RICE UNIVERSITY
VOLCANIC ROCKS OF THE RIO CAUCA VALLEY, COLOMBIA, S. A.
by
José Maria Jaramillo Universidad Nacional, Medellin, Colombia
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Arts
Thesis Director's Signature:
May, 1976 ABSTRACT
Miocene volcanic and related intrusive rocks have been investigated in the Rio Cauca valley of western Colombia. This valley is formed along the Romeral and related fault zones during tectonic adjustments between the continental plate east of the Romeral fault and dominantly oceanic rocks in the Cordillera Occidental. The Combia Formation is predominantly a sequence of volcanic and pyroclastic rocks formed in a graben-like de¬ pression along the Rio Cauca valley south of Medellin. Flows of the Combia Formation are generally tholeiitic ac¬ cording to most criteria, as are basic dikes that cut the formation. Other shallow intrusive rocks, however, are mostly calcalkaline by the same criteria, and Plio-Pleis- tocene flows in the adjoining Cordillera Central show ideal calcalkaline trends. One dike cutting the Combia Formation shows alkaline tendencies. The close association in time and space of these di¬ verse rocks in the Rio Cauca Valley implies ease of access to volcanic source regions. Furthermore, the crust beneath the Cauca Depression is inferred to be mostly continental on the basis of the diverse chemistry of the
associated suites. Continental crust presumably extends to the western edge of the Cauca Depression, where it ad¬
joins a dominantly oceanic crust of the Cordillera Occidental. ACKNOWLEDGEMENTS
In this work I have received academic advice, criti¬ cism, and assistance from Dr. John J. W. Rogers, thesis advisor. His understanding and interest are fully appre¬ ciated and acknowledged. Drs. R. R. Schwarzer and B, N.
Powell read the manuscript twice, and their criticism and advice was very helpful. Dr. Schwarzer is acknowledged for introducing me to the multivariate approach used in this study. The clustering technique was made available through Dr. J. L. Gevirtz of Interscience, Inc. and Rice University. Expenses for this study were provided by:
The Walter B. Sharp Fund of Rice University; Robert A.
Welch Foundation Grant C-009 to Drs. John A. S. Adams and
John J. W. Rogers; the Latin American Scholarship Program in American Universities; ICETEX, Ministerio de Educacion de Colombia; COLCIENCIAS; Universidad Nacional de Colombia (Sede de Medellin). TABLE OF CONTENTS Page
INTRODUCTION 1
GEOLOGIC SETTING 3 Statement of the Problem 7 ROCKS INVESTIGATED 9 Combia Formation 9
Stratigraphy 19 Petrography 11
Intrusive Rocks of the Cauca Valley (Excluding Dikes) 14
Basic Dikes of the Cauca Valley 19
Nevado del Ruiz and La Olleta Volcanic Rocks 18
Summary 19 CHEMISTRY 21
Methods of Investigation 0 1
Results 22 Discussion 23 Harker Diagrams 23 Alkali-Silica Diagrams 25
AFM Diagrams 26 S102~Fe0*/Mg0 Diagrams 26
Ti02~Fe0*/Mg0 Diagrams 28 MULTIVARIATE ANALYSIS 29
CONCLUSIONS .' 33 List of Figures
1. Major Features of Colombia.
2. Fault zones of Western Colombia, location of combia Formation and Plio-Pleistocene Volcanoes. Distribution of Precambrian massiffs.
3. Map showing the aerial distribution of the Combia Formation, intrusive bodies and dikes. Northern portion of the Cauca Depression.
4. Map showing sample location. (Cauca Depression only).
5. Stratigraphic sections of the combia Formation.
6. photomicrograph. Hyperstene phenocryst with an outer rim of clinopyroxene. combia Formation. Plane po¬ larized light.
7. Photomicrograph. Euhedral hyperstene phenocryst with Pigeonite core, combia Formation. Polarized light.
8. Photomicrograph. Euhedral plagioclase phenocryst in a quartz K- feldspar fine-grained matrix. Cauca Depression intrusive. Polarized light.
9. Photomicrograph. Micrographie intergrowth of quartz and K-feldspar. Groundmass. Cauca Depression intru¬ sive. Polarized light.
10. Photomicrograph. Augite phenocryst with interstitial magnetite crystals. Basic dikes. Quebrada Popala. Plane polarized light.
11. Photomicrograph. Fluidal texture, phenocrysts are Mg-rich olivines. Nevado del Ruiz. Polarized light.
12. Photomicrograph. Strongly oxidized ignimbrite flow. Nevado del Ruiz. Plane polarized light.
13. Harker diagram: combia Formation. 14. Harker diagram; Basic Dikes.
15. Harker diagram; Cauca Depression intrusive rocks.
16. Harker diagram; Nevado del Ruiz, Cordillera central.
17. Alkali-Silica diagram; combia Formation.
18. Alkali-Silica diagram; Basic Dikes.
19. Alkali-Silica diagram; Cauca Depression intrusive rocks.
20. Alkali-Silica diagram; Nevado del Ruiz.
21. AFM diagram; Combia Formation.
22. AFM diagram; Basic Dikes.
23. AFM diagram; Cauca Depression intrusive rocks.
24. AFM diagram; Nevado del Ruiz.
25. Silica- Fe0*/Mg0 diagram; Combia Formation.
26. Silica- Fe0*/Mg0 diagram; Basic Dikes.
27. Silica- FeO*/MgO diagram; Cauca Depression intrusive rocks.
28. Silica- Fe0*/Mg0 diagram; Nevado del Ruiz.
29. Ti02 ~ Feo*/MgO diagram; all rocks investigated.
30. Dendrogram derived from cluster analysis of volcanic rock suites,world wide. LIST OF TABLES
1. X-ray Fluorescence Analysis Operating Conditions,
2. Chemical Analysis.
3. Suites used for Cluster Analysis (Figure 30). 1
INTRODUCTION
The present work is a study of the Combia Formation, its associated intrusive rocks, and several lava flows of the Nevado del Ruiz and La Olleta volcanoes of the Cor¬ dillera Central of Colombia. The Combia Formation occurs along the Valle del Rio Cauca of Western Colombia between the Cordilleras Central and Occidental (Figure 1). This thesis reports the results of field and petrographic studies, chemical analysis for major elements, and a dis¬ cussion of the relationship between petrology and tectonics in the area. The area studied (Figures 2 and 3) encompasses approxi¬ mately 1400 square kilometers. It is situated approximately
30 kilometers south of Medellin and is bounded on the north by the town of Titiribi, on the south by Riosucio, on the west by Jardin, and on the east by Damasco. The volcanoes of Nevado del Ruiz and La Olleta are located on the axis of the Cordillera Central west of Manizales, between the
Departments of Caldas and Tolima. Previous work on the Combia Formation was carried out by Grosse (1926), who published an extensive description of its stratigraphy, petrography, and field relationships.
Duran (1937) published two chemical analyses of samples collected by Grosse. Van der Hammen (1958) studied the flora of the sedimentary units of the Combia Formation and Figure 1. Major Features of Colombia. FIGURE 1 2 indicated a Middle Miocene age for the formation. Figure 2. Fault Zones of Western Colombia, location
of Combia Formation and Plio-Pleistocene
Volcanoes* Shades^ areas correspond to Precambrian massiffs. FIGURE 2 3
Geologic Setting
The Combia Formation occurs in a graben-like de¬ pression between the Romeral and Cauca fault zones of west¬ ern Colombia. The Romeral and Cauca faults are major zones of movement separating the Cordilleras Central and Occi¬ dental (Estrada, 1972). To the east of the Romeral fault, the Cordilleras Central and Oriental and the Llanos Orientals are underlain by Precambrian crystalline rocks, which crop out in the eastern portion of the Llanos Orientales but are overlain by lower Paleozoic sedimentary rocks toward the west. The Cordilleras Central and Oriental and related massifs show complex Paleozoic and Early Mesozoic deforma¬ tion around cores of these Precambrian rocks.
To the west of the Cauca Depression is a complex and little-understood terrane consisting largely of sedimentary and volcanic rocks evolved in an oceanic setting (Case et al., 1971). The Romeral fault has, thus, been hypothe¬ sized as having formed along the continental margin against which oceanic material was accumulated (Estrada, 1972). Geophysical evidence (Case et al., 1971) confirms a transi¬ tion from continental to dominantly oceanic crustal charac¬ teristics across this presumed margin. The following summary describes the geology of Colombia in terms of a model of westward accretion of crustal mater¬ ial. The information is obtained from a variety of sources, including a summary of the Andes by Gansser (1973), a summary of the geology of Colombia in a thesis by Estrada 4 (1972), and in the papers of Burgl (196.7) , Radelli (1967), and Campbell (1974), and from geophysical studies by J. E.
