic during the Pleistocene and basaltic-andesitic to andesitic during the Holocene. All these rocks are the result of low-dipping slab subduction-related magmatism. Alignment of cones and structures follow NE-SW, N-S and E-W directions. The youngest faults are associated with the E-W Morelia-Acambay Fault System, with historic earthquake activity. The volcanic complexes, El Estribo and La Muela, have cone collapses to the north that correspond with the direction of maximum regional extension associated with the E-W fault system. The 3.9 Ma history of monogenetic volcanism around the Pátzcuaro Lake demonstrates that monogenic fields can be long-lived volcanic features.

Felsic-intermediate magmatism and brittle deformation in Sierra del Tzirate (Michoacán-Guanajuato Volcanic Field)

The Tzirate volcanic complex (TVC) is located in the central part of the Michoacán-Guanajuato volcanic field (MGVF), central Mexico. It is characterized by a series of andesitic-dacitic domes and by active faulting of the Morelia-Acambay fault system. Its age and the relationship between the magmatic processes that generated the TVC rocks and the evolution of the local stress field were determined with field work, structural analysis, 40Ar-39Ar dating, geochemistry and petrography of the lavas. The dacitic volcanism of the TVC occurred during the Pleistocene (1 Ma) with the eruption of lavas that formed a series of domes (SiO2 = 63–69 wt.%) aligned in NNE-SSW and NW-SE directions. The area is covered by andesitic-basaltic cinder-lava cones and andesitic lava cones of Pleistocene-Holocene age (2.5 Ma – 0.011 Ma) produced by monogenetic volcanism of the MGVF that surrounds the TVC.The TVC magmas produced rocks that can be classified in four mineral groups. One group contains olivine and pyroxene phenocrysts. The second biotite and amphibole euhedral phenocryst, and the other two contain biotite euhedral phenocryst, amphibole microphenocrysts and biotite subhedral crystals surrounded by a rim of plagioclase microcrysts. The diversity of textures in biotite crystals and the linear trends of major and trace elements between La Huacana granite and the less evolved lavas of the TVC suggest that assimilation of a granitic body in the crust might have influenced the evolution of the TVC rocks.The results of the structural analysis show that when the minimum principal stress was vertically oriented the rise of magma to the surface is hindered, favoring the entrapment of magma in shallow magma chambers. The magmatic assimilation process possibly occurred during the same phase, forming the felsic magmas. Subsequently, the local stress field became extensional, and the vertical principal maximum stress developed normal or extensional faults that affected the magma chamber and served as plumbing system for the rise of felsic lava.

Geology and radiometric dating of Quaternary monogenetic volcanism in the western Zacapu lacustrine basin (Michoacán, México): implications for archeology and future hazard evaluations

The Zacapu lacustrine basin is located in the north-central part of the Michoacan-Guanajuato volcanic field (MGVF), which constitutes the west-central segment of the Trans-Mexican Volcanic Belt. Geological mapping of a 395 km2 quadrangle encompassing the western margin of the basin, 40Ar/39Ar and 14C radiometric dating, whole-rock chemical and petrographic analyses of volcanic products provide information on the stratigraphy, erupted volumes, age, and composition of the volcanoes. Although volcanism in the MGVF initiated since at least 5 Ma ago, rocks in the western Zacapu lacustrine basin are all younger than ~ 2.1 Ma. A total of 47 volcanoes were identified and include 19 viscous lava flows (~ 40 vol.%), 17 scoria cones with associated lava flows (~ 36 vol.%), seven lava shields (~ 15 vol.%), three domes (~ 6 vol.%), and one maar (~ 2 vol.%). Erupted products are dominantly andesites with 42 km3 (~ 86 vol.%) followed by 4 km3 of dacite (~ 8 vol.%), 1.4 km3 of basaltic trachy-andesite (~ 3 vol.%), 1 km3 of basaltic andesite (~ 2 vol.%), and 0.14 km3 of rhyolite (~ 0.3 vol.%). Eruptive centers are commonly aligned ENE-WSW following the direction of the regional Cuitzeo Fault System. Over time, the high frequency of eruptions and consequent accumulation of lavas and pyroclastic materials pushed the lake’s shore stepwise toward the southeast. Eruptions appear to have clustered through time. One cluster occurred during the Late Pleistocene between ~ 27,000 and ~ 21,300 BC when four volcanoes erupted. A second cluster formed during the Late Holocene, between ~ 1500 BC and ~ AD 900, when four closely spaced monogenetic vents erupted forming thick viscous ‘a’a to blocky flows on the margin of the lacustrine flats. For still poorly understood reasons, these apparently inhospitable lava flows were attractive to human settlement and eventually became one of the most densely populated heartlands of the pre-Hispanic Tarascan civilization. With an average eruption recurrence interval of ~ 900 years during the Late Holocene the western Zacapu lacustrine basin is one of the most active areas in the MGVF and should hence be of focal interest for regional volcanic risk evaluations.

