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Journal of South American Earth Sciences 73 (2017) 65e77

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Journal of South American Earth Sciences

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The transition between shortening and extensional regimes in centralMexico recorded in the tourmaline veins of the Comanja Granite

Edgar Angeles-Moreno a, b, Angel Francisco Nieto-Samaniego a, *,Francisco Jesús Ruiz-Gonz�alez b, Gilles Levresse a, Susana Alicia Alaniz-Alvarez a,María de Jesús Paulina Olmos Moya a, b, Shunshan Xu a, Raúl Miranda-Avil�es c

a Centro de Geociencias, Universidad Nacional Aut�onoma de M�exico, Quer�etaro, Mexicob Posgrado en Ciencias de la Tierra, Centro de Geociencias, Universidad Nacional Aut�onoma de M�exico, Quer�etaro, Mexicoc Departamento de Minas, Metalurgia y Geología, Universidad de Guanajuato, Ex-Hacienda San Matías, C.P. 36020, Guanajuato, Guanajuato, Mexico

a r t i c l e i n f o

Article history:Received 23 September 2016Received in revised form30 November 2016Accepted 1 December 2016Available online 3 December 2016

Keywords:Comanja GraniteTourmaline veinsGranite exhumationUplift Mesa CentralMexican Basin & Range

* Corresponding author. Centro de Geociencias, Unide M�exico, Blvd. Juriquilla No. 3001, Quer�etaro, 7623

E-mail address: afns@geociencias.unam.mx (A.F. N

http://dx.doi.org/10.1016/j.jsames.2016.12.0040895-9811/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

In central Mexico, there is a major angular unconformity separating two lithologic groups. Below theunconformity, the rocks display shortening deformation structures produced by the Laramide orogeny,overprinting those shortening structures there are normal faults related to the Cenozoic Basin and Rangetectonics. Above the unconformity, the rocks are affected only by the Basin and Range tectonics, dis-playing mainly normal faults and in minor amount, lateral-oblique faults. We analyzed the ComanjaGranite in the Sierra de Guanajuato, it is a large pluton that contains tourmaline veins. The Granite lacksof the shortening structures that pervasively affected the Mesozoic host-rocks; for this reason, we inferthat the granite was formed after the main shortening event. We determined that the emplacement ofthe Comanja Granite took place between 51.0 ± 0.3 Ma and 49.5 ± 0.8 Ma, and that the tourmaline veinsof the granite were formed at ~51.0 Ma. At the microscopic scale, the tourmaline veins contain threekinds of tourmaline, called T1, T2, and T3, according to the order of their formation. The T1 tourmalinesare brown colored and appear affected by brittle-ductile (D1) and brittle (D2) deformations. The cata-clasites formed during D2 overprinted the brittle-ductile structures of D1, indicating the transition fromdeeper to shallower levels. In contrast, T2 and T3 tourmalines were not involved in those deformations.At the outcrop scale, we identified two slickensides in the tourmaline veins. The older, related to D1, isstrike-slip with a small thrust component and the younger, related to D2, is normal with obliquecomponents. The T1 tourmalines (which are deformed) were formed before the lateral-thrust faulting,whereas the T2 and T3 tourmalines, which are not deformed, were deposited after the faulting occurredin the veins. Our interpretation is that T1 tourmalines were deposited in the later phases of the ComanjaGranite emplacement, with the minimum principal compressional stress (s3) being vertical, whereas theT2 and T3 were formed during the exhumation process of the Comanja Granite, with the minimumprincipal stress (s3) being horizontal. We infer that the lateral-thrust faulting represents the transitionbetween the Laramide shortening and the Basin and Range extension in the Sierra de Guanajuato. Thereare Paleogene intrusive bodies in the southern Mesa Central that were exhumed before the Oligocene.This observation strongly suggests that the uplift of the Sierra de Guanajuato could represent a wideevent in the southern Mesa Central of Mexico.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

In central Mexico there are two groups of rocks with a major

versidad Nacional Aut�onoma0, Mexico.ieto-Samaniego).

angular unconformity between them (Nieto-Samaniego et al.,2005, 2007). Below the unconformity, pre-Cenozoic rocks aredeformed by shortening. They exhibit structures varying fromschist foliation to folds and thrust. Those deformation structuresare related to at least two shortening events (Martini et al., 2016);the last one is attributed to the Laramide orogeny (Nieto-Samaniego et al., 2007; Trist�an-Gonz�alez et al., 2009; Cu�ellar-

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C�ardenas et al., 2012). Above the unconformity, Cenozoic rocks areaffected by brittle normal faults with some oblique and lateralfaults, which have been related to the Basin and Range tectonics(Henry and Aranda-G�omez, 1992; Nieto-Samaniego et al., 2007;Trist�an-Gonz�alez et al., 2009). The knowledge about the transitionbetween the shortening and extension is very poor; only twostudies are focused on this point: Trist�an-Gonz�alez et al. (2009)presented a study about the tectonomagmatic evolution of thecentral and eastern Mesa Central (MC), focusing on the magmaticevolution more than on the structural or tectonic analysis. Botero-Santa et al. (2015) studied the structural evolution of the northernSierra de Guanajuato. They inferred a transtensional phase arguingthat the intrusion of the Comanja Granite is the evidence, because itrequired a large volume for the emplacement. The stratigraphicrecord of the Mesa Central indicates that emplacement and exhu-mation of some plutonic bodies, like the Comanja Granite, roughlyoccurred during the transition between Laramide shortening andthe Basin and Range extension (Nieto-Samaniego et al., 2007).

