Introduction

Monogenetic volcanoes are small-volume eruptive centers (≤1 km³) that experience a single eruption in their lifespan (Valentine and Gregg 2008). Typical monogenetic volcanoes are basaltic, and the most common type is a scoria cone (Valentine and Connor 2015). Basaltic scoria cones are formed by the uninterrupted ascent of small magma batches to the surface, usually with scarce or no processes of assimilation and fractional crystallization due to the rapid ascent of magma (McGee and Smith 2016; Murcia and Németh 2020), leading to primitive compositions in some cases (Smith and Németh 2017). Nevertheless, stagnation of magma and processes of assimilation and fractional crystallization can occur, which leads to evolved magmatic compositions ranging up to dacitic and rhyolitic (e.g., Borrero et al. 2017; Ross et al. 2017; Murcia et al. 2019; Sosa-Ceballos et al. 2021; Sánchez-Torres et al. 2019, 2022).

In general, monogenetic volcanism can be found in any tectonic setting (Németh 2010; Le Corvec et al. 2013; Kereszturi et al. 2014) and monogenetic volcanoes can either form volcanic fields or occur as clusters around composite volcanoes (Schaaf et al. 2005; Cañón-Tapia 2016; Schonwalder-Ángel et al. 2018). A volcanic field consists of an area occupied by various volcanic centers (Valentine and Connor 2015) that can vary from just a few volcanoes (e.g., as at Pijaos field in Colombia; Velandia et al. 2021) to hundreds of centers (e.g., as at Michoacán-Guanajuato field in México; Osorio-Ocampo et al. 2018) and a field can remain active millions of years (Németh 2010).

The volcanoes considered here are located in the Central Cordillera of Colombia and are part of the San Diego–Cerro Machín Volcano-Tectonic Province (SCVTP; Fig. 1a-b), the northernmost Andean volcanic chain (cf. Sánchez-Torres et al. 2022). The volcanoes are situated between the city of Manizales (~400,000 inhabitants) and the town of Neira (~30,000 inhabitants), along the Tapias and Guacaica rivers (Fig. 1c). Their discovery adds them to the monogenetic volcanoes recognized across the SCVTP, adding them to those grouped in the monogenetic volcanic fields of Samaná (Murcia et al. 2019; Sánchez-Torres et al. 2019, 2022), Villamaría-Termales (Botero-Gómez et al. 2018; Osorio et al. 2018; Salazar-Muñoz et al. 2021), and Pijaos (Velandia et al. 2021) (Fig. 1b).

Figure 1
figure 1

A Map of Colombia showing the location of the San Diego Cerro–Machín Volcano-Tectonic Province within the red polygon. B Image of the San Diego–Cerro Machín Volcano-Tectonic Province and location of the Las Margaritas volcanoes. C Geological map of Las Margaritas volcanoes zone. Digital elevation model (DEM) from Alos Palsar satellite. Abbreviations SMVF: Samaná monogenetic volcanic field; VTMVF: Villamaría-Termales monogenetic volcanic field; PMVF: Pijaos monogenetic volcanic field; RV: romeral volcano; CBV: Cerro Bravo volcano; NRV: Nevado del Ruiz volcano; PSRV: Paramillo de Santa Rosa volcano; CV: Paramillo del Cisne volcano; QV: Paramillo del Quindío Volcano; NSIV: Nevado de Santa Isabel volcano; NTV: Nevado del Tolima volcano; CMV: Cerro Machín volcano

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This study presents the results of mapping, plus petrographic, geochemical, and geochronological analyses of two newly identified volcanoes, named here Las Margaritas 1 and Las Margaritas 2. These analyses are used to determine the landforms as well as their mineralogical and chemical compositions. We use this data to infer the different magmatic processes that occurred during magma ascent, the emplacement ages of the landforms, and therefore their eruptive histories. This work highlights that effusive and evolved monogenetic volcanism is rather common and therefore, it should be considered in the classification schemes of this type of volcanism.