Paleomagnetic constraints on the ages of the Holocene Malpaís de Zacapu lava flow eruptions, Michoacán (México): Implications for archeology and volcanic hazards

Four monogenetic Holocene lava flows located within the Michoacan-Guanajuato Volcanic Field, Mexico, were sampled for paleomagnetic dating. These flows (namely, El Infiernillo, Malpais Las Viboras,...

B16 メキシコ火山帯中央部、ミチョアカンーグアナフアト火山地域で活動したマグマの Sr, Nd, Pb 同位体組成 : 脈状で不均質な起源マントルモデルへの制限

B16 メキシコ火山帯中央部、ミチョアカンーグアナフアト火山地域で活動したマグマの Sr, Nd, Pb 同位体組成 : 脈状で不均質な起源マントルモデルへの制限

Temporal and compositional evolution of Jorullo volcano, Mexico: Implications for magmatic processes associated with a monogenetic eruption

The 1759–1774 eruption of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico, produced lavas that range in composition from basalt to basaltic andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr–Nd–Pb–Hf–Os, and O isotopes in olivine separates) of the Jorullo lavas and tephras spanning the duration and compositional range of the eruption, to further constrain the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of the Jorullo magmas. This study presents the first Hf, high precision Pb and comprehensive oxygen isotope measurements for Jorullo volcanic rocks. All samples have arc-like trace element patterns with enrichments in large ion lithophile elements (e.g. Ba, Rb, and Pb) and depletions in fluid immobile elements (e.g. Nb, Ta). In addition, the samples show variations in 87Sr/86Sr (0.7038–0.7040), 143Nd/144Nd (0.51280–0.51285), 176Hf/177Hf (0.28297–0.28300), 206Pb/204Pb (18.62–18.66), 207Pb/204Pb (15.57–15.59) and 208Pb/204Pb (38.34–38.43). Osmium isotope signatures are, with one exception, more radiogenic than the depleted and primitive mantle (187Os/188Os=0.1231–0.1616). Oxygen isotope ratios of olivine phenocrysts (δ18.OSMOW=5.70–6.02‰) show limited variation, but are isotopically heavier than normal mantle olivine. The samples define two geochemical groups: high-MgO samples with higher 87Sr/86Sr, lower 143Nd/144Nd and 176Hf/177Hf, and a positive correlation of Sr and Pb isotopes; and low-MgO samples displaying lower 87Sr/86Sr but higher 143Nd/144Nd and 176Hf/177Hf than the former group, and a negative correlation of Sr and Pb isotopes. The high-MgO group comprises most of the early tephra and lavas, whereas the low-MgO group includes most of the late tephra and lavas. These compositional variations are inconsistent with shallow level contamination, but rather are interpreted to reflect mantle source heterogeneity. Trace element and isotopic signatures are consistent with North Mexican Extensional Province (NMEP) mantle metasomatised by subduction components composed of sediment- and oceanic crust-derived hydrous fluid. The temporal–compositional variations observed in Jorullo magmas are inferred to result from a combination of variable degrees of fractional crystallization of magmas produced by tapping a progressively less metasomatised mantle source that is vertically and/or laterally heterogeneous.

