RICE UNIVERSITY
STRUCTURAL INTERPRETATION OF THE TINAQUILLO PERIDOTITE AND ITS COUNTRY ROCK, COJEDES STATE, VENEZUELA
by
MARINO OSTOS ROSALES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE
MASTER OF ARTS
APPROVED, THESIS COMMITTEE:
Dl . xx. vx. xxvc mxxcuiaui Professor of Geology, Chairman
3 1272 00288 9929
Houston, Texas April, 1984 ABSTRACT
STRUCTURAL INTERPRETATION OF THE TINAQUILLO PER I DOT I TE AND ITS COUNTRY ROCK, COJEDES STATE, VENEZUELA
MARINO OSTOS ROSALES
The Tinaquillo Complex is bordered to the south by the
Tinaco Complex, and to the north is separated by a thrust fault from the Cordillera de la Costa belt. The Tinaquillo
Complex, consisting mainly of harzburgite and metagabbro, was formed at high temperature (up to 1400°C) and presumably at great depth in the upper mantle. The Tinaco Complex consists of meta-igneous and metasedimentary rocks. The
Tinaquillo Complex was juxtaposed onto the Tinaco Complex at intermediate crustal levels at temperatures of about 650°C.
Structures in the Tinaquillo harzburgites indicate that the juxtaposition was caused by northwesterly directed thrusting.
The metamorphism in the Tinaco Complex is clearly not of the contact type, as has been suggested previously, but is a regional metamorphism. Although good age dates are not available, it is suggested that the juxtaposition of the two complexes occurred during Late Paleozoic times, probably related to collision of a volcanic island arc with South
America. The juxtaposition of both complexes onto the Cordillera
de la Costa belt is probably a Late Cretaceous or Tertiary
event and may be related to a collision of another volcanic island arc with South America. ACKNOWLEDGMENTS
The work involved in this thesis was completed with the cooperation of several persons whom I wish to acknowledge. First I wish to thank Dr. Hans G. Avè
Lallemant for his patience, conscious criticism and guidance.
My thanks to Miriam Hidalgo, my wife, who helped me endure our separation during my stay in Houston. I am also thankful to my parents for their moral support. A very special acknowledgment to the Universidad
Central de Venezuela that granted me the scholarship to study at Rice University. 1 I TABLE OF CONTENTS
** Page INTRODUCTION 1
Objectives 1
Study Area 1
REGIONAL GEOLOGY 4
Mafic and Ultramafic Belts' in the Caribbean
Mountains System 5
Cordillera de la Costa Belt 6
The Sebastopol Complex 6 The Caracas Group 6
The Caucagua-El Tinaco Belt 10
Tinapu Schist 11
LOCAL GEOLOGY 12
Ultramafic and Gabbroic Unit 12
Dunite, Harzburgite, and Pyroxenite 13
Serpentinite 14
Gabbros 15
Amphibole Gneiss, Amphibolite, and Feldspar-
quartz Gneiss 16
Amphibole Gneiss 17
Amphibolites 17
Feldspar-quartz Gneiss 18
Calcareous Phyllites and Marbles Unit 18
Calcareous Phyllites 18
Marbles 19 t II Page METAMORPHISM 20
Regional Metamorphism 20
Cordillera de la Costa Belt 20 The Caucagua-El Tinaco Belt 22
Local Metamorphism 23 The Tinaquillo Peridotite Complex 23
The Tinaco Complex 24
Gabbroic Rocks 25
Gneisses and Amphibolites 27
The Las Mercedes Formation 28
Contact Metamorphism 29
Geothermometry 32
Petrologic Interpretation 37
STRUCTURAL GEOLOGY 43
Regional Structural Geology 43
Local Structural Geology 46
Phases of Deformation in the Tinaquillo Peridotite Complex 46
Phases of Deformation in the Tinaco Complex. . 53 Phases of Deformation in the Las Mercedes
Formation 54 General Structural Interpretation 56
Textures and Microfabrics 60
Textures and Microfabrics in the Tinaquillo
Peridotite Complex 60 Ill
Page
Paléopië’Zometry 67
Petrofabric analysis 72 Conclusions based on the olivine
fabrics 83
Textures suggesting the sense of
the shear 84 Textures and Microfabrics in Rocks from the
Tinaco Complex 88
Gabbroic Rocks 88
The Tinaco Complex 95
Petrofabric Analysis 95
Conclusions based on the quartz fabrics ... 98
Textures and Microfabrics in the Las Mercedes
Formation 98
Petrofabric analysis 99
Conclusions based on the quartz fabrics . . . 101
TECTONIC MODELS 103 Plate Tectonic Models of Venezuela and the
Caribbean 103
Analysis of Age Dates in Venezuela 107
Tectonic Setting of the Tinaquillo and
Tinaco Complexes Ill
SUMMARY AND CONCLUSIONS 116 BIBLIOGRAPHY 122 » IV LIST OF FIGURES Figure ' '** Page
la. Tectonic Belts in the Caribbean Mountains
System 2
lb. Localization of the study area 3 2. Poles to the foliation in the Tinaquillo and
Tinaco complexes and in the Las Mercedes
Formation 47
3. Mineral lineations in the Tinaquillo and
Tinaco complexes 48
4. Fold axes and poles to axial planes in the
Tinaquillo and Tinaco complexes 50 5. Fold axes and poles to axial planes in the
Las Mercedes Formation. Poles to fracture
cleavage in the Tinaquillo complex 52 6. Photomicrography of a kinked enstatite ... 62
7. Photomicrography of a bent enstatite .... 64
8. Photomicrography of exsolution lamellae
along enstatite . 65
9. Photomicrography of different olivine
textures 66
10. Recrystallized grain size vs. stress for
olivine 68
11. Recrystallized grain size vs. stress for
enstatite 69
12. Fabrics of 100 X, Y, and Z axes of olivine
(Sample VT-82-lb) 74 V
Figure Page
13. Fabrics of"100 X, Y, and Z axes of olivine
(Sample VT-82-2) 76
14. Fabrics of 100 X, Y and Z axes of olivine
(Sample VTO-82-22) 78
15. Fabrics of 100 X, Y and Z axes of olivine
(Sample VTO-82-40) , ‘ 79
16. Fabrics of 100 X, Y, and Z axes of olivine
(Sample VT0-82-55) 81
17. Fabrics of 100 X, Y, and Z axes of olivine
(Sample VTO-82-123) 82
18. Textures suggesting the sense of the shear. . 86
19. Photomicrography of garnet porphyroclasts in
gabbroic rock at the southern contact with
the Tinaquillo complex 89
20. Photomicrography of crown texture in gabbroic
rock from the Tinaquillo complex 62
21. Photomicrography of a gabbroic rock from the
Tinaquillo peridotite showing mortar texture. 63
22. Photomicrography of a gabbroic rock with
pyroxene porphyroclasts replaced by
hornblende 64
23. Fabric of 200 c-axes of quartz from samples
of the Tinaco Complex 67
24. Fabric of 200 c-axes of quartz from samples
of the Las Mercedes Formation 100 » VI Figure Page
25. Interpretative orientation to the principal
axes of strain and stress 102 26. Plate Tectonic Models 114 * VII
LIST OF TABLES
Table * ■** Page 1. Orthopyroxene and clinopyroxene analyses of the Sample VTO-82-99 33
2. Orthopyroxene and clinopyroxene analyses of
the Sample VTO-82-87 34
3. Geothermometry 36
4. Interpretative Structural Geology ..... 58 t VIII LIST OF PLATES
Plate ' ■** 1. Geologic Map In pocket
2. Interpretative Structural Cross Section A-A’ In pocket
3. Interpretative Structural Cross Section B-B' In pocket t 1
INTRODUCTION
■"* Objectives
The major objective of this research was to ascertain the emplacement mechanism of the Tinaquillo Peridotite
Complex; whether it was emplaced as a crystal mush, as a hot solid slab, or a relatively cold slab. A second objective was to determine the structural relationship between the Tinaquillo peridotite and the country rock, and on the basis of the structures to reconstruct the kinematic history of both assemblages. This interpretation yields constrains with which the various tectonic and plate-tectonic models.prposed for the area are tested.