Geological setting

The northwestern margin of South America has been subjected to the collision of oceanic plateaus and island arcs alternating with subduction episodes from the Late Cretaceous to the present day, associated with the interactions between the South American, Caribbean, Farallon/Nazca plates, and the Panamá-Chocó arc (e.g., Kerr and Tarney 2005; Villagómez et al. 2011; Villagómez and Spikings 2013; León et al. 2018; Montes et al. 2019; Zapata et al. 2020). From the late Miocene to the present day, the oblique subduction (re)initiation and collision/subduction of aseismic ridges of the Nazca plate as well as slab flattening have controlled the upper plate architecture and the tectono-magmatic evolution of the continental arc in the northwestern Colombian Andes (e.g., Wagner et al. 2017; Jaramillo et al. 2019; León et al. 2021). The current evidence of this active magmatic arc is the San Diego–Cerro Machín Volcano-Tectonic Province (SCVTP), a volcanic chain that hosts at least 10 polygenetic volcanoes as well as tens of monogenetic edifices grouped into three monogenetic volcanic fields: Pijaos, Villamaría-Termales, and Samaná (Botero-Gómez et al. 2018; Osorio et al. 2018; Monsalve et al. 2019; Murcia et al. 2019; Sánchez-Torres et al. 2019, 2022; Vargas 2020; Monsalve-Bustamante et al. 2020; Salazar-Muñoz et al. 2021; Velandia et al. 2021) (Fig. 1b). The magmatic affinity of the SCVTP is typically calc-alkaline and compositions vary from basaltic to rhyolitic (e.g., Osorio et al. 2018; Murcia et al. 2019; Sánchez-Torres et al. 2019, 2022; Salazar-Muñoz et al. 2021; Velandia et al. 2021). Monogenetic volcanism started at 2 Ma in the Villamaría-Termales field (Botero-Gómez et al. 2018; Murcia et al. 2019; Salazar-Muñoz et al. 2021) and is likely still active in the Samaná field (Murcia et al. 2019; Sánchez-Torres et al. 2019, 2022).

Methods

Volcano distribution and fieldwork

An area of about 120 km2 was searched for evidence of volcanic constructs. Mapping then focused on the two identified volcanoes for which a digital elevation model (DEM) of 12 * 12 m resolution was used to define their morphology and limits. Field mapping, outcrop descriptions, and sample collection were then carried out at each volcano. Unfortunately, due to tropical weathering and to the thick (3-6 m) younger pyroclastic fall deposits that cover the region, we were able to collect only one single fresh sample per volcano, outcroping in a quarry and in a landslide, respectively. These samples were used for petrography, geochemistry, and dating.

Petrography

Thin sections were prepared for the two samples at TecLab laboratories, Manizales, Colombia, and point counting (~500 points) using a 1-mm grid spacing was used to determine the groundmass/crystal ratio and the proportion of each crystal phase. Following González (2008) and Murcia and Németh (2020), crystal sizes were defined as follows: phenocrysts = >0.5 mm and microphenocrysts = 0.5–0.05 mm. Microlites (˂0.05 mm) were considered part of the groundmass. A petrographic microscope at the Instituto de Investigaciones en Estratigrafía (IIES) at the Universidad de Caldas (Colombia) was used to carry out this analysis.

Whole-rock chemistry

Both samples were analyzed for major oxides and trace elements. Sample preparation for major oxides followed the fused disc method in which samples in powder are mixed with a lithium borate flux and then fused at 1100 °C in a muffle furnace in a Pt-Au crucible. The discs were then measured via X-ray fluorescence spectroscopy (XRF) using an X-ray Philips PW-2400 XRF spectrometer. For trace elements, multi-acid digestion was employed on powders and then the solutions were analyzed via inductively coupled plasma mass spectrometry (ICP-MS). Both analyses were performed at ALS laboratories (Medellin, Colombia) using the OREAS 24b standard (https://www.oreas.com/crm/oreas-24b/). Loss on ignition was determined from the weighed samples, and iron was reported as total Fe2O3. The chemical data was plotted in the software GCDkit (Janoušek et al. 2006).

