Geochemistry and petrogenesis of oligocene felsic volcanic rocks from the Pinos Volcanic Complex, Mesa Central, Mexico

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Highlights

  • The PVC is represented by mainly felsic volcanic rocks.

  • Partial melting of the middle-upper continental crust generated the felsic volcanic rocks.

  • Magmatic events occurred in the PVC during middle Oligocene in an extensional regime.

Abstract

The Pinos Volcanic Complex (PVC) is located in the San Luis Potosi Volcanic Field (SLPV) and southern region of the Mesa Central that includes Oligocene rhyolites and trachydacites. These felsic volcanic rocks are characterized by porphyritic textures with a main mineralogical arrangement of phenocrystals of quartz, potassium feldspar (sanidine), plagioclase and biotite embedded in a vitreous matrix with microcrystals of quartz. They display peraluminous, high-silica, alkaline (not peralkaline) composition, with flat heavy rare earth element concentrations and negative Eu anomalies values. They are enriched in large-ion lithophile elements (LILEs; Rb, Ba, K, Th, Ce) relative to highfield-strength elements (HFSE; Nb, Zr, P, Ti). Trace element ratios trend highlights variable degree of partial melting rather than the role of fractional crystallization. The petrogenetic modelling of trace elements reveal that the felsic volcanic rocks are consistent with their formation via partial melting of the middle/upper continental crust. Multi-dimensional tectonic discriminant diagrams and magmatic model indicate an extensional tectonic environment for the genesis of the felsic volcanic rocks.

Introduction

The Mesa Central province (MC) is mainly located at the central−northern Mexico and is bounded to the N and E by the Sierra Madre Oriental (SMOr) and toward the W by the Sierra Madre Occidental (SMOc; Fig. 1a; Nieto-Samaniego et al., 2007). The MC has been defined as an elevated plateau and is divided into two main regions. In the southern region, the topography is higher than 2000 masl (metres above sea level) whereas the northern region displays below 2000 masl. In addition, it provides a geological record from the Mesozoic to Quaternary (Nieto-Samaniego et al., 2007).

The activity of major fault systems during the Cenozoic explains that northern region is characterized by advance stage of erosion with alluvial−lacustrine basins, which shows low magmatic evidence from the Oligocene and Quaternary ages and southern region that is mainly covered by Paleogene−Neogene volcanic rocks with normal faults (Nieto-Samaniego et al., 1996, 1999, 2007).

The main volcanic activity of the MC is concentrated at the southern region (Fig. 1b). Previous studies of Cenozoic volcanic rocks at the southern region of the MC suggest that the occurrence of partial melting and fractional crystallization processes for the formation of the continental crust in an extensional setting originated the felsic rocks in this region (Orozco-Esquivel et al., 2002; Aguillón-Robles et al., 2014; Sieck et al., 2019; Torres-Sánchez et al., 2019). Nevertheless, there are still areas where petrological and geochemical characteristics of the felsic volcanic rocks is still poorly constrained and that is the case of the Pinos Volcanic Complex (PVC).

The PVC is located at the east part of the southern region of the MC, near the Zacatecas and San Luis Potosi cities that is mainly composed of Oligocene felsic volcanic rocks (Aranda-Gómez et al., 2007; Tristán-González et al., 2009). Previous studies performed in the PVC have focused mainly on its geology, geochemistry and mineralization of the Pinos dome (Aguillón-Robles et al., 1996; Rodríguez-Ríos, 1997), K–Ar geochronology and the magnetic polarity of red conglomerates/beds and trachyte (Aranda-Gómez et al., 2007), stratigraphy and K–Ar geochronology of the rhyolites (Tristán-González et al., 2009). A recent study demonstrated that the felsic volcanic rocks originated in a strike -slip fault system (Rodríguez-Ríos et al., 2013). Although the petrogenesis of these felsic volcanic rocks in such an igneous unit is crucial for determining the origin, evolution and tectonic setting detailed studies are lacking. Therefore, a systematic study is necessary to provide a new perspective on the magmatic processes as well as tectonic setting of the PVC.

