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INTRODUCTION
Grajales et al. (2020) discussed geochronological and geochemical data from Paleogene arcs developed at three locations of the northwestern South America: Panama-Choco Block, which they called northern Western Cordillera (WC), southern WC, and Central Cordillera (CC), now juxtaposed onto the Colombian Andes (Figure 1). They reported geochemical data from the southern WC and geochronological data already published and discussed by Barbosa-Espitia et al. (2019) (Figure 1). These data were combined with previously published geochemical and geochronological data, and interpreted to result from a single, continuous Paleogene arc along the paleocontinental margin of the Colombian Andes, which formed as a result of the subduction of the Farallon Plate in a transtensional tectonic setting. This paleogeographic and tectonic interpretation contrasts with well-established models for double subduction of the Farallon and Caribbean plates proposed for the origin of Paleogene arc-like rocks in Colombia, Ecuador and Panama (e.g., Cardona et al., 2018, Barbosa-Espitia et al., 2019; Vallejo et al., 2020). The double- subduction hypothesis is based on a large and diverse data set including field work, paleomagnetic, geochronologic, geochemical and isotopic studies (e.g., Restrepo and Toussaint, 1990; Estrada, 1995; Chiaradia, 2009; Pindell and Kennan, 2009; Bayona et al., 2012; Montes et al., 2012; Boschman et al., 2014; Wright et al., 2016; Cardona et al., 2018; Barbosa-Espitia et al., 2019; Montes et al., 2019; Vallejo et al., 2019, 2020). We welcome discussions on the tectonic and paleogeographic evolution of the Northern Andes during the Paleogene to improve our understanding of this complex orogenic system involving interactions of several tectonic plates. However, the profound implications of the model proposed by Grajales et al. (2020) for ore deposits, hydrocarbon prospectivity, and geohazards, requires a careful examination of the presented data and interpretations.
Figure 1 Tectonic setting of the northern Andes showing information not taken into consideration or wrongly interpreted by Grajales et al. (2020) for their paleogeographic and tectonic model: Contemporaneous Paleogene arc-like rocks derived from continental and island arcs, tectonic boundary proposed by Duque-Caro (1990) for the Panama-Choco Block, Cretaceous rocks assumed as Paleogene and included in interpretations, location of samples already published by Barbosa- Espitia et al. (2019) (numbers in the map).
TEMPORAL AND GEOCHEMICAL CONSTRAITNS
Grajales et al. (2020) compiled several whole rock geochemical data and Paleogene geochronological dates from magmatic rocks of the Panama-Choco Block, southern WC (Timbiquí Complex), and CC to establish a temporal and magmatic framework for their work (Grajales et al., 2020, table 4 and supl. mat.) (Figure 1). However, the authors made some mistakes when compiling the data. Grajales et al. (2020) presented geochronological data for the Antioquia, Ibagué, Sabanalarga and Buga batholiths with ages between ~44 and 59 Ma, which they consider crystallization ages obtained by the U/Pb method (Grajales et al., 2020, table 4). However, after a revision of such data it is clear that Grajales et al. (2020) erroneously included 40Ar/39Ar ages, as well as zircon and apatite fission-track ages reported by Villagómez and Spikings (2013) and ANH- UCaldas (2011), as if they were U/Pb. These Ar/Ar and fission-track apparent ages represent post crystallization cooling to temperatures below ~350 ⁰C and cannot be considered as crystallization ages. Although Grajales et al. (2020) presented a corrigendum for these inconsistencies, geochemical data from the Anserma Gabbro, Mistrato Pluton and Buga Batholith are still included in the Paleogene magmatism of the CC (Grajales et al., 2020, figures 4 and 5, suppl. mat.) (Figure 1). The Buga Batholith, as well as the Mistrato Pluton were dated as Cretaceous U/Pb zircon ages; (Pardo-Trujillo et al., 2020; Villagómez et al., 2011), whereas the Anserma Gabbro was dated in the same temporal range using whole rock K/Ar ages (Maya, 1992). The wrong assumption of these plutonic bodies as Paleogene in age, which is clearly incorrect, creates serious conflicts in the interpretation of the tectonic setting of the Northern Andes during Cretaceous and Paleogene time. According to several authors, during Cretaceous and Paleogene time a series of tectonic events took place in the paleocontinental margin of NW South America with consequences for arc magmatism and sedimentary basins infill (e.g., Villagómez et al., 2011; Montes et al., 2019; Pardo-Trujillo et al., 2020; Bayona et al., 2021). Furthermore, Grajales et al. (2020) reported six zircon U-Pb ages (samples 1246-1, 1246-4, 40-006-CPC, 90-005-CPC, 40-007-CPC, CLM-0447-P) already published by Barbosa-Espitia et al. (2019) without citation or citing ANH-GRP (2014) (Grajales et al., 2020, tables 1 and 4). Compiled U/Pb ages for the samples X, XX, APO-0056-LG, CLM-0376-R, CDG-0255-P (Grajales et al., 2020, table 4), are not reported in Cardona et al. (2018), or ANH-GRP (2014), which citation is in fact ANH-GRP (2011); therefore, it is unknown where geochronological data of these samples come from. The incorrect assumptions of Grajales et al. (2020) and lack of care with the data compilation brings confusion instead of shedding light on the origin and evolution of the Paleogene arc magmatism in the Northern Andes.