Case and co-workers (e.g., Case et al., 1971). Unlike the southern Andes of Peru and Chile that have evolved on Precambrian crust to a present, abnormal, thick¬ ness of 70 km (e.g., Pichler and Zeil, 1972), the northern
Andes have grown laterally by accretion of oceanic material to the Guayana shield. The portion of the Andes for which this accretion is apparent extends northward from the
"Amotape cross", a complex cross-over of the Andean orogen and a major structural feature involving the Carnegie Ridge, the Huacabamba depression, the Amazonas depression, and the
Romanches trench (Gansser, 1973). The lateral growth in the northern Andes of Colombia is relatively well documented, as described below, by the following features; 1) ages of magmatic activity; 2) timing of deformational activity; and 3) patterns of sedimentation. The oldest crystalline rocks of Colombia are found in the high grade metamorphic terranes composing the cores of the Santa Marta (2), Santander (3), Puerto Berrio (4), and Garzon (7) massifs (see figure 2). Rb/Sr and K/Ar ages de¬ termined by MacDonald and Hurley (1969), Pinzon et al.
(1962), Tschans et al. (1969) and Tschans (1974) are in the range of 900 to 1300 m.y. for these rocks. A middle Ordo¬ vician event is recorded in intrusive granitic rocks in the Santander massif and in the Llanos Orientales (data from
Creole Oil Co. cited by Estrada, 1972, and Irving, 1975) . 5 Rifting along the Rio Magdalena and Rio Cesar valleys is indicated by intense magmatism during Triassic to Jurassic time forming alkaline granites, extensive lava flows, and pyroclastic rocks (Irving, 1975, and Radelli, 1967). Epi- zonal granodioritic batholiths were emplaced along the axis of the Cordillera Central by late Cretaceous. (K/Ar dates from Botero, 1963, and Irving, 1975, vary between 69+/- 3 m.y. and 79+/- 3 m.y.). Paleocene to Oligocène plutons are known on the western side of the Cordillera Central and in the Cordillera Occidental (Schwinn, 1969, and Botero, 1969, cited by Irving, 1975).
Folding and metamorphism have also shifted toward the west with time (Campbell, 1974). Radelli (1967) identifies
Caledonian and Hercynian deformational events in the Sierra Nevada, Guajira, Santander, and Quetame massifs, and Barrero et al. (1968) demonstrate folding and metamorphism of tur- bidites and basalts in the Cordillera Central before intrus¬ ion of Triassic granites. Rocks of Mesozoic to Paleocene age in the Cordillera Occidental have been affected by low- grade metamorphism (Campbell, 1974). Sedimentary patterns generally record deeper water sedimentation toward western Colombia within rocks of each recognizable age. Shallow-water marine sediments and con¬ tinental red beds were deposited on the continental plat¬ form east of the Rio Magdalena during the Upper Paleozoic to Triassic and Jurassic time (Burgl, 1967), while deep sea sediments, turbidites, and spilitic basalts were formed 6 on the continental slope and oceanic basin west of the river.
Three facies can be distinguished in the Middle to Upper Cretaceous sea that covered most of the Andes. These facies ranged from epicontinental sediments in the east through miogeosynclinal to eugeosynclinal materials in the west
(Estrada, 1972).
The Middle Cretaceous transgression was terminated by Late Cretaceous at the same time that a deep trench (the Bolivar Geosyncline) was formed west of an embrionic Cor¬ dillera Occidental (Duque, 1972). Fluviatile and lacustrine conditions existed in the Cauca Depression, allowing the formation of thick coal deposits, first in the south, near Cali, and then in the north (Departamentos Caldas, Antioquia, Cordoba, and Sucre) (Radelli, 1967). By Middle Miocene the deep trench (Bolivar Geosyncline) in the west disappeared and became the site of shallow-water deposition (Duque, 1972) while extension and active volcanism took place in the Cauca Depression. This volcanism is represented by the Combia and La Paila formations in the Cauca Depression and the
Honda Formation in the Rio Magdalena valley (Grosse, 1926;
Radelli, 1967; Van Houten and Travis, 1968) (see Figure 2). The main orogenic event of the Colombian Andes appar¬ ently took place at Middle Miocene time, as shown by a sedimentary hiatus in the stratigraphic columns of those
Cenozoic basins which have been studied in some detail (e.g., Duque, 1972; van der Hammen, 1958). From Pliocene 7 to the present the Cordillera Oriental has been continuously uplifted, folded and thrust over the horizontal lying
Paleozoic to Recent sedimentary cover of the Guayana shield, while epirogenic uplifting has taken place to the west,
raising the Cordilleras Central and Occidental to their pre¬ sent size. An active volcanic belt has been formed along
the axis of the Cordillera Central. This shift of the vol¬
canic centers from the Cauca Depression toward the axis of the Cordillera Central, that is from west to east, is at variance with the general picture just drawn that indicates a progressive displacement of the geological activity to¬ ward the west and continuous growth of the continental plate
in the same direction. The volcanoes of the Cordillera Cen¬
tral rise upon newly dissected plateaus, uplifted during the last Andean event, (Plio-Pleistocene) to about 3000 meters. From these plateaus the volcanic cones rise to ca.
5600 meters. The height of the volcanoes is then only about
2000 to 2600 meters. The northern volcanoes (Nevado del Ruiz - Tolima) are built up by andesitic to dacitic lava flows and welded tuffs and their most recent activity has been the eruption of large amounts of volcanic ashes that
today cover most of the Cordillera Central and adjacent re¬
gions (Hermelin, 1975, and personal communication; field trips).
Statement of Problem
The Cauca depression is a structurally complex zone at 8 the western edge of the continental crust in western Colombia. This zone is bounded by two major faults of un¬ certain nature: the Romeral fault to the east, along the base of the Cordillera Central; and the Cauca fault to the west, along the base of the Cordillera Occidental. The purpose of this study was to investigate the petrography and chemical trends exhibited by the Middle to Late Miocene volcanic rocks of the northern part of the Cauca depression to obtain a better understanding of the nature of this zone. The Nevado del Ruiz volcano along the axis of the Cordillera Central was investigated for comparison with the rocks in the Cauca depression. The lack of chemical data for these and other volcanic areas of the northern Andes has pre¬ vented regional comparison of the volcanic activity and interpretation of tectonics based on such information. This study is the first systematic investigation of the petro¬ graphy and chemical composition of the volcanic rocks of the northern Andes Figure 3. Map showing thé aerial distribution of the
Combia Formation, intrusives and dikes.
Northern portion of the Cauca Depression.
Numbers correspond to the following in¬
trusive bodies of the Cauca depressions, in¬
cluding dikes:
1. Alto Corcobado 10. Alto Chelines 2. Sierra Vetas 11. Alto Sillon 3. Sierra Candela 12. Alto Melindres 4. Morro Redondo 13. Cerro Bravo 5. Quebrada Popala Dikes 14. Morro Alegre 6. Cerro Tusa 15. Alto Buenavista 7. Alto La Mina 16. Rio Poblanco Dikes 8. Alto Doraditas 17. Cerro Negro 9. Alto Pueblano 18. Pluton de Marmato -K
lVlNiaiDDO Y « 311 i a » O 3 9
ROCKS INVESTIGATED
Investigations have been made of the Combia Formation, some of the intrusive bodies of the Cauca valley, basic dikes of the Cauca valley, and a suite collected from the
Nevado del Ruiz volcano of the Cordillera Central. Samples were collected during two months of field work in the summer of 1974 (see locations shown on Figure 4). One week was spent collecting in the Nevado del Ruiz and La Olleta areas (Figure 2). Conventional petrographic methods were used in the in¬ vestigation of samples. Reflected light studies of opaque minerals were not made. Plagioclase compositions were de¬ termined by Michel-Levy methods for crystals that showed sufficient albite twinning for the method to be useful.
X-ray diffraction analyses of selected minerals followed standard procedures for determination of crystallographic spacings. Compositions of plagioclase and pyroxene and the general nature of certain fine-grained groundmasses were roughly estimated by x-ray diffraction of whole-rock pow¬ ders. Combia Formation
The Combia Formation consists of volcanoclastic sedi¬ ments interbedded with basaltic and andesitic lava flows. The formation lies on top of Middle Oligocène to Lower Miocene fluviatile to lacustrine sediments (Antioquia Figure 4. Locations of Samples Studied.
(Cauca Depression only) Riosucio
*■ 10 Formation) deposited in a tjraben like depression within the
Romeral and Cauca fault zones (Figure 2 and 3).