Geochronology

K/Ar dating was carried out at ActLabs (Ontario, Canada) after separating glassy groundmass portions (<1 mm). For the K analysis, the sample aliquot was weighed in a graphite crucible with lithium metaborate/tetraborate flux and fused using a LECO induction furnace. The fusion bead was then dissolved with acid. For the Ar analysis, sample aliquots were weighted in an Al container, and degassed at ~100 °C for 2 days.

Determination of radiogenic argon content was performed twice using an MI-1201 IG mass spectrometer with the isotope dilution method and 38Ar as a spike. Two standards (P-207 Muscovite and 1/65 “Asia” rhyolite matrix) were measured for 38Ar spike calibration (actlabs.com).

Results

Fieldwork and geochronology

Las Margaritas 1 volcano (5°10′28.4″ N, 75°27′15.2″ W, 2327 m asl) corresponds to a 150-m-high dome coulée with a basal diameter of ~1.8 km (Fig. 1c). Las Margaritas 2 volcano (5°11′8.018″ N, 75°27′2.971″ W, 2641 m asl) corresponds to a 10-20-m-thick lava flow length of ~3.3 km with the lowest point at 2131 m asl (Fig. 1c). The two volcanoes overlie, and are younger than Cretaceous Quebradagrande Complex, Paleocene intrusive rocks, and the Pliocene-Pleistocene Aranzazu volcaniclastic sequence (Fig. 1c). The area is also crossed by three faults: Manizales, Guacaica, and Tapias faults, plus the El Perro fault system (Fig. 1c).

Field relationships indicate that Las Margaritas 2 sits on top of Las Margaritas 1. Las Margaritas 1 coulée comprises massive rocks, occasionally displaying spheroidal weathering patterns (Fig. 2a). In hand sample, the rock has a porphyritic texture with phenocrysts of amphibole, plagioclase, and biotite embedded in a gray glassy groundmass (Fig. 2b). Las Margaritas 2 lava flow comprises massive rocks with columnar jointing (Fig. 2c). The rocks have a porphyritic texture with phenocrysts of amphibole and plagioclase (but no biotite) embedded in a gray glassy groundmass (Fig. 2d). Las Margaritas 2 volcano contains lithic fragments of Las Margaritas 1 (Fig. 2e).

Figure 2
figure 2

A Spheroidal weathering in Las Margaritas 1 outcrop. B Textural characteristics of Las Margaritas 1. C Cooling joints of Las Margaritas 2 outcrops. D Textures of Las Margaritas 2. E Lithic clast of Las Margaritas 1 volcano in a sample from Las Margaritas 2 volcano

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Ages obtained from the groundmass yielded 0.77 ± 0.04 and 0.80 ± 0.05 Ma, for Las Margaritas 1 and Las Margaritas 2, respectively (Table 1). These indicate that the volcanoes formed during the Pleistocene, with the two eruptive events being closely separated in time.

Table 1 K/Ar geochronology test results. The uncertainty of the calculated ages falls within the 2σ error
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Table 2 Whole-rock chemistry for Las Margaritas 1 and Las Margaritas 2 volcanoes. LOI loss on ignition
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Petrography

The Las Margaritas 1 volcano is characterized by a porphyritic texture and contains 35 vol.% phenocrysts and microphenocrysts in a cryptocrystalline to microcrystalline groundmass of plagioclase, amphibole, and Fe-Ti oxides (Fig. 3a). Phenocrysts and microphenocrysts are of plagioclase (20 vol.%), amphibole (9 vol.%), and biotite (6 vol.%). Plagioclase is mainly euhedral (Fig. 3a), varies between 0.2 and 5.2 mm in size, and can often be found forming glomerocrysts with amphibole and biotite (Fig. 3a). Many plagioclase crystals display zonation (Fig. 3b) and a few show a sieve texture characterized by a dusty zone in the crystal rim (fine sieve) (Fig. 3c). Amphibole is euhedral to subhedral, varies between 0.2 and 1.4 mm in size, is oxidized, and is often found with resorption features (Fig. 3d) and poikilitic texture. Biotite is subhedral (Fig. 3a, d), and its size varies between 0.4 and 1.2 mm. Fe-Ti oxides also appear as inclusions in plagioclase and amphibole.

