undergraduate thesis tectono-magmatic analysis of the
TRANSCRIPT
UNIVERSITY OF LOS ANDES
FACULTY OF SCIENCES
DEPARTMENT OF GEOSCIENCES
Undergraduate thesis
TECTONO-MAGMATIC ANALYSIS OF THE JUANAMBÚ CANYON, SOUTHWESTERN
COLOMBIAN ANDES.
SOFIA MARGARITA DELGADO BALAGUERA
MAY, 2018
UNIVERSITY OF LOS ANDES
FACULTY OF SCIENCES
DEPARTMENT OF GEOSCIENCES
Undergraduate Thesis
Tesis de Pregrado
TECTONO-MAGMATIC ANALYSIS OF THE JUANAMBÚ CANYON, SOUTHWESTERN
COLOMBIAN ANDES.
Tesis que presenta SOFIA MARGARITA DELGADO BALAGUERA para optar al grado de Geocientífico por la
Universidad de los Andes
Fdo. Sofia Margarita Delgado Balaguera
V°B° del Director V°B° del Codirector
Yamirka Rojas-Agramonte Andrés Ignacio Rodríguez
Bogotá, 21 de Mayo de 2018
AGRADECIMIENTOS
A mi familia que siempre está conmigo, a todas las personas que me acompañaron durante mi
estadía en la universidad y a las que permitieron el desarrollo de este proyecto de grado. Agradezco
a mis directores de tesis Yamirka Rojas-Agramonte y Andrés Ignacio Rodríguez por acompañar el
desarrollo intelectual de este estudio y por confiar en mis capacidades. Especialmente agradezco
el seguimiento y la contribución de los conocimientos de Nicolás Pérez-Consuegra y de Marco
Antonio Rodríguez Ruíz, sin los que no hubiera sido posible realizar este trabajo y por enseñarme
la importancia del método geocientífico e involucrarme con el planteamiento de preguntas de
investigación y la resolución de las mismas.
Gracias al extenso apoyo de José María Jaramillo y a GMas Lab: Laboratorio de Soluciones
Geocientíficas, por instruirme en la preparación de muestras y permitirme realizar los análisis
químicos correspondientes al proyecto. A Ivette Cucunubo, por aguantarme tantos días en el
laboratorio de la universidad y ayudarme con la elaboración de láminas delgadas.
Abstract
This contribution integrates petrological and field relation constrains from the geological record of the
Juanambú River Canyon and provides another important record of the Late Cretaceous to Miocene tectono-
magmatic evolution of the southwestern margin of the Colombian Andes. Furthermore, the first 1:25.000
geological cartographic chart is presented within this project and it could be enhanced and used as a basis
for the territorial planning, the evaluation of natural resources and the recognition of geological risks. Two
intrusive bodies were recognized in the study area: First, a Late Oligocene to Early Miocene phaneritic-
tonalitic rock body mainly composed by plagioclase with complex zonation, variable quantities of mafic
minerals and lesser amounts of quartz. Second, a Middle to Late Miocene porphyritic intermediate to acid
rock intruding the tonalitic bodies and mainly constituted by plagioclase and quartz, and in lesser amounts
hydrated amphibole and biotite filling interstitial spaces. Granitoids are characterized by high contents of
silica 𝑆𝑖𝑂2(66 − 70 𝑤𝑡. %), with a metaluminous to slightly peraluminous composition (𝐴𝑆𝐼 = 0.96 −
1.03). The analyzed samples exhibit a calc-alkaline character and show an intermediate to acid composition,
ranging from granodiorites to granites. Tectonic classification diagrams also suggest a volcanic arc affinity
related to a subduction tectonic setting. Eastward migration of the magmatic arc has been suggested for
northern and central segments of the Colombian Andes, nevertheless, in the southern Juanambú Canyon
region the longitudinal displacement of magmatism is not recognized. The observed spatial framework in the
canyon could be related to a late migration of the magmatism over a stationed axis for southern parts of the
Colombian Andes or to a subduction edge with convex center and concave extremes geometrical
configuration which induced a less prominent slab dip to the southern segment represented by this region.
Resumen
Esta contribución integra relaciones de campo y un análisis petrológico partiendo del registro geológico del Cañón
del Río Juanambú, el cual provee otro importante ejemplo de la evolución tectono-magmática desde el Cretácico
Tardío al Mioceno temprano a lo largo del margen sur occidental de los Andes colombianos. Además, el proyecto
presenta el primer mapa geológico a escala 1:25000 de la zona el cual puede ser mejorado y usado como base para
el planeamiento territorial, la evaluación de recursos naturales y el reconocimiento de riesgos geológicos. Dos
cuerpos de roca se diferenciaron en la zona de estudio: Primero, un cuerpo fanerítico – tonalítico de edad Oligoceno
Tardío a Mioceno temprano, principalmente compuesto por plagioclasa con zonación compleja, cantidades variables
de minerales máficos y cuarzo en menores cantidades. Segundo, un cuerpo porfirítico de intermedio a ácido con
edades de Mioceno Medio a Tardío cuya composición principal se basa en plagioclasa, cuarzo y en menor cantidad
cristales de anfíbol y biotita rellenando espacios intersticiales en la roca. Los granitoides se caracterizan por un alto
contenido de óxido de sílice 𝑆𝑖𝑂2(66 − 70 𝑤𝑡. %) con una composición metaluminosa a ligeramente peraluminosa
(𝐴𝑆𝐼 = 0.96 − 1.03). Las muestras analizadas exhiben un carácter calco-alcalino y muestran una composición
intermedia a ácida, variando desde granodiorita a granito. Los diagramas de clasificación tectónica también sugieren
afinidad de arco volcánico asociado a un ambiente de subducción para estas rocas. Hacia los segmentos central y
más al norte del área trabajada se ha sugerido una migración hacia el este del arco magmático, sin embargo, hacia
el segmento sur en el Cañón del Juanambú no se reconoce un desplazamiento longitudinal del magmatismo. Se
sugiere por tanto que el escenario magmático observado en el cañón puede estar relacionado con una migración
tardía del arco magmático a través de un eje estacionario, o a un eje de subducción con configuración convexa hacia
el centro y cóncava hacia los extremos que induce un buzamiento menos prominente de la placa hacia los bordes.