Stratigraphy A generalized stratigraphic section of the Combia For¬ mation is difficult to draw because of the extreme variabil¬ ity and limited lateral extension of the various litholog¬ ical units. Nevertheless, some general characteristics can be extracted from the three individual columns in Figure 5.
First, it appears from these columns and field obser¬ vations that the lower part of the Combia Formation is a cobble size basal conglomerate that lies on top of the upper unit of the Antioquia Formation. The maximum angular discordance between these two formation does not appear to exceed 10 degrees. (The difference in tectonic style of these two formations is caused by the much greater compe¬ tence of the conglomerates, breccias and lava flows of the Combia Formation relative to the ductile shales and thin sandstone beds of the Antioquia Formation). Second the size (5 to 20 cm in diameter) and composi¬ tion (amphibolites, serpentinized harzburgites, diorites, gabros, green schists, cherts) of the clasts of the basal conglomerate indicates derivation from exposed pre-Oligocene basement of the Antioquia Formation in the general region of the Combia outcrop area.
Third, this conglomerate is overlain by a complex se¬ quence of interbedded lava flows, mud flows, tuffs, aglom- Figure 5. Stratigraphie Sections of Combia Formation. GOO v v Lovu How
\a o o C o ngf ornorcifo
[> o Q O o O S00 o o <> Conglomérat*
lava f I o v/
100
Breed a
000
Lova flows • Broccia 200
Breccia T uff Broccia 100 Tuff _ . • Broccia Tuff Tuff Lava flow Co.i'jlonorolo
FIGURE 5 0 5 mm j— 1
Figure 7. Photomicrograph. Euhedral hyperstene pheno- cryst with pigeonite core, (monoclinic, 2V less than 30 degrees) Combia Formation. Polarized light. 0 10 mm L
Figure 6. Photomicrograph. Hyperstene phenocryst with
an outer rim of clinopyroxene. Plagioclase
phenocryst with glassy inclusions, left hand.
Combia Formation. Plane polarized light. 11 erates, and very thin beds, sandstones and clays. Each of these lithologic units have very limited lateral extent which indicates that they were formed in a zone of very high relief.
Fourth, the upper, volcanoclastic, portion of the for¬ mation was probably two to three times thicker when depos¬ ited than it is now. This fact can be inferred from the exposure of domes and shallow plugs of intrusive porphyries that crop out above the Combia Formation.
Petrography: As shown in the stratigraphic columns (Figure 5), more than two thirds of the Combia Formation consists of pyroclastic material, including ash, tuff and ignimbrite. This pyroclastic material is intensely weath¬ ered, presumably to such extent that present composition would not be representative of original composition. For this reason, petrographic and chemical studies of the Com¬ bia Formation have concentrated on flow rocks. Flow rocks constitute about one third of the exposed Combia Formation and consist of basalts, basaltic andesites, and andesites interbedded with pyroclastic material. (The nomenclature is based on conventional petrographic classifications: basalts are predominantly mafic and their mafic phenocrysts are mostly pyroxene; basaltic andesites are more felsic than mafic and their mafic phenocrysts are predominantly amphibole; andesites are mostly plagioclase and contain little mafic material). Basalts: Basalts are subdivided into three types de- 12 pending on the variety of the major phenocryst. Plagioclase basalts contain few mafic phenocrysts, and their plagioclase penocrysts are large, generally occurring as glomerocrysts.
Hypersthene basalts and augite basalts contain phenocrysts of those two minerals respectively. Plagioclase is the most conspicuous mineral in all rocks of basaltic composition. Crystals commonly show extreme nor¬ mal and oscillatory zonation, with compositions ranging from
Angg to An^. Crystals in glomerocrysts commonly do not show the extreme zonation found in large single crystals.
Augite occurs as subhedral to anhedral crystals and is the dominant pyroxene in most rocks. Large crystals (in the augite basalts) commonly show some zonation and thin exsolution lamellae.
Hypersthene generally occurs as euhedral crystals, some of which are surrounded by a rim of augite (Figure 6). Broad exsolution lamellae of augite parallel to (001) are common. In some rocks, the hypersthene crystals have a core of pigeonite (Figure 7). Poikilitic textures are also common, with the hypersthene enclosing euhedral to subhedral opaque crystals. Hypersthene does not occur in the ground- mass of most of the basalts of the Combia Formation.
Olivine occurs in a few rocks as small, rounded cry¬ stals.
Opaque minerals are abundant as euhedral crystals in all rocks. No investigation has been made of their miner¬ alogy. 13 The groundmass of the basalts of the Combia Formation consists of small plagioclase laths with augite and fine¬ grained alteration products (apparently former glass).
Opaque minerals are abundant. Hypersthene is present in a few samples. The glass is commonly replaced by palagonitic and other clayey material. Intergranular and intersertal textures are common.
Basaltic Andesites: Basaltic andesites are less abun¬ dant than the basalts. Most samples contain hornblende phenocrysts. The hornblende invariably shows some reaction to biotite and a colorless amphibole plus opaques and con¬ tains inclusions of apatite and opaques. Hypersthene is absent from the basaltic andesites. Plagioclase phenocrysts occur as single crystals, commonly showing a zonation from
Angg to ^35* The groundmass is an interserta; mixture of plagioclase laths, minor augite, and a fine-grained, quartzo-feldspathic material. Andesites: Andesites are the less abundant rocks in the lava flows of the Combia Formation. The rocks are charac¬ terized by relative sodic plagioclase glomerocrysts, some¬ times containing interstitial clinopyroxene crystals, horn¬ blende phenocrysts and biotiote, euhedral quartz phenocrysts are rare. The groundmass consists of a granophiric inter¬ growth of quartz and feldspar. Most andesitic rocks show some signs of propilitization. 14 Intrusive Rocks of the Cauca Valley (Excluding Dikes)
A large number of intrusive bodies are present in the general outcrop area of the Combia Formation. Stratigraphy indicates emplacement slightly later than the Combia Forma¬ tion. Locations of the principal bodies are as follows
(Figure 3);
Occurrence Rock Types laccoliths
Alto Corcobado (1) hornblende andesite Sierra Candela (2) hydrothermally altered andesite Sierra Vetas (3) hydrothermally altered andesite Alto Nudo (4) hornblende andesite
Morro Redondo (5 ) hornblende andesite sills
Alto Doraditas ( 7) basaltic andesite Cerro Tusa (north slope) augite and hypersthene andesites near Sierra Vetas and hydrothermally altered Sierra Candela basaltic andesite
Cerro Braro (north slope) augite andesite domes and small plugs
Cerro Tusa (6) hornblende andesite
Morro Alegre (14) hornblende andesite Alto Chelines (10) biotite andesite
Alto Organos (9) biotite andesite 0 4 mm 1 -, i —j
Figure 8. Photomicrograph. Euhedral plagioclase phenocryst extremely zoned, fine grained
matrix consists of quartz and K-feldspar. Cracks in the plagiocase crystal were pro¬
bably originated during emplacement. Cauca Depression intrusive, Cerro Negro (11).
Polarized light. 0 1 mm i. i
Figure 9. Photomicrograph. Micrographie intergrowth
of quartz and K-feldspar, groundmass.
Cauca Depression intrusive, Polarized light. 15
Occurrence Rock Types domes and small plugs (continued)
Alto Buenavista (15) biotite andesite Alto La Mina (7) hornblende andesite
Alto Silloncito hornblende andesite
Cerro Negro (17) dacite
Farallones (17) dacite volcanic necks Alto Sillon- Melindres (11, 12) strongly altered horn blende-biotite andesite
Cerro Bravo (13) hornblende andesite
The preceding list indicates that hornblende andesite occurs in the largest number of intrusive bodies. Horn¬ blende andesite is also present in the largest bodies, thus causing it to be the dominant rock of the shallow intrusive assemblage. The whole assemblage of intrusive rocks has the following general petrographic characteristics:
Hornblende generally occurs as large, elongated pheno- crysts (as much as 10-15 mm long) or as inclusions in the large plagioclase phenocrysts. The hornblende crystals are commonly rimmed by an alteration zone of colorless amphibole, opaques, and biotite and generally contain inclusions of apatite and opaque minerals. The hornblende pleochroism is brown-pale brown-dark green. Hornblende does not occur 16 in the groundmass of most rocks.
Plagioclase occurs as laths in the groundmass and also as phenocrysts in most rocks. Large crystals show normal and oscillatory zoning (see Figure 8) from AngQ to An^g. Minor clustering is present in some rocks, but glomerocry- stic aggregates are absent.