In this study, we present whole–rock major, trace and rare earth elements and mineral compositions of the felsic volcanic rocks from the PVC. These new data allow us to determine their petrogenesis and constrain the tectonic setting of the PVC in the southern region of the MC.

Section snippets

Geological background

The San Luis Potosi Volcanic Field (SLPV) is located at the southern region of the MC (Fig. 2a). It consists of felsic−mafic volcanism and shows a range from the Eocene to Quaternary. The initial volcanism in the SLPVF began during the Oligocene (~33 Ma) and this event is mainly characterized by high silica rhyolites and large volume ignimbrites. In contrast, the youngest volcanic event at the SLPVF is represented by less voluminous mafic to intermediate lava composition (Aguillón-Robles et

Field relationships

The units of PVC are represented by different lithologies (Fig. 3). The oldest unit corresponds to the Barbechos (Fig. 4a), which displays flow foliations indicating the geometry of the spills. Overlying the previous unit is Barbechos 2, which consists of the trachydacites (Fig. 4b) with vitrophids (Fig. 4c and d) at their base and indicates that the structure was formed by successive lava flows. At the top of the trachydacite, develops foliations up > 60° (such as 34° NE-SW and strike of 65°

Sampling and analytical methods

For the present study, we collected representative felsic volcanic rock samples from the PVC. Modal composition were determined by point counting on thin section using a Leica petrographic microscope and a PELCON Automatic point counter. In order to obtain a representative mode, approximately 1000 points per sample were counted. Modal composition data are summarized in Table 1.

Whole-rock major element analysis for 20 samples was performed by a wavelength dispersive X–ray Fluorescence

Petrography

The petrography analysis reveals that felsic volcanic rocks from the PVC are mainly characterized by porphyritic textures with no presence of vesicles (Table 1). In some samples, it can be observed some degree of Fe–Ti oxides in the vitreous matrix with microcrystals of quartz (Table 1; Fig. 5a−b). The main mineral assemblage consists of phenocrysts of euhedral to subhedral potassium feldspar (12–36%; Fig. 5c−d) that displays diameters of 0.1–1 mm, xenoform of quartz (1–9%; Fig. 5c−d), euhedral

Alteration effects

Values of loss-on-ignition (LOI) for the felsic volcanic rocks from the PVC display a range from 0.57 to 2.80%, except one sample (P-08) that shows high values 4.17% (Table 2). Therefore, the effect of post-emplacement alteration on elemental mobility could be considered for only some samples. The Zr concentration have been considered as an alteration-independent index for geochemical diversity, principally for the significant correlation with other elements and therefore can be used to test

Conclusions

Based on the results and observations the following conclusions can be summarized:

  • 1.

    The PVC is constituted by felsic volcanic components of trachydacite and rhyolite compositions.

  • 2.

    The felsic volcanic rocks from the PVC show porphyritic textures with a main mineral assemblage of phenocrysts of potassium feldspar, quartz, plagioclase and biotites.

  • 3.

    The petrogenetic modelling indicated that the felsic volcanic rocks from the PVC were most likely generated via partial melting of the upper-middle

Declaration of competing interest

This paper has not been published elsewhere, nor is it under consideration of any other journal. It is an original contribution, authors have contributed to its development, and there is no conflict of interest.

Acknowledgement

Both authors (KGAF) and (DTS) are thankful to the National Council of Science and Technology (CONACYT), Mexico for their Master and Doctoral fellowship [grant #932894 and #336677], respectively. SKV is grateful to Newton Advanced Fellowship award–The Royal Society, UK for the grant [NA160116]. We are grateful to Dr. Pradip Singh and an anonymous reviewer for helpful comments on an earlier version of this paper.

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