Study Area
The Tinaquillo Peridotite and its country rock are part of the Caribbean Mountains System, which follows the
Caribbean coast in northern Venezuela (Figure la). The area has been mapped twice (Mackenzie, 1960 and Bellizia,
1967). On the basis of these previous studies a 5 km wide,
NE-trending strip across the complex and its country rock was chosen for the present detailed study (Figure lb). t 2 T 3 REGIONAL GEOLOGY
Menendez (1966) divided the Caribbean Mountains into four tectonic belts which have characteristic structural features. Three major faults, which trend east-west, separate the four belts which are from north to south (Figure la).
(1) The Cordillera de la Costa belt is bound to the north by the Caribbean Sea and to the south by the La Victoria fault zone. This belt is composed of rocks of the Jurassic-Cretaceous Caracas Group which overlie granitic basement of Precambrian or Paleozoic age exposed in the cores of large open folds.
(2) The Caucagua-El Tinaco belt is bound to the south by the Santa Rosa normal fault. This belt consists of a granitic basement of Paleozoic age overlain by a younger volcanic-sedimentary sequence which has undergone low grade metamorphism. The upper part of the Caracas Group is locally thrust over this belt.
(3) The Paracotos belt is bound to the south by the south-dipping Agua Fria thrust fault. This belt consists in its totality of the Paleocene Paracotos Formation which has a very constant foliation parallel to the Agua Fria thrust fault (Menendez, 1966). (4) The Villa de Cura belt consists of volcaniclastic
and volcanic (olivine basalts) rocks, metamorphosed to the blueschist facies, probably during the Late Cretaceous ! 5 (Navarro, 1983). This belt has been interpreted as an allochthonous block (Bell, 1968; Maresch, 1974). The Villa de Cura sequence underlies volcanic rocks of the Tiara
Formation (Menendez, 1966) of unknown age and it underlies unconformably the flysch sequence of the Guarico Formation of Paleocene age. To the south of the Villa de Cura belt occurs a flysch basin of Paleocene to Eocene age overlain by a non-flysch basin of Miocene to Pliocene age (Beck,
1978). Bell (1968) defined four more tectonic belts all occurring in these basins.
Mafic and Ultramafic Belts in the Caribbean
Mountains System.
Bellizia (1967) suggested that the mafic and ultramafic rocks of the Caribbean Mountains System occur in two relatively ill-defined belts. The northern belt coincides with the Cordillera de la Costa belt. The southern belt, in which the Tinaquillo Peridotite Complex is included, occurs within the Caucagua-El Tinaco belt. These ultramafic complexes occur as lenses concordant with the structure of the country rock.
Bellizia (1967) pointed out that there are some mafic and ultramafic bodies outside of these belts. They occur
as olistoliths in the southern flysch basin, and along fault zones in the Villa de Cura belt. 6 Cordillera de la Costa Belt
The Cordillera de la Costa belt consists of granitic gneisses of the Sebastopol Complex, which have been interpreted as belonging to older basement, overlying meta-sedimentary rocks of the Caracas Group, and mafic to ultramafic rocks.
The Sebastopol Complex
The Sebastopol Complex consists of granitic gneisses probably of igneous origin. All authors (e.g.: Dengo,
1951) have interpreted this complex as being the basement of all the metamorphic sequences in the Cordillera de la
Costa belt. A whole-rock Rb/Sr age of 425 m.y. was determined by Hess (1968), which may date a Silurian orogenic event. The Sebastopol Complex was correlated to the Tinaco Complex by Wehrmann (1972).
The Caracas Group
The Caracas Group is the dominant lithostratigraphic unit of the La Cordillera de la Costa belt. While there is a lithologic uniformity of the Caracas Group throughout
Venezuela, the proportions of particular lithologies may differ considerably from place to place. The lithostratig¬ raphic interpretation is consequently confused and may be even invalid (Gonzalez de Juana et al., 1980). The Caracas
Group has been subdivided into several formations : Las 7 Brisas Formation, Antimano Formation, Pena de Mora Formation,
Las Mercedes Formation, and Tacagua Formation.
The characteristic lithologies of the Las Brisas
Formation are quartz-feldspar-mica schist and gneiss, quartz- sericitic schist, and minor marble (Gonzalez de Juana et al., 1980). Its lower contact with the Sebastopol Complex is unconformable and the upper contact with the Las Mercedes
Formation is transitional (Dengo, 1951). Kovach et al. (1977) did a whole-rock Rb/Sr determination on a schist which yielded an age of 270 m.y. In the marbles some fossils have been found at several localities but the fauna only gives an age range from Late Jurassic to Cretaceous. Important fossils are Pelecypodes indicating a shallow marine environment (Urbani, 1982).
The Antimano Formation consists of marbles which are difficult to differentiate from marbles of the conformably overlying Las Mercedes Formation (Wehrmann, 1972). Dengo
(1951) described a similar sequence of marbles interlayered with mica schist and in some parts of the formation with
garnet amphibolites, epidote amphibolites, eclogitic
amphibolites, and locally glaucophanites.
The Pena de Mora Formation was defined by Wehrmann
(1972) as an igneous metamorphic complex which in part at
least is the lateral equivalent of the Las Brisas Formation.
Kovach et al. (1977) acquired a whole-rock Rb/Sr age of
220 + 20 m.y. from a gneiss of this formation.
The Las Mercedes Formation is a sequence of calcareous t 8 graphite schists with lenses of marble in the lower part of the sequence (Wehrmann, 1972). The precise age of this formation is unknown. Mackenzie (1966) found some fossils restricting the age only to the Mesozoic. Furrer and Urbani
(1973) identified some foraminifera indicative of shallow marine water.
The Tacagua Formation consists of a sequence of epidote-sericite schists which had a volcanic source (Dengo,
1951). Locally it is interbedded with calcareous quartz graphite schists and calcareous albite schists. Dengo (1951) interpreted the lower contact with the Las Mercedes Formation as comformable but Morgan (1969) and Wehrmann (1972) considered this formation as a lateral equivalent of the Las
Mercedes Formation.
The Caracas Group has been interpreted as a southward transgressive sequence. This sequence was deposited on a stable continental shelf; the source area of the detritus must have been granitic (Menendez, 1966; Bell, 1972). The continental margin is supposedly of the Atlantic type. Talukdar and Loureiro (1982) interpreted the Caracas Group as a mixture of rocks deposited in different environments; some were deposited on the Atlantic-type margin, other rocks were deposited in a deep ocean basin near a volcanic island arc which must have existed to the north.
In the Caracas Group there are different associations of igneous rocks grading from ultramafic to felsic. The 9 characteristic lithologic types are amphibolites, eclogitic amphibolites, glaucophanites, serpentinites, biotite augengneisses, and granitic rocks. The mafic and ultra- mafic rocks, which are interlayered with the metapelitic sequences, have been interpreted as dismembered and metamorphozed ophiolites (Talukdar et al., 1979; Ostos,
1980; Talukdar and Loureiro, 1982). The mafic rocks have a variable mineralogy and based on textural and some geochemical data are thought to have a polymetamorphic history (Navarro, 1977; Talukdar et al., 1979; Ostos, 1980).
Based on studies of relict grains, the first metamorphic event was a high P/T event causing blueschist and eclogite metamorphism. The felsic rocks were not affected by this metamorphic event. Talukdar and Loureiro (1982) related this metamorphic event to a progressive northward subduction of the oceanic part of the South American plate beneath an active primitive island arc which ultimately collided with the continent during the Cenomanian. The second metamorphic event of intermediate P/T was of greenschist to amphibolite metamorphic facies. Talukdar and Loureiro (1982) related
this event to a south-dipping subduction zone north of the extinct arc (Upper Cretaceous to Lower Oligocène). This
second event has a related magmatic activity which is preserved in the islands north of Venezuela (Santamaria and
Schubert, 1974). 10
The Caucagua-El Tinaco Belt
Menendez (1966) recognized in the Caucagua-El Tinaco
Belt a lithologic association formed by hornblende gneisses and some trodhjemitic rocks which he described as the basement of this belt and he included these rocks in the
Tinaco Complex, In the area of interest the Tinaco Complex consists of two formations (The La Aguadita Formation and the Tinapu Schist). Elsewhere, the Caucagua-El Tinaco belt has other formations of post-Caracas Group age.