GEOCHEMISTRY AND ITS RELATIONSHIP WITH MIGRATION AND EMPLACEMENT OF INTRUSIVES
Grajales et al. (2020) used the temporal and apparent compositional similarities of the arc-like rocks of the Panama-Choco Block and Timbiquí Complex (Figure 1) to propose continuation of the Paleogene arc-like magmatism along the entire WC. They argue that all these rocks belong to the Panama-Choco Block, citing Zapata-García and Rodríguez-García (2020). Barbosa-Espitia et al. (2019) suggested that the contemporaneous Paleogene arc-like rocks from the northern WC and southern WC have different origin and cannot be related on the basis of new and preexistent geochemical and isotopic data. Following the simplistic interpretation of Zapata-García and Rodríguez-García (2020), Grajales et al. (2020) proposed that the geochemical differences in arc-like rocks from the Panama-Choco Block and CC are related to subduction of the Farallon plate and basaltic magma emplacement related to tensional fractures. This explanation, however, is inconsistent with the extensive magmatism in the Panama-Choco Block represented by the Paleogene Mandé Batholith, which is a large tholeiitic-calc-alkaline pluton emplaced under low- pressure conditions (Barbosa-Espitia et al., 2019) and separated from the Cretaceous WC rocks by a tectonic boundary (Duque-Caro, 1990) (Figure 1). The Mandé Batholith is similar to plutonic outcrops in Panama and northern Colombia (Acandí Region), which show geochemical signatures akin to intra oceanic arc settings (e.g., Salazar et al., 1991; Wegner et al., 2011; Montes et al., 2012; Cardona et al., 2018; Sánchez-Celis et al., 2018; Barbosa-Espitia et al., 2019). Moreover, Grajales et al. (2020) used Sr/Y vs. MgO diagram to propose emplacement of arc-like rocks within the Panama-Choco Block (northern WC) in a continental border. However, Grajales et al. (2020) did not discuss that the fields in this diagram are a function of the degree of fractional crystallization of minerals within the magma, which in turn depend on pressure. In their diagram, it is evident that fractional crystallization of arc-like rocks from the Panama-Choco Block (northern WC) occurred under lower pressure conditions, given by the decrease of Sr/Y with increasing MgO (Figure 2), which suggest the presence of plagioclase in the magma. This is in stark contrast with the behavior of the southern WC and CC arc-like rocks, which show an increase in Sr/Y when MgO decreases, a feature that indicates the presence of amphibole ± garnet and crystallization under intermediate to high pressure conditions (Figure 2). Contrasting Sr/Y behavior between the Panama-Choco Block and the southern segment of the WC arc-like rocks was already noted by Barbosa-Espitia et al. (2019), who used these geochemical differences along with isotopic data and previous provenance studies within contemporaneous units in Ecuador (Vallejo et al., 2009) to propose a continental arc origin for the igneous suites of the southern WC compared to the island arc origin for the Panama-Choco Block. Geochemical evidence clearly suggests contrasting tectonic settings for Paleogene igneous rocks of the Panama-Choco block and southern portions of the WC.
Figure 2 Sr/Y vs. MgO diagram by Grajales et al. (2020) reinterpreted in this comment. Red arrow represents a decrease in Sr/Y with differentiation due to the presence of plagioclase in the magma, implying crystallization under lower pressure conditions. Blue arrow represents an increase in Sr/Y with differentiation due to the presence of amphibole ± garnet and plagioclase suppression, implying crystallization under intermediate to high-pressure conditions in a thick crust, characteristic of continental arcs. North Western Cordillera samples are from the Panama-Choco Block. Dotted circles represent outliers.