INDEX
1. Introduction ........................................................................................................................................................... 1
2. Geological Setting .................................................................................................................................................. 4
2.1. Paleogene Magmatism in Western Colombia ..................................................................................................... 7
2.2. Miocene Magmatism in Western Colombia ....................................................................................................... 7
3. Methods and Analytical techniques ........................................................................................................................ 8
3.1. Petrography and Whole rock analyses ............................................................................................................... 8
4. Results ................................................................................................................................................................... 9
4.1. Field Relations ................................................................................................................................................. 9
4.2. Petrography................................................................................................................................................... 15
4.2.1. Porphyritic Intermediate to Acid Composition ........................................................................................... 15
4.2.2. Phaneritic Tonalite Composition ................................................................................................................ 19
4.3. Whole Rock Geochemistry .............................................................................................................................. 20
4.3.1. Major Elements ....................................................................................................................................... 20
5. Discussion ............................................................................................................................................................ 24
5.1. Magma origin ................................................................................................................................................ 24
5.2. Magmatic Evolution ....................................................................................................................................... 25
6. Conclusions .......................................................................................................................................................... 27
7. References cited ................................................................................................................................................... 30
1
1. INTRODUCTION
Geological mapping is the fundamental for territorial planning, the recognition of threats related to geological
events and the modeling of economic resources. Understanding origin, distribution and dynamics of
convergent plate margins is fundamental to decode the continental orogenesis and coupling of subduction
(Jordan et al., 1983). The Paleogene-Neogene Andean geometrical configuration, coupled with its
magmatism and tectonics, are controlled predominantly by variables such as the age of the subducting plate,
the subduction angle, plates convergence rate and thermal variations in the crust (Pilger, 1984). Geological
data suggest that different segments of the western edge of the Andean Cordillera have experienced
decoupled tectonic evolution, associated with changes in these variables (Ramos, 2010; Romeauf et al., 1995)
(Fig. 2). Some subduction zones of the northern active margin in the Andean Cordillera show contrasting
models of convergence dominated by oblique, orthogonal and flat-slab subduction segments (Echeverri et
al., 2015; Taboada et al., 2000; Ramos, 2010; Villagómez and Spikings, 2013; Gutscher et al., 1999). In
Colombia, the temporal and spatial distribution of magmatism together with the sedimentary record has
been controlled by orogenic processes related to the formation of the Northern Andes (Echeverri et al.,
2015). However, the mechanisms, distribution and factors that have controlled the magmatic and orogenic
development of the Andean mountain system during the Cenozoic remain enigmatic.
The Colombian Andes register a continuous magmatic arc activity from Paleozoic to recent times, and it is
recorded in meta-igneous, volcanic and plutonic rocks exposed along the Western and Central cordilleras.
(Aspden et al., 1987; Villagómez et al., 2011). The magmatic and orogenic history of the Andes towards
southwestern Colombia during the Cenozoic has been linked to different theories (Fig. 2), from de occurrence
of a continuous orthogonal subduction of shallow angle during the Miocene (Echeverri et al., 2015) to the
collision/subduction of the submarine cordillera of Carnegie with South America (Villagómez and Spikings,
2013; Gutscher et al., 1999) and changes in the plates directions and rates of convergence (Taboada et al.,
2000; Somoza and Ghidella, 2012). In southwestern Colombia, at the Nariño department (Fig. 3), previous
works in the area identify at least two pulses of magmatism during the Cenozoic, one of them exposed as a
tonalitic intrusion during the Oligocene and other exposed by porphyritic igneous rocks of tonalitic to granitic
composition during the Miocene (Murcia and Cepeda, 1991; Leal-Mejia, 2011). Therefore, the igneous record
arising in this area suggests a continuous magmatism at least from the Late Oligocene to the Miocene related
to the subduction of the younger Nazca Plate beneath the South American plate, after the Farallon plate split
(Aspden et al., 1987; Echeverri et al., 2015).
2
Subduction of shallow angle during the Miocene is proposed by Echeverri et al., (2015) as an orogenic model
for southwestern Colombia to sustain a fore-arc strong subsidence, changes in the intra-arc environment
with increasing deformation and eastern migration of the magmatic arc. On the other hand, Gutscher et al.,
(1999) inferred a continuation of the Carnegie Ridge beneath the southwestern Colombian margin according
to relative plate motions and active subsidence consistent with migration of the ridge during the Miocene.
By contrast, other authors suggested that a Neogene reorientation of convergence direction resulted in an
orthogonal subduction regime due to the break-up of the Farallon plate into the Nazca and Cocos plates that
triggers Cenozoic plutonism associated with the Nazca plate subduction (Taboada et al, 2000; Aspden et al.,
1987; Meschede and Barckhausen; 2000).
Magmatism studies in the northern and central segments of the Colombian Andes have been developed by
different authors (Aspden et al., 1987; Echeverri et al., 2015; Villagómez et al., 2011), whereas in southern
parts the lack of geological studies is evident. The Juanambú River Canyon was selected as the study area due
to its unexplored topography and the lack of geological works. The Juanambú Canyon is a morphological
depression located in southwestern Colombia, at the Nariño department (Fig.1 and 3). The river flows from
east to west until it reaches the Patía River, from which it is a tributary. Also, it actively migrating across its
flood plain and the channel extends longitudinally on the order of 80 km (Guerrero, 2014). The incised
topography of the Juanambú river allows the exposure of Cretaceous bedrocks together with the
sedimentary and magmatic record, and the recent extensive volcanic activity which has contributed to the
modeling of relief in the area (Murcia and Cepeda, 1991; Royo, 1942).
This contribution provides new cartographic, petrographic and geochemical constrains from magmatic units
exposed along of the Juanambú Canyon area. The main goals of this project are the following: To identify the
geological units which give the shape to this poorly constrained morphological depression. To recognize
timing, spatial distribution and if there is a tectonic and geochemical differentiation of the Cenozoic igneous
units. To quantify the depths and temperatures of the plutons to understand better the exhumation
mechanisms and to address the Late Paleogene to Miocene geologic and tectonic history of Southwestern
Colombia as essential part of the Northern Andes submitting it to a critical review of previously works in the
Juanambú River Canyon. Based on these results, the study aims to find a model that best suits the geological
features of the region and discuss around the dominant convergence model in this segment of Southwestern
Colombia.