The La Aguadita Formation The La Aguadita Formation is according to Menendez
(1966) the oldest formation of the Caucagua-El Tinaco belt.
It consists of hornblende gneisses, biotite gneisses, amphibolites, and some granitic rocks, overlain by metaconglomerates and some lenses of marble. This formation is assumed to be derived from volcanic and volcaniclastic rocks (Menendez, 1966). All these rocks are intruded by four trondhjemite plutons which caused migmatization of the
country rock.
Hess (in: Martin Bellizia, 1968) published K/Ar ages
on biotite and hornblende of 112+ 3 m.y., 117+ 3 m.y. and
204+ 10 m.y., respectively. These ages may represent uplift
ages and the 204 m.y. may be a minimum age of the metamorphic event. Urbani (1982) made a copy of unpublished
data of Hess; these data are geochronological analyses made 11 by Stan Hart (M.I.T.) in minerals from the gabbroic rock close to the contact with the Tinaquillo Peridotite Complex.
The method used was K/Ar and the ages are as follows: plagioclase: 191+ 15 m.y,; hornblende: 235+ 13 m.y.; and pyroxene: 684+ 55 m.y.. Hart made a marginal note stating that the pyroxene may have absorbed Ar from the environment during its crystallization.
The base of the Tinaco Complex is not exposed; Bellizia and Rodriguez (1976) believe that the La Aguadita Formation overlies Precambrian basement.
The Tinapu Schist
The Tinapu Schist overlies the La Aguadita Formation; the contact is transitional. Menendez (1966) described this formation as a sequence of quartz-sericite-albite- muscovite schist, quartz-albite-chlorite schist, and metaconglomerate. This formation is overlain unconformably by the Las Placitas Formation which consists of undeformed basaltic pillow lavas and tuffs, probably of post-Caracas
Group age (Menendez, 1966). 12 LOCAL GEOLOGY
In the field area three major rock units are recognized
(Plate 1). In the center occurs the Tinaquillo Peridotite
Complex. To the north it is in thrust contact with calcareous phyllites and marbles, which previous authors have correlated with the Las Mercedes Formation (Mackenzie,
1966). To the south the Tinaquillo Peridotite Complex is in apparent conformable contact with high-grade metamorphic rocks which generally are included in the Tinaco Complex (Mackenzie, 1960, 1966).
Ultramafic and Gabbroic Unit
The Ultramafic and Gabbroic unit occurring in the central part of the study area is part of the Tinaquillo
Peridotite Complex as defined by Mackenzie (1960). This unit is well-exposed; locally it is covered by red-colored
lateritic latosols. This soil is apparently unable to
sustain the growth of high vegetation. The thickness of
this unit in the area is estimated to be 2.1 km.
In the study area, the Tinaquillo Peridotite consists
of approximately 90% ultramafic rocks and 10% metagabbros.
In this area a gabbroic layer at the sourthern contact is
included in the Tinaco Complex as previously mapped by
Mackenzie (1960). The ultramafic rocks consist of
approximately 75% harzburgite, 20% dunite, and 5% 13 serpentinite. A few pyroxenite and amphibolite veins are volumetrically insignificant.
Dunite, Harzburgite, and Pyroxenite
The ultramafic rocks in most outcrops contain large orthopyroxene porphyroclasts which are flattened parallel to the foliation and strongly elongate within the foliation plane. This orthopyroxene elongation forms a well-defined lineation. The size of the orthopyroxene is generally 1.5 cm, but in one case an orthopyroxene crystal was found which had a length of 6 cm and a width of 1 cm. Black spinel crystals up to 2 mm large occur in all the rocks.
They are only slightly flattened parallel to the foliation and elongate parallel to the orthopyroxene lineation.
Clinopyroxenes are rare, but occur in almost all the samples.
According to the classification of Jackson (1968) most of these rocks can be termed harzburgites (80%) and few dunites (20%).
The dunites are dark green on fresh surfaces; on altereted surfaces they are orange colored. A foliation is clearly observable in most outcrops with the exception of those in which a fracture cleavage is developed or where the dunite is partially serpentinized.
The harzburgites are dark green on fresh surfaces but weather to dark orange colors. On weathered surfaces the
orthopyroxene crystals stick out enhancing the foliation
and the lineation. An attempt was made to relate the volume 14 of orthopyroxene porphyroclasts to stratigraphic or structural position in the complex. Percentages of orthopyroxene were obtained on slabbed samples, but no such relationship was established although such a relationship can not be ruled out, but this analysis has to be carried out in the field.
Infrequent pyroxenite veins (maximum thickness 3 cm) occur in the dunite and harzburgite. They often show well- developed open to isoclinal folds, the axial planes of which parallel the foliation in the dunite and harzburgite.
The pyroxenites are black and consist mainly of orthopyroxene with minor spinel. Locally, they are partly or completely transformed to amphibolite.
Serpentinites
All the ultramafic rocks are to some degree
serpentinized. Only approximately 5 percent of the ultramafic rocks are more than 50% serpentinized, warranting the name of serpentinite. Serpentinites are restricted to four areas.
a. The strongest serpentinization took place near the
thrust contact of the ultramafic rocks with the phyllites
and marbles to the north. In three localities the following
sequence was observed from south to north: massive unfoliated serpentinite, a foliated serpentinite, and a
breccia which consists mostly of fragments of serpentinite
and to lesser degree fragments of the phyllite, exposed north 15 of the fault contact. The foliated serpentinite does not
show slickensides; thus the sense of the displacement along
the thrust fault could not be established. The
serpentinization probably occurred contemporaneously with the thrusting event.
b. Close to NW-striking strike-slip faults (Plate 1)
the ultramafic rocks are strongly, but not completely
serpentinized along foliation and fracture cleavage planes. c. At the contact with the gabbroic rocks (Plate 1),
the ultramafic rocks are also strongly serpentinized as
shown by the occurrence of opal and magnesite veins which
cut the ultramafic rocks in a random pattern.
d. Near the contact with the Tinaco Complex
serpentinization took place as well. At these contacts the
ultramafic body is completely altered and it has a cloddy
aspect and texture, caused by random cross cutting veins of
opal and magnesite.
Generally, the massive serpentinites are dark olive
green and they have an aphanitic texture. The foliated serpentinites instead have lepidoblastic texture even when
they are aphanitic. Their color is light green and they may
contain some talc, because the rocks upon touch, feel greasy.
Gabbros
The gabbros occur as concordant lenses in the ultramafic
rocks; they underly approximately 10 percent of the area.
The majority of these bodies occur in the structurally lowest 16 part of the ultramafic sequences, close to the thrust contact with the metasedimentary sequence.
Mapping of these rocks was difficult because they often occur on topographic highs due to their resistance to weathering. But frequently only scattered blocks on hill¬ sides with no outcrop indicate that they occur underneath the soil cover. Therefore, the contacts are frequently inferred.
The gabbros on fresh surfaces are dark green to black and the foliation is difficult to see, but in weathered gabbros a pronounced gneissic to blastoporphyritic gneissic texture can be seen. A well-defined layering is also often visible. The mafic layers consist mainly of hornblende; in two outcrops garnet was found. The felsic layers consist of strongly altered plagioclase.
Amphibole Gneiss, Amphibolite, and Feldspar-Quartz
Gneiss.
South of the Tinaquillo Peridotite Complex occur amphibole gneisses, amphibolites, and feldspar-quartz gneisses, which traditionally have been placed in the Tinaco
Complex. The gabbros of the Tinaquillo Complex were previously included in the Tinaco Complex (Mackenzie, 1966), but they have some characteristics which clearly separate them from the Tinaco rocks; thus in this study, as discussed above, they are placed in the Tinaquillo Complex. 17
Amphibole Gneiss Amphibole gneisses occur in a 500 m wide belt south of the Tinaquillo Peridotite Complex. Although these rocks look like tectonites, they have been described by Mackenzie
(1960) as contact metamorphic rocks of the Tinaquillo
Peridotite Complex.