Hypersthene occurs as euhedral phenocrysts and is common only in relatively mafic rock types, such as the sills of Alto Nudo and Alto Doraditas. Augite is a common groundmass mineral and also occurs as phenocrysts in most rocks, although its abundance is not great. Quartz constitutes up to 10 percent of the more acid plutons in the neighborhood of La Pintada (Cerro Negro and Farallones). Quartz phenocrysts are commonly euhedral and show only minor rounding and no embayment. Irregular ex¬ tinction is developed by cataclasis in the vicinity of the Romeral fault zone (samples SP-2 and SP-3).
The groundmass shows normal variations throughout the assemblage of shallow intrusive rocks. Ophitic textures are found in some of the mafic rocks types, and a grano- phiric matrix is common in the more acid types (see Figure 9). Basic Dikes of the Cauca Valley
Mafic dikes occur in various portions of the Combia outcrop area. The main areas are: Quebrada Popala (north of Bolombolo): The rocks are 0 3 mm J k
Figure 10. Photomicrograph. Augite phenocryst with
interstitial magnetite crystals.
Basic dikes, Quebrada Popala. Plane polarized light. 17 fine-grained diabases consisting of about 70 percent pla- gioclase laths (An^^gQ) an(* verY large, poikilitic, augite with interstitial opaque crystals (Figure 10). The suite consists of a thick (200 m) dike and numerous smaller ones.
The thick dike shows a thin horizontal layering of alterna¬ ting mafic and felsic minerals where it is cut by Popala
Creek in good outcrops just north of Bolombolo. Rio Poblanco (near La Pintada): Porphyritic and aphyric plagioclase-bearing basalts occur in a swarm of dikes of variable thickness (0.5 to 10 m). The plagioclase forms large, euhedral, twinned (albite law) phenocrysts with zoning restricted to an outer rim of more sodic composition.
These phenocrysts commonly contain abundant opaque, and glassy, inclusions. Augite and serpentinized olivine occur as phenocrysts in two samples. The groundmass is subophitic to intersertal and consists of small plagioclase laths in¬ terstitial augite and olivine in a red-to-orange altered matrix. Small, acicular crystals are very common in the groundmass of all rocks (not identifiable by optical methods).
Fine-grained opaque minerals are very abundant in all of these rocks. Alto Combia: Two augite-rich basalt dikes cut the
Combia Formation at this location. The dikes are about 2 m thick and are very vesicular toward their margins. Large augite phenocrysts (15 to 20 mm long) constitute from 30 to 50 percent of the rock: these crystals are euhedral to subhedral, zoned, commonly broken, and show undulatory 18 extinction. Large olivine crystals make up about 5 percent of the rock and are commonly replaced by iddingsite or ser¬ pentine. The groundmass consists of plagioclase, sanidine, clinopyroxene, opaques, and unidentified fine-grained ma¬ terial. Amygdules contain calcite and zeolite.
Nevado del Ruiz and La Pileta Volcanic Rocks
The Nevado del Ruiz and La Olleta volcanoes are the northernmost Quaternary volcanoes of the Andes. Very little has been published yet about their stratigraphy and petrol¬ ogy, and the very brief visit made to them by the present writer provided no opportunity for detailed study. Lava flows, however, were sampled along the roads from the base to the top of the volcanoes, thus providing somewhat of a cross-section through the eruptive rocks.
The rocks of both volcanoes are andesites character¬ ized by great variation in ferromagnesian mineral content. A variety of phenocrysts are set in a groundmass of fluidal, subophitic, intersertal, or hyaline textures consisting of plagioclase laths, both clino- and orthopyroxenes, very fine-grained opaques and glass (Figure 11). Plagioclase is a common phenocryst in all rock types and occurs as single, euhedral, zoned crystals commonly containing glassy inclusions of the groundmass in the cores of the crystals.
Olivine is Mg-rich and occurs as small, euhedral pheno¬ crysts; it is abundant in some rocks and also occurs in a hornblende andesite in a strongly oxidized ignimbrite flow 0 1 mm 1 — ^
Figure 11. Photomicrograph. Fluidal texture, phenocrysts
are Mg-rich olivines. Nevado del Ruiz volcano. Polarized light. 0 3 mm
1 «
Figure 12. Photomicrograph. Strongly oxidized ignimbrite
flow, euhedral olivine crystals are completely covered by iddingsite, dark rim around olivine.
Nevado del Ruiz volcano. Plane polarized light. 19 (Figure 12). Hypersthene forms euhedral to subhedral cry¬ stals with an outer rim of clinopyroxene and is a minor constituent of all rocks except those rich in olivine. Au- gite occurs as small subhedral crystals or as a rim around hypersthene. In one sample from La Olleta, augite crystals are surrounded by acicular hornblende crystals. Hornblende occurs as small, acicular crystals, is commonly zoned, and shows a strong pleochroism from pale brown to reddish brown.
Biotite occurs as small flakes in the most acidic rocks and shows pleochroism from pale brown to brownish red.
Summary
The primary characteristics of the rocks investigated in this study are summarized below: Combia Formation:
occurrence: lava flows in pyroclastic sequence above comglomeratic unit. rock types: basalts, basaltic andesites, andesites, altered ash. textures: porphyritic, glomeroporphyritic. minerals: plagioclase, augite, hypersthene, hornblende, opaques.
remarks: hypersthene shows augite exsolution
lamellae parallel to (001); hornblende is green and has reaction rims.
Intrusive Rocks (except Dikes):
occurrence: laccoliths, plugs, necks, domes, 20 shallow plutons.
rock types: mostly hornblende andesite, some other andesites and dacites. textures: porphyritic.
minerals: plagioclase, augite, hypersthene, hornblende, biotite, quartz, alkali feldspar,
remarks: plagioclase forms single, zoned cry¬ stals; hornblende is green to brown and
shows reaction rims; quartz is euhedral, Basic Dikes: occurrence: wide range in thickness up to 200 m.
rock types: diabase, plagioclase- and pyroxene¬ bearing basalt.
textures: diabasic, porphyritic. minerals: plagioclase, augite, olivine, opaques. Nevado del Ruiz and La Olleta: occurrence: flows from Quaternary volcanoes, rock types: andesites. textures: porphyritic, fluidal, hyaline,
minerals: plagioclase, olivine, hypersthene,
augite, hornblende, biotite, opaques, remarks: olivine is Mg-rich and euhedral; horn¬
blende is pale brown to reddish brown and
forms small, acicular crystals. 21
CHEMISTRY
Methods of Investigation
Chemical investigations were made of samples collected during the 1974 field season. Collected samples consisted of large blocks of rock broken down in order to obtain apparently unweathered cores. The resulting 2- to 3- kilogram samples were crushed in a steel jaw crusher and disk grinder in the Mineral Dressing Laboratory of the U. Nacional in Medellin. An aliquot of the ground material was reduced to -200 mesh powder in a tungsten carbide vial on a Spex Mixer at Rice University.
Major elements, except sodium, were determined by x-ray fluorescence methods at Rice University. Analyses were made of undiluted pressed-powder pellets following pro¬ cedures outlined by Novitsky-Evans (1974). Operating con¬ ditions with the Norelco vacuum spectrometer are shown in Table 1. Calibration curves were prepared primarily from USGS standards, and corrections were made for instrumental drift with an internal standard. Analytical procedures were identical to those discussed by Novitsky-Evans (1974), who also discusses accuracy and precision. Sodium was determined by neutron activation analysis at the Nuclear Science Center of Texas A and M. University. The procedure followed is described by Novitsky-Evans (1974).
Estimated error is + 5 percent of the amount present. 0 • g o O O O o O o o ■H (1) CO CO CO CO cri CO CO B W
g 0 CO CO CO CO to Ü CD (D CD 0 0 0 0 fd >* >H >H a >H » >H >
• • P H H CO a) H H 0 -P » £ £ H £ •iH £ o C 0 0 0 •P o -P 0 •H P H H I—1 0 H £ H •P 0 PM PM pM •Hi PM •H PM H u Ü Ü ns CO CO 0 O o p H 0 rd -P 0 -P a •H co 0 EH EH EH PM cu PM EH -P >i-P Q Q o •H Q ■IH Q rd p 0 W W M PI < PI W U U T3 0 OA 0 fd in in in in m in in i S HI CM i—i H CM H H to 1 1 1 l 1 i I 1 *r| CQ y* in o o O o o O >i 0 « in in H A fd 0 1 EH fd •P CD EM tr> p p p P p P p p u a u O u U o B rd EH
-P 0 0 (D CO CO g -H CM o CM o CD X O CM o CM o o o H O •H H •r| 0 tn fd n w — CO < EH PM a o « 22 Reported water contents are weight loss upon fusion with sodium tungstate.