3
Figure 1. Google Earth Photo of the study area, Juanambú Canyon and nearly municipalities.
Figure 2. Proposed models of convergence in the Southwestern Colombian, Andean Cordillera. A) Proposed
shallow subduction for the Southwestern Colombian margin (Echeverri et al., 2015). B) Inferred positions of
Carnegie Ridge prolongation (Gutscher et al, 1999).
4
2. GEOLOGICAL SETTING
Colombian Andes are made up of three main mountain ranges: The Eastern, Central and Western Cordilleras
with approximate N-S orientation and separated by the inter Andean geomorphologic depressions of the
Magdalena and Cauca-Patía Valleys (Aspden et al., 1987; Jaramillo et al., 2017; Villagómez et al., 2011). The
Central Cordillera includes autochthonous and para-autochthonous terranes constituted by Paleozoic
metamorphic basement rocks of continental and oceanic affinity intruded by numerous Mesozoic magmatic
arc products (Jaramillo et al., 2017; Villagómez et al., 2011; Villagómez and Spikings, 2013). The basement of
the Western Cordillera represents the geological record of the allochthonous oceanic terrane accreted during
the Upper Cretaceous composed by mafic, ultramafic and marine sediments intruded by several Cenozoic
magmatic pulses (Aspden et al., 1987; Villagómez et al., 2011). The Romeral fault system is interpreted as an
extinct subduction zone that separates bedrock of the Central and Western Cordilleras and it is constituted
by the Cauca-Almaguer, Silvia-Pijao and San Jerónimo faults (Murcia and Cepeda, 1991; Aspden et al., 1987;
Rodríguez and Arango, 2013; Chicangana, 2005).
The Colombian Massif is crossed by the Romeral fault system, which separates the bedrocks of the Central
and Western Cordilleras and represents the tectonic limit of the oceanic terranes accreted to the continental
margin. Towards the east of the Romeral fault is exposed a group of graphite schists, slates and quartzites
called the Metamorphic rocks of Buesaco, which has been interpreted as a remnant of the Arquia Complex
by Ruíz-Jiménez et al., 2012. These para-autochthonous complex is defined as a tectonic mélange associated
with collision/subduction processes during the Mesozoic (Rodríguez and Arango 2013; Ruíz -Jiménez et al.,
2012; Villagómez et al., 2011). In addition, the Diabásico Group emerges towards west of the Romeral fault,
constituted by units of basalts, diabases and marine sediments related to a phenomenon of accretion,
subduction and obduction of terranes in the Mesozoic (Murcia and Cepeda, 1991). The Metamorphic rocks
of Buesaco as well as the Diabásico Group are intruded by plutons and Cenozoic subvolcanic rocks and are
unconformably covered by Pliocene to Holocene deposits of volcano-clastic and sedimentary components
(Murcia and Cepeda, 1991) (Fig. 4).
Along the southwestern western margin of Colombia, different authors have reported an eastward migration
of the Paleogene-Neogene magmatic arc (Echeverri et al., 2015; Villagómez et al., 2011). Nonetheless, this
longitudinal Miocene migration remain poorly constrained at the Nariño department segment. Therefore,
the Juanambú canyon area is an important target in order to evaluate previous geological models for the
magmatic evolution of the Colombian Massif.
5
Figure 3. Distribution of the Cenozoic Magmatism and Arquía Complex along the Colombian Western and Central
Cordilleras. The red box locates the study area. CC: Central Cordillera. WC: Western Cordillera. EC: Eastern Cordillera. CP:
Cauca-Patía Depression. MV: Magdalena Valley.
6
Figure 4. Cartographic chart of Juanambú Canyon developed based on field relations on a 1:25.000 scale.
7
2.1 Paleogene Magmatism in Western Colombia
The Paleogene magmatism in Colombia has been described as a continental arc product related to the
subduction of the Farallon plate and subsequent subduction of the younger and buoyant Nazca plate during
the Paleogene-Neogene boundary (Aspden et al., 1987; Taboada et al., 2000; Echeverri et al., 2015; Bayona
et al., 2012). Magmatic activity during the Paleogene in the Colombian Andes has been mainly assigned to
the Central Cordillera. However, evidence of plutonism has been found to a lesser extend towards the
Western Cordillera and the Cauca-Patía basin (Fig. 3). A variety of intrusive rocks crop out in the Western
Cordillera with variation from diorite to tonalite in compositions. They intrude volcano-sedimentary
sequences with Mesozoic-Cenozoic ages, whereas towards the Central mountain range the batholiths and
stocks have fluctuations in composition from granodiorites to tonalites and intruded metamorphic basement
rocks (Aspden et al., 1987; Bayona et al., 2012; Bustamante et al., 2017; Leal-Mejia., 2011). Along the
Juanambú river canyon, intrusive rocks of tonalitic composition intruding Cretaceous rocks of oceanic affinity
are exposed (Fig. 4).
2.2 Miocene Magmatism in Western Colombia
Plutonic bodies in Colombia during the Miocene are exposed along the Western Cordillera, the Cauca-Patía
depression and the Central Cordillera (Fig. 3) and are associated with the subduction of the Nazca Plate
against the South American Plate. Intrusives rocks ranging from diorites to granodiorites are located at the
Central and Western Cordilleras and are intruding the metamorphic basement and rock bodies of oceanic
affinity. In the inter-Andean valley, hypabyssal bodies of diorite and quartz-diorite composition are exposed,
cutting rocks with oceanic affinity (Aspden et al., 1987; Leal-Mejia, 2011). Along the Juanambú river canyon
sub-volcanic and intrusive rocks intrude both the Metamorphic rocks of Buesaco and the Cretaceous rocks
of the Diabásico Group (Fig. 4). Particularly, this magmatism is ascribed to the occidental oceanic
metallogenetic province of Colombia (Salinas et al., 1999) and it is extensively associated with the formation
of important ore deposits along the western margin of the country which are subject of exploration and
mining.