As the gabbroic rocks in the ultramafic body, the amphibole gneisses occur on topographic highs; they are well-exposed on the southern Teta de Tinaquillo. Macroscopi- cally, they resemble the gabbroic rocks in the ultramafic complex but the blastoporphyritic texture is much more common; the gneisses are also nematoblastic.
Amphibolites
South and conformably overlying the amphibole gneisses occur amphibolites. Generally, the amphibolites are inter- layered with feldspar-quartz gneisses, but locally (e.g. south of Las Mesas) they occur as massive bodies. The amphibolites are dark green, when fresh, but turn brown upon alteration. Hornblende, white mica, and epidote are recognized in hand specimen.
In the mixed amphibolite-felsic rock sequence the layer
thickness of the amphibolite is between 1 and 5 cm with a mean of 2 cm, whereas the feldspar-quartz layers have
thickness of 4 to 30 cm, averaging about 15 cm. The
amphibolite layers are often boudinaged. It is quite
possible that the large, massive amphibolites represent ♦ 18 mega-boudins, but poor outcrop makes It impossible to ascertain this.
Feldspar-Quartz Gneiss
The feldspar-quartz gneiss occur as layers within the amphibolites. In hand specimen feldspar, quartz, white mica, and hornblende are recognized. The composition of these rocks suggest that the protolith of the feldspar- quartz gneiss is an immature sandstone.
Calcareous Phyllltes and Marble Unit
Outcrops in the calcareous phyllite and marble unit are scarce and where exposed, the rocks are strongly weathered.
The only outcrops occur along the roads; elsewhere the terrain has grown over completely by a tropical jungle.
The lithologic characteristics of this unit are very similar to those of the Las Mercedes Formation in the
Cordillera de la Costa belt as defined by Dengo (1951), and thus this unit has been correlated with the Las Mercedes
Formation.
Calcareous Phyllites
In general, the calcareous phyllites have a gray color on fresh surfaces and on altered surfaces they are red. Two
types are recognized in the field: (a) Calcareous quartz- muscovite phyllite which occurs more frequently near the I 19 thrust contact; (b) Calcareous quartz-graphite phyllite which gradationally changes into the marble. The quartz- graphite phyllite is generally crosscut at variable angles by veins of calcite and siderite.
Marbles
Marbles are a minor constituent of this unit. The marble layers on the map (Plate 1) were mapped from air- photos; they occur on ridges which are generally unreachable because of the tropical vegetation. In the outcrops studied they are related to the second type of phyllites described above.
The thickness of the marble bedes is at the most 10 cm.
They are aphanitic, blueish gray on fresh surfaces, and they are red on weathered surfaces. These marbles contain
locally pyrite crystals. t 20
METAMORPHISM
Regional Metamorphism
The Cordillera de la Costa Belt
Until recently the metamorphism in the Cordillera de la
Costa belt was interpreted as gradational, the grade of metamorphism increasing from south to north. In a study of the Las Brisas Formation in the La Victoria area, Seiders
(1962) described minéralogie assemblages corresponding to the green-schist facies; instead north of Caracas the rocks are in the amphibolite facies (Dengo, 1951). Gonzalez de
Juana et al. (1980), however, recognized that this gradation was very imperfect, and that the minéralogie assemblages are variable from region to region. Thus, the picture is not as simple as was believed previously because the metamorphic facies distribution appears very heterogeneous. Students and faculty of the Universidad Central de
Venezuela have carried out geologic studies in the La
Cordillera de la Costa belt and on Margarita Island. These studies show that at least two different periods of regional metamorphism have affected the rocks in this belt (Blackburn and Navarro, 1977; Navarro, 1977; Navarro, 1978; Talukdar et al., 1979; Ostos, 1980; Navarro, 1981; Talukdar and Loureiro, 1982).
The first metamorphic event is represented by rare garnet amphibolites, eclogites, and glaucophane rich rocks. In many rocks one can recognize relict grains which clearly 21 have formed during this event. Even the rocks mentioned above have undergone retrograde metamorphism. The evidence of this first metamorphic event is both textural as well as geochemical.
Navarro (1981) analyzed garnets, amphiboles, pyroxenes, and white micas in eclogitic rocks of Margarita Island; the
cores of these crystals have MgO/FeO values, which are very
different from those of the rims. In the same rocks barroisite and actinolite occur which must have formed at
different regimes of P/T; also paragonite and phengite co¬ exist in equilibrium in the same rocks. The same textural evidence of two periods of crystal growth is present in some garnets in the amphibolites; also
a retrograde metamorphism has occurred by which the garnet
is replaced by chlorite. In other rocks glaucophane is
rimmed by barroisite and/or actinolite and cores of
barroisite may have rims of actinolite (Ostos, 1980).
Talukdar and Loureiro (1982) described several
varieties of amphibolite consisting of garnet, omphacite,
glaucophane, actinolite, zoisite, epidote (clinozoisite to
pistacite), rutile, and white mica some of which represent the high P/T eclogite and blueschist facies and others
belong to the greenschist facies of intermediate P/T. The
second metamorphic event is the most wide spread in the
Cordillera de la Costa belt.
Talukdar and Loureiro (1982) believe that the first
metamorphic event took place during the period of 130 - 95 22 m.y. ago (Valanginian to Cenomanian), and the second meta- morphic event had its climax during the Upper Cretaceous
(85 - 60 m.y.) with associated calcalkaline igneous activity, but the orogenic event continued to the Oligocène (60 - 30 m.y.). The activity during this last part consisted essentially of post-metamorphic deformation and minor acid intrusions.
The Caucagua-El Tinaco Belt Menendez (1966) described the mineralogical assemblages of the La Aguadita Gneiss and concluded that it was meta¬ morphosed to the almandine amphibolite facies probably at low P/T conditions. He also studied the minéralogie assemblages of the Tinapu Schist and interpreted it as having been metamorphosed to the greenschist facies. The change in metamorphic facies is an unsolved problem; it may be the result of a regional gradient caused by different geothermal gradient or the formations were metamorphosed at
different structural levels. This metamorphic event may not be correlated with the
greenschist to amphibolite metamorphic facies in the La
Cordillera de la Costa belt. The few radiometric dates
(See Regional Geology) are not very reliable, but they may
indicate that this event is much older. 23 Local Metamorphism
The Tinaquillo Peridotite Complex
The Tinaquillo Complex consists mainly of two litho¬ logies, ultramafic rocks and gabbros, in which textures occur indicating a polymetamorphic history. The gabbroic rocks have the same minéralogie assemblage as the gabbroic rock south of the ultramafic complex which were mapped as part of the Tinaco Complex. Therefore their metamorphism will be discussed with the Tinaco Complex.
The ultramafic rocks are metamorphic tectonites. They have porphyroclasts of mostly orthopyroxene but also of clinopyroxene and olivine in a mylonitic matrix of fine¬ grained olivine and pyroxenes. The bimodal grain size distribution suggest that the rocks underwent at least two metamorphic events, during the first of which the large grains grew, and during the second the fine-grained material. In the harzburgites, a pargasitic amphibole occurs invariably but as minor constituent (less than 1%).
The pargasite appears to have formed during the second metamorphic event.
Holloway and Ford (1975) experimently established the upper stability limit of pargasite + water at a temperature of 1100°C and 15 Kb pressure, and they pointed out that the most crucial factor at these conditions is the amount of fluorine available, but pargasitic amphibole also breaks down between 840°C and 1025°C at pressures of 250 to 800 t 24 bars to the following anhydrous crystalline phases and vapor: Diopside + Olivine + Nepheline + Anorthite + Spinel + Water (Deer et al., 1980). These conditions have probably
been attained during the second metamorphic event.
Low temperature retrograde effects are ubiquitous in
the Complex as was described in the local geology; the
serpentinization is most pervasive near the thrust fault
contacts. Antigorite is the characteristic mineral; brucite
was not found. At the asbestos mines (Plate 1) the foliated
serpentinites are cut by chrysotile, magnesite, and opal veins. The orientation of the antigorite defining the
foliation may indicate that it formed during thrusting.