Results
All chemical data are reported in Table 2. Many of the data are also shown on conventional plots in Figures
13 to 29. All reported and plotted values are in weight percent of the oxide. Total iron is reported as F©2°3 •’•n
Table 2, and recalculation to FeO is made for many of the figures. CHEMICAL ANALYSIS •H ! •H •P U P«4 ftf O U B ftf O £ -H OOCMCNOOOCMOË H O WHChSü^lziBœ CM CO PU H h) PU H h3 PU H h) (1) coro CO « LO X HHHCDtnrtfcMftfcNis PU H LO PU H H PU P) 00 PU 1 H h) 1 PU en H h) i PU 1 H 1 I l 1 I LO CM LO H LO LO LO co LO LO r* en r*- 00 LO LO LO LO LO HO H OLOVOCMCO r- o LO CM LO en H O^ LO H en CO CM O • • • • • • • o en LO en o CO o en H O LO o LO LO OCMLO^OCN^CncO o i> 00 o 00 co CO 00 CM LO • • • • • • • H CM o c- H LO r- 00 H 00 co H LO LO H LO LO O in CMCOHCOCOOO^CO H LO «—i r* 1—1 LO LO o • • • • • • • en LO LO en 00 co 00 00 H LO LO en 00 co H H lO CM en 00 LO i—I H CO O'*H en O00H • • • • • t • LO co 00 CM LO LO CM LO H l> co co CM C0 CM co CM H CO LO OCM • • • • • • • LO 00 H 00 LO 00 00 LO 00 en i— en LO en CM i—1 00 LO co CO • • • • • • t CM CM o LO o O CM LO o en co H LO 00 i—1 CM 00 CO O CM • • • • • • • CM CM 00 CM H CM CM LO CM CM co CM «—i CM en H CM 00 • • • • • • • en CM co CM o c- H no LO O • • en en 00 1—1 en C'¬ LO LO en C'¬ LO en c- o en en H LO 00 en en LO H en en LO en en CTi • • • • ♦ • • o O 00 o o CM « O o O H o o o ftf o r^ r^ O H o LO CM CM • • • • • • • ra H o H H CM r- U CM co O LO CD U LO X co PQ PQ PQ PQ H $ CO CM | (X 1 • l pcî CO EH 1 PU 1 1 1 I O CMCMO B LO HHHO tnrdCMfüCM p en en en co CM H O to H LO o LO en 00 LO HH O00^00H(7iC0LO LOOLOO^OOHCM OfOOOCT>CX)COCMI> LO H CM O co LO • LO LO CM LO CMO^HLOCOOCM LO H OCO^COCOCOCnCD CMCOvOCO^tlOHCO • • • o o 00 CM H o co o en co o 00 o en • LO o • en LO • • • • H LO en 1—1 00 o r- H LO CM r- H LO r- LO H o co oo H • r- LO CO H • H en • • • • * 00 LO co H H O H H H ^en •—i r- o H H CM H H co 00 • H H o en • en CM H • • • • co C- CM co LO O CM Hcoen O oLO CM LO LO H co • CM • co CM O • • • • H r*en en H 00 en LO LO H H O H en en G\ 00 O^(TllO • • • H H LO H CM O CMLO^ • H H co CM • o o en 00 H r» o en r* o CM LO LO 00 H O en 00 o r- co H s o o o H cri 00 • en co o t • r- I* CO Hl en KO O CM LO 00 ^ • « • • O « • o ftf CM O 00 CM ^p0707CMV0v0m CM H ^P r» VO 00 in ao 00 CM VO 00 0 00 <7* CM 0 m ^p l • • • • t • • • • • • rtf CM 1—1 O œ 0 0 O CM 00 «—1 0 •rl P in 1—1 HI H 0 in VO 00 00 00 in r- H H 0 00 CD H 00 VO VO H 00 CM 0 1 • • • • t • • • • • u r- 0 CM 00 07 CD 00 H r- 00 0 ^P i—1 H ao H 0 ^P O VO CM H CM VO ao -p vO 0 in H 00 GO 07 P CO 00 H • 1 • • • • # • • # £3 • < CM 00 H ^p 00 l> H H r- 0 Q in H H CD OHO^OnhO^ O LO Ln^HCM ft ^H<ÿp00^pr-CMH 00 rl Q ID H H O'* *73 0 0 0 H l/l p HH 0 0 •rl in 00 O CM O 00 00 00 CD 0 r- O 1—1 H O r* in •rl 0 P ** 00 vO r* H VO 00 P in 00 CM H CM vo 00 CM VO H P vo r^ X» X 1 • • • • • • • • S3 • • 1 • • • • • • • • S3 • • 0 H O ft CM H VO H 00 H H 00 O ft H 1—1 00 CM in l> H CM ^P 0 0 Q O P in H H CTi + a in H H CD 0 0 0 rtf CM •rl H 0 XI 0 rtf O H ft CM VO O rp 00 a\ 00 07 CM 1—1 CD H VO H VO r^ 00 H 00 CD XI 00 H CM 07 H VO O CM P CM ^P H CM H CD VO vo O P 00 00 rd 0 1 • • • • • • • • Sz; • • 1 • • • • • • • • S3 • • EH •rl CM 00 0 in O VO 07 H CM CTi O ft CM H H VO H CM O Q in H H 0* •f a in H H CD rtf rtf a, H 00 r- O r* 00 0 in H CD o a> r- [•'•oo^ocor-' en œ CM in O in 00 CD 00 CD r- CD O CM ^ 1 I • • • ft CM H vo H • 00 H H O H rtf (N H LO (N ^ h H CN VO O P in H H CD •73 ft IT) H iH CD I rtf U XI 0 13 00 CD 00 CD 0 00 CD ^P r* 07 O r- 07 VO 00 00 0 00 in ^P in m 1—1 CM 0 CD in CD H H CD in vo O 00 H 0 00 00 00 00 O o CM CM 0 * rtf co rtf 0 ro 00 0 co co S3 *d CM CMO O 0 *Ü CM CMO O O . \ •rl O O CM CM O O 0 CMO Ë O •H O O Chemical properties of the rocks investigated are de¬ scribed by means of the diagrams discussed below: Harker diagrams: Figures 13 to 16 show Harker diagrams for the following suites: Combia Formation (Figure 13), basic dikes (Figure 14), other intrusive rocks (Figure 15), Nevado del Ruiz (Figure 16). Standard product-moment correlation coefficients are calculated for each diagram. With respect to each suite, the following observations can be made: 1. The lava flows of the Combia Formation (Figure 13) show a large scatter, with considerable variation in oxide percentages at a given silica content. This variation is reflected in the low "r" values. The scatter may be caused by: 1) the addition or subtraction of phenocrysts (the rocks are highly porphyritic); 2) hydrothermal alteration or weathering, which is unlikely because only petrographi- cally "fresh" samples are plotted; or 3) a lack of genetic relationship between samples. 2. The variation diagrams for the basic dikes are shown in Figure 7. Because there is a marked difference in min¬ eral composition within the three groups of dikes, a diff¬ erent symbol was used for each group. The dikes of the Rio Poblanco exhibit and iron en¬ richment and aluminum and calcium depletion over a small range of silica variation. The aluminum and calcium trends Figure 13. Harker Diagram; Combia Formation. 'r' is the correlation coefficient. For a sample of this size (18 analysis) a correlation coefficient, ' r', equal or larger than 0.40, in absolute value, is significantly different from zero (null hypothesis) at 5% confidence level and ' r' major or equal than 0.54 at 1% confidence level. FIGURE 13 1 6 8 ^ O F e203 1 2 O O 86 O 4 L J L 50 60 8_ 1 2 - 71 2 S- r -.67 It02 *oO ° « * 50 50 ' ■ * S i02 50 60 Si02 Figure 14. Harker Diagram; basic dikes, (+) Quebrada Popala ( • ) Rio Poblanco (O) Alto Combia FIGURE 14 18 «» AI203 16 « + a + • + * 14 O J » » CaO o f 99 9 .i t J- 52 53 54 4 7.45 Si02 Figure 15. Harker Diagram; Cauca Depression intrusive rocks except dikes. 'r' is the correlation coefficient. For a sample of this size (15 analysis) a correlation coefficient 'r* equal or larger than 0.44, in absolute value, is significantly different from zero (null hypothesis) at the 5% confidence levels and 'r' equal or larger than 0.59 at the 1% confidence level FIGURE 15 #- I 12 10 F 6203 8 L 91 o • 00 2 1 12 v 10 CaO r 89 • ° 8 L o % O 6 000 4 I 2 Ti O. r -.01 >»»> o *«»•« 1 « 5 0 60 ! » «V » • 50 6 0 S i 02 Si02 Figure 16. Harker Diagram; Nevado del Ruiz. ' r' is the correlation coefficient. For a sample of this size (13 analysis) a correlation coefficient, 'r', equal or larger than 0.48, in absolute value, is significantly different from zero (null hypothesis) at the 5% confidence level, and * r' equal or larger than 0.63 at 1% confidence level. FIGURE 16 M g O Fe203 4 - *. r --90 «b 0 *0*«0 0 r -.95 • °* J I I I 8* N a2 O O 1 0 L • O r -.20 CaO al % 0 0 0 0* «0 r -.87 4 - J L 55 60 65 70 T i O' r -.8 7 S i02 î»*#» 0a» L. ± 5 5 60 65 70 S iO~ 24 are almost parallel, which could be explained by subtraction or anorthite from the melt by precipitation of Ca-rich plagioclase; (calcic plagioclase glomerocrysts are the most abundant phase in the rocks). The reasons for the iron enrichment are not so obvious and will be discussed later in this section. The dikes of the Quebrada Popala show very little variation with increasing silica content. Lack of varia¬ tion may be attributed to the fact that analyzed samples were much larger than the scale of fine horizontal layering showed by these dikes. The Alto Combia dikes show a very different chemical composition from all other rocks in this study. The Alto Combia dikes have the lowest SiC>2 and AI2O3 contents and the highest MgO and K2O contents of the investigated rocks. This distinct chemical composition clearly reflects the un¬ usual mineralogy of these dikes, which consist of very large augite phenocrysts and olivine and spinel phenocrysts in a groundmass of augite, plagioclase, sanidine (determined by x-ray diffraction of the whole-rock powder) and altered glass. These rocks are chemically and mineralogically similar to the absarokites of Yellowstone Park, Wyoming, USA (Iddings, 1895); see also Joplin (1968; Table 1, analyses 1 and 2, p. 276). 