8
3. METHODOLOGY AND ANALYTICAL TECHNIQUES
Field work was executed during 12 days in the Juanambú River Canyon. This study presents the first
approximation to the geological cartography of the region in 1:25000 scale (Fig. 4) taking into account field
observations and the “Mapa geológico de la Plancha 410-La Unión” in 1:100.000 scale (Murcia and Cepeda,
1991). Digitalization of the map was carried out in ArcGIS® for desktop. Several samples were taken during
the field trip; however, ten samples were carefully chosen to perform petrological analyses. Despite of having
the aim to develop EPMA analysis and determine the P-T conditions for two of the selected samples, the
results were not obtained for the final deadline of this project. Table 1 shows the analysis carried out for each
of the samples selected during the study.
3.1 Petrography and Whole rock geochemistry
Seven samples from de Juanambú Canyon intrusive and porphyritic bodies were selected for petrographic
description and four were selected for whole-rock geochemistry. For each petrographic analysis, counting of
300 points in 10X optical microscope magnification was executed at Los Andes University (Colombia) to
determine the modal abundance of minerals. Powdered whole-rock samples were obtained by grinding the
rock in an Agatha mill, major elements were determined by the glass beads method using the proportion of
1 g of powder sample diluted in 7 g of lithium borate (𝐿𝑖2𝐵4𝑂7) with a Bruker S4 Explorer X-ray spectrometer
by X-ray fluorescence (XRF) at the “GMas Lab: Laboratorio de Soluciones Geocientíficas” in Bogotá, Colombia.
The results were obtained as weight percentages of oxides or elements, with an uncertainty of ± 0,02%.
Sample ID N W Petrography XRF
Juanam-3 1,443031 -77,173781 X
Juanam-4 1,448889 -77,182781 X X
Juanam-6 1,442414 -77,173250 X
Juanam-7 1,433050 -77,147439 X X
Juanam-8 1,433050 -77,147439 X X
Juanam-10 1,445150 -77,145269 X
Juanam-12 1,442550 -77,138911 X
Juanam18-24 1,448730 -77,183420 X
Juanam18-30 1,442530 -77138920 X
Juanam18-33 1,455490 -77,148250 X
Table 1. Analyses carried out for each sample and their respective location.
9
1. RESULTS
4.1 Field Relations
Field word was carried out in the Colombian Massif along the Juanambú River Canyon from N1° 25’ to N1°29’
in the Nariño department (Fig. 3 and 4). Wandering along the Canyon allow to reconstruct a geological history
of the units which compose this depression. Basement, volcanic and plutonic units exposed in the region are
mostly deformed showing the tectonic control related to the high-strain zone and the prominent strike-slip
component of the Romeral Fault System. A schematic and generalize reconstruction of field relations and
evidence photos are presented within the study area (Fig. 4.1 to 4.6).
The history is presented starting from the oldest geological unit to the youngest one identified: The geological
units include from the east green and black schists, quartzites and rocks with gneiss texture. This unit is
named the Buesaco Metamorphic rocks (Murcia and Cepeda, 1990) (Fig. 4.1). Towards the west of the unit,
increasing of the deformation and the presence of secondary folds in the schists were observed suggesting
an approach to a fault zone (Fig. 4.1). Some authors attribute a Cambrian-Ordovician age to these rocks
(Murcia and Cepeda, 1991). However, Rodríguez and Arango (2013) presents this unit as part of the Arquía
Complex which is geologically associated to metamorphic rocks with both sedimentary and igneous basic
protoliths and is limited to the east by the Silvia-Pijao Fault and to the west by the Cauca-Almaguer Fault
(Ruíz-Jiménez et al., 2012).
The Metamorphic rocks of Buesaco are found in fault contact with sheared and metamorphized diabases and
basalts of the cretaceous Diabásico Group (Fig. 4.2). Additionally, in some areas this unit presents
interbedded with grey claystones. The Cretaceous rocks in the Juanambú Canyon were interpreted as part of
the allochthonous Calima terrane, product of the Caribbean Large Igneous Province juxtaposed to South
America during late Cretaceous times (Restrepo and Toussaint, 1996; Kerr et al., 1997).
The para-autochthonous Arquía Terrane and the cretaceous Diabásico Group allochthonous terrane are
intruded by plutonic holocrystalline, phaneritic and fine to medium-grained stocks of tonalitic composition,
some outcrops present several groups of joints and are highly altered and weathered. The plutonic rocks
exhibit mafic enclaves supporting they are intruding the Diabásico Group (Fig. 4.3). Furthermore, towards
the regions of El Tambo and La Llanada, near the study area, similar intrusive rocks dated by the whole rock
10
K/Ar method yielded an age of 26.3 ± 1.8 Ma (De Souza et al., 1984) and by zircon U/Pb an age of 23.4 ± 0.5
Ma (Leal-Mejía, 2011), confirming the Late Oligocene-Early Miocene intrusion of these stocks.
The Buesaco Metamorphic rocks, the Diabásico Group and the tonalitic igneous bodies are intruded at the
same time by hypabyssal, holocrystalline and porphyritic stocks ranging from diorite to granodiorite
compositions. Hypabyssal rocks present several groups of joints and in some cases, are highly altered. Also,
in these porphyritic bodies, mafic enclaves are observed and particularly an enclave exhibit a contact
between the tonalitic intrusive unit and the black schist (Fig. 4.4), which shows that the intrusion of the
hypabyssal rocks occurs after the emplacement of the Cretaceous rocks and after the intrusion of the plutonic
tonalitic stocks. In addition, some radiometric dates of similar bodies near this area were reported by K/Ar
method yielded ages of 13 ± 3 Ma (Álvarez and Linares, 1979) and 9.9 ± 0.8 Ma (Leal-Mejía, 2011).
The magmatic record during the Late to Middle Miocene is the previous episode from the Late Neogene to
Quaternary extensive volcanic activity associated to different eruptive sources as the Galeras, Doña Juana,
Morasurco, Ánimas and Bordoncillo volcanoes (Pardo et al., 2018; Murcia and Cepeda, 1991). The
metamorphic basement and the igneous plutonic rocks are unconformably covered by deposits related to
volcanic activity and sedimentation processes (Fig. 4.5 and 4.6). In the region, diluted and concentrated
pyroclastic density current deposits interbedded with debris avalanches and lahars were differentiated
considering previous descriptions of Murcia and Cepeda (1991) and tuff occurrences as El Gigante Tuff
Formation near the Animas Volcano mentioned by Pardo et al., (2018). Lastly, alluvial deposits of interbedded
polymictic conglomerate with claystones and epiclastic deposits associated to erosion and re-working of
some of the volcanoclastic units were found (Murcia and Cepeda, 1991).