Winkler (1979, pag. 156) established a series of re¬
actions in ultramafic rocks at fluid pressures of 2 Kb. The
presence of the antigorite without brucite may be indicative of a high water content in the fluid phase at temperatures
between 525°C and about 50°C at 2 Kb. Chrysotile must have formed by percolation of hydrothermal fluids along the fault
planes, but it must have been formed later than the
formation of fracture cleavage.
The Tinaco Complex
The Tinaco Complex can be subdivided into two litho¬
logic zones: 1. Gabbroic rocks in contact with the Tinaquillo
Peridotite Complex which are similar to those within the
Complex, and 2. Bocks to the south of the pyroxene bearing
gabbroic rocks, consisting of gneisses and amphibolites. 25 Gabbroic Rocks
The gabbroic rocks within the Tinaquillo Peridotite
Complex and directly south of it have the following meta- morphic minerals: Plagioclase (Andesine) + Diopside +
Hornblende + Hypersthene + Garnet with magnetite and ilmenite as accessory minerals. This metamorphic assemblage can be divided into two types depending on textures and structures; each type represents a different metamorphic event. The metamorphic assemblage which represents the first metamorphism is: Plagioclase + Diopside + Hypersthene +
Garnet. These minerals occur as porphyroclasts in the rocks.
Turner (1981) has shown that the zonal sequence in the granulite facies can be based on the relative amount of hornblende and biotite with respect to pyroxenes. An
increase in pyroxenes and garnet is to be correlated with
increasing metamorphic grade, decrease in water fugacity,
increase in temperature, and perhaps an increase in pressure
as well. Turner (1981) listed a few of the many recent
estimates of temperature and pressure that have been
proposed for the granulite facies. Thus, it is suggestive
that the Tinaquillo gabbros were formed at temperatures of
700° to 900°C and pressures of less than 15 Kb, in the granulite facies of the low pressure type of Miyashiro
(1979). The plagioclase porphyroclasts are andesines; the
An-content of plagioclase in the granulite facies should be
much higher. These plagioclases probably have been I 26 transformed during the second metamorphic event.
The metamorphic assemblage which formed during the second metamorphic event consists of: Plagioclase
(Andesine) + Brown Hornblende. These occur as finer-grained neoblasts surrounding the porphyroclasts. Miyashiro (1979) established that the calcic amphiboles in metabasites of the epidote - amphibolites and amphibolite facies are hornblendes, and that brown hornblende occurs at the highest temperature of the amphibolite facies. Miyashiro's (1979) correlation of the composition of the plagioclase with pressure establish
that the Tinaquillo metamorphic assemblage may have formed
at low pressures. Winkler (1979) shows that during high grade metamorphism,
basaltic rocks recrystalize to form a hornblende-almandine
garnet association (garnet if the pressure is high enough);
these minerals form mainly from clinopyroxene, hypersthene, and/or olivine. On the basis of the textures these
transformations are clearly established in the rocks studied
here. Turner (1981) characterized the almandine zone of the
amphibolite facies by the assemblage: Hornblende-Andesine- (epidote-almandine-biotite). Nevertheless, Miyashiro (1979)
regards that the formation of garnet is strongly dependent
on the original composition (Ferrous iron/Magnesium). As
garnet has not formed during the second metamorphic event in the present rocks, this association was formed at
conditions transitional between the biotite and garnet zone
in the amphibolite facies. The pressure was probably not 27 high enough as to form garnet or the original rock did not have the required composition.
Gneisses and Amphibolites
The more southerly zone of the Tlnaco Complex consists of plagioclase-quartz gneiss, amphibole gneiss, and amphibolite, which can be placed in two different meta- morphic assemblages: 1. Plagioclase-quartz rocks which consist of the following metamorphic minerals: Plagiocase (Oligoclase) + Quartz + Muscovite + Epidote + Microcline +
Green Hornblende + Sphene with apatite and allanite as accessory minerals. 2. Mafic rocks which consist of the following metamorphic minerals: Plagioclase (Oligoclase) +
Green Hornblende + Epidote + Quartz + Microcline + Biotite
+ Chlorite + Sphene + Garnet with zircon and allanite as accessory minerals. Two chemical reactions are useful to establish the metamorphic conditions of the plagioclase-quartz rocks: 1. the beginning of melting in the simplified granite system and 2. the breakdown of muscovite in the presence of quartz, even if no pelitic rocks exist. Helgeson et al. (1978) established the univariant equilibrium of the breakdown of muscovite in the presence of quartz and Turner (1981) has shown approximately the beginning of the melting in the granite system. Based on these studies the maximum temper¬
ature and pressure at which these assemblages may have
formed are 600°C and 4 Kb, respectively. These metamorphic \ 28 conditions are of medium grade according to Winkler (1979) or of the amphibolite facies of Turner (1968). The plagioclase has a lower anorthite content than the Tinaquillo gabbros and it is associated with epidote, which is indicative of a lower temperature of metamorphism (Winkler, 1979) than in the gabbroic rocks of Tinaquillo.
This is also in agreement with the disappearance of pyroxenes and the appearance of quartz in these rocks
(Miyashiro, 1979). On the basis of the composition of plagioclase associated with epidote the metamorphic event was of the medium to low pressure type of Miyashiro (1979).
Because garnet was found in only one sample and the plagioclase is not an andesine, the assemblage seems better representative of the biotite zone than the garnet zone of Turner (1981).
The Las Mercedes Formation
The Las Mercedes Formation has a calcareous lithology with a small pelitic component, but to study its meta¬ morphism, it is considered as a calcareous rock. The characteristic metamorphic assemblage is: Calcite +
Dolomite + Quartz + Graphite + Muscovite with pyrite and haematite as accessory minerals. Winkler (1979) described the association calcite- dolomite-quartz-white mica as characteristic of marls subjected to very low-grade metamorphism but he pointed out 29 that these rocks usually contain graphite which probably reduced the activity of water in favor of methane in the fluid phase. Turner (1981) extends the stability field of calcite-dolomite-quartz in siliceous magnesian marbles up to the amphibolite facies where tremolite appears.
According to Winkler (1979) this assemblage is stable at the boundary of the low to medium grade of regional meta¬ morphism.
Hewitt (1973) reversed the reaction Muscovite + Calcite
+ 2Quartz = K-feldspar + Anorthite + CO2 + H2O at XQQ2 =
0.50 + 0.06 over the pressure range 2 to 7 Kb. At 6 Kb fluid pressure, the equilibrium bracket is 584° + 4°C and
= 5 the equilibrium curve is symmetrical about XçQ2 °* *
French (1966) has shown that the presence of graphite is indicative of methane as a fluid phase and that it probably would constitute a high percentage of the fluid phase. If this is the case, the temperatures in these rocks were probably higher than the ones established by Hewitt (1973) because he studied the effect of both H2O and CO2 in the fluid phase. According to Miyashiro's (1979) classification, this association was formed at low-pressure.
Contact Metamorphism
Mackenzie (1960) studied the Tinaquillo Peridotite
Complex and he interpreted it as a 3 km thick sill, which
formed by the intrusion of an ultrabasic magmatic crystal 30 mush. The Tinaco Complex overlies the Tinaquillo Peridotite
Complex to the south. Near the contact it consists of plagioclase-garnet-pyroxene-hornblende gneisses of the granulite facies, but toward the south the grade of meta¬ morphism decreases to epidote amphibolite and even green- schist facies (Mackenzie, 1960). He took it as proof for a contact metamorphic aureole caused by the hot (crystal mush) intrusion of the ultrabasic complex. He also considered the tabular bodies of metagabbro in the complex as xenolith of the country rock, but he did not substantiate that with geochemical evidence. Mackenzie (1960) proposed that the body intruded from northwest to southeast during an orogenic event. On the basis of the alignment of enstatite crystals, he suggested that the crystals rotated with their tabular shape into the foliation or flow plane and with their longest dimension into the direction of the tectonic transport. Based on the occurrence of garnet and pyroxene, and some optical measurements of the pyroxenes Mackenzie (1960) estimated that the temperature of the intrusion was between 800° to
1000°C, and in the country rock between 550° and 650°C. Green (1963) analyzed the alumina content of the
enstatite crystals. He found zonation in the porphyroclasts with high values in the core decreasing toward the rim. The
orthopyroxenes in the matrix have similar low alumina
contents as the rims of the porphyroclasts. He interpreted this as a change in the pressure regime during the 31 emplacement. Green (1967) suggested that the peridotite is of the
Alpine-type which equilibrated initially at high pressure at which time spinel formed in the dunite, and re¬ equilibrated at high temperature but lower pressure to cause the contact aureole. This suggests a diapiric intrusion of the body.