3. The intrusive rocks of the Cauca depression (ex¬ cept dikes) are shown in Figure 15. These rocks have a larger range of silica variation and show considerable less Figure 17. Alkali-Silica diagram; Combia Formation (Q) Shaded zone is the boundary between alkaline and subalkaline rock series after Schwarzer and Rogers, 1974. Hexagons represent average compositions of basalt (B) , basaltic andesite (BA), andesite (A) and rhyolite (R) from the Cascades, after Carmichael et al., 1974. Symbols with vertical bar Q» correspond to K20/Na2Û vs. Silica, same scale. FIG U R E 1 7 © 1 5 0 60 70 Figure 18, Alkali-Silica Diagram; Basic dikes (X) . Shaded zone is the boundary between alka¬ line and subalkaline rock series after Schwarzer and Rogers, 1974. Hexagons re¬ present average compositions of basalt (B), basaltic andesite (BA), andesite (A) , and rhyolite (R) from the Cascades, after Car¬ michael et al., 1974. Symbols with ver¬ tical bar p|C) correspond to K20/Na2Û vs. Silica, same scale. NO2 FIGURE 13 Si0 2 Figure 19. Alkali-Silica Diagram? Cauca Depression Intrusive Rocks (Except Dikes). Q Shaded zone is the boundary between alkaline and subalkaline rock series after Schwarzer and Rogers, 1974. Hexagons re¬ present average compositions of basalt (B), basaltic andesite (BA), andesite (A), and rhyolite (R) from the Cascades, after Car¬ michael et al., 1974. Symbols with verti¬ cal bar ($) correspond to vs. Silica. Same scale. FIGURE 19 50 55 60 65 70 s i o2 Figure 20. Alkali-Silica Diagram; Nevado del Ruiz Volcano (Û). Shaded zone is the boundary between alkaline and subalkaline rock series, after Schwarzer and Rogers, 1974. Hexagons represent average compositions of basalt (B), basaltic andesite (BA), andesite (A), and rhyolite (R) from the Cascades, after Carmichael et al., 1974. Symbols with vertical bar (Æ) correspond to K20/Na20 vs. Silica. Same scale. FIGURE 20 25 scatter (higher "r" values) than the rocks of the Combia Formation. Compared with the Combia Formation, the intrus¬ ive rocks have higher Na20 and 1^0 contents and lower TiC>2, AI2O3, total iron, MgO, and CaO at the same level of SiÛ2 content. 4. The rocks of the Nevado del Ruiz (Figure 16) ex¬ hibit much less scatter than any of the suites of the Cauca depression (higher "r" values). This low scatter could be explained by the fact that the rocks of this single volcano are clearly more cogenetic than the other suites or by the lack of fractionation as shown by the scarcity of phenocrysts. Alkali-silica diagrams: Figures 17 to 20 are total alkalis versus silica diagrams for: Combia Formation (Figure 17), basic dikes (Figure 18), intrusives of the Cauca depression (Figure 19), and the Nevado del Ruiz (Figure 20). These diagrams show that all of the rocks in this study are in the subalkaline field except for the Alto Combia dikes, which are in the alkaline field. The total alkali content of all suites increases with in¬ creasing silica content. In the lower part of each alkali-silica diagrams, Figures 17 to 20, the ratio KjO/l^O has been plotted (symbols with vertical bar). The K20/Na20 ratio of the Combia Formation is almost constant with variation in sil¬ ica content and is less than 1.0, with some very low values. The same low ratio is also found in the basic dikes, except Figure 21. AFM Diagram; Combia Formation (O)» A = Na2<3 + ^0, F = total iron as FeO, M = MgO (Weight percent). Dashed lines* A and B,enclose the Cal- alkaline field, after Kuno# 1968. FIGURE 21 F Figure 22. AFM Diagram; Basic Dikes (X). A = Na2Û + K2O, F = total iron, as Fed M = MgO (Weight percent). Dashed lines, A and B, enclose calcalkaline field, after Kuno, 1968, FIGURE 22 F Figure 23. AFM Diagram; Cauca Depression intrusive rocks.Q A = Na2Û + K2O, F = total iron as FeO, M = MgQ . (Weight percent). Dashed lines, A and B, enclose calcalkaline field after Kuno, 1968. FIGURE 23 F Figure 24. AFM Diagram; Nevado del Ruiz Volcano Cordillera Central. A A = Na2Û + K2O, F = total iron as FeO M = MgO. (Weight percent). Dashed lines# A and B# enclose the cal calkaline field after Kuno, 1968. FIGURE 24 F Figure 25. Silica-Feo*/MgO Diagram, Combia Formation. (O) FeO* = Total iron as FeO. Dashed line is an arbitrary boundary be tween tholeiitic (TH) and calcalkalic (CA) rock series, after Mryashiro, 1974 The equation of this line is : SiC>2 (percent) = 6.4 X FeO*/MgO + 42.8, F IGU?v E 25 70 65 T s « O 60 O / O 55 50 / O J J » «. 12 3 4 FeO*/ MgO 26 for the Alto Combia dike, which has a ratio greater than 3.0. The K20/Na2Û ratios of the other intrusive rocks of the Cauca depression vary between 0.68 and 2.38 ; Lefevre (1973) has referred to subalkaline rocks in Peru with K2Û/Na20 greater than 1.0 as shoshonites, although many writers restrict the term to alkaline rocks. The K20/Na2Û ratios of the rocks of the Nevado del Ruiz are less than 1.1 and are typical of calcalkaline rock series. AFM diagrams: Figures 21 to 24 show the AFM diagrams of the Combia Formation, the basic dikes, other intrusives of the Cauca depression, and the Nevado del Ruiz. The broken lines (A and B) delineate the fields of iron enrich¬ ment trends and calcalkaline trends (no iron enrichment) of Kuno (1968). The Combia Formation (Figure 21) and the basic dikes (Figure 22) show iron enrichment, whereas the other intrusive rocks (Figure 23) and the rocks of the Nevado del Ruiz (Figure 24) do not show iron enrichment. SiO‘>-FeO*/MgO diagrams ; The differences between rock series that do or do not show iron enrichment can be shown on a SiC>2-FeO*/MgO diagram (Miyashiro, 1974, 1975). Fig¬ ures 25 to 28 show these diagrams for: Combia Formation (Figure 25, the basic dikes (Figure 26), other intrusive rocks (Figure 27), and the Nevado del Ruiz (Figure 28). The broken line on these diagrams is Miyashiro's separation of suites that show iron enrichment (tholeiitic trend, TH) from those that do not show iron enrichment (calcalkaline trend, CA). In al plots, FeO* is total iron calculated Figure 26. Silica - FeO*/Mgo Diagram. Basic Dikes. (*) FeO* = Total iron as FeO. Dashed line is an arbitrary boundary between tholeiitic (TH) and calcalkalic (CA) rock series, after Miyashiro, 1974. The equation of this line is: SiC>2 (percent) = 6.4 X FeO*/MgO + 42,8 FIGURE 26 / 65 _ / / / C A T H CM / O mmm / CO / * #v* 50 / / / i 8 8 i 12 3 4 FeO* / MgO Figure 27. Silica-Fe0*/M9° Diagram; Cauca depression intrusive rocks (Q). FeO* = Total iron as FeO. Dashed line is an arbitrary boundary between tholeiitic (TH) and calcalkalic (CA> rock series, after Miyashiro, 1974. The equation of. .this line is: SiC>2 (percent! = 6.4 X FeO*/MgO + 42.8, FIGURE 27 70 65 60 55 50 F o O * / ÏA c O Figure 28. Silica-FeO*/MgO Diagram; Nevado del Ruiz Volcano, Cordillera Central (A). FeO* = total iron as FeO. Dashed line is an arbitrary boundary between tholeiitic (TH) and calcalkalic (CA) rock series, after Miyashiro, 1974. The equation of this line iss SiÛ2 (percent) = 6.4 X FeO*/MgO + 42.8. FIGURE 2 C 70 ! / 65 L /9 / C A ^ A TH A / A / 60 AA A / A Û / / CM 55 L o A / / CO 1 3 4 FoO / MgO 27 as FeO. According to