Chichinautzin Volcanic Field Research Articles

Geochemistry, Sr–Nd isotope composition, and tectonic setting of Holocene Pelado, Guespalapa and Chichinautzin scoria cones, south of Mexico City

Holocene Pelado, Guespalapa and Chichinautzin monogenetic scoria cones and associated lava flows located within the Sierra del Chichinautzin Volcanic Field (SCVF) at the southern margin of Mexico City were mapped and sampled for mineralogical and chemical analyses. With the exception of Parı́cutin volcano in western Mexico, few scoria cones in the Trans-Mexican Volcanic Belt have ever been sampled in greater detail. Chemical analyses of rocks indicate that mafic products (e.g. Guespalapa and Chichinautzin) from individual volcanoes in the Sierra del Chichinautzin are characterized by substantial chemical variability, whereas high-silica andesite volcanoes (e.g. Pelado) are very uniform in composition. These findings have important bearings for regional tephrochronology. As a whole, rock compositions form a continuous coherent calc–alkaline suite, explicable by polybaric fractional crystallization±assimilation associated with successive stagnation at different depths along the ascent path. Trace element and Sr–Nd isotope analyses point toward a <1-km-scale heterogeneous (enriched/depleted) mantle wedge underneath the SCVF. The recently proposed plume-origin for these rocks is not in accord with our data. Instead, magma origin is discussed in relation to the tectonically complex subduction process of the oceanic Cocos Plate underneath the continental North American Plate.

Radiocarbon ages of Holocene Pelado, Guespalapa, and Chichinautzin scoria cones, south of Mexico City: implications for archaeology and future hazards

Pelado, Guespalapa, and Chichinautzin monogenetic scoria cones located within the Sierra del Chichinautzin Volcanic Field (SCVF) at the southern margin of Mexico City were dated by the radiocarbon method at 10,000, 2,800–4,700, and 1,835 years b.p., respectively. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco around 1,665±35 years b.p. The new dates indicate that the recurrence interval for monogenetic eruptions in the central part of the SCVF and close to the vicinity of Mexico City is <2,500 years. If the entire SCVF is considered, the recurrence interval is <1,700 years. Based on fieldwork and Landsat imagery interpretation a geologic map was produced, morphometric parameters characterizing the cones and lava flows determined, and the areal extent and volumes of erupted products estimated. The longest lava flow was produced by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each eruption range between 54 (Chichinautzin) and 80 km2 (Pelado); and total erupted volumes range between 1 and 2 km3/cone. An average eruption rate for the entire SCVF was estimated at 0.6 km3/1,000 years. These findings are of importance for archaeological as well as volcanic hazards studies in this heavily populated region.

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.

First Partial Melting Inversion Model for a Rift-Related Origin of the Sierra de Chichinautzin Volcanic Field, Central Mexican Volcanic Belt

We present the geochemical and isotopic characteristics of Pleistocene to Holocene olivine-bearing mafic rocks from the Sierra de Chichinautzin (SCN) volcanic field, located in the central part of the Mexican Volcanic Belt (MVB). Some have geochemical characteristics of primary magmas, and their MORB-normalized patterns are similar to those displayed by extension-related mafic rocks. The SCN primary magmas show a limited range in 87Sr/86Sr (0.70348-0.70397; average(n = 7) = 0.70370 ± 0.00019), and 143Nd/144Nd (0.51279-0.51294; average(n = 6) = 0.51288 ± 0.00006).~New mineral and whole-rock chemical data allow estimation of eruption temperatures and assure the near-primary nature of the studied samples. Equilibrium temperatures for the primary magmas were in the range 1070-1150°C. Trace-element concentration data for near-primary magmas from the SCN are used to develop a partial melting inversion model, the first for any area of the MVB. The source composition calculated by this inversion method shows an enrichment in highly incompatible elements (e.g., [CLa/CYb]n ∼2.2), where []n represents mande-normalized values), without a decoupling between LILE, REE, and HFSE. This relationship supports a rift-related origin for SCN volcanism. The calculated REE composition of the mantle source is comparable to that observed in mande xenoliths of central Mexico. Concentration ratios of incompatible elements have also been used to infer 7-16% as the approximate range in degree of partial melting of the mantle. These results reinforce the hypothesis that SCN mafic magmatism reflects partial melting of an enriched lithospheric mantle in an extensional tectonic setting, precluding participation of the subducted Cocos plate in the genesis of these magmas.

A rock-magnetic and paleointensity study of some Mexican volcanic lava flows during the Latest Pleistocene to the Holocene

Eleven Late Quaternary lava flows were sampled in the Chichinautzin volcanic field of central Mexico to determine their magnetic characteristics and absolute paleointensity. The samples studied cover a geological time interval of approximately 0.39 My to 2000 years. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Continuous susceptibility measurements with temperature in most cases yield reasonably reversible curves with Curie points close to that of almost pure magnetite, which is compatible with low-Ti titanomagnetite resulting from oxi-exsolution. Judging from the ratios of hysteresis parameters, it seems that all samples fall within the pseudo-single domain grain size region, probably indicating a mixture of multidomain and a significant amount of single domain grains. Forty-two samples belonging to six independent cooling units yielded acceptable absolute paleointensity estimates. The NRM fractions used for paleointensity determination range from 0.34 to 0.97 and the quality factors varies between 4.5 and 97.8, being normally greater than 5. The obtained virtual dipole moment values are higher than those recently reported for the past 5 My and to the present day geomagnetic field strength. Individual paleointensity of around 2000 BP is substantially higher than the present day intensity, which is in broad agreement with worldwide archeomagnetic results.

Magnetic properties of lake sediments from Lake Chalco, central Mexico, and their palaeoenvironmental implications

Lake Chalco (99.0°W, 19.5°N) in the Basin of Mexico, was formed during the Pleistocene after the emplacement of the Chichinautzin volcanic field that closed the former drainage system. The lake sediment record has been influenced by a number of factors, including glacial–interglacial cycles, local volcanism, erosion of soils and anthropogenic disturbances. The magnetic properties of the lake sediments and the associated tephra layers of the last 16500 yr have been studied. It is found that the magnetic properties of the Lake Chalco sediments are very distinctive. Magnetic concentration varies by a factor of 1000 and magnetic stability also varies over an extremely wide range. The predominant magnetic mineral is titanomagnetite in addition to an imperfect antiferromagnetic phase, possibly goethite. An unusually large range of coercivities is found in certain of the tephras. Down-core variations in magnetic properties closely follow climatic/environmental changes previously established by other proxy methods. The late Pleistocene and late Holocene lake sediments display a higher concentration of magnetic minerals than the early–middle Holocene sediments. In the non-volcanic sediments, fluctuations in the magnetic concentration reflect changes in both the intensity of erosion, as represented by ferrimagnetic and paramagnetic minerals, and by the maturing of soils, as represented by geothite. Copyright © 2000 John Wiley & Sons, Ltd.

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.