Lopez Eyzaguirre (1972) analyzed seven serpentinized peridotite and fourteen gabbro samples from the Tinaquillo Complex. The range of values of Ti, Ni, and Sr of the gabbroic rocks are in the range of mid-oceanic ridge basalts (Pearce and Cann, 1973; Schedegger, 1973; Pearce,
1976; Sun et_ al. , 1979).
Philpotts et al. (1972) analyzed the rare-earth element content of one sample of peridotite from the Tinaquillo
Peridotite Complex. They found a pattern with low values of the light rare-earth elements. On the basis of this analysis, Green et al. (1979) interpreted the Tinaquillo Complex as the residue of the upper mantle after extraction of a small portion of melt.
Thayer (1980) suggested that the proportion of gabbroic rocks in the complex was underestimated and that the rocks of the "contact aureole" around the peridotite probably were part of the same complex. Thayer (1980) pointed out that the field relations of the peridotite and gabbros in the
Tinaquillo Complex are very similar to those in others ophiolitic complexes except for the strong foliation and the » 32 absence of original magmatic textures in the Tinaquillo
Complex.
Geothermometry
The petrographic study of the ultramafic rocks from the
Tinaquillo Complex shows in almost all samples paleoblasts of enstatite, and in few samples paleoblasts of diopside, and minor olivine in a strongly recrystalized matrix of olivine with minor pyroxene. Two samples were chosen to determine the chemical composition of the paleoblasts, and of the matrix. The compositions were determined with the
ETEC Autoprobe-0901 at Rice University. The pyroxenes porphyroclasts are zoned and the composition of the rims corresponds generally to that of the small recrystalized grains. The analyses are presented in Tables 1 and 2 along with calculations of the cations on the basis of 6 oxygens atoms.
From the chemical composition equilibrium temperatures were calculated using several two-pyroxene methods (Wood and
Banno, 1973; Wells, 1977; Lindsley and Andersen, 1983).
Differences in temperature obtained by the various methods
are partly due to inherent differences among the geo¬
thermometers and to large extent due to disequilibrium between the minerals in the rock. Several authors have commented on disequilibrium between mineral phases in the
upper mantle to explain the homogeneity of the mid-oceanic
ridge basalts. White and Schilling (1978) suggested that TABLE 1 ORTHOPYROXENE AND C LINOPYRO XENE ANALYSES OF SAMPLE VTO-82-99 CM O Y* *- o t- r- Y- a CC z - < T“ Y- 13 9 CM N Y— CD T* O Y— T** <0 co eft to Y* o a H Z CM CC O 2 CC Q co 2 CO CC CC oc 2E o o a Ul H >• X 1 1 1 1 M-S Mx o o o O O O O o o o X a X X a X a X X X Q. a X Q. X a cx X a Op* Cp* CL .Y— CO CM q CD* CM to VF o o> CD q O O to Y- to CO CD to CD to Y- CD o a> CO o K CD to Y~ to IO Y» to Y-* r- to Y- CD to to to to to to CM e • • • • e • o O to CM CM CM co O d o Y* 0) ** ft. q CO q 0> CM o> to W o CO to q q CO CO CM to o> CD d CM CM CM CM ft- CM o d <*e « o • t • • » CO d Y- q ft- o CD o CO CO CO o O CM CO *>. o NT* r* ft. o d q q CO o K o d p O o CO K o o H CM ft e ft ft • « • o O 9 o O Y- o o d o CM Y» K o o CD o t- CM o o €0 o O o Y- 2 Y- CO Y- o CO o CD e ft e ft e ♦ ft ft ft ft • S CM CM CO to CO CM to CM CO * o> N « CM CO q Y- to Y» CD CD h to * o CM CM o> 9 CO «0 CM CM ft- < CD CO CO CM CO • • ft ft ft • e s m M CM O O 9 CM CM CD <1 O CO CM CM CM h «0* CO T CD to CO CD CM CO 9 to CD CO 1* CO * Y“ u. o CM o • » ft • e • e ft ft » CM to O to Y- CD o CD Y* co CO CD «1 CM CO CO to CO to q CD Y* Y- CO 0) CD Y- ft- s CO CO Y* CO CO ft- K q CO Y- Y“ to N * to * CO CO 9 B « B ft ft ft ft 9 o CD 9 9 O T" Y* to o Y“ CO O o CO q o q Y“ 9 Y» z d o o Y- Y* o o q Y- CM o o d q d o IB CM ft ft ft e B « ft CD CD CO CO o ** o o q o h q CD d d Y- o d CM K o o CO o o Y- CM Y“ o CM CO CM o o o V CM «0 B t B ft B B B ft Y- CD CD N o> CD 9 ** CM 9 o CD 9 to 9 r» h» q to 1- to o q CO 9 9 o Y- -1 o to CM o T" to 9 9 9 < Y- o d Y- Y* o q Y» o O CO CM o Y* ft ft % ft B ft B ft CO o z o X > III «0 O CO Ik CO h CO o z < o O h z z ILI ID < Y Y- q q CD Y- ft- to to Y“ q CO CO Y» CO 9 to CO CD CO 9 o CO q Y- CO r* K r» Y“ to 9 Y- 9 O BB> Y" ' o q CO K Y* q 9 CD 9 CO to B B ft ft ft ft o o d Y- o Y* O 9 CO d CM q CM d q O Y- ft- CM o CM o CM Y— CO Y“ Y* CO to Y- Y» d Y- Y- to d o — o Y* d Y- Y- Y- o < ft • B ft B B o o o o o o CO CO o o o Y* o o Y* o o Q q 9 o d o Y- b q d q o o o o d o o o Y* o o d q o o d q o H Y*» ft ft ft B B • ft CD 9 O CD q O 9 d O 9 * o o CO * N o CO co K q o o CD o. o d CO q d q CD CD CM o CO CM o o o Ü. o O CD Y* ♦ o co Y- • ft B B ft ft ft ft • Y~ o o o o o o o o q o o d o o o o o o o o o o CM o Y- ■M* CO o o Y- Y- CD CO o o o CM o UL -ft CM Y“ Y» o d Y*“ o • ft B B ft ft B B ft B B CO CO o o o q O CM CM q d o d q d co q o to o o d CO d q o o CO o o d * o o CD d q o to o o o o o o s o o c • ft » ft i vS €0 o CO q o Y- CO co Y- CO CO K 9 to co ft* d o d q h- Y- q K q CD CD N o ft- CD q o CD co K Y- * T* q CD q to T- s Y*“ 9 ft • ft ft o q d d q o Y“ CO O CM o Y- 9 o o o o q 9 d q O o o CD o d q to d o o CO o o to o CO q o o o d q d ft ft ft • « *Y- CO o CM d 9 Y» d q ft- 9 q 9 d q o 9 Y- CD d 9 q CD O 00 o d CM CO q Y— CO o o o o o Y- o Y- CM o Y* K q Y- o d IB ft B ft ft ft * °. o co co q o K q o Y- o d O d CO q CO CO o O d q o o o N Y- q d d q d q O o o o CM o o o d d q z o CM « ft ft ft
TOTAL 3.999 3.999 3.999 3.999 3.999 3.999 4.018 4.006 4i027 4.007" 4.061 4.031 33 FeO= Total iron as FaO; C= cores C-R= point between core and rim; M-Ss medium size porphyroclast; Mxr matrix. TABLE 2 ORTHOPYROXENE AND CLINOPYROXENEANALYSES OF SAMPLE VTO-82-87 O CLH a o> < t ^ CMO ^ COOC ^ Œ ID T- CM CDO W NO CM COOC CM O) CD K r- t- O a Ul 2 o 2 IL 5 rjr X X x | w O n O o K o ■» O <*> o a . X O o a X o O S. t O £ « o. * CM X a a x ^ a a X x a K a w
T O T A L 3.999 3.999 3.961 3.999 3.991 4.056 4.088 3.981 4.016 4.011 4.036 FeOr Total iron as FeO; Cr core; Rr rim; C-R* point between core and rim; Mxr matrix 34 35 disequilibrium exist on a microscopic scale between neighboring grains, but that at a mesoscopic scale the mantle is homogeneous. Wood (1979) however, suggested that the upper mantle is inhomogeneous, and that fluids derived from the lower mantle rich in incompatible trace elements intruded the upper mantle and gave rise to homogeneous mid- oceanic ridge basalts.