11
Figure 5.1 Field reconstruction of the Juanambú Canyon geological sequence. Cretaceous Stage.
Figure 5.2 Field reconstruction of the Juanambú Canyon geological sequence. Cretaceous terranes accretion.
*(Jaramillo et al., 2017).
Thrust Fault*
7 km
7 km
12
Figure 5.3 Field reconstruction of the Juanambú Canyon geological sequence. Early Magmatic Stage.
Figure 5.4 Field reconstruction of the Juanambú Canyon geological sequence. Late Magmatic Stage.
7 km
7 km
13
Figure 5.5 Field reconstruction of the Juanambú Canyon geological sequence. Deposition, erosion and uplift (?).
7 km
14
Figure 5.6 Field reconstruction of the Juanambú Canyon geological sequence. River Incision Stage.
Terrace Deposits
7 km
15
4.2 Petrography
4.2.1 Porphyritic Intermediate to Acid Rocks
Description is based on several samples, showing variations in modal compositions from diorite to
granodiorite (Table 2). These inequigranular rocks intrude the Metamorphic rocks of Buesaco and the
Diabásico group and are in contact with intrusive rocks of tonalitic composition. The crystals size varies from
few millimeters to 1 centimeter and the main composition consists on plagioclase, hornblende and variable
presence of biotite, pyroxenes and quartz embedded in an aphanitic and holocrystalline matrix that
predominates over phenocrysts in most samples, in order of 40% to 60% (Table 2). The main components of
the matrix are plagioclase and in few quantities quartz. In numerous specimens a slight alignment of biotite
and amphibole (Fig. 7-E) is observed indicating the possible structural control of regional faults.
Sample ID Classification Pl Qtz Kfs Amp Bt Px Matrix
Juanam-3 Diorite 29 -- 2 10 7 1 51
Juanam-4 Granodiorite 19 12 3 16 -- -- 50
Juanam-6 Quartz-diorite 24 6 1 12 2 1 54
Juanam-7 Quartz-diorite 28 7 -- 12 12 -- 41
Juanam-8 Quartz-diorite 33 5 -- 11 8 -- 43
Juanam-10 Diorite 22 -- 2 15 -- 3 58
Table 2. Modal composition (volume %) of porphyritic samples analyzed under petrographic microscopy.
Plagioclase
Crystals vary from idiomorphic to sub-idiomorphic and in several samples, present a prismatic habit.
Inclusions of amphibole, pyroxene and biotite are common, polysynthetic twinning is observed and some
periclinal twins can be distinguish. The zoning of the plagioclases is normal and, in some cases, appear
convolute or oscillatory (Fig. 6-B and 7-C). Possible hydrothermal alteration is exhibit as sericite (Fig. 7-F),
oxides and in lesser amounts as potassic feldspar, filling interstitial spaces and veins (Fig. 7-B).
16
Amphibole
Idiomorphic to sub-idiomorphic phenocrystals of green amphibole disseminated in the matrix, as inclusions
in plagioclases (Fig. 7-A) and in some cases altered to biotite. Crystals habit is generally tabular or prismatic.
Hornblende crystals show partially pyroxene core replacement (Fig. 6-C). Grains present highly pleochroism,
simple twinning is observed, and zonation is not very common.
Quartz
Two samples are depleted in quartz, it appears as euhedral (Fig. 6-D) to anhedral crystals of low modal
percentage integrated in the matrix. Monocrystalline sub-idiomorphic to xenomorphic phenocrysts with
rounded faces and right-angle extinction are observed and, in some sections, polycrystalline quartz
aggregates (Fig. 6-E) with inter-granular suture contacts and wavy extinction are presented as product of a
possible deformational event.
Biotite
Idiomorphic to sub-idiomorphic primary phenocrysts disseminated in the matrix with tabular elongated
habit, high birefringence colors and medium pleochroism. Secondary biotite (Fig. 6-A) is observed in some
samples as anhedral smaller grain crystals surrounding plagioclase, replacing amphibole borders or following
a slight alignment along the matrix, and is accompanied by sericite, calcite and potassic feldspar.
Pyroxene
Xenomorphic crystals of small size disseminated in the matrix (Fig. 6-F), as inclusions within plagioclase or
replacing amphibole cores (Fig. 7-D), crystals present parallel extinction, lower pleochroism and in some
cases a prismatic habit with cleavage lines perpendicular to elongated axis.
Opaque minerals
In thin section is possible to differentiate magnetite and hematite crystals disseminated in the sample and as
exsolution in biotite.
17
Figure 6. A) Primary and secondary biotite (Juanam-7). B) Convolute and normal zonation in plagioclases (Juanam-7). C) Plagioclase-
amphibole-biotite intergrowth and pyroxene trapped in an amphibole core as a clot texture (Juanam-7). D) Quartz phenocryst and minerals
of the porphyritic composition (Juanam-7). E) Plagioclase altered to sericite and polycrystalline quartz aggregates (Juanam-6). F) Euhedral
amphiboles and pyroxenes suspended in the matrix (Juanam-10). Pl: Plagioclase. Amp: Amphibole. Qz: Quartz. Px: Pyroxene. Bt1: 1ry
Biotite. Bt2: Secondary Biotite. Ser: Sericite. Cal: Calcite.
18
Figure 7. A) Amphibole inclusion on plagioclase (Juanam-3). B) Hydrothermal alteration to potassic feldspar and biotite
(Juanam-3). C) Convolute zonation in plagioclase and mafic minerals aligned (Juanam-6). D) Pyroxene trapped in amphibole
core with sericite alteration (Juanam-6). E) Aligned amphibole and biotite surrounding plagioclase (Juanam-8). F) Seritization
and polysynthetic twin in plagioclase, opaque mineral (Juanam-10). Pl: Plagioclase. Amp: Amphibole. Qz: Quartz. Px: Pyroxene. Bt1:
1ry Biotite. Bt2: Secondary Biotite. Ser: Sericite. Cal: Calcite.