Figure 3
figure 3

Photomicrographs showing the main petrographic characteristics of Las Margaritas 1 volcano (AL-01). A Typical porphyritic texture of the rock with plagioclase, amphibole, and biotite phenocrysts, and a cryptocrystalline to microcystalline groundmass. B Zonation in plagioclase phenocryst. C Sieve texture in the rim of a plagioclase crystals. D Resorption texture is given by the presence of the groundmass inside the subhedral amphibole. NX: crossed nicols, N //: parallel nicols. Abbreviations: Pl: plagioclase, Amp: amphibole, Bt: biotite

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The Las Margaritas 2 volcano is characterized by a porphyritic texture (Fig. 4a) with 34 vol.% phenocrysts and microphenocrysts in a glassy to microcrystalline groundmass of plagioclase, amphibole, Fe-Ti oxides, and apatite (Fig. 4a). Many of the smaller plagioclase crystals are aligned (Fig. 4b). Phenocrysts and microphenocrysts are of plagioclase (19 vol.%) and amphibole (15 vol.%). Plagioclase is euhedral to subhedral (Fig. 4b) and varies between 0.1 and 4 mm in size. This mineral occasionally displays twinning (Fig. 4c), zonation (Fig. 4c), glomerocrysts with amphibole (Fig. 4d), and a coarse sieve texture (Fig. 4a). Amphibole is euhedral (Fig. 4c-d) and varies between 0.5 and 5 mm in size. It displays resorption (Fig. 4c) and oxidation rims (Fig. 4c-d). Fe-Ti oxides and apatite are also found as inclusions in the crystals.

Figure 4
figure 4

Photomicrographs showing the main petrographic characteristics of Las Margaritas 2 volcano (AL-02). A Typical porphyritic texture of the rock with plagioclase and amphibole phenocrysts, and a glassy to microcrystalline groundmass. Note the sieve texture in the core of a plagioclase crystal. B Fluidal texture in euhedral plagioclase. C Zoned and twinned plagioclase phenocryst, and oxidation rim and resorption in amphibole. D Glomerocryst of plagioclase and amphibole. Note the oxidation rims in amphibole. NX: crossed nicols, N //: parallel nicols. Abbreviations: Pl: plagioclase, Amp: amphibole, Ap: apatite

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Whole-rock chemistry

Major oxides and trace elements are reported in Table 2. Both volcanoes are andesitic in composition with a calk-alkaline signature (Fig. 5a-b), varying between medium and high potassium. Trace elements indicate an enrichment in light rare earth elements (LREE) in comparison to heavy rare earth elements (HREE) (Fig. 5c). Las Margaritas 2 has a higher enrichment in REE (Fig. 5c) and several other trace elements (Table 2) in comparison to Las Margaritas 1. Trace element profiles also show negative Th, Nb, Ta, and Ti anomalies, and a positive Sr anomaly in Las Margaritas 1 (Fig. 5d). Eu/Eu* values vary between 0.81 and 0.97, i.e., the samples display small negative Eu anomalies.

Discussion

Las Margaritas are two effusive and monogenetic centers (cf. Murcia and Nemeth 2020) with complex geometrical characteristics that could be related to the shape of the feeder vent, the duration of lava discharge (Mériaux et al. 2022), the shape of magma conduit, and the topography conditions (Murcia and Nemeth 2020).

Mapping indicates that the smaller Las Margaritas 1 couleé was erupted first, and the larger Las Margaritas 2 lava flow second (Fig. 2e). The K-Ar ages for the two volcanoes are similar and within error; thus, there is a possibility of polymagmatic monogenetic volcanism (cf. Brenna et al. 2010; Van Otterloo et al. 2014; Németh and Kereszturi 2015; Kugaenko and Volynets 2019) with two different magma batches ascending from the same reservoir to feed Las Margaritas 1 and 2, respectively. This would make Las Margaritas 1 and Las Margaritas 2 two units comprising a single monogenetic polymagmatic volcano. Apart from this, the ages highlight the existence of volcanism as recently as 0.8 Ma in a region where volcanism was not recognized until this study.