The temperatures obtained are given in Table 3. The highest temperatures of equilibration have been found in the core of pyroxene porphyroclasts of sample VTO-82-87; these temperatures are (depending on the method used) from
1367°C to 1454°C. Other pyroxene cores yielded temperatures of 885°C to 1051°C. The rims of several pyroxene crystals yielded paleotemperatures of 682°C to 998°C, while the neoblast pyroxenes (the fined-grained reerystalized crystals) yielded temperatures of 576°C to 711°C.
From the textural evidence, it is clear that the ultra- mafic rocks underwent at least two metamorphic events: a high temperature (^;1400oC) and a low temperature event
(^650°C). The cores of smaller pyroxene porphyroclasts, and the rims of larger ones yield intermediate temperatures (sslOOO°C). These temperatures could be explained in two alternative ways: 1. they represent a separate thermal event, and 2. the chemical composition of these pyroxenes has been partly changed by the low-temperature event. TABLE 3 GEOTHERMOMETRY O 2 H Ul o x CO 2 a < -J til 10 » o O o z O CD < z CO CO 0> K CO ” 2 JO Û -J CO Ui X z < u UJ CO o o Ui X z x> CO CO 0» XI CM o JO m X x T" X m o o 0 fiC UI o oc UI h- CO CO ■M* o V— K 1 CO Y~ in T- a < O ? 2 a o X IL Zs o o o tO CO 0) o CM CO N» CO T* o CO CO o 0> o a X 0> o co co a> a o x X O » a ? S IL oc < CM K * K CO O O CO o CO co CD CM V CM * CO CO 0> 0) 0) CO CO w xl o & * X c • z <->» X OLCM HO O 2 «> < z T* K f- CO K o in K o o 00 CO h* K CM CO o O a tt 0 GC ui UI CO co o> T» o CM CO 1 in ■* o O) r* o CO CO T- o o> 00 m o K T CO 00 10 ° x 2 < UI z 2 ° 2 & o a x CO o CM 00 O) o> T- K ID O o K N O) CO CO CO CO N» CO CD CO co £ 2 O ui a 2 ° N UI o O X 2 V — m CM N* 0> o CM CO CM » O O CO N- K CO K N* m * CO * CO «- » r» CO X CL o o K» CO CO *- CM o co CO O K K CO N. to O K CO CO co co in CO K z o a X < O a ücc2 ï az - X<5 CO CO 00 CO s K 00 CM CO H U w 05 m 5° ui ^ o CO X œ O JO O w o 5* Ui UI z O H - o 2 5 * UJ QC _» u. O Œ < ° 2 H a x UI a H UJ o < * D Œ oc . UI CO o a §5 5 J I S > S s 5 o Q. CO f“ < cc U. UI < § Ui cc E ° o s < < ~ o •" Si UI 5 < a H ui g a § H D • a CO UJ < £ 2 5 O 0) UJ UI H o UI tL O < UI ui 2 36 ACCORDING TO LINDSLEY AND ANDERSEN (1983). t 37
Petrologic Interpretation
The Tinaquillo Peridotite Complex underwent a high temperature event (%1400°C) which may be related to the crystalization of the peridotite in the upper mantle where the spinel is the stable phase or related to recrystalli¬ zation processes during solid-state flow and partial melting in the upper mantle. Haggerty (1979) pointed out that the transition of spinel to garnet as stable phases occurs at 1400°C and 25 Kb. Dawson (1980) established the stability of ..the spinel phase down to approximately 120 km depth. Many different mantle geotherms have been proposed from which the depth equivalent to 1400°C can be derived.
Anderson (1980) suggested a temperature profile of the upper mantle (dry lherzolite) which indicates that at a depth of about 130 km a temperature of 1400°C is reached.
Nicolas et al. (1980) stated that it is usually possible to distinguish mantle peridotites from deformed ultramafic cumulates by the following considerations: 1.
The spinel in ultramafic cumulates is commonly associated with olivine; it is euhedral to anhedral and opaque in thin section. The spinel from mantle peridotites is generally aluminous and is red-brown to brown in thin section. 2. In strongly depleted rocks, diopside is absent or scarce owing to its removal during partial melting but magmatic diopside in ultramafic cumulates is usually intertitial and has curved grain boundaries. Thus, the 38
Tinaquillo peridotite may represent a depleted upper mantle peridotite from which only a small percentage of melt was extracted so as to preserve the clinopyroxenes.
Jaques and Green (1980) carried out melting experiments on Tinaquillo peridotite samples from which they first removed 40% olivine. Clinopyroxene disappeared completely after 12% of partial melting at a pressure of 12 Kb and a temperature of 1275°C, and after 15% partial melting at 15
Kb and 1325°C. The melts at these conditions (spinel peridotite facies) ranged from olivine tholeiite through tholeiitic picrite to komatiite.
Philpotts et al. (1972) obtained a light-rare-earth depleted pattern from a sample of the Tinaquillo peridotite.
They interpreted the peridotites as cumulates or residues related to tholeiitic basalts, but they might have undergone subsequent metamorphism. Therefore, the partial melts extracted from the Tinaquillo peridotite must have been primitive to be in agreement with the data of Philpotts et at. (1972) and Jaques and Green (1980).
Presnall and Helsley (1982) proposed a model for the origin of hot spots that depends on the existence of major- element heterogeneities in the mantle. They proposed that depleted peridotite returns into the deep mantle at sub¬ duction zones, and that at certain depth it becomes gravitationally unstable, and tends to rise diapirically through undepleted peridotite. In their model the diapirism is initiated between 100 to 150 km depth, which is 1 39 equivalent to 30 to 50 Kb.
Ross et al. (1980b) studying the tectonite suite of the
Vourinos Complex in Greece proposed a kinematic model in which the first period of deformation was at 60 km depth
(20 Kb). In such a model, the Tinaquillo peridotite might have undergone partial melting at approximately 120 km depth
(40 Kb) whence it started to rise in a mantle diapir. The first metamorphic event recognized in the Tinaquillo peridotite may be related to this diapiric up welling according to the models of Presnall and Helsley (1982) and
Ross et _aT. (1980b). Structures that may be related to this event are all destroyed; only relicts such as the abundant enstatite and less abundant diopside, spinel, and olivine porphyroclasts may be related to this event.
The diapir may have continued rising to shallow levels, possibly to the Mohorovicic discontinuity where separation of the melted fraction may have occurred to form the gabbros. The information, discussed above, combined with data on the orogenic belt in which the Tinaquillo peridotite occurs, may yield constrains for the tectonic setting at which the Tinaquillo peridotite originally formed.