19
4.2.2 Phaneritic Tonalite Composition
Description is based on sample Juanam-12, the rock was classified as a tonalite following the QAPF
Streckeisen diagram for plutonic rocks. These equigranular rocks intrude Cretaceous rocks of oceanic affinity
belonging to the Diabásico group and basement rocks and are in contact with porphyritic rocks of
intermediate to felsic character. The crystals size varies from few millimeters to 1 centimeter and its main
composition consists on plagioclase and quartz (Fig. 8-B). The rock is holocrystalline and shows a phaneritic
and fine to medium-grained textures. This sample is mainly rich in mafic accessory minerals as biotite and
hornblende and hydrothermal alteration is mainly presented as sericite and secondary biotite. An enclave
mainly composed of amphibole with partially sericitization is observed in this sample (Fig. 8-D).
Sample ID Classification Plagioclase K-Feldspar Quartz Amphibole Biotite
Juanam-12 Tonalite 44,2 3,1 35,3 8,9 8,5
Table 3. Modal composition (volume %) of tonalitic sample analyzed under petrographic microscopy.
Plagioclase
Crystals are idiomorphic to sub-idiomorphic and the habit is short prismatic. The zoning of the plagioclases is
normal and, in some cases, appears convolute. The twinning is in the albite law and polysynthetic twins are
presented too. Phenocrysts are surrounded by fine grained amphiboles and biotite (Fig. 8-A). Inclusions of
mafic minerals are observed, and, in some cases, inclusions show alignment.
Amphibole
Xenomorphic to sub-idiomorphic crystals of green amphibole as inclusions or surrounding plagioclases and
in some cases altered to biotite. Crystals habit is generally tabular or prismatic. Zoning and twinning cannot
be distinguished in this samples because of the grain size. In some cases, amphibole may be altered to
secondary biotite. Mafic enclave is mainly composed of sub-idiomorphic, prismatic, olive-green and high
pleochroic amphiboles.
Quartz
Sample shows xenomorphic crystals and the modal abundance remains lower than plagioclase.
Polycrystalline quartz is observed with inter-granular suture contacts and wavy extinction.
20
Biotite
Xenomorphic to sub-idiomorphic primary phenocrysts surrounding and as inclusions (Fig. 8-C) in plagioclase
is presented with tabular elongated habit, fine grain-size and medium pleochroism. In some cases, secondary
biotite is replacing amphibole or following inclusions alignment.
Opaque minerals
In thin section is possible to differentiate magnetite and hematite crystals disseminated in the sample and as
exsolution in biotite.
Figure 8. A) Plagioclase zonation and surrounded by amphibole and biotite (Juanam-12). B) Mainly minerals of the tonalitic composition
facies (Juanam-12). C) Aligned inclusions of amphibole and biotite in plagioclase (Juanam-12). D) Mafic enclave showing sericitization
(Juanam-12). Pl: Plagioclase. Amp: Amphibole. Qz: Quartz. Px: Pyroxene. Bt1: 1ry Biotite. Bt2: Secondary Biotite. Ser: Sericite. Cal: Calcite.
21
4.3 Whole rock geochemistry
4.3.1 Major Elements
Both the porphyritic and the phaneritic rocks show similar major element geochemistry, samples evidence a
sub-alkaline acid composition and are characterized by high contents of 𝑆𝑖𝑂2 showing variations from 66 −
70 𝑤𝑡. % (calculated at 100% anhydrous basis). The phaneritic sample presents the highest percentage of
silica oxide. With respect to representative chemical values of acid rocks, samples show in general high
contents of 𝑇𝑖𝑂2 (0.247 − 0.399 𝑤𝑡. %), 𝐹𝑒𝑂 (2.28 − 3.73 𝑤𝑡. %), 𝑀𝑔𝑂 (1.35 − 1.92 𝑤𝑡. %) and
𝐶𝑎𝑂 (3.47 − 4.89 𝑤𝑡. %). Rocks present relatively low contents of 𝑀𝑛𝑂 (0.031 − 0.068 𝑤𝑡. %), and
standard contents of 𝐴𝑙2𝑂3 (15,27 − 16,89 𝑤𝑡. %), 𝐾2𝑂 (1.31 − 2.26 𝑤𝑡. %) and 𝑁𝑎2𝑂 (2.73 −
4.66 𝑤𝑡. %) (Table 4). All samples of the porphyritic rocks plot in the total alkalis classification diagram for
plutonic rocks as quartz-diorites/granodiorites and sample of the intrusive phaneritic rock classify as granite
(Fig. 9-A).
Discrimination diagram based on 𝐾2𝑂 and 𝑆𝑖𝑂2 contents plot all the samples in the calc-alkaline field (Fig.
9-B) and this nature is confirmed by the AFM diagram (Fig. 9-D). In the Alumina Saturation Index diagram,
the rocks exhibit values of Alkalinity Index from 𝐴/𝑁𝐾 = 1.6 − 2.3 and Alumina Index from 𝐴/𝐶𝑁𝐾 =
0.96 − 1.03, plotting in metaluminous field except one that reaches the peraluminous space. Sample from
phaneritic acid rock body indicates a peraluminous magma, however, samples from both bodies remain very
close to the metaluminous/peraluminous boundary (Fig. 9-C).
The 𝑅1 vs 𝑅2 show two samples of the porphyritic acid rocks and the phaneritic tonalite rock body plotting
into the Pre-Collision field, conversely, one sample of the hypabyssal rocks falls in the Mantle Fractionates
field and two more of the same nature plot in the Mantle Fractionates – Pre-Collision limit (Fig. 9-E). Leal-
Mejía (2011) defined some intrusive rocks with similar mineral assemblages and geochemical features from
Arboleda and La Llanada municipalities, near the study area, as diorites and quartz-diorites of medium-K calc-
alkaline character and metaluminous affinity related to Pre-plate Collision and Post-Collision uplift tectonic
settings.