Petrography

Products from the volcanoes contain plagioclase, amphibole, and biotite as phenocrysts and microphenocrysts (Figs. 3a, 4a), although Las Margaritas 2 lacks biotite. Based on the mineral characteristics, it can be suggested that:

  1. (i)

    The larger crystals were formed in early crystallization stages under slow cooling conditions, low nucleation rates, and slow magma ascent (Cox et al. 1979; López and Bellos 2006), while smaller crystals were formed at more superficial levels, with more rapid cooling conditions, higher nucleation rates, and faster magma ascent (López and Bellos 2006).

  2. (ii)

    The microlites within the groundmass were related to the final stages of crystallization and the syn-eruptive phase, in which the magma undergoes fast decompression, degassing, and exsolution of H2O due to a rapid cooling (Renjith 2014).

Equilibrium textures in the minerals are apparent from aggregates of multiple interlocking euhedral crystals (both microphenocrysts and phenocrysts) forming glomerocrysts. This texture represents the aggregation of crystals during convection or stagnation processes (Vance 1969; Hogan 1993; Seaman 2000). Zonation in plagioclase is interpreted to be caused by convection driven by small-scale physical-chemical perturbations or by differential reaction between the crystal and the melt (Best 2003; López and Bellos 2006; Viccaro et al. 2010; Renjith 2014); this allows CaAl-NaSi diffusion within the crystals (Shcherbakov et al. 2010).

Disequilibrium textures in the minerals include the coarse sieve texture in plagioclase (Figs. 3c, 4a), which suggests decompression processes that caused dissolution; when an unaltered rim occurs, it indicates that the crystal ascended to a new equilibrium zone that allowed the formation of the new rim (Viccaro et al. 2010, 2012; Renjith 2014). Resorption textures in amphibole (Figs. 3d, 4c) are the result of melt incorporation in the crystals due to rapid cooling during magma ascent, or decompression processes (Gill 2011). Amphibole oxidation rims (Fig. 4c-d) were likely caused by the response to water loss during slow magma ascent due to the decompression processes (Rutherford and Hill 1993; Ridolfi et al. 2008). The poikilitic texture of plagioclase in amphibole formed because of the differential nucleation and growth rates (López and Bellos 2006). The amphibole’s lower nucleation rate allowed it to grow more and incorporate a previously formed plagioclase crystal (Winter 2001; López and Bellos 2006).

In addition, the lack of biotite in Las Margaritas 2 suggests a more rapid ascent in the superficial levels, preventing biotite crystallization (Castro and Dingwell 2009). The lack of amphibole oxidation rims in Las Margaritas 1, and the presence of them in Las Margaritas 2, can be related to fast ascent rates at deeper levels for Las Margaritas 1 (Hall et al. 2004; Ridolfi et al. 2008) that did not allow the reaction rims to form, but a slower ascent rate at superficial levels that allowed biotite to crystallize.

We suggest that the first stage of crystal formation happened at depth, involving equilibrium conditions between the crystals and the melt, with low nucleation and ascent rates, which allowed the phenocrysts to form. During this stage, the magma had different stagnation and small-scale convection processes that formed glomerocrysts and compositional zoning in the plagioclase phenocrysts. The second stage of crystallization happened at lower depths and was characterized by the formation of microphenocrysts related to higher nucleation and ascent rates. During this stage, convection processes caused zonation in the microphenocrysts. Disequilibrium textures were formed due to decompression, more rapid cooling, and a decrease in water concentration. The final stage happened near the surface and corresponds to the last stage of crystallization. This stage was characterized by decompression processes, very rapid cooling, and high nucleation rates that generated the microlites of the groundmass.

