Volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, Mexico

ABSTRACT This work presents the volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, including the Serdán-Oriental basin and Cofre de Perote-Citlaltépetl volcanic range, two contrasting Quaternary volcanic fields. The first comprises dominant monogenetic volcanism of bimodal composition including isolated rhyolitic domes and tuff rings, basaltic andesite maar volcanoes, cinder and lava cones, and an active caldera complex (Los Humeros). The second is dominated by large composite polygenetic volcanoes of andesitic-dacitic composition, including the shield-like compound Cofre de Perote volcano, La Gloria and Las Cumbres complexes, and the active Citlaltépetl stratovolcano. Mapping units include a pre-volcanic basement made of metamorphic (Paleozoic), and sedimentary (Jurassic-Cretaceous); intrusive (Miocene) rocks; multiple basaltic-andesitic lava flows and rhyolitic domes; volcaniclastic sequences (debris avalanches and lahars; and pyroclastic deposits (block-and-ash flows, ignimbrites, fallouts, and surges) (Miocene-Holocene). This map provides a comprehensive view of the highly diverse volcanism, which may reference for future research work.


Introduction
The easternmost sector of the Trans-Mexican Volcanic Belt (TMVB) is a large natural volcanic laboratory made of two contrasting volcanic areas, the Serdán-Oriental basin (SOB) and the Cofre de Perote-Citlaltépetl volcanic range (CPCVR) (Figure 1). Both areas host a large diversity of volcanic landforms formed by intense and highly variable volcanism since the Miocene (10.5-8.9Ma), in terms of eruptive style and composition (effusive to explosive and magmatic to phreatomagmatic and basaltic to rhyolitic). As a result, volcanoes range from small monogenetic (cinder and lava cones, domes, maars, tuff rings, and small shields) to large polygenetic volcanic landforms (stratovolcanoes, compound volcanoes, calderas, and dome complexes), formed by a broad range of deposit types including lava flows, domes, volcaniclastic sequences (debris avalanches and lahars), and pyroclastic deposits (ignimbrites, fallouts, block-and-ash flows, and surges). The relatively young age and climatological conditions of these areas permit the preservation of the original volcanic landforms, making this sector an exceptional volcanic region.
The SOB is a broad closed basin (2,300 m.a.s.l.), hosting isolated basaltic andesite cinder and lava cones, maar volcanoes, small to medium-sized rhyolitic domes and tuff-rings, pyroclastic sequences, and an active caldera (Los Humeros). In contrast, the CPCVR comprises two andesitic-dacitic composite volcanoes (Citlaltépetl, or Pico de Orizaba, and Cofre de Perote) and two volcanic complexes (La Gloria, Las Cumbres) (Figure 2a), which form a pronounced physiographic boundary between the SOB and the Gulf of México Coastal Plain (GMCP) (Figure 1), with a nearly 1 km relief height. Continuous degradation and sedimentation processes associated with these large and unstable volcanic ranges have promoted repeated sector collapses toward the GMCP (Carrasco-Núñez et al., 2006). This work presents the volcanic geology of this sector in a regional map and cross-sections, showing the distribution, chronostratigraphy, and composition of the main volcanic units, which may serve as an important reference for future work in this region.

Field methods and data compilation
Cartographic work consisted of the digital mapping of an extensive fieldwork database (>150 sites with stratigraphic and structural descriptions) into a 154 × 119 km area by using GIS software. Geographical data from each site were recorded using a handheld GPS in UTM coordinates under the WGS84 datum. Representative samples were selected for further petrographic and geochemical characterization (61 new analyses from 550 compiled: data source available in the repository). Also, 104 isotopic ages obtained by different dating methods were compiled into the map (references and dating methods are cited in the main map), leading to identifying distinct lithostratigraphic units and constructing the regional chronostratigraphy.
Furthermore, in addition to our regional analysis, lithological contacts and volcano-structural features were redefined by imagery analyses of the geological maps and regional structural works by Fitz-Díaz et al. (2018) and Norini et al. (2019).