In this contribution, we document the age and type of defor-mation occurred in the Sierra de Guanajuato during the transitionbetween shortening and extension. Our study includes a detailedmap of the faults and tourmaline veins in the southeastern zone ofthe Comanja Granite, and also an analysis of mineralogy andstructures observed in the infill veins.

2. Analytical techniques

2.1. Mineral separation

Two dike samples (C2, TC32) and one sample of themain granitebody (GC-08) were analyzed. The samples were crushed, powderedand sieved (200e50 mesh). Heavy mineral fractions with zirconswere obtained by panning. Final mineral separates were hand-picked under a binocular microscope. Zircons were mounted inepoxy resin together with the standard proposed by NationalInstitute of Standards and Technology (NIST), and polished. Laserablation target points were selected after cathodoluminiscencesample recognition in order to identify zircon cores and re-growthzones. 20 to 30 target points were selected in the most externalzones of zircons for dating the latest crystallization phases. Somepoints were located in zircon cores to obtain additional data ofinherited zircons.

2.2. U-Pb age dating

The zircon samples were analyzed at the laser ablation systemfacility at Laboratorio de Estudios Isot�opicos (LEI), of the Centro deGeociencias, Universidad Nacional Aut�onoma de M�exico. The lab-oratory consist of a Resonetics excimer laser ablation workstation(Solari et al., 2010), coupled with a Thermo XII-Serie quadrupoleICPMS. The laser ablationworkstation operates a Coherent LPX 200,193 nm excimer laser and an optical system equipped with a longworking-distance lens with 50e200 mm focus depth. The ablationcell is He-pressurized capable of fast signal uptake and washout.The drill size employed during this workwas of 30 mm, and the drilldepth during the analysis is about 20e25 mm, for a total mass ab-lated during each analysis of ~70e80 ng. Seventeen isotopes arescanned during each analysis including those necessary for U-Pbdating (lead, uranium and thorium), as well as detailed monitoringof major and trace elements such as Si, P, Ti, Zr and REEs. For eachanalysis, 25 s of signal background are monitored, followed by 30 sof signal with laser firing with a frequency of 5 Hz and an energydensity of ~8 J/cm2 on the target. The remnants 25 s are employedfor washout and stage repositioning. A normal experiment involvesthe analysis of natural zircon standards, as well as standard glasses.

NIST standard glass analyses are used to recalculate the zircons Uand Th concentrations. Repeated standard measurements are inturn used for mass-bias correction, as well as for calculation ofdown-hole and drift fractionations.

2.3. Microprobe analysis

Samples described in this paper were collected from variousgenerations of tourmaline veins in the Comanja Granite. Allelemental analyses of tourmaline were obtained from polished thinsections using a CAMECA SX-100 electron microprobe at the Ore-gon State University (Oregon, USA). Analyses were conducted usinga 15 kV accelerating voltage, 10 nA beam current and 5 mm beamdiameter with counting times between 10 and 30 s. Raw data werecorrected using a stoichiometric PAP correction model (Pouchouand Pochoir, 1985) to a suite of natural and synthetic standardsby microprobe software, which also provides estimates of lowerlimit of detection for each element.

3. Geologic context

The Sierra de Guanajuato (SG) is located at the southwesternlimit of the Mesa Central physiographic province, central M�exico(Fig. 1). The first map of the SG was published by Martínez-Reyes(1992). He proposed two groups of lithostratigraphic units: theolder group is Mesozoic and consists of Jurassic-Lower Cretaceousmarine volcano-sedimentary strata, a plutonic suite includingtonalite and diorite, zones of basaltic and doleritic dike swarms,and an outcrop of ultramafic rocks; all of these rocks weredeformed by, at least, two shortening events (e. g., Echegoy�en-S�anchez et al., 1975; Martínez-Reyes, 1992; Lapierre et al., 1992;Ortiz Hernandez et al., 2003; Mortensen et al., 2008; Martiniet al., 2011). Resting unconformably on the volcano-sedimentaryrocks appears La Perlita fossiliferous limestone of Albian age,which was deformed by only one shortening event (Quintero-Legorreta, 1992; Martini et al., 2013). The younger group consistsof continental Cenozoic formations which overlie the Mesozoicrocks by angular unconformity, and are affected by normal faults,indicating an extensional tectonic regime (Botero-Santa et al., 2015;Nieto-Samaniego et al., 2016). The base of the Cenozoic cover is aconglomerate with few intercalated lava flows and pyroclastic de-posits. This conglomerate has been separated into two formations:(a) The Guanajuato Conglomerate that crops out in the GuanajuatoCity (Fig. 1) (Edwards, 1955; Echegoy�en-S�anchez et al., 1970); itsolder age was obtained from a lava flow at 49 ± 1 Ma (K-Ar inplagioclase), which is intercalated towards the lower part of theconglomerate (Aranda-G�omez and McDowell, 1998). Theconglomerate rests under a volcanic unit dated at 33.53 ± 0.48 Ma(Ar-Ar in K-feldespar, Nieto-Samaniego et al., 2016). (b) The secondformation is the Duarte Conglomerate (Martínez-Reyes, 1992;Botero-Santa et al., 2015) that crops out along the SW front of theSierra de Guanajuato. The Duarte conglomerate contains, in thelower part, rhyolitic lava flows of 53.4 ± 0.33 Ma (U-Pb in zircon,LA-ICPMS; Olmos-Moya, 2016) and underlies the Bernalejoandesite of 31.36± 0.24Ma (U-Pb in zircon, LA-ICPMS, Botero-Santaet al., 2015). The Duarte conglomerate and Bernalejo andesite arecovered by a pile of rhyolitic pyroclastic flows, andesite lavas andsome rhyolite flows, which are Oligocene to Miocene in age(Olmos-Moya, 2016).

