Sierra Chichinautzin Research Articles

Geology and geochemistry of Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoes in the Sierra Chichinautzin Volcanic Field, south of México City

This study focuses on the geology and geochemistry of three closely-spaced monogenetic volcanoes that are located in the NE sector of the Sierra Chichinautzin Volcanic Field near México City. Pelagatos (3020 m.a.s.l.) is a small scoria cone (0.0017 km 3) with lava flows (0.036 km 3) that covered an area of 4.9 km 2. Cerro del Agua scoria cone (3480 m.a.s.l., 0.028 km 3) produced several lava flows (0.24 km 3) covering an area of 17.6 km 2. Dos Cerros is a lava shield which covers an area of 80.3 km 2 and is crowned by two scoria cones: Tezpomayo (3080 m.a.s.l., 0.022 km 3) and La Ninfa (3000 m.a.s.l., 0.032 km 3). The eruptions of Cerro del Agua and Pelagatos occurred between 2500 and 14,000 yr BP. The Dos Cerros eruption took place close to 14,000 yr BP as constrained by radiocarbon dating. Rocks from these three volcanoes are olivine-hypersthene normative basaltic andesites and andesites with porphyritic, aphanitic, and glomeroporphyritic textures. Their mineral assemblages include olivine, clinopyroxene, and orthopyroxene phenocrysts (≤ 10 vol.%) embedded in a trachytic groundmass which consists mainly of plagioclase microlites and glass. Pelagatos rocks also present quartz xenocrysts. Due to their high Cr and Ni contents, and high Mg#s, Pelagatos rocks are considered to be derived from primitive magmas, hence the importance of this volcano for understanding petrogenetic processes in this region. Major and trace element abundances and petrography of products from these volcanoes indicate a certain degree of crystal fractionation during ascent to the surface. However, the magmas that formed the volcanoes evolved independently from each other and are not cogenetically related. REE, HFSE, LILE, and isotopic (Sr, Nd, and Pb) compositions point towards a heterogeneous mantle source that has been metasomatized by aqueous/melt phases from the subducted Cocos slab. There is no clear evidence of important crustal contributions in the compositions of Pelagatos and Cerro del Agua rocks. The Sr-isotopic composition of Dos Cerros, however, indicates a small degree of crustal contamination.

Changes in community structure of ectomycorrhizal fungi associated with Pinus montezumae across a volcanic soil chronosequence at Sierra Chichinautzin, Mexico

Ectomycorrhizal (ECM) communities of Pinus montezumae Lamb. var. montezumae were studied across a volcanic soil chronosequence at Sierra del Chichinautzin, Mexico, to investigate differences in community structure at sites with different soil quality. Study sites were located on volcanic soils of different ages. Both aboveground and belowground fungal communities were compared for 3 years. Species abundance, richness, and diversity were compared among sites. Significant differences were found among the three sites for sporocarp abundance and biomass but not for total ECM root tips. The more diverse sporocarp community was found at the youngest, less fertile site. Dominant species at the sporocarp and ECM morphotype levels differed across the chronosequence. The largest proportion of sporocarp species was site-exclusive, which suggests that aboveground communities depend upon soil conditions. Contrarily, 85% of ECM morphotypes were present at least at two sites. The poor correspondence between the aboveground and belowground views of the ECM fungal community may be due to differences in sampling methodology and level of identification. Inocybe and Laccaria species were abundant fruiters at all sites, and as multisite fungi, they may be tested for inoculum efficiency to be used in reforestation programs in the surroundings of Mexico City.

Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone, southeast of México City

The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich (up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.% microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western Mexico, were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt.

Re and Os concentrations in arc basalts: The roles of volatility and source region fO 2 variations