Integration of the lithostratigraphic framework and map construction
Mapping volcanic geology is always a challenging work due to the complex nature of volcanic systems, which commonly records facies variations, overlapping eruptive styles, and temporal changes (Németh & Palmer, 2019). Mapping is even more complicated when mapping on a regional scale because of the large diversity of volcanic landforms, eruptive products, compositions, and spatio-temporal variations, which is the case in this work. To solve this, we constructed a regional lithostratigraphic framework based on the lithology, eruptive style, origin, and stratigraphic relations. For this, we integrated field and chronological data along with the cartographic database. This approach allowed us to combine, in some cases, individual rock units into single lithostratigraphic units. In the cases where the lithostratigraphic units include rocks from multiple volcanoes or vents, and to summarize their description and facilitate their identification in the map, we preferred to use general descriptions (as a Group) based on the dominant composition, age, and type of volcanic product (e.g. andesitic lavas, rhyolitic domes, ignimbrites) rather than assigning formal names. A stratotype locality was assigned in individual lithostratigraphic units from a particular vent source, and volcanic facies were featured following Németh and Palmer (2019).
By following this methodology, and after careful analysis, a comprehensive regional lithostratigraphic column was constructed comprising 36 lithostratigraphic units. For practical purposes, and due to space limitations, the descriptions of the lithostratigraphic units presented are deliberately summarized. However, the reader is referred to the cited bibliography for additional information.
J.-Jurassic shale and limestone The regional basement is overlain by a Jurassic sedimentary sequence, which mostly corresponds to highly-deformed shales and limestones forming the Sierra Madre Oriental (SMO) province (Viniegra-Osorio, 1965), also referred as the Mexican fold and thrust belt (Fitz-Díaz et al., 2018). Outcrops are limited northwest of LHVC and in the Las Minas region.

K.-Cretaceous limestone
This unit corresponds to the Cretaceous SMO sequence, composed of clayey limestones with flint intercalations that laterally grades to reef facies (Viniegra-Osorio, 1965). Outcrops are widespread and are unconformably overlain by the Cenozoic volcanism ( Figure 2a).

Middle-Early Pleistocene
Qa3.-Andesitic, trachyandesitic and dacitic lava flows and domes Group (Cofre de Perote, La Gloria). Trachyandesitic lava flows and andesitic-trachyandesitic-dacitic domes are associated with the early activity of the Cofre de Perote shield-like compound volcano, forming medial facies in a stratovolcanodominated system (Németh & Palmer, 2019) around the main volcano edifice. A 40 Ar/ 39 Ar age is reported in 0.51 ± 0.06 Ma , which is consistent with the K/Ar age range of 0.47 ± 0.2 Ma to 1.3 ± 0.12 Ma of Cantagrel and Robin (1979). These lavas correlate with those derived from La Gloria complex (LGC) at its central edifice and eastern distal facies, where lavas intercalate with volcaniclastic deposits (ring plain).
Qtrav.-Puente Nacional travertine deposits Late Pliocene-Pleistocene travertine rocks are located to the southeast of Xalapa city and Citlaltépetl volcano and around Acatzingo town. These are characterized by recrystallized and massive textures (SGM, 2010b). Stratotype: Puente Nacional (40 km southeast of Xalapa city, north of San José Chipila).