Miyashiro's classification, the Combia Formation (15 of 18 samples) and the basic dikes are thol- eiitic, whereas the other intrusive rocks of the Cauca de¬ pression and the Nevado del Ruiz are calcalkaline. The reasons for the generation of an iron-enrichment versus non-iron enrichment trend have been discussed by a number of writers. Osborn (1962) indicates that the con¬ trolling factor is the oxygen fugacity of the melt. Under high oxygen fugacity, precipitation of magnetite drives the liquid toward a more silica-rich composition, thus genera¬ ting a calcalkalic series. Low oxygen fugacity will drive the liquid toward a more iron-rich composition. Carmichael et al., (1974) indicates that Osborn's hy¬ pothesis is not a viable mechanism based on the Ti-Fe oxides of volcanic rocks of the Cascades, from which Osborne derived his hypothesis. Taylor (1969) had also shown that the vanadium contents of some calcalkalic rocks are in dis¬ agreement with what one would expect if, in fact, the pre¬ cipitation of magnetite controlled by a high oxygen fugacity were the main mechanism by which the rocks had been derived. Miyashiro (1975), using a larger amount of data than those available to Taylor (1969) on the vanadium content of differentiated volcanic rocks, indicates that vanadium does follow the trend predicted by Osborn's hypothesis and in¬ dicates that differences between his findings and those of Taylor (1969) are due either to sampling problems or to a multiple origin for these trends. 28 Liquids derived by partial melting of oceanic crust show either iron enrichment trend or non-iron enrichment trend depending upon conditions under which melting occurs. Green and Ringwood (1968) have shown that partial melting of basaltic oceanic crust at low pressures (less than 15 kilobars) generated liquids that exhibit an iron enrichment trend, while those melted at higher pressures (26 to 35 kilobars) could generate liquids that do not show any iron enrichment trend. These higher pressure melts have higher K 0 contents depending upon the degree of melting. Melting 2 of ecolgite at 30 to 40 kilobars will produce liquids with no iron enrichment. Ti02 - FeO*/ MgO Figure 29 shows trends exhibited by the combia For¬ mation, the basic dikes and the other intrusive rocks, and the volcanic rocks of the Nevado del Ruiz on a diagram of Ti02 versus FeO* / MgO. The combia Formation and the basic dikes show a maximum in the intermediate rocks and then decrease toward lower Ti02 values at higher FeO* /MgO ratios. The other intrusives of the Cauca Depression and the rocks of the Nevado del Ruiz show a continuous decrease in Ti02 with increasing FeO*/ MgO, a typical trend exhib¬ ited by calcalkalic rock series (Miyashiro, 1975). Figure 29. Ti02“Fe0*/Mg0 Diagram; All rocks investigated. (FeO* = Total, iron, as FeO)*. FIGURE 29 FeO*/ Mg O - Co mbi a F. Intrusives Cauca Depression ° Rio Poblanco + Q. Popala *—— Nevado del Ruiz 29 MULTIVARIATE ANALYSIS Two-dimensional plots of petrochemical data, of the kind used in the preceding diagrams, have proven very useful but may lead to contradictory or insufficient con¬ clusions. consequently, it is advisable to develop methods of analysis by which rock suites can be compared using all available data simultaneously. For the purpose of this investigation, an effort has been made to compare the lavas of the combia Formation, the intrusive rocks of the cauca depression, and the volcanic rocks of the Nevado del Ruiz with well-known suites of volcanic rocks from other areas. The method used is cluster analysis of the coefficients of the first and second principal component axes. The closest approaches to this study in the geologic literature are those of Le Maitre (1968) and Cann (1971). Davis (1973) explains the statistical methods used here. Principal component axes for the three rock suites studied here are for 26 well-known rock suites were cal¬ culated using Biomedical computer Programs (Dixon, 1971) available at I.C.S.A., Rice university. The input data were the weight percentages of Si02# TiC^# AI2O3, total Figure 30, Dendrogram of relationships between rock suites derived from cluster analysis of the first and second principal component axes. Suites investigated are identified in Table 3 Similarity Coefficient EH 00 g LO • PM O a\ » rH Pi O 00 00 rH rH 4^. rd O PM G\ VO VO * o rd VO Pq » rH i—i o\ cr> r- VO ■P H rH rH o • a\ -P CTk CD o pq VO •w in PM H CD rH in o a > >i VO o> CO o\ 13 tf 0 0) •H o •H rH tf U ^ rH a rH U tf O ^ tf 0) o S H u •H •wr rd CD CD -P Ch rd o rtf M S CO •H & g o CO tf vo 5 C7> 0 PM pq U CQ CO rd rd tf O u rd tf tf •H •rH tn o p U 00 rd rd s a CD U EH 1—1 rH tf Î3 o rd o ü 0 Ü rd Ü Ü o O o u +J u P u U •P u P u u •H o § tf tf < *c tf tf tf tf tf tf tf tf PM H |3 CD CD S3 Q rtf *d Tf tf 'd O Pi a •H tf tf tf tf •rf tf tf tf tf tf to EH H rd -p rd rd rd rd -P rd rd rd rd rd H O > rH a rH rH rH rH tf rH rH iH rH rH tn W S3 to o CO CO CO CO O CO CO CO CO CO >H EH pq H o H H H H a H H H H H a tf S3 tf a pq Î3 EH O — CO H 00 ID EH VO Hi pq • EH iH CNI oo in VO 00 cr> o rH OH H rH rH rH 13. Thingmuli, V. Iceland TH Ridge Carmichael (1964b) ID to CLASSIFICATION TECTONIC REFERENCE FOR SUITE LOCATION KUNO (1968) ENVIRONMENT CHEMICAL INFORMATION •H ■H •H •H ">w* rH rH rH H CM en U Ai cd -P •P & S PQ «H HrH I—IrHCM ^ mvooo p P P •H ■rH •rH tx> tn p P P fd fd fd a a a EH rH rH rH IS cd fd fd M -P -P -p -P o a P P p P H IS (1) CD W • • EH vo r* 00 Q-mode cluster analysis of the coefficients of the first and second principal component axes together was performed to obtain the dendrogram shown in Figure 30. Table 3 shows the rock suites used for comparison; data for these suites are from papers cited in Appendix 1. Despite the relatively small size of the sample (29 suites), several significant conclusions may be drawn from this dendrogram (Figure 30) : 1. Two major groups of volcanic rock suites can be defined (Groups A and B), the coefficients of similarity which is zero, the correlation coefficients is minus one. The volcanic suites in Group A belong to the alkaline rock 31 series by most criteria (Schwarzer and Rogers, 1974)? the suites in Group B belong to the calcalkaline series. 2. Group B may be subdivided into subgroups (Bl and B2) . This subdivision of the subalkaline rock series does not correspond to the subdivision proposed by Kuno (1968) in his diagram of total alkali vs. silica, i.e., high- alumina basalt and tholeiite. This subdivision also does not appear to correspond to differences in tectonic environments, for both subgroups contain suites from island arcs, continental margins, and oceanic ridges. 3. The negative correlation between Groups A and B may indicate that the magmas for these two groups have originated under very different conditions, possibly at different depths. 4. With respect to the present study, the combia Formation (28) is linked in the dendrogram with the Hachijo-Jima suite, izu Arc, Japan (3), and the Mount Misery volcano (16) of the Caribbean. The K20 content of the combia Formation is higher that in the other suites, but all suites exhibit iron enrichment. The intrusive rocks of the Cauca depression (27) and the volcanic rocks of the Nevado del Ruiz (26) are linked together in the dendrogram (Figure 30). This relationahip indicates a close correlation in major element chemistry 32 despite the appearance of minor, but probably significant, differences in mineralogy and chemistry described in the preceding sections. 