In the Sierra de Guanajuato is the Comanja Granite. It is a largepluton which intrudes the Mesozoic rocks and rests below theCenozoic volcanic cover. The granite is formed of massive bodiesand dikes, and the composition varies from biotite granite togranodiorite. The mineralogy consists of quartz (Qz) þ K-Feldspar(K-Feld) þ plagioclase (Pl) þ biotite (Bt) and accessory minerals as

Fig. 1. Regional geologic map showing the location of the study area. SMOc Sierra Madre Occidental, MC Mesa Central, SMOr Sierra Madre Oriental, FVTM Transmexican VolcanicBelt, A Aguascalientes, C Comanja, L Mineral de La Luz, D Duarte, G Guanajuato, SMA San Miguel de Allende, FB El Bajío Fault, Gvr Villa de Reyes Graben.

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zircon, apatite and opaque minerals. The cooling age of theComanja Granitewas previously determined at 53.11 ± 0.27Ma (Ar-Ar in biotite, Botero-Santa et al., 2015); similar dates (49.5 ± 1.5 to51 ± 1.3 Ma) were obtained using K-Ar method in biotite by Steinet al. (1994).

4. The Comanja Granite

4.1. Field and petrographic description

The name Comanja Granite was introduced by Echegoy�en-S�anchez et al. (1970). These authors described the unit as a bath-olitic granite body exposed from Comanja to the Mineral de la Luz(Fig. 1). The first complete map of the granite exposures was pub-lished by Martínez-Reyes (1992) and the main descriptions come

from studies in the Comanja area (Yta and Chiodi, 1987; Quintero-Legorreta, 1992; Botero-Santa et al., 2015). The Comanja Graniteforms a ~45 km long and 2e7 kmwide quasi-continuous outcrop ofgranitic rocks (Fig. 1). In the Comanja area, Yta and Chiodi (1987)reported compositional variations from alkaline granite to quartzrich granite, and the presence of alkali-feldspar syenite xenoliths.Quintero-Legorreta (1992) reported a calc-alkaline compositionbased on mineralogy Qz þ K-feld > Pl (albite-oligoclase) with bio-tite as ferromagnesian mineral. Petrographic description of Botero-Santa et al. (2015) in the same area is Qz þ K-feldspar þ oligoclase >> biotite.

In the Comanja Granite it is very noticeably the presence oftourmaline veins with cataclastic or brittle-ductile structures. Also,there are faults or shear zones adjacent to those veins. The tour-maline veins do not appear in the Oligocene to Miocene units, they

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only are emplaced into the granite and in the contact metamorphichalo with the Mesozoic rocks. Because the tourmaline veins wereemplaced just after the intrusion, their internal structures mustregister the earlier phases of deformation of the Comanja Granite.

We studied the Comanja Granite in the zone located around LaEstancia (Fig. 2). There, the exposed area of the granite is roughlyelliptic, 12 km long and 3.5 km wide, with the major axis orientedNW-SE. The exposed area of granite is ca. 30 km2 and themaximumelevation difference is ca. 350 m. The granite intrudes marinemesozoic rocks (the Sierra de Guanajuato volcanosedimentaryComplex) developing locally a metamorphic halo. The granite restsunconformably below andesitic lavas of Miocene age (Fig. 2).

In the center, the Comanja granite is mainly formed of wideintrusive bodies, more or less being homogeneous, which containsome dikes. In contrast, near the borders are abundant dikes. Thewide intrusive bodies are homogeneous, commonly presentstextural variations and without significant changes in the miner-alogical composition. They are leucocratic phaneritic rocks, formedof quartz þ oligoclase þ orthoclase >>biotite. Under the micro-scope the texture is holocrystalline, phaneritic, hypidiomorphic,with quartz (50%) > oligoclase (30%) > orthoclase (20%), the fer-romagnesic mineral is biotite. Acicular tourmaline and zircon wereidentified as accessories.

4.2. Dikes

We grouped all the dikes of the Comanja Granite in four groups,according to the mineralogical composition, texture and the cross-cutting relationships.

The first group is formed of pegmatite dikes of coarse phaneritictexture, they contain miarolitic circular cavities with centimetriccrystals (Fig. 3A). We observed many patches of pegmatite andschlieren structures, both are sub-parallel to the magmatic folia-tion. The mineralogy of the pegmatite dikes is

Fig. 2. Geologic map of the study area. ME Mangas de la Estanc

orthoclase þ quartz þ biotite >> tourmaline.The second group is formed of NW-SE leucocratic aplite dikes,

with fine-grained phaneritic texture with quartz þ K-feldespar þ biotite (Fig. 3B). These dikes were observed cuttingboth the magmatic foliation and the pegmatitic dikes.

The third group is formed of K-rich feldspar dikes. Theycommonly have nuclei of tourmaline and K-feldspar alterationhalos (Fig. 3C, D, 3E). The dikes are formed of K-feldspar(55%) >> quartz (30%) >> oligoclase (10%)þ5% formed of horn-blende, tourmaline, biotite and allanite; as accessory minerals weidentified sphene, ilmenite, apatite and zircon. Frequently, thesedikes are associated with zones of cataclasite formed oftourmaline þ quartz. In some cases, thin dikes could be confusedwith alteration zones around the quartz-tourmaline veins, but indetail, we could see the thin zone of quartz þ K-feldspar that formsthe dike. We observed dikes emplaced in the granite with cata-clastic deformation in the core and ductile deformation in thecontact with the granite. The dikes of the third group cut the dikesof the second group.We dated the sample C2 at 51.0± 0.6Ma (U-Pb,LA-ICPMS in zircon) (Fig. 4A, Table 1 in supplementary data).