Geological and environmental controls on the change of eruptive style (phreatomagmatic to Strombolian-effusive) of Late Pleistocene El Caracol tuff cone and its comparison with adjacent volcanoes around the Zacapu basin (Michoacán, México)

The 28,300year BP (cal 32,300 BP) El Caracol tuff cone complex is one of the few phreatomagmatic volcanoes in the scoria-cone dominated Plio-Quaternary Michoacán-Guanajuato Volcanic Field (MGVF). It displays a shallow circular crater of ~1km in diameter that is filled with several meter-thick lava flows and is located between two NE–SW trending normal faults dipping NW. It lies directly on top of Pliocene lavas of the San Lorenzo shield volcano that forms part of a tectonic horst (topographic high) separating the Zacapu lake basin (1980m) in the south from the Río Angulo river valley (1760m) to the north. Detailed study of the tephra sequence indicates that the eruption occurred in two stages: 1) Weak phreatomagmatic, in which about 0.1–0.5km3 dense rock equivalent (DRE) of magma was issued within ~1 to 3months at the rate of 4–40m3/s, and 2) purely magmatic (Strombolian-effusive) during which the vent shifted slightly its position toward the NW, forming a small scoria cone (~100m high) on the crater rim of the tuff cone. From this scoria cone lava flows were issued, first into the tuff cone crater occupying its bottom, and subsequently toward the NW, down the outer flank of the tuff cone and into the plain, where they reached a distance of ~3.5km. During this stage ~0.6km3 DRE of magma was produced at the rate of ~4m3/s in a period of ~5months.Although El Caracol displays many features that are characteristic for a phreatomagmatic vent, its morphology, types of deposits, and its complex process of formation makes it strikingly different from the more typical case of the ~21,000year BP (cal 25,300 BP) Alberca de Guadalupe maar volcano, situated not far at the SE margin of the Zacapu basin. The latter was solely phreatomagmatic during the course of its eruption and is formed in its entirety by surge and fallout breccias consisting largely of xenolithic material. In contrast, at El Caracol the hydrogeological environment (namely the low groundwater flow gradient) did not allow for sustained water/magma interactions at ideal conditions (ratio of 0.2). Hence, the system soon dried out due to insufficient groundwater flow toward the eruption site and the eruption turned purely magmatic. This study shows that despite of its proximity to a large water reservoir (Zacapu lake basin) and similarities with Alberca de Guadalupe maar in regard to age, climate, magma composition, and tectonic environment, several other specific parameters play a crucial role in shaping a monogenetic phreatomagmatic volcano. The case of El Caracol (phreatomagmatic style followed by magmatic during the course of a monogenetic eruption) has been reported from other volcanic fields around the globe where it might have similar causes. Furthermore, the general scarcity of this type of volcanoes in the Michoacán–Guanajuato Volcanic Field implies that the conditions required for their formation are rarely met in this region.

The ∼AD 1250 effusive eruption of El Metate shield volcano (Michoacán, Mexico): magma source, crustal storage, eruptive dynamics, and lava rheology

Medium-sized volcanoes, also known as Mexican shields due to their andesitic composition and slightly higher slope angles in comparison to Icelandic shields, occur across the Trans-Mexican Volcanic Belt and represent nearly one third of all volcanic edifices in the Michoacan-Guanajuato Volcanic Field (MGVF). Many questions about their origin and eruptive dynamics remain unanswered. Here, we focus on El Metate, the youngest (∼AD 1250) monogenetic shield volcano of the MGVF and the most voluminous (∼9.2 km3 dense rock equivalent) Holocene eruption in Mexico. Its eruptive history was reconstructed through detailed mapping, geochemical analysis (major and trace elements, Sr-Nd-Pb isotopic data), and rheological study of its thick andesitic flows. Early and late flow units have distinct morphologies, chemical and mineralogical compositions, and isotopic signatures which show that these lavas were fed by two separate magma batches that originated from a heterogeneous mantle source and followed distinct differentiation paths during their ascent. Thermobarometry calculations constraining the conditions of crystallization indicate a temporary storage of the last erupted magma batch at a depth of ∼7–10 km. Lava rheology was estimated using petrographic characteristics, geochemical data, and flow dimensions. The magma viscosity increased from 102–103 Pa s prior to eruption through 106–108 Pa s during ascent, to 109–1011 Pa s during lava emplacement. Though magma viscosity was quite high, the eruption was purely effusive. The explosive eruption of such a large magma volume was probably avoided due to efficient open system degassing (outgassing) of the magma as it ascended through the uppermost crust and erupted at the surface.