Three tectonic setting in which the Tinaquillo peridotite may have formed are: a mid-oceanic ridge, a
volcanic island arc, or an active continental margin. As mentioned before, Thayer (1980) assumed that the Tinaquillo
peridotite is an ophiolite, formed originally at a mid-
oceanic ridge. In the Tinaquillo Complex only depleted 40 peridotite and gabbroic layers are present, and the gabbros have undergone a granulite facies metamorphism. This meta¬ morphism probably occurred after the separation of the gabbroic melt from the peridotite, close to the Mohorovicic discontinuity as Dawson (1980) has suggested to explain the basic granulites. However, a normal ophiolite sequence has many more rock types which are not present in the Tinaquillo
Complex. This discrepancy may be explained by subsequent dismemberment during its later emplacement, or the magma chamber was deep, small, and related to a slowly spreading ridge which did not have a homogeneous supply from the lower mantle as to develop the whole sequence (Robson and Cann,
1982). If the few chemical analyses of the gabbros (Lopez
E., 1972) show them to be primitive are representative, a mid-oceanic ridge environment may be the most logical site at which the Tinaquillo Complex has formed. Ophiolites or ophiolite-like complexes may also form in volcanic island arcs. Sr isotope values from volcanic arc rocks are generally higher than those from mid-oceanic ridges indicating that island-arc magmas cannot be derived by simple partial melting of the oceanic crust in the sub¬ ducted lithosphere. Miyashiro (1982) established that the
Sr isotopes from island-arc volcanics are similar to those of mantle derived basaltic rocks of intraoceanic islands and continental regions. This leads to the possibility that arc magmas may be derived by partial melting of upper mantle material, such as water-rich peridotite in the upper mantle 41 wedge aboyé a subduction zone. Therefore, the Tinaquillo peridotite might represent the depleted peridotite in an island arc system and the only way to differentiate it from one related to mid-oceanic ridges may be to look at the total assemblage of rocks in an orogenic belt (magmas series, sedimentary rock sequences, metamorphism, etc.).
The same evidence used for a volcanic-island-arc model by Miyashiro (1982) can be used for an origin in an active continental arc. A major difference would be the different proportions of specific magma types present in the belt
(more calcalkaline) and the associated sedimentary sequences and their metamorphism.
To discriminate between the tectonic settings mentioned above, it is necessary to better know the history of the whole belt. The juxtaposition of the Tinaquillo peridotite with the Tinaco Complex and their metamorphism must have represented the climax of the orogenesis which affected the
Caucagua-El Tinaco belt. This metamorphism is a low pressure type. Miyashiro (1982) classified orogenic belts and established that the low pressure belts lying on the continental side may be regarded as representing the deeper part of a volcanic arc zone of an island arc or active continental margin.
The rocks of the Tinaco Complex are interpreted as a metamorphosed volcanic-sedimentary sequence (Menendez, 1966).
The southern part of the complex was intruded by syntectonic \ 42 felsic magmas. Therefore, the Tinaquillo peridotite could be a fragment of the upper mantle root of a normal island arc and the Tinaco Complex could be a low P/T metamorphic belt of a volcanic arc zone. It is also possible that these complexes represent an Andean-type arc. These models would be more complex because island arcs generally have paired metamorphic belts, and it is not certain that such belt of the same age as the Tinaquillo and Tinaco Complexes exists.
A complete understanding of this belt can only be obtained through more precise chemical characterization of the mafic rocks and an extensive geochronological study. 1 43
STRUCTURAL GEOLOGY
Regional Structural Geology
The regional structure in the Cordillera de la Costa belt and the Caucagua-El Tinaco belt are not well understood, partly because of the complexity of the structures and partly because of lack of good radiometric and paleontological dates. It is clear that much more work is needed, but some generalities can be made. In the Cordillera de la Costa belt the ages of the basement (Sebastopol Complex) and the overlying Caracas Group are very scarce. As discussed previously (See Regional
Geology) the basement may have undergone an orogenic event during the Silurian. On scarce fossil evidence the Caracas
Group may be of Late Jurassic to Cretaceous age; some fossils are indicative of shallow marine environment, but Urbani and Furrer (1976) found some fossils typical of a deep water environment and they give an age of Late Cretaceous.
In the Cordillera de la Costa belt rocks are included which may represent oceanic crust, and they were deformed with the sedimentary and partly volcaniclastic sequences of the Caracas Group. These rocks are eclogites (Dengo, 1951;
Morgan, 1969; Navarro, 1977, 1978; Maresch, 1973), serpentinites (Gonzalez de Juana, 1968 and Maresch, 1973), ortho-amphibolites on the Los Monjes Islands (Beets, 1972), pillow lavas on Curacao (Beets, 1972), and lamprophyres and diabases on the Gran Roque Island which have an ocean 44 affinity (olivine tholeiites) and have K/Ar ages of 114 m.y. to 130 m.y. (Santamaria and Schubert, 1974). Coleman (1977) interpreted this association of rocks as representative of convergent plate boundaries. Maresch (1973) and Talukdar and Loureiro (1982) have proposed plate tectonic models to explain the evolution of the Caribbean
Mountains System, which will be discussed at the end of this chapter.
The Cordillera de la Costa belt has a polymetamorphic history as discussed before (See Regional Metamorphism); the last metamorphic event occurred from Late Cretaceous to Early Oligocène (Talukdar and Loureiro, 1982) and was thought to be synchronous with the formation of antithetic south-directed thrusts. Since the Oligocène the southern border of the Caribbean and northern Venezuela have under¬ gone several subsequent deformations. Neogene sediments off the coast of Venezuela are folded (Case, 1975). East- west trending faults such as the Caribbean fault system and the La Victoria fault system are young features, as also east-west trending antiforms and synforms (Dengo, 1951;
Wehrmann, 1972).
Few radiometric dates are available of the Caucagua-
E1 Tinaco belt, south of the La Cordillera de la Costa belt
(Menendez, 1966). There are indications of a metamorphic event in the basement during the Triassic and a later uplift during the late Early Cretaceous. Menendez (1966) described an overlying gently metamorphosed volcanic-sedimentary 45 sequence of post-Caracas Group age. He described the structures of this belt as consisting of open folds, shallow dipping, east-west striking foliations and related thrust faults. Menendez (1966) and Mackenzie (1966) recognized that part of the Caracas Group was overthrust to the south onto the youngest sequences in this belt.
Mackenzie (1966) stated that all the metamorphic rocks in the southern part of the Caucagua-El Tinaco belt are bounded to the south by a north-dipping high angle reverse fault trending N75E. The rocks south of this fault are
Senomanian to Paleocene sediments interbedded with basalts.
The northern contact of the Caucagua-El Tinaco belt is a fault along which the basement (La Aguadita Formation) and the Tinaquillo Peridotite Complex were thrust northward over black phyllites and marbles which were correlated with the Las Mercedes Formation (Mackenzie, 1960, 1966). Based on the orientation of the enstatite porphyroclasts he proposed that the Tinaquillo Peridotite Complex was emplaced
from northwest toward southeast.
Mackenzie (1966) interpreted all the structures in the belt as having formed during the same orogenic event. He
suggested that the regional metamorphism of the Sebastopol
Complex and the intrusion of the Tinaquillo peridotite were
contemporaneous and took place during a Late Cretaceous
(Coniacian) orogenic event and that the younger, post- metamorphic structures were the consequence of the same
stress field. 46 Local Structural Geology
The contact of the Tinaquillo Peridotite Complex and the Tinaco Complex appears to be concordant; that is, major structures in both complexes are parallel to the contact
(Plate 1). These structures are represented by an intense foliation and by tabular bodies of gabbroic composition in the Tinaquillo Complex lying parallel to the foliation.
The contact between the Tinaquillo Complex and the Las
Mercedes Formation to the north is a thrust fault. This thrust fault is of regional extent and it is called the
Manrique thrust. A characteristic detail of this contact.is that the ultramafic body is strongly serpentinized along this fault.
Phases of Deformation in the Tinaquillo Peridotite Complex D^: First Deformation
The oldest structures observable in the field are folds and intense foliation parallel to fold axial planes. The foliation (Si) is characterized by strongly flattened pyroxene crystals (generally orthopyroxene); the foliation has a general east-west strike and it dips shallowly to the south (Figure 2). On the foliation plane one can observe
in general a very well-developed mineral lineation which primarily is the result of parallelism of elongate ortho¬ pyroxene crystals. This lineation plunges to the southeast
(Figure 3). The folds are only visible where pyroxenite Figure 2: Equal-area, lower hemisphere projection of poles to the mylonite foliation (S^) in the harzburgite and dunite (circles) and in the serpentinite (S5) (triangles) in the Tinaquillo Peridotite Complex, the Tinaco Complex, and the Las Mercedes Formation. 47
E
THE TINAQUILLO PERIDOTITE COMPLEX
E
THE TINACO COMPLEX
E