Fourth group is formed of green color granodiorite dikes ofporphyric texture; the matrix is aphanitic to cryptocrystalline withchloritic alteration (Fig. 3D). Phenocrysts arequartz þ plagioclase >> K-Feldespar þ hornblende. We identifiedepidote, sphene, zircon and apatite as accessory minerals. The dikeshave disequilibrium textures showing embayed quartz crystal andthe hornblende is replaced by chlorite and muscovite. The grano-diorite dikes cut the tourmaline veins. We dated the sample TC-32 at 49.5 ± 0.8 Ma (U-Pb, LA-ICPMS in zircon) (Fig. 4B, Table 1 insupplementary data).

The first dike group was emplaced to the end of the maingranitic intrusion under semi-plastic conditions, some with diffuseborders. The second group of dikes was emplaced in fractures withwell-defined borders. The third group of dikes represents a minor

ia, SJO San Juan de Otates, OAR Ojo de Agua de Los Reyes.

Fig. 3. Examples of the four groups of dikes. A: Pegmatitic dike in the main granite body. B: Aplite dikes intruding the main granitic body. C: K-rich feldspar dike with tourmalineveins. The tourmaline (dark) and the “horses” (white) into the vein showing brittle-ductile shear structures. D: Detail of K-rich feldspar dike with tourmaline vein. E: K-rich feldspardike with tourmaline veins and alteration zones. F: Detail of granodiorite dike.

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magma pulse associatedwith themain tourmaline event, emplacedunder fragile-ductile conditions. The dikes of the fourth group arehypabyssal bodies emplaced at shallow depth, under brittle con-ditions and without associated tourmaline event.

4.3. The age of the Comanja Granite

Different isotopic ages have been obtained: Múgica-Mondrag�onand Jacobo-Albarr�an (1983) reported two K-Ar ages in biotite,55 ± 4Ma and 58 ± 5Ma; Zimmerman et al. (1990) two K-Ar ages inbiotite of 53 ± 3Ma and 51 ± 1Ma; Stein et al. (1994) reported threeK-Ar ages in biotite 52.9 þ 2.7 Ma, 51 þ 1.3 Ma and 49.5 þ 1.5 Ma;Botero-Santa et al. (2015) reported one Ar-Ar plateau age in biotiteof 53.11 ± 0.27 Ma, one Ar-Ar isochron age in orthoclase of53.63 ± 0.75 Ma, and one U-Pb age of 51.7 þ 0.2/-0.8 Ma using LA-ICPMS in zircon. In our study we dated the sample GC-08 obtaining51.0 ± 0.3 Ma using U-Pb LA-ICPMS in zircon (Fig. 4C, Table 1 insupplementary data).

Using the mean values of the U-Pb ages obtained in our studyand considering the dikes as part of the granite, we calculated that

the Comanja Granite emplacement last 1.5 m.y. If we include theanalytical errors, themaximum emplacement time is 2.6m.y. Theseages span into a reasonable time lapse for the emplacement of alarge pluton as the Comanja Granite (Chesley et al., 1993;Schaltegger et al., 2009).

5. Faults and tourmaline veins in the study area

5.1. The major fault systems

The major fault systems in the study area are the El Bajío faultsystem and the Villa de Reyes Graben (Fig. 1). Both are formed ofCenozoic normal faults and have multiple reactivation phases(Nieto-Samaniego et al., 2007).

The El Bajío fault system is located west of the study area (Fig. 1).The accumulated dip-displacement of the Oligocene units has beenestimated of 1200 m near Le�on, Guanajuato, and the verticaldisplacement of middleMiocene units in El Cubilete range is ca. 500m (Nieto-Samaniego et al., 2007). The system is formed of threesub-systems of faults: (a) A northwest trending arrangement of

Fig. 4. UePb isotopic ages from the Comanja Granite and dikes. The left column shows concordant UePb isotopic analyses of zircons. Central column represent the distribution ofthe analyzed zircons ordered by age; rectangles indicate the zircons used for mean age calculation, the discarded ages appear in grey. In the right column the mean weighted agesobtained from the younger zircons, with discordance <20%, which form a coherent group and have mean weighted age in agreement with the stratigraphic position. Analyticalresults are given in Table 1 (in supplementary data).

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linked fault segments forming a large fault zone that extends fromLe�on to the Presa Duarte. This fault zone constitutes the maincontact between the Mesozoic and Cenozoic rocks (Fig. 2); fielddata collected along this fault correspond to minor associated faultsand show amean strike of N58�Wand dip of 58�SW. (b) The secondsub-system is formed of many overlapped or soft-linked NW in-dividual faults that generally dip to the SW. Some of those struc-tures appear buried by lithologic units recording different phases ofactivity (Fig. 2). (c) The third subsystem does not crop-out, it isburied by the alluvial deposits that fill the El Bajío basin. The ex-istence of this subsystem has been inferred because the volcanicunits in the foot-block are truncated and they appear below the fill-basin deposits, evidencing normal displacement of hundreds ofmeters (Ramos-Leal et al., 2007) (Fig. 2).