QXig.-Xáltipan Ignimbrite (LHVC)
This is a pumice-rich, rhyolitic pyroclastic sequence composed of two flow units bounded by a basal and an intercalated thin pumice fallout layer. This unit´s recent cartographic and stratigraphic studies reveal that this ignimbrite formed during a continuous, multi-phase eruption, with ca. 290 km 3 of DRE ejected magma (Cavazos-Álvarez & Carrasco-Núñez, 2019;, ranking this eruption as the largest in the TMVB. A previous K/Ar date provided an age of 460 ka (K-Ar, Ferriz & Mahood, 1984); however, combined 40 Ar/ 39 Ar and U/Th dating provided an age of 164 ± 4.2 ka (Carrasco-Núñez et al., 2018). This unit is related to the formation of the ca. 17 km-diameter Los Humeros caldera. A stratotype locality is referred adjacent to the Xáltipan town (northern LHVC). Nevertheless, the complete stratigraphy of this unit can be better assessed by following the two stratotype sections described in Cavazos and Carrasco-Núñez (2020) since this is a composite, facies-variating ignimbrite (from proximal outflow sheet to distal valley-pond; outcrops are all around Los Humeros caldera).
Qba-b.-Basaltic andesites and basalts Group (Cofre de Perote-Naolinco, Eastern LCC-LGC) This unit consists of lava flows from the northeastern vent cluster of the Cofre de Perote and the Naolinco volcanic field, with an age range of 40-43 ka ( 14 C; Siebert & Carrasco-Núñez, 2002). This unit also includes some small scoria and lava cones located east of LCC-LGC.
QLCa.-Las Cumbres debris avalanche. It consists of poorly-sorted, heterolithic, altered boulders with jigsaw-fractures and imbricated blocks and clayey minerals (Carrasco-Núñez et al., 2006;Rodríguez, 2005). It was formed by the sector collapse of Las Cumbres volcano and transformed downstream to debris flow and fluviatile facies. 14 C dating provides a minimum age of 40 ka (Rodríguez, 2005). Stratotype: 12 km northeast from Las Cumbres summit.
QCvc.-Cofre Perote volcanoclastic deposits. This unit includes two different deposits derived from sector collapses of the Cofre de Perote compound volcano (Carrasco-Núñez et al., 2006;. Los Pescados debris flow deposit consists of a massive and heterolithic mixture of boulders and jigsaw-fractured blocks supported by a silt-rich matrix, dated at 44 ka ( 14 C). Stratotype: Los Pescados river, next Jalcomulco town. Also, it includes the Xico debris-avalanche deposit dated between 11-13 ka ( 14 C), which is composed of heterolithic, boulder-to-gravel-sized lithic clasts within a sand-silty matrix, showing hummocky topography. Downstream transformation to fluvial deposits is observed in both deposits at the distal facies. Stratotype: 2 km northwest of Xico town.
Qp.-Undifferentiated pyroclastic and alluvial deposits This unit includes pyroclastic deposits of different sources, reworked pumice deposits, and soil layers.

Summary and concluding remarks
The geologic cartography of the SOB and CPCVR records the highly diverse Quaternary volcanism of the easternmost sector of the TMVB. The SOB is featured by monogenetic bimodal volcanism comprising basaltic andesite maar volcanoes (e.g. Alchichica, Aljjojuca, Atexcac, Tecuitlapa, Preciosa), cinder and lava cones (e.g. El Brujo), large isolated rhyolitic domes (e.g. Cerro Pizarro, Las Derrumbadas), tuff rings (e.g. Cerro Pinto, Tepexitl), and the active Los Humeros volcanic complex, the largest caldera in the TMVB. In contrast, the CPCVR comprises large andesitic-dacitic polygenetic volcanoes, including the shield-like compound Cofre de Perote volcano, La Gloria and Las Cumbres complexes, and the active Citlaltépetl (Pico de Orizaba) stratovolcano, with subordinate monogenetic volcanism.
The map and cross-sections show the spatio-temporal distribution of the volcanism and its relationship with the structural framework. The most recent volcanism is everywhere, so no systematic variations across the volcanic arc are observed through the entire easternmost sector of the TMVB. This work may serve as a useful reference for further studies, not only for volcanological purposes but also for petrology, geophysics, hydrogeology, geothermal exploration, and even volcanic hazards, as this is regarded as an active volcanic area.

Software
Geo-referencing and digitization of map elements was made using ESRI© ArcGIS 10.2. The final map layout and pictures were produced using CorelDRAW X7.

Geolocation information
The map presented in this work has the shape of a box delimited by the coordinates −98.000°E -−96.777°E, 18.735°N -18.808°N (WGS84 datum).