22
Figure 9. Diagrams of geochemical classification and discrimination of the samples collected from de Juanambú Canyon igneous
rocks. A) Chemical classification and nomenclature of plutonic rocks based on total alkalis versus silica (TAS) diagram (Cox et al.,
1979). B) K2O versus silica diagram for subdivision of subalkalic rocks (Peccerillo & Taylor, 1976). C) Alumina saturation index
diagram for plutonic rocks (Shand 1943). D) AFM plot (Irvine and Baragar, 1971) for the intrusive rocks of the Juanambú Canyon. E)
R1 vs. R2 geotectonic plot (Batchelor and Bowden, 1985) for the intrusive rocks of the Juanambú Canyon.
23
Figure 10. Discrimination trace diagrams of samples collected in this study. F) Sr-Y plotted against Y diagram (Defant & Drummond, 1990).
G) Rb plotted against SiO2 diagram (Pearce, Harris, & Tindle, 1984). VAG: Volcanic Arc Granites. syn-COL G: Syn-Collision granites.
Sample Juanam-4 Juanam-7 Juanam-10 Juanam18-24 Juanam18-30 Juanam18-33
𝑆𝑖𝑂2 68,908 65,676 68,744 67,927 69,025 66,947
𝑇𝑖𝑂2 0,342 0,327 0,342 0,399 0,247 0,285
𝐴𝑙2𝑂3 15,273 16,575 15,665 15,639 15,907 16,899
𝐹𝑒𝑂 3,209 3,731 3,151 3,704 2,276 2,868
𝑀𝑛𝑂 0,055 0,068 0,051 0,057 0,034 0,031
𝑀𝑔𝑂 1,487 1,92 1,349 1,746 1,79 1,696
𝐶𝑎𝑂 4,078 3,984 4,105 4,515 3,467 4,893
𝑁𝑎2𝑂 3,971 4,438 3,672 2,731 4,601 4,113
𝐾2𝑂 1,795 1,977 2,111 2,263 1,729 1,307
𝑃2𝑂5 0,149 0,18 0,159 0,167 0,092 0,134
Total 96,058 95,145 96,198 95,444 96,892 96,305
Ba 2095,9 3660,1 1852,6 2076,2 2517,5 2684,5
Cr 23,7 46,7 21,3 39,4 48,7 35
Cu 68 52 60,3 105,9 95,6 118,2
Ni 21,8 40,5 23 22,1 47,4 38,2
Pb 0 51,1 0 0 0 41,2
Rb 508,3 615,3 135,4 1183,2 1076,1 0
Sr 809,4 2350,3 500,8 565,1 1720,8 1924,4
Zn 85,6 95,9 209,6 237,8 108,9 66,2
Zr 157,9 157,6 159,1 160,4 160,4 157,9
Y 0 27,2 33,7 21,7 0 0
Table 4. Major and some trace elements compositions of the Juanambú Canyon samples rock (wt. %).
24
Discrimination diagram based on 𝑆𝑟 and 𝑌 contents plot two of the samples in the normal arc magmas field
and one of them plotted outside the classification realms (Fig. 10-F), nevertheless, three of the samples lack
of 𝑌 contents and did not achieve in this plot. On the other hand, the 𝑅𝑏 against 𝑆𝑖𝑂2 diagram yielded two
of the samples in the syn-Collision granite field and one on the Volcanic Arc - Collision granites boundary.
Three of the samples did not achieve in this plot due to both the lack of 𝑅𝑏 and the high contents of this
element which yielded outside the diagram.
5. Discussion
5.1 Magma Origin
Petrographic features observed in several samples as the presence of secondary or late-stage
hydrated minerals such as biotite and hornblende amphibole filling interstitial spaces and the variety
of zonation-types presented by the plagioclases are commonly found in the products associated to
the formation of Cordilleran-type batholiths (Castro et al., 2010). A temperature of more than 1000°C
is necessary to reach an equilibrium between hydrated phases, thus, the presence of water-rich
components suggest an input of water supplied which allows to satisfy the undersaturated content
of the granodiorite and tonalitic magmas composition (Castro et al., 2010). In addition, there is a
geological consensus about the calc-alkaline character of granitoids associated to destructive plate
margins and the origin of Cordilleran batholiths (Sheth, Torres-Alvarado & Verma, 2010; Frost et al.,
2001). Intrusive pulses found in the Juanambú Canyon are the result of magmatic assimilation
processes with volcanic arc affinity in a subduction-related tectonic setting indicated in the Pre-
Collision field diagram. On the other hand, the continent-oceanic collision regime and the contents
of rubidium in the rocks allow to determine and indicate the temporal relations of the magmatic
pulses with the major deformation event. The collisional granitoid rpaleegime indicates at least a syn-
or a post-collision magmatism related to the Farallon and succeeding Nazca Plate subductions.
25
5.2 Magmatic Evolution
Information of the geological constitution of the Juanambú Canyon was presented in this project as the first
detailed cartography in 1:25.000 scale supported by previous maps and field relations determined in the
morphological depression. The geological constraints allow to recognize the canyon as an important target
to develop studies and go deeper into the tectono-magmatic evolution and the dominant styles of subduction
in the northwestern margin of South America. The black schists can be identified as the oldest geological unit
in the area and are interpreted as part of the Late Cretaceous para-autochthonous Arquía Complex (Ruíz-
Jiménez et al., 2012; Rodríguez & Arango, 2013). Towards the west, meta-igneous and meta-sedimentary
rocks of oceanic character are exposed, which according to the literature and previous studies in the Western
Cordillera, belong to the Cretaceous allochthonous Diabásico Group (Murcia & Cepeda, 1991; McCourt et al.,
1984; Marriner & Millward., 1984). Schists show an increasing deformation to the west and previous reports
account for a fault contact between the Diabásico group and the Buesaco Metamorphic rocks (Murcia &
Cepeda, 1991; McCourt et al., 1984; Aspden et al., 1987).