The Villa de Reyes Graben is a major NE structure that cuts theSierra de Guanajuato near the study area (Fig. 1). In the central partof this graben the main displacement took place before theMiocene (Trist�an-Gonz�alez,1986). In the intersection zone betweenthe two major systems, the El Bajío faults show well-defined scarpswhereas the morphologic expression of the Villa de Reyes Grabendisappears, indicating that the El Bajío fault displays the latestdisplacements.

5.2. The tourmaline veins

In the study area the Comanja Granite contains tourmaline veins

(or faulted tourmaline veins) which appears in four main localities:One is near La Estancia, the veins trend WNW with dips to the NEand SW; the second locality is to the west of Mesa El Gallo, wherethe veins trend NW-SE to WSW-ENE and dip to the NE (Fig. 2). Thetwo other zones are more discreet. One is located east of the CerroVerde, in that place the faults and veins strike NW-SE with dips tothe SW; the other is located in the northern part of the studied area,the faults and veins strike NW-SE and dip to the NE (Fig. 2). Wedescribe in more detail the outcrops located in La Estancia and tothe west of the Mesa El Gallo.

The observed structures are hydrothermal breccias, veins withfault planes, and anastomosing shear zones. The veins are formedof black tourmaline with fragments of granite and quartz. There arehydrothermal breccias that form subvertical poorly defined planes(Fig. 5A) and a few subhorizontal veins of tourmaline (Fig. 5B). Themost common structures are sheared veins associated with faults,which contain well-defined striated planes (Fig. 5C). Most of thosestructures dip with moderate to high angles (Fig. 5D). The shearedveins exhibit slickensides, breccias, cataclasite, very thin bands ofpseudotachylite and ribbons of quartz (Fig. 5E).

Tourmaline veins were formed close to the end of the ComanjaGranite emplacement and are absent in other lithologic units, withexception of theMesozoic rocks in the contact with the granite. Thetourmaline deposit took place between 51 and 50 Ma because thetourmaline veins are younger than the Comanja Granite (51.0 ± 0.3Ma), and dikes of the fourth group (49.5 ± 0.8 Ma) cut the veins.

Fig. 5. Field examples of tourmaline veins. A: Breccia of tourmaline located near La Estancia. B: subhorizontal tourmaline vein in La Estancia; the circles show zones where youngerfaults displace the tourmaline vein; commonly the subhorizontal veins have no evidence of shear. C and D typical tourmaline veins with slickensides; the veins display cataclasiteand shear structures. E: Tourmaline vein with ductile deformation; a, b and c show details of cataclasite with ductile-deformed strips and recrystallization tails of quartz.

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Thin sections of tourmaline veins show the presence of threetourmaline generations, recognized by the crystal habit, color,texture, crystal overgrowth and the chemical composition deter-mined by electron microprobe (EMP) analysis. The older tourma-lines (T1) are dark brown, anhedral to subhedral and constitute themicrocrystalline groundmass (Fig. 6A and B). Within the shearzones there are cataclasites and angular to subangular granite andquartz fragments (Fig. 6A). They are considered as the older tour-malines because younger tourmaline crystals grow from this browngroundmass, or appear as euhedral crystals above it. Tourmalines ofthe second generation (T2) are brown at low magnifications andshow bluish tonalities at high magnification. Crystals are euhedralto subhedral with acicular habit, filling fractures and commonlygrowing from T1 crystals; also appear within quartz crystals orgranite fragments (Figs. 6B and 5C). Most of the observed T2

crystals are euhedral and do not show evidence of cataclasis. Thetourmaline crystals of the third generation (T3) commonly arelarger than crystals of T1 and T2; most of them are prismatic,euhedral and blue or brown colored (Fig. 6D). The T3 crystalsappear on the quartz, on crystals of previous tourmalines, or ongranite fragments. As the T2 tourmalines, T3 do not show evidenceof cataclasis.

Based on the petrographic study, we selected T1, T2 and T3crystals (28 T1, 16 T2 and 25 T3) for microprobe analysis on thinpolish sections (Table 2 in supplementary data). Data were pro-cessed using WinTcac program (Yavuz et al., 2013) with normali-zation to 6 silicon atoms, based on the formula XY3Z6 (T6O18)(BO3)3V3W proposed by Henry et al. (2011), where:

X ¼ Na1þ, K1þ, Ca2þ or vacancy; Y¼ Feþ2, Mg2þ, Mn2þ, Zn2þ,Ni2þ, Co2þ, Cu2þ, Al3þ, Feþ3, Cr3þ, V3þ, Ti4þ, Li1þ; Z ¼ Al3þ, Feþ3,

Fig. 6. Micrographs of the tourmalines T1, T2 and T3. A: tourmaline vein of T1 deformed by cataclasis. B: Cataclasite tourmaline T1 with crystals of T2 tourmaline growing from T1crystals; note that T2 tourmaline is not affected by the cataclasis. C: Detail of tourmaline T2 growing from crystals of the T1 tourmaline. D: Crystals of tourmaline T3.

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Cr3þ, V3þ, Mg2þ, Fe2þ; T ¼ Si4þ, Al3þ, B3þ; B¼ B3þ; V¼ OH1�, O2�;see the results in Table 3 (in supplementary data).

In Fig. 7A the samples fall in the alkali and X-site vacancy groups.Most of the T1 are alkali tourmalines, whereas T2 and T3 samplesare more scattered in alkali and X-site vacancy groups (Fig. 7A). Thealkali tourmalines are Fe rich and correspond to schorl variety withfew exceptions of dravite composition (Fig. 7B), whereas the tour-malines of the X-site vacancy group are foitite (Fig. 7C).