Geochemistry

Las Margaritas volcanoes

The sub-alkaline andesitic composition with the medium K calc-alkaline affinity of the Las Margaritas volcanoes is typical of subduction zone magmatism (e.g., Gill 1981; Best 2003; Rollinson and Pease 2021). The evolved character of the Las Margaritas magmas is evidenced not only by the SiO2 contents (60–61 wt.%), but also by the low values of Fe2O3 (~6 wt.%), MgO (3 wt.%), and CaO (5 – 6 wt.%) in comparison to a primitive magma. Las Margaritas volcanoes show a general enrichment of large-ion lithophile elements (LILE) such as Ba, K, and Sr and a depletion of high-field strength elements (HFSE) such as Nb, Ta, Th, and Ti (Fig. 5d). These anomalies can be related to the contribution (or not) of terrigenous subducted sediments (Pearce and Peate 1995) or differentiation processes during magma ascent (Rollinson 1993). Negative anomalies in Nb can also be related to clinopyroxene fractionation, while negative anomalies in Ti can be evidence for amphibole and Fe-Ti oxide fractionation (Fig. 5d) (Rollinson and Pease 2021). The negative Eu anomaly (Fig. 6) can indicate plagioclase crystallization at depth and posterior fractionation (Rollinson, 1993). In the multi-elemental diagrams (Fig. 5c-d), Las Margaritas 1 and 2 display similar trends; however, Las Margaritas 2 shows more enrichment, especially in the REE diagram (Fig. 5c). This enrichment can be explained by a lower degree of partial melting, since REE enrichment decreases as the percentage of melting increases (Winter 2001). The presence of biotite in Las Margaritas 1 is evidence of crystallization conditions at shallow levels and temperatures lower than 800°C (Rutherford and Devine 2003).

Figure 5
figure 5

Geochemical diagrams with data normalized volatile-free. A Total alkali vs. SiO2 diagram (Le Bas et al. 1986). B AFM diagram (Irvine and Baragar 1971). C Rare earth elements (REE) normalized to chondrite (Nakamura 1974). D Extended trace element diagram normalized to chondrite (Sun 1980). As reference: Pijaos monogenetic volcanic field (PMVF; Velandia et al. 2021), Villamaría-Termales monogenetic volcanic field (VTMVF; Borrero et al. 2009; Botero-Gómez et al. 2018; Osorio et al. 2018; Salazar-Muñoz et al. 2021), and Samaná monogenetic volcanic field (SMVF; Borrero et al. 2017; Monsalve et al. 2019; Sánchez-Torres et al. 2019, 2022)

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Figure 6
figure 6

Major and trace element Harker diagrams. As reference: Guacharacos and El Tabor volcanoes from the Pijaos monogenetic volcanic field (PMVF; Velandia et al. 2021), Gallinazo, Tesorito, La Negra, La Laguna, La Esperanza, and Santana volcanoes from the Villamaría-Termales monogenetic volcanic field (VTMVF; Borrero et al. 2009; Botero-Gómez et al. 2018; Salazar-Muñoz et al. 2021), and El Escondido, San Diego, Piamonte, Pela Huevos, Morrón, Guadalupe, and Norcasia volcanoes from the Samaná monogenetic volcanic field (SMVF; Borrero et al. 2017; Monsalve et al. 2019; Sánchez-Torres et al. 2019; Sánchez-Torres et al. 2022). The horizontal axis is SiO2 everywhere except for the bottom left diagram

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Las Margaritas volcanoes vs. other monogenetic fields

Las Margaritas volcanoes show similar geochemical behavior in comparison to monogenetic volcanic fields nearby. This is apparent from the positive correlations of K2O, Ba, and Rb vs. SiO2 (Fig. 6). Furthermore, the negative correlations of MgO, V, and CaO/Al2O3 vs. SiO2 show the fractionation process of ferromagnesian minerals principally (Martínez et al. 2014). This is reinforced by the horizontal linear trend in the Nb/Th vs. SiO2 diagram (Fig. 6). Fractional crystallization processes are also evident in the Yb/Dy vs. SiO2 diagram (Fig. 6), where the arrow direction mainly indicates an increase in amphibole fractionation (cf. Sánchez-Torres et al. 2022). Moreover, the positive Eu anomaly found in some volcanoes of the Samaná and Villamaría-Termales fields (Fig. 6) can indicate plagioclase accumulation at more superficial levels, while the negative Eu anomaly found in the Las Margaritas volcanoes can indicate plagioclase fractionation.