Two subduction zone-related intrusive bodies were identified in the area: First, a tonalitic body which intrude
both components of the basement. Furthermore, the influence of the strike slip component of the Romeral
Fault System in the region could worked as a road to transfer silicic melts to low crustal levels. Previous
studies around the area established Late Oligocene – Early Miocene ages for similar rocks (De Souza et al.,
1984; Leal-Mejía, 2011). Therefore, this pulse is related to a magmatic peak from the subduction of the
Farallon plate and its coeval rupture during the same period (Echeverri et al., 2015). Second, a more dioritic-
granodioritic pulse rises cutting the cretaceous basement and the ancient tonalitic pulse. This intrusive
trapped highly deformed xenolith, as well as enclaves founded in the tonalitic pulse. Particularly, an enclave
which exhibited a contact between the tonalitic intrusive unit and the black schists shows that the intrusion
of the hypabyssal rocks occurs after the emplacement of the Cretaceous basement rocks and after the
intrusion of the plutonic tonalitic stocks. On the other hand, Middle to Late Miocene ages have been
attributed to similar bodies near this area (Álvarez and Linares, 1979; Leal-Mejía, 2011). Thus, the second
magmatic pulse could be part of the magmatic peak emplaced along the Cauca-Patía Valley and the Central
Cordillera due to the youngest Nazca Plate subduction after the Farallon plate break-up (Echeverri et al.,
2015).
26
An exhumation event in the study area was recognized based on the unconformity presented between the
Late-Miocene intrusive rocks and the Quaternary deposits along the Juanambú Canyon. The depth of the
intrusive bodies could be used as a proxy to determine if an exhumation pulse was developed after the Late
Oligocene – Early Miocene intrusion. The P-T emplacement conditions of the rocks were not calculated but
the research question related to an exhumation event remains open and mineral analyses must be
developed.
By means of the spatial distribution, Echeverri et al., (2015) proposed an almost 40 km eastward migration
of the Miocene magmatism in southwestern Colombia. The migration was conceived after the weakening of
the dominant oblique-type subduction beneath southwestern Colombian margin in most of the Oligocene,
which was triggered by the rupture of the Farallon plate and results in the beginning of an orthogonal
subduction type and the subsequent shallowing of the subduction angle during the Miocene, which allow the
eastward displacement of the magmatism. Nevertheless, this work reveals that at least in the Juanambú
Canyon area the eastward migration of the magmatism is poorly constraint, on the basis of spatial
observations where the ancient pulse is intruded by the youngest one. Therefore, the shallowing subduction
model proposed by Echeverri et al., (2015) for southwestern segment of the Colombian margin should be
reevaluated at least in the Juanambú Canyon and other southern segments where a magmatic arc migration
is not presented or is delayed.
On the other hand, the inferred continuation of the Carnegie Ridge proposed by Gutscher et al., (1999) and
the time of influence are widely discussed. The collision and the exhumation rates clouted by the Carnegie
ridge range from 1 to 15 Ma (Shumway, 1954; Villagómez et al., 2011) and in the southwestern margin of
Colombia this model is coupled with relative plate motions and active subsidence consistent with migration
of the ridge during the Miocene (Gutscher et al., 1999). Nevertheless, the geochemical signature of the
magmatic response which has been associated to this aseismic ridge subduction (Bourdon et al., 2003) is not
consistent with those determined for the Juanambú Canyon rocks (Normal Volcanic Arc). Hence more
geochemical studies must be done in order to determine if this southwestern segment of Colombia was
influenced by the collision of the Carnegie Ridge.
However, Echeverri et al. (2015) report in the geological map that at southern parts of its studied segment
and moreover from this work there are some poorly identified Oligocene and Miocene magmatic pulses.
These bodies do not show a longitudinal migration as well. Consequently, another scenario should be
27
evaluated where the eastward migration is not orthogonal but is over a stationed axis and the Juanambú
intrusive rocks and southern ones where emplaced on this axis. This idea could explain the absence of
migration at the south of Colombia. Another idea which could explain the stationary magmatism during the
Miocene is likely to be a geodynamical model proposed by Schellart (2017), where the mantle or the plate
subduction in a range of at least 200 Ma induces a shallowing in the subduction angle taking in consideration
a convex center in the subducted slab and a concave configuration away from it (Fig. 11). The further
geometrical configuration of the slab allows a pronounced shallowing to the center and a less pronounced
slab dip to the end of the edge as in the case of the study segment. In the Juanambú segment the subduction
of the Nazca Plate started at Late Oligocene – Early Miocene times suggesting an earlier phase of the
proposed model. Migration in northern parts of the segment could be earlier than at southern parts of the
Colombian margin. Nevertheless, both ideas stablished above must be better sustained by means of more
field, petrological, geophysical and geodynamical constraints.
6. Conclusions
The geological cartographic chart presented in this project is the first developed in 1:25.000 scale at the
Juanambú River Canyon and it could be enhanced and used as a fundamental basis for the territorial planning,
the evaluation of natural resources and the recognition of geological threats. The identified units at this
segment are another important record of the Late-Cretaceous to Miocene evolution of the Northern Andes.
The intrusive bodies found in the canyon are volcanic arc products associated with a subduction tectonic
setting. Two pulses were identified: First, a tonalitic pulse of Late Oligocene to Early Miocene age associated
to a magmatic peak coeval with the Farallon Plate split. Second, a diorite to granodiorite pulse which intrudes
the previous one during Middle to Late Miocene and is related to another magmatic peak product of the
Nazca orthogonal subduction. In northern segments of the Colombian Andes has been identify a 40-km
eastward migration of the Paleogene-Neogene arc magmatism, whereas in the study region these magmatic
peaks are superimposed. Consequently, in southern segments of the Colombian Andes the Miocene
evolution of the subduction zone must be revaluated. The unconformity observed between the Late-Miocene
intrusive rocks and the Quaternary deposits allow to recognize at least an exhumation event in the study
area which coincides with the Miocene exhumation of the southern Colombian Andes segment. Depths and
temperatures of the plutons were not calculated due to the lack of time to develop the EPMA analyses. The
28
geological features of the Juanambú Canyon allow to determine that the subduction model which best suits
the Late-Paleogene to Neogene tectonic history of southwestern Colombian Andes is the one proposed by
Echeverri et al., (2015), moreover, in southern segments the model must be revaluated to justify the
distribution of the intrusive bodies and the superimposed magmatic peaks.
Figure 11. Geometrical configuration edge evolution for the South American margin since Jurassic Times
proposed by Schellart (2017).
29
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