Condiciones de cristalización y diferenciación de las lavas del volcán El Metate (Campo Volcánico de Michoacán-Guanajuato, México)

El Metate is a shield volcano located in the southern sector of the Michoacan-Guanajuato Volcanic Field, one of two largest monogenetic volcanic fields of the Transmexican Volcanic Belt. It was active c. 4.700 ± 200 years B.P and emitted about fifteen calcalkaline lava flows showing variable differentiation degrees. Temperatures calculated from mineral-liquid geothermobarometers for olivine, plagioclase and pyroxene, suggest that olivine was the earliest fractionating phase (1232–1198 °C), followed by plagioclase (1162–1126 °C), orthopyroxene (1147–1027 °C) and clinopyroxene (1147–1018 °C). Pressure estimations indicate that crystallization started at ~7 kbar and progressed up to surface levels. Water contents in the melts during crystalliztion of plagioclase is estimated at ~1.6%. Temperatures calculated on the basis of Al content in amphibole, provide a crystallization range between 995 and 922 °C, at an average pressure of 3.5 kbar and water contents between 5.2% and 6.9%. Although these values could agree with a scenario where amphibole represents a late crystallization phase along the previous fractionating sequence, the systematic presence of disequilibrium textures, which are also observed occasionally in other phases, suggest that other possibilities such as open-system crystallization cannot be discarded.

40Ar/39Ar dating, geochemistry, and isotopic analyses of the quaternary Chichinautzin volcanic field, south of Mexico City: implications for timing, eruption rate, and distribution of volcanism

Monogenetic structures located at the southern and western ends of the Chichinautzin volcanic field (Trans-Mexican Volcanic Belt, Central Mexico) yield 40Ar/39Ar ages ranging from 1.2 Ma in the western portion of the field to 1.0–0.09 Ma in the southern portion, all of which are older than the <0.04 Ma age previously established for the entire volcanic field. These new ages indicate: (1) an eruption rate of 0.47 km3/kyr, which is much lower than the 11.7 km3/kyr previously estimated; (2) that the Chichinautzin magmatism coexisted with the Zempoala (0.7 Ma) and La Corona (1.0 Ma) polygenetic volcanoes on the southern edge of Las Cruces Volcanic Range (Trans-Mexican Volcanic Belt); and confirm (3) that the drainage system between the Mexico and Cuernavaca basins was closed during early Pleistocene forming the Texcoco Lake. Whole-rock chemistry and Sr, Nd, and Pb isotopic data indicate heterogeneous magmatism throughout the history of Chichinautzin activity that likely reflects variable degrees of slab and sediment contributions to the mantle wedge, fractional crystallization, and crustal assimilation. Even with the revised duration of volcanism within the Chichinautzin Volcanic Field, its eruption rate is higher than most other volcanic fields of the Trans-Mexican Volcanic Belt and is comparable only to the Tacambaro-Puruaran area in the Michoacan-Guanajuato Volcanic Field to the west. These variations in eruption rates among different volcanic fields may reflect a combination of variable subduction rates of the Rivera and Cocos plates along the Middle America Trench, as well as different distances from the trench, variations in the depth with respect to the subducted slab, or the upper plate characteristics.

Reconstruction of the volcanic history of the Tacámbaro-Puruarán area (Michoacán, México) reveals high frequency of Holocene monogenetic eruptions

The 690 km2 Tacambaro-Puruaran area located at the arc-front part of the Michoacan-Guanajuato volcanic field in the Trans-Mexican Volcanic Belt (TMVB) records a protracted history of volcanism that culminated with intense monogenetic activity in the Holocene. Geologic mapping, 40Ar/39Ar and 14C radiometric dating, and whole-rock chemical analyses of volcanic products provide insights to that history. Eocene volcanics (55–40 Ma) exposed at uplifted blocks are related to a magmatic arc that preceded the TMVB. Early TMVB products are represented by poorly exposed Pliocene silicic domes (5–2 Ma). Quaternary ( 30 km) may be related to high rates of magma production at depth and a favorable tectonic setting.