COMPARISON BETWEEN PREVIOUSLY PROPOSED PALEOGEOGRAPHIC AND TECTONIC MODELS FOR THE NORTHERN ANDES AND PANAMA, AND THAT PROPOSED BY GRAJALES ET AL. (2020)
In addition to the rocks studied by Grajales et al. (2020), the paleogeographic and tectonic settings of the Northern Andes during Paleogene times involve arc-like rocks of the costal arc in northern Colombia (Santa Marta and Guajira massifs); San Blas Complex in Panamá; and Silante and Macuchí arcs in Ecuador (Figure 1). All these rocks have been studied by several authors with different techniques including paleomagnetism, provenance, geochemistry, geochronology and isotopic analyses (e.g., Restrepo and Toussaint, 1990; Estrada, 1995; Chiaradia, 2009; Pindell and Kennan, 2009; Vallejo et al., 2009; Cardona et al., 2011; Bayona et al., 2012; Montes et al., 2012, Boschman et al., 2014; Cardona et al., 2014; Salazar et al., 2016; Wright et al., 2016; Cardona et al., 2018; Montes et al., 2019; Vallejo et al., 2019, 2020). As a result of these works, a robust model was proposed and refined in recent studies (Cardona et al., 2018; Barbosa-Espitia et al., 2019) (Figure 3a and 3b). This model suggests that the Paleogene arc-like rocks in the region were generated by subduction of the Caribbean and Farallon plates. According to this model, the N-NW movement of the Caribbean plate resulted in subduction beneath the northern paleocontinental margin of Colombia generating the arc-like rocks in the Central Cordillera, as well as Santa Marta and Guajira massifs (Cardona et al., 2011; Bayona et al., 2012; Cardona et al.,2014, Salazar et al., 2016; Bustamante et al., 2017; Cardona et al.,2018). On the other hand, the subduction of the Farallon plate generated the arc-like rocks of southern Colombia and Ecuador, as well as Panamá, including its accreted sector in northern Colombia (Cardona et al., 2018; Barbosa-Espitia et al., 2019) (Figure 3a and b). In contrast, Grajales et al. (2020) place their data into an in-situ model, without a discussion on how the data support this model or why they discarded the Caribbean and Farallon plates double-subduction model. The Grajales et al. (2020) model implies that the southern sector of the Panama-Choco Block was already accreted to the continental margin during Early Paleogene and that the oblique subduction of the Farallon plate would have generated simultaneous magmatism in the CC and WC (Figure 3c and d). However, this interpretation contradicts thermochronological, structural, and paleomagnetic data from Colombia and Panama suggesting that the Panama-Choco Block was accreted during the Neogene to north western Colombia (MacDonald, 1980; Suter et al., 2008; Farris et al., 2011; Barat et al., 2014; Piedrahita et al., 2017; León et al., 2018). This model neither can explain the Paleogene magmatism in the Santa Marta and Guajira massifs (Cardona et al., 2011, 2014; Salazar et al., 2016), high-pressure metamorphic rocks associated to suture zones reported in western Colombia (Bustamante and Bustamante, 2019; Avellaneda-Jiménez et al., 2020; Bustamante et al., 2020) and faults that bound the Panama-Choco block and southern sector of the WC (Figure 3c). All these data and studies must be considered and fully discussed to construct an integrative-alternative in situ paleogeographic model. Without the consideration of these data, the in situ model proposed by Grajales et al. (2020) lacks support and reliability.
Figure 3 Comparison between double-subduction and single-subduction models for the northern Andes and Panama. a) Previously proposed double-subduction paleogeographic model [after Cardona et al. (2018)] and b) tectonic model [after Barbosa-Espitia et al. (2019)]. c) Single- subduction paleogeographic and d) tectonic model proposed by Grajales et al. (2020). The tectonic model proposed by Grajales et al. (2020) do not take into consideration geographic separation between the Mande Batholith and the Timbiquí Stock. These plutons are separated by hundreds of kilometers and presumably fault bounded, as Grajales et al. (2020) correctly show in the paleogeographic model on c.
The lack of care with the compilation of geochemical and geochronological data and poor discussion of geochemical data presented by Grajales et al. (2020), along with the extensive and diverse set of data presented during the last four decades indicate that a double subduction system controlled by the convergence of the Caribbean and Farallon plates beneath the north Andean block during Paleogene, is the more plausible explanation for the contemporaneous arc-like magmatism, currently located in disparate segments of the northwestern Andes.
CONCLUSIONS
We reviewed the data presented by Grajales et al (2020) and found serious inaccuracies in the compilation and interpretation of the data. In order to propose a new Paleogene paleogeographic and tectonic model for the complex geologic history of the Northern Andes, all published data must be evaluated, and discussed considering the limitations of the data. If the data is not accurately interpretated, then the subsequent models and conclusions will be erroneous. New ideas and interpretations to understand the complex geology of the Northern Andes must be based on consideration and citation of previous works, and use of extant and new data. Models not supported by data bring confusion to relevant aspects of the geologic history of Colombia such as the timing of ore deposit formation, closure time of fore arc basins in western Colombia, and post Paleogene tectonic events such as the collision of the Panama-Choco Block.