In Fig. 7B there are samples of the same generation dispersedfrom Fe rich to Mg rich, and there are tourmalines from differentgenerations with similar composition. We suggest that thecompositional superposition among T1, T2 and T3 occurs becauseyounger tourmalines grew from older tourmalines, resulting in theobserved compositional changes during crystal growth. Fig. 7D il-lustrates the phenomenon: A single (T2) crystal sampled in thebase and final parts exhibits different compositions. The lowermostparts have alkali-schorl composition, similar to the T1 tourmalinesbecause crystal grew from a T1 tourmaline. In the tip, the crystalschange to X-vacancy-dravite, which reflects a change of composi-tion of T2 fluid with respect to T1 fluid composition.

In summary, samples show that hydrothermal system respon-sible for the tourmaline veins evolved from alkali Fe-rich phase (T1)to X-vacancy Mg-rich phase (T2). The compositional variations ofT3 in Fig. 3A suggest the third event of tourmalines also grewpartially from T1 and T2 tourmaline crystals. This behavior could beinterpreted like the evolution from proximal-intermediate to distallocation of the hydrothermal system (Pirajno y Smithies, 1992).

6. Deformation events

The progressive shallow levels of deformation were evidencedby the mineralization events and structures observed in the tour-maline veins. The older tourmaline (T1) was deformed by the D1deformation phase under brittle-ductile conditions together with

parts of the granitic host rock. Some crystals of quartz appearsstretched forming ribbons and strips of recrystallization (Fig. 8A).At microscopic scale we observed clasts of T1 tourmaline formingsigma-delta structures, some of them in cataclastic regime andothers with recrystallization tails (Fig. 8B).

During the deformation phase D2, most of the veins weresheared in cataclastic regime, with low or null crystal-plastic strain.Within the veins, the cataclastic zones cut the ductile deformedareas. We identify protocataclasite mainly formed of angular clasts;cataclasite formed of very fine material with angular clasts(Fig. 6A); and small zones of ultracataclasite formed of dark ma-terial with few or without crystals (Fig. 8A). Pseudotachylite ispresent forming very thin bands of black material, which isisotropic under polarized light (Fig. 8A).

The D3 deformation phase produced the observed brittle faultsthat cut the tourmaline veins. This faulting event took place after ofthe deformation that occurred within the tourmaline veins.

The T2 tourmaline is observed without cataclastic deformation,it forms acicular crystals growing from T1 tourmaline (Fig. 6B andC). The last tourmalinization event T3 was less intense than T1 andT2, and represents the final hydrothermal activity related with theComanja Granite. T3 tourmaline is scarce and was not observedparticipating in the cataclastic deformation. The crystals areeuhedral and appear isolated onto T2 and T1 tourmaline crystals(Fig. 7). These relationships permit to reconstruct the followingsequence of events: After the emplacement of the main granitebody, a pneumatolytic-hydrothermal episode formed the F-richtourmaline T1. The presence of undeformed subhorizontal veinsindicates that the hydrothermal fluids did not reach the surface.The D1 deformation phase took place under brittle-ductile condi-tions. The ductile deformation that affects only the quartz indicatesa moderate temperature and low strain rate. The T1 tourmalineveins were intensely sheared, in contrast to the granite that pre-sents scarce faults or shear zones. We interpreted that veins acted

Fig. 7. Composition of the tourmalines. A: most of T1 belong to alkali group whereas most of T2 belong to X-site vacancy group indicating an evolution of the hydrothermal events.B: Li-Fe-Mg tourmaline classification diagram (modified from Hawthorne and Henry, 1999) of the x-vacancy group analysis, where most of the tourmalines are schorl. C: Li-Fe-Mgtourmaline classification diagram (modified from Hawthorne and Henry, 1999) of the alkali group analyzes, most of the tourmalines are foitite; B and C diagrams show the Fe-richcharacter of all tourmalines. D: example of the change in composition along the T2 crystals; from the base (star) to the tip of the crystal (triangle) the composition changes fromalkali to x-vacancy and from Fe-rich to Mg-rich. Plots were elaborated with WinTcac program (Yavuz et al., 2013).

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Fig. 8. Shear structures and fault rocks within the tourmaline veins. A: microscopic shear zone with bands of quartz, cataclasite and pseudotachylite; the ductile-deformed quartzindicates a low rate of deformation, and probably higher temperature than the indicated by the cataclasite; the pseudotachylite is formed at very high strain rate, much more thanthe needed to form cataclasite and ductile-deformed quartz. We interpret that the veins were sheared at different depths. B: delta-type structure with the core formed of T1tourmaline, formed under fragile-ductile conditions. The circle indicates a sigma-type structure of quartz.

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as planes of weakness accommodating most of deformation. Thedikes of the fourth group cut the tourmaline veins, indicating theD1 deformation phase and T1 occurred after the emplacement ofthe main granite body (51.19 ± 0.33 Ma), and before the emplace-ment of the fourth group of dikes (49.5 ± 0.8 Ma). The D1 defor-mation phase in the study area was also inferred from the lateral-thrust slickensides observed in the tourmaline veins of La Estan-cia and Mangas de la Estancia zones. The inversion of these fault-slip data indicates a lateral-thrust fault regime with N35�W prin-cipal shortening (Fig. 9A). We consider this faulting corresponds tothe D1 deformation phase (brittle-ductile) because it is the olddeformation registered in the tourmaline veins. The D1 deforma-tion phase is coeval with the last intrusions of the Comanja Graniteindicating their emplacement was under shortening-

Fig. 9. A: Stereographic projections of tourmaline veins with lateral-thrust slickensides. Thdominant strike-slip displacement of the faults indicate a transpressional regime. B: Girdles owith the main shortening axis near-vertical and the dominant normal-slip displacement of thfrom La Estancia and Mangas de la Estancia; the poles roughly follow a great circle sugges

transpressional regime. These events coincide with the end of theLaramide orogeny deformation in central Mexico, considering theages proposed by Cu�ellar-C�ardenas et al. (2012).