33 CONCLUSIONS The Middle to Late Miocene volcanic rocks of the northern part of the Cauca depression belong primarily to two different subalkaline rock series: 1) the tholeiitic rock series, including the combia Formation and the dikes of Quebrada Popala and Rio Poblanco; and 2) the calcalkaline rock series, including a variety of small intrusions into the combia Formation. The Alto Combia dike is an alkaline rock, with high K O content, K O/Na.O > 1 (wt. per cent), 2 2 * and a mineral and chemical composition very similar to that of absarokite as described by Joplin (1965, 1968). Intrusive rocks in the Cauca depression have K^ /Na^O > 1 and are similar to rocks labeled shoshonitic in Peru by Lefèvre (1973). The presence in a narrow zone of volcanic to sub- volcanic rocks that belong to different rock series and that were formed nearly synchronously indicates that at least this part of the Cauca depression was a very permeable zone that allowed easy egress toward the surface of dif¬ ferent batches of magma generated under different condi¬ tions and/or different depths. Extension in the Romeral 34 fault zone because of deflection around the Antioquia batholith probably contributed to this ease of access and caused the continued and extensive igneous activity. In contrast to the possibility of rifting and development of a thin, suboceanic crust, several lines of evidence indicate a considerable thickness of sialic crust beneath the Cauca Depression, including: 1) synchronous formation of several different rock series, as has been observed in some areas of thickened crust (e.g., Kamchatka, New Guinea; Jakes and White, 1971); and 2) K^O > 1% in most rocks of the tholeiitic series (see discussion by Rogers et al., 1974). The conclusion that a continental crust exists beneath the cauca Depression supports the interpretation of Case et al. (1971) that the crust is about 35 km thick in this area. Restrepo and Toussaint (1973, 1974) believe that pieces of oceanic crust (serpentinized peridotites, gabbros, and basalts) formed west of the Western Cordillera have been thrust over the Cauca Depression and Central Cordillera. This study also supports the establishment of the boundary between the oceanic and continental crust at the Cauca fault (Grosse, 1926; Case, 1971; and Campbell, 1973). The Plio-Pleistocene rocks of the central cordillera 35 adjacent to the Cauca depression are quite different from rocks in the depression. All rocks in the central Cordillera are calcalkaline and highly differentiated (Si02>55 per cent). Several features appear to indicate that these rocks have formed by partial melting of eclogite in the manner described by Green and Ringwood (1968) including: 1) the flat trend, without iron enrich¬ ment, on an AFM diagram; and 2) SiC^ in the range of 55 to 63 per cent. Depth of origen calculated from Dickinson and Hatherton plots (1967) is approximately 150 km, which coincides with the observation of earthquake foci beneath the central Cordillera at depths of about 100 to 150 km (Ramirez, 1969). 36 Appendix 1 References to Table 3 1. west Kurile islands Gorshkov, G.S., 1970 "Volcanism and the Upper Mantle, investigations in the Kurile island Arc. XV, 385 pp. Plenum press, N. Y. 2. Nicaragua Volcanoes McBirney, A.R. Volcanic History of Nicaragua. Univ. of California Pub. in Geol. Sci. vol. 55. T-3-4. 3. Hichijo-Jima Volcano Issihiki, N., 1963. Petrology of Hachijo-Jima volcano Group, Seven Izu islands, Japan. Tokyo Univ. Fac. Sci. Journal, Vol. 15, p. 91-134. T.ll 4. Usu volcano Oba, Y., 1966. Geology and Petrology of Usu Volcano, Hokkaido, Japan: Hokkaido Univ., Fac. Sci., J., Ser. 4, Vol. 13, p. 185-236. 5. Asama volcano Aramaki, S., 1963. Geology of Asama Volcano: J. Fac. Sci. Tokyo Univ. Sec. 2 Vol. 14. Table 21. 6. Hakone volcano. Kuno, H., 1960. Petrology of Hakone Volcano and the adjacent areas, Japan. Geol. Soc. Am. Bull., Vol. 61, 0. 957-1020. 7. Paricutin Volcano Mooser, F., 1958. Active Volcanoes of Mexico, catalog of the active volcanoes of the world, including Solfatara Fields, Part VI. ed. by I. V. Association. 8. Aoba volcano Warden, A. J., 1970. Evolution of Aoba Caldera, New Hebrides Bull, volcanol., Vol. 34, T-3, p. 107-140. 37 9. New Georgia volcanoes Stanton, R. L., and Bell, J. D., 1969. Volcanic and associated rocks of the New Georgia group, British Islands protectorate. Overseas Geol. Min. Res. (G.B.), Vol. 10, p. 113-145, T-l. 10. New Guinea Morgan, W. R., 1966. A note on the petrology of some lava types from East New Guinea. J. Geol. Aust., Vol. 13, p. 583-591. 11. Raoul Island volcano Brothers, R. N., and Searle, E.J., 1970. Geology of Raoul island, Kermadec Group, S. W. Pacific. Bull. Volcanol., Vol. 34, p. 7-17. T-l. 12. izu-Hakone Arc Kuno, H., 1950. petrology of Hakone volcano and the adjacent areas, japan, Geol. Soc. Am. Bull., Vol. 61, p. 957-1020. 13. Thingmuli Carmichael, I.S.E., 1964b. The petrology of Thingmuli, a Tertiary volcano in Eastern Iceland, J. Petrol., Vol. 5, p. 435-460. 14. crater Lake Volcano Williams, H., 1942. The Geology of Crater Lake National Park, Oregon. Carnegie Institute Wash. Pub. 540, p. 1-157. 15. Medicine Lake Volcanoes Anderson, L. A., 1941. volcanoes of the Medicine Lake Highland, California. Univ. Calif. Bull. Dept. Geol. Sci., Vol. 25, p. 347-442. 16. Mt. Misery Volcano Baker, P. E., 1968. Petrology of the Mt. Misery Volcano, St. Kitts, West indies. Lithos, vol. I, p. 124-150. 17. Skye Island Thompson, R.N., Esson, J., and Dunham, A.C., 1972. Major Elemental and Chemical Variation in Eocene Lavas of the isle of Skye, Scotland, J. Petrol., Vol. 13, p. 219-253. 38 18. Kilimanjaro Volcano King, B.C., 1965. petrogenesis of the alkaline igneous rock suites of the volcanic and intrusive centers of Eastern Uganda, J. Petrol., vol. 6, p. 67-199 19. Gulf of Aden Gass, I.G., Mallick, D.I.J., 1968. Jebel Khazia: an upper Miocene strato-volcano of Comenditic Affinity on the South Arabian coast. Bull, volcanol., Vol. 32, p. 33-38. Cox, K.G., Gass, I.G., Mallick, D.I.J., 1970. The peralkaline volcanic Suite of Aden and Little Aden, South Arabia. J. Petrol., Vol. 11, p. 433-461. 20. Nandewar volcano Abbott, M.J., 1969. Petrology of the Nandewar volcano, N.S.W., Australia, Contr. Mineral, and Petrol., Vol. 20, p. 115-134. 21. Gough Island Le Maitre, R.W., 1962. Petrology of volcanic rocks Gough Island, South Atlantic, Bull. Geol. Soc. Am. 73, Table 10, p. 1330. 22. Central Kenya, Mildly Alkaline Series Saggerson, E.P., 1970, The structural control and genesis of alkaline rocks in central Kenya. Bull. Volcanol., Vol. 34, p. 38-76. 23. central Kenya Strongly Alkaline Series. Op. cit. (22) 24. Tristan de cunha Baker, P.E., Gass, I.G., Harris, P.E. and Le Maitre, R.W., 1964. The Volcanological Report of the Royal Soc. Exp. to Tristan da Cunha.î Phil. Trans. R. Soc. Ser. A, Vol. 256, Table 6, p. 530. 25. Hawaii, Alkaline Rocks Only MacDonald, G.A., 1968. composition and origin of Hawaiian Lavas. Geol. Soc. Am. Mem. 116, p. 477-522. and Katsura, T. 1964. Chemical composition of Hawaiian J. Petrol., Vol. 5, p. 82-133. 39 26. Nevado Del Ruiz volcano. Cordillera Central, Colombia, South America (This study) 27. Acid intrusives, cauca Depression, Colombia, South America (This study) 28. Combia Formation, Cauca Depression, Colombia, South America (This study) 29. Davis Mountains, West Texas, USA Schwarzer, R.R. (unpub. data) Cook, B.G. (unpub. data) McKay, G.A., Rogers, j.j.w. (1970). Ignimbrites of the seven Spring Formation, Barrilla Mountains, Western Texas, Bull. Geol. Soc. 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