The less evolved volcanoes in the region are those of the Pijaos field, which have basaltic to basaltic-andesite compositions and high MgO values (SiO2: 52–54 wt.%, MgO >10 wt.%; Velandia et al. 2021). They are followed by the Villamaría-Termales field, which has a basaltic-andesite to dacite composition (SiO2: 56–69 wt.%; Borrero et al. 2009; Botero-Gómez et al. 2018; Murcia et al. 2019), and the Las Margaritas volcanoes which have an andesitic composition (SiO2: 60–61 wt.%, Table 1). The most evolved volcanoes are those of the Samaná field, which range up to dacitic compositions (SiO2: 61–70 wt.%; Borrero et al. 2017; Monsalve et al. 2019; Murcia et al. 2019; Sánchez-Torres et al. 2019, 2022). These different compositions can be explained by different degrees of crystal fractionation, the type of mineral phases that took part in this process for each volcano, and the variable crustal assimilation processes (Cavell 2020).

Compositional variations within each volcano demonstrate individual crystallization histories for each one. Yet they mostly behave as a group on the Nb/Th vs. Nb/Zr diagram, which can indicate that the magmas came from the same crustal magma reservoir (Gómez-Vasconcelos et al. 2020). Therefore, Las Margaritas volcanoes can be related to the magmatic accumulation zone in the crust (20–35 km depth) that has been proposed as feeding the SCVTP, based on seismic tomography (Londoño 2016; Murcia et al. 2019) and supported by geothermobarometric calculations (Salazar-Muñoz et al. 2021; Sánchez-Torres et al. 2022). Reservoirs at lower crustal (33–36 km) or at mid-crustal levels (23–26 km) have also been proposed in other monogenetic fields, such as the Wulanhada Volcanic Field, northern China (Luo et al. 2022).

Fault controls

The volcanic zone where the Las Margaritas units were emplaced is controlled by the N-S Manizales fault and NE-SW El Perro fault system (Fig. 1c), which around the Las Margaritas volcanoes are classified as NE-trending reverse faults. There is also a clear E-W drainage trend in the area (Fig. 1c), which aligns with the E-W elongated morphology of the Margaritas volcanoes. Shear relations and drainage displacements associated can be used to propose the strike-slip movement of the faults in the Las Margaritas area (Fig. 7a). Displacement of the Guacaica river generated by the El Perro fault reveals sinistral movement, and curvature of the Manizales fault generated by the Tapias fault reveals the dextral movement of the latter (Fig. 1c). Thus, the interaction between the El Perro and Tapias faults may have generated a dilatant zone, which allowed magma ascent (Fig. 7b). Alternatively, an en-echelon fault array linked to the E-W compression in the region generated elongated fractures, with a 45° angle to the shear zone, thus allowing magma ascent (Fig. 7c).

Figure 7
figure 7

Structural geological models proposed for the Las Margaritas volcanoes. A Scheme representing the drainages structural control that were used to identify the kinematics. B Proposal 1: scheme representing a dilatancy zone formed by the interaction between the El Perro and Tapias faults, which allowed the magma emplacement. C Proposal 2: scheme representing an en-echelon system linked to the acting E-W compression in the region, which allowed the magma emplacement

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Conclusion

Las Margaritas 1 and 2 are two newly identified monogenetic eruptive centers in the Manizales region of Colombia that add to the list of monogenetic effusive activity involving evolved compositions that are little studied worldwide. In particular, our study highlights the existence of Quaternary volcanism (~800 ka) in a location where it was not recognized prior to this study. The reason why these magmas erupted effusively is still unclear, while mineral textures and the evolved composition suggest fractionation related to magma stagnation. This shows that monogenetic volcanoes are formed not only by rapid ascent of small batches of magma with primitive compositions, but also by magmas taking off from crustal reservoirs.