Olivine and spinel compositions, major elements (including ferric and ferrous iron), S, Re and Os contents have been measured for a suite of primitive (most >6 wt% MgO) basalts from the Trans Mexican Volcanic Belt (TMVB), including the western Mexican volcanic belt, the Michoacan-Guanajuato Volcanic Field, Sierra Chichinautzin, Pico de Orizaba region, Palma Sola, San Martin Tuxtlas, and the eastern alkaline province (EAP). Sulfur contents at sulfide saturation were calculated to determine whether the measured S contents are representative of sulfide saturated liquids. Most of the samples have S contents much lower than expected for sulfide saturation. A few have higher contents than calculated perhaps due to the presence of sulfate in the measured total sulfur (i.e., more oxidized samples). Comparison of the TMVB samples along with previously analyzed MORB, OIB, BABB and arc samples reveals a continuum of Re and Os contents that is best explained by variation in oxygen fugacity—and thus sulfide stability—in the source region. High Re and Os magmatic suites are best explained by derivation by melting of oxidized mantle, where sulfide is no longer stable and Re and Os behave incompatibly, whereas low Re and Os magmatic suites are derived from melting of relatively reduced mantle where sulfide is stable, and Re and Os behave compatibly. Intermediate examples abound, and arc magmas span a wide range of Re and Os concentrations due to variation of fO 2 in the source during genesis of arc magmas. Low Re magmatic suites are furthermore potentially affected by volatility which can lower Re by a factor of 3–5.

Geochemical Evidence for Mantle Origin and Crustal Processes in Volcanic Rocks from Popocatépetl and Surrounding Monogenetic Volcanoes, Central Mexico

Elemental, isotopic, and mineral compositions as well as rock textures were examined in samples from Popocatepetl volcano and immediately surrounding monogenetic scoria cones of the Sierra Chichinautzin Volcanic Field, central Mexico. Magma generation is strongly linked to the active subduction regime to the south. Rocks range in composition from basalt to dacite, but Popocatepetl samples are generally more evolved and have mineral compositions and textures consistent with more complicated, multi-stage evolutionary processes. High-Mg calc-alkaline and more alkaline primitive magmas are present in the monogenetic cones. Systematic variations in major and trace element compositions within the monogenetic suite can mostly be explained by polybaric fractional crystallization processes in small and short-lived magmatic systems. In contrast, Popocatepetl stratovolcano has produced homogeneous magma compositions from a shallow, long-lived magma chamber that is periodically replenished by primitive basaltic magmas. The current eruption (1994–present) has produced silicic dome lavas and pumice clasts that display mingling of an evolved dacitic component with an olivine-bearing mafic component. The longevity of the magma chamber hosted in Cretaceous limestones has fostered interaction with these rocks as evidenced by the chemical and isotopic compositions of the different eruptive products, contact-metamorphosed xenoliths, and fumarolic gases. Popocatepetl volcanic products display a considerable range of Sr/Sr (0 70397–0 70463) and eNd (þ6 2 to þ3 0) whereas Pb isotope ratios are relatively homogeneous (Pb/Pb 18 61–18 70; Pb/Pb 15 56–15 60).

Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin, Central Mexican Volcanic Belt

The Sierra Chichinautzin Volcanic Field is a key area for understanding the origin of the controversial Mexican Volcanic Belt (MVB). This is due to its recent volcanic activity, which is located at the front of the Central MVB. Although a direct genetic relationship between the volcanism of the MVB and the subduction of the Cocos plate is accepted by most authors, geological, geophysical and geochemical key features of the MVB depart from those of typical magmatic arcs. This fact has promoted a debate on the nature of the complex mantle beneath the MVB: (1) slab-induced convection in the mantle wedge that causes advection of asthenospheric mantle and (2) anomalously hot mantle related to the eastward channeling of plume-related material. This is a petrological study of the Sierra Chichinautzin volcanism, using mineralogical data from selected representative samples, together with their geochemistry, to refine the existing models of magmatic processes. The integrated petrological analyses have been used to define four main groups of rocks in Sierra Chichinautzin: (1) Ol-mafic rocks, (2) Opx-intermediate rocks, (3) Opx-felsic rocks, and (4) Qtz-intermediate/felsic rocks. Moreover, two different types of mantle-derived primitive mafic magmas have been described based on their mineralogical and geochemical features: (1) OIB-like mafic magmas (presence of olivine+plagioclase phenocrysts; higher incompatible element contents with no HFSE/LILE negative anomalies), and (2) mafic magmas derived from a metasomatized mantle (no plagioclase phenocrysts and more forsteritic olivine; HFSE/LILE negative anomalies). We prove that the diversity of magmas (basalts to dacites) in Sierra Chichinautzin is un-related to fractional crystallization processes from the mafic magmas. Rather, magma mixing phenomena between mafic magmas and two different crustal-derived felsic magma types (Opx-dacite and Qtz+Bt+Amp+Pl+Cpx+Opx-dacite) explain the textural, mineralogical, and geochemical signatures of most of the Sierra Chichinautzin rocks. There is a strong genetic relationship between the mafic and felsic end members of each mixing trend. The relatively anhydrous OIB-like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions, generating Opx-bearing felsic rocks. On the contrary, hydrous mafic magmas derived from a metasomatized mantle, promote basaltic crustal partial melting under high water fugacities, yielding magmas with a complex hydrous phenocryst assemblage. Several important geodynamical implications for the origin of the MVB magmatism can be extracted from this petrological study: (1) the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath Sierra Chichinautzin; (2) present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath Sierra Chichinautzin; and (3) the negative Nb anomaly of most of the Sierra Chichinautzin rocks (andesites and dacites) is due to magma mixing, and therefore is not a signature of the role of fluids from the subducting Cocos plate. These geochemical features, together with the geophysical and tectonic scenario of the Sierra Chichinautzin, indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot OIB-like mantle, and cast further doubts on a significant role of the subducting Cocos plate in the origin of this magmatism.

Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic belt

Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt (TMVB), the Sierra Chichinautzin volcanic field (SCN) is a key area for the understanding of this controversial volcanic province. Volcanic activity has built more than 220 monogenetic volcanoes (shields, scoria cones, thick lava flows, and hydromagmatic structures) during the last 40,000 years, for a total volume of about 470 km 3. The SCN basalts are geochemically similar to OIBs, while the intermediate and felsic volcanic rocks show a calc-alkaline trend and abundant evidence for magma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for detecting volcanic center alignments. The tectonic data, together with geophysical evidence, confirm active general N–S extensional conditions with a strike–slip component for the SCN area, the same structural setting that prevails in the rest of the Central TMVB. Extensional tectonics, a negative regional Bouger gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity suggest a rift-type setting involving the upwelling of anomalous mantle beneath the Central TMVB. The combined petrological, structural and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanism.

Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions

The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatepetl, Iztaccihuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichinautzin that forms the southern terminus of the Valley of Mexico. The Chichinautzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts.

The water balance for the Basin of the Valley of Mexico and implications for future water consumption

The Basin of the Valley of Mexico is a closed basin of 9600 km2, where average annual precipitation (1980–85) is 746 mm (226.7 m3/s). Calculated actual evapotranspiration is 72–79% of the precipitation. The surrounding mountain ranges of the Sierra de Las Cruces, Sierra Nevada, and Sierra Chichinautzin are the main recharge areas for the enclosed Basin, in decreasing order.

Affinity among mountain ranges in Megamexico: A phytogeographical scenario

The concept of Megamexico as a phytogeographic unit has been introduced by Rzedowski (1991) on the basis of distribution patterns of genera. Until now precise information on the resemblance between plant communities in the proposed Megamexico was scarce. This phytosociological study documents the affinities between plant communities within Megamexico, based on studies from mountain ranges in Mexico (Sierra Chichinautzin, Sierra Nevada) and Guatemalan mountain ranges (Sierra de los Cuchumatanes, Cadena Volcánica). Data, collected in the style of Braun-Blanquet during extensive fieldwork, were pooled into a single data set, and analyzed by using (1) TWINSPAN, for plant community classification, (2) ordination (DCA), (3) alpha log series biodiversity-index to measure intergroup diversity, and (4) information on the altitudinal distribution of the vegetation belts. Classification showed that four communities were common to the two groups of mountain ranges, namely, alpine bunchgrass-land, pine forest, fir forest and mixed forest. Along the altitudinal gradient Mexican communities are distributed 200 m higher than their Guatemalan equivalents. This appears to be an illustration of the ‘Massenerhebungseffekt’. DCA showed that the first axis represents a set of minor differences of closely related ecological factors (e.g., temperature, precipitation) and the second a humidity gradient. The biodiversity index showed that the Mexican mixed forest was significantly more diverse than the Guatemalan mixed forest. From the present results, ecological conditions among mountain ranges in Megamexico differed significantly. To conclude, phytogeographical units in Megamexico can better be defined on basis of both historical and ecological characteristics of the communities.

Monogenetic vulcanism in sierra Chichinautzin, Mexico

The variation in the activity patterns of the Chichinautzin volcanic rocks is discussed. This sequence of lavas and pyroclastic deposits is located in the central part of the Mexican Volcanic Belt, directly south of Mexico City, and is typical of its Quaternary monogenetic vulcanism.