The cataclastic shearing of D2 deformation phase, occurred in abrittle regime, was superposed to the brittle-ductile deformationphase D1. The cataclasitc event did not affect the tourmaline T2, orthe tourmaline event was active after the main cataclasis. It isnoticeably that many pseudotachylite bands were formedcontemporaneously with cataclasites evidencing a very high strainrate. We observed an intense normal faulting superimposed to thelateral-thrust event. Normal faulting affects the same tourmalineveins affected by the lateral-thrust faulting and without exemption,the normal-oblique slickensides overprint the lateral ones. Invert-ing the fault-slip data of La Estancia and Mangas de la Estancia, we

e “beach ball”, the eigenvectors with the main extensional axis near-vertical and thef tourmaline veins with normal-oblique slickensides. The “beach ball”, the eigenvectorse faults indicate an extensional regime. C: Poles and countering of the tourmaline veinsting that all veins rotated around an axis oriented 290.5�/0.8� (trend/plunge).

E. Angeles-Moreno et al. / Journal of South American Earth Sciences 73 (2017) 65e77 75

obtained a normal-oblique fault regime with the principal hori-zontal extension oriented N23�E (Fig. 9). We associate the D2deformation phase with the normal faulting, because occurred onthe same veins affected by the lateral-thrust faulting and before thelast tourmaline T3 that is not affected by the cataclasis. Within theveins, the younger tourmaline T3 is not deformed although normalfaulting continued long after. An extensional phase D3 has beenwell documented in all the Mesa Central of Mexico from Eocene to,at least, the Miocene (Trist�an-Gonz�alez et al., 2009; Nieto-Samaniego et al., 2007).

The sequence of tourmaline / semi-ductilefaulting / tourmaline / brittle faulting suggests that hydrother-mal events were contemporaneous with the granite exhumationprocess. The progressive unloading depressurized the granite andcould produce successive hydrothermal episodes. The hydrother-mal fluids moving throughout fractures and faults at high pressurecould promote displacements by reducing the effective normalstress. Our observations are in agreement with the occurrence offeedback among exhumation, tourmalinization and faulting events(Fig. 10).

7. Discussion

Cu�ellar-C�ardenas et al. (2012) documented that deformation ofthe Laramide Orogeny has been still active after 62 Ma in San Luis

Fig. 10. Proposed evolution of the stress state during the transition from shortening to extediminished until it becomes the minimum compressive stress. si are the principal stresses, astresses acted horizontal in NE-SW direction. The magnitude of the stress ratio (s2�s3)/(s1�condition can be assumed at the end of Laramide orogeny. B: The ending of Laramide shortento the Poisson ratio (n). Granites have Poisson ratios between ca. 0.1 and ca. 0.33 (Gercek, 200change because it is the lithostatic pressure. This situation implies that the stress in NE-SWcompatible with the observed lateral-thrust regime with the s1 stress oriented NW-SE (Fig.and in the whole Mesa Central. This situation implies the stress state evolve until the NE-SW(s3) in C.

Potosí, which is located 100 km to the north of the study area. In themapped area, the older undeformed rocks are rhyolitic lavas of53.4 ± 0.33 Ma intercalated in the Duarte Conglomerate (Olmos-Moya, 2016). Then, the shortening deformation of the Laramideorogeny finished between 62 and 53 Ma (Thanetian-Selandian) inthe Sierra de Guanajuato, giving a post-tectonic character to theComanja Granite.

Some authors proposed Eocene strike-slip faulting in the MCbefore the Oligocene extensional phase (Trist�an-Gonz�alez et al.,2009; Botero-Santa et al., 2015), but nowhere the Eocene lateralfaults have been documented. The strike-slip phase was inferredfrom the displacements observed in outcrops of Mesozoic strata(Trist�an-Gonz�alez et al., 2009), by using paleomagnetic directionsin Eocene and Oligocene rocks (Andreani et al., 2014), or arguingthat a transitional strike-slip regime is needed to change from ashortening regime to extension (Botero-Santa et al., 2015). In thiscontribution we document lateral-thrust faults in the ComanjaGranite that were active during the Ypresian (51 Ma). These faultsrepresent a transient phase between the shortening (s3 vertical)and extensional (s1 vertical) tectonic regimes. The large volumeoccupied for the Comanja Granite, the presence of not-shearedhorizontal tourmaline veins and the results of fault-slip inversion(Fig. 9) are stronger arguments for the granite emplacement withs3 vertical. The magma may have ascended with s3 horizontal, butfor installing the large granite body in the crust, s3 has to change to

nsion in the Sierra de Guanajuato. Our model assumes that tectonic compression wasnt it is assumed s1 > s2 > s3 � 0. A: During the Laramide shortening, the s1 principals3) is > 0.5 due to high vertical stress produced by overburden (Lisle et al., 2006); thising implies that s1 diminishes, the s2 diminish slower because it changes proportional7); we assumed that s2 variates 27% of the s1 variation. The vertical stress (s3) does notdirection (s1 in A) becomes smaller than the NW-SE stress. The new configuration is9). C: The extension regime has been widely documented in the Sierra de Guanajuatoprincipal compression (s1 in A), becomes the minimum principal compressive stress

E. Angeles-Moreno et al. / Journal of South American Earth Sciences 73 (2017) 65e7776

vertical. In the literature, the favoredmechanisms for emplacementof large plutons are laccolith inflation and sill-stacking (e. g.,Vigneresse et al., 1999; Menand, 2011). For both mechanisms thelocal s3 must be vertical (Menand, 2011). The general (regional)tectonic stress regime could be different, but it tends to be changedto vertical s3 due to the intrusion process itself (Vigneresse et al.,1999). It is clear that large plutons can be intruded in differenttectonic regimes; however, a regional s3 vertical is more favorablefor magma accumulation.

Within the Sierra de Guanajuato the extension continued duringthe Oligocene and Miocene. Fig. 9C shows that poles of tourmalineveins fall along a great circle that have a pole oriented 290.5�/0.8�

(trend/plunge). This axis is the line around which the tourmalineveins rotated. Because the tourmaline veins are of Eocene age, weinterpreted this axis as the rotation tilting axes around which thewhole range tilted to the NE throughout the Oligocene-Neogene.

The evolution of the state of stress in the Sierra de Guanajuatoduring the emplacement-exhumation of the Comanja Granite isillustrated in the Fig. 10. For the Laramide shortening regime withthe stress state of Fig. 10A, the vertical stress (sv ¼ lithostaticpressure) is s3. The maximum principal compression s1 is hori-zontal and oriented ~ NE-SW, as has been inferred from the strike ofthrusts and folds mapped in the Laramide deformed rocks (Martiniet al., 2016; Botero-Santa et al., 2015). The stress difference (s1-s3)must be the adequate to touch the envelope line of the Mohr circle.According to Byerlee (1978), the envelope line was drawn using aslope of 0.85, considering a depth lesser than 10 km. The end of theLaramide orogeny implies diminishing of the s1 stress, and s2decreasing due to the Poisson ratio (n) of the granite. The Fig. 10Bshows the situation of strike-slip faulting with a minor thrustcomponent observed in the tourmaline veins. The NE-SW stress(s1) decreases and it is transformed into s2; in this way, we havethe s1 oriented ~ NW-SE, as was obtained from fault-slip inversion(Fig. 9A). As s1 and s2 decreased, the radius of the Mohr circle di-minishes and does not touch the envelope line. The field observa-tions show that lateral-thrust faults was coeval with intensehydrothermal activity responsible of tourmaline veins formation.The presence of hydrothermal fluids in the faults is equivalent tointroduce a pore pressure in the system, and the Mohr circle ismoved to the left. Introducing the pore pressure it is possible theslip on the fault planes (Fig. 10B). The Fig. 10C corresponds to theextensional regime. The magnitudes of s1 and s2 decrease untilthey are lesser than sv. The Mohr circle is small but it can touch theenvelope line with or without pore pressure.

The Comanja Granite is not an isolated case of exhumed intru-sive of Eocene age: in the Sierra de Catorce, S. L. P., there are graniticbodies of ca. 45 Ma (Mascu~nano et al., 2013); in Pe~n�on Blanco, S. L.P., there is a granite dated in 50.94 ± 0.47Ma, with tourmaline veinsvery similar to Comanja Granite (Aranda-G�omez et al., 2007 (Fig. 1).The outcrops of intrusive bodies similar to the Comanja Granitesuggest that the uplift and/or exhumation of the Sierra de Guana-juato could occur in a wide area of the southern Mesa Central ofMexico, although not necessarily at the same time. We mustconsider that the end of the Laramide shortening has not the sameage in the whole MC.

8. Conclusions

The emplacement of the Comanja Granite lasted 1.5e2.4 m y. Itinitiated with the intrusion of the main body at 51.0 ± 0.3 Ma andculminated with the intrusion of the granodiorite dikes at49.5 ± 0.8 Ma. A large amount of tourmaline veins were formed atca. 51.0 Ma coeval with lateral faults with a minor thrustcomponent.

We recognized three phases of tourmaline, T1, T2 and T3 in

chronological order, which vary in composition from Fe-rich alka-line schorl to Mg-rich X-site vacancy tourmaline. We inferred threephases of deformation: D1 took place under brittle-ductile condi-tions and produced lateral faulting with small thrust components.D2 was normal faulting with oblique components that overprintedthe structures of D1 and formed cataclasites and pseudotachylites.Both phases of deformation are recorded within the tourmalineveins and affected only the T1 tourmalines. D3 consists of brittlefaults that cut the tourmaline veins. Our interpretation is that T1tourmalines were formed in the last phases of the emplacement ofthe Comanja Granite, whereas the T2 and T3 were formed after thecataclasis (D2) and during the exhumation process of the ComanjaGranite.

Prior to the intrusion of the granite, the tectonic regime in theMesa Central was horizontal shortening associated with the Lar-amide orogeny. The lateral-thrust faulting (D1) occurred after theLaramide shortening and before the Basin and Range extension. Wepropose that D2 represents the transition between the Laramideand Basin and Range tectonic regimes.

The exhumation of the Comanja Granite initiated at ca. 51 Ma(Ypresian). This event could be awide process in the southernMesaCentral because there are other Paleogene plutons which wereexhumed before the Oligocene.

Acknowledgments

This research was financed by PAPIIT-UNAM project IN104014.We thank Juan Tom�as V�azquez for their valuable assistance insamples preparation. The paper was reviewed by Thierry Calmusand Gabriel Ch�avez, to whom the authors are most grateful.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jsames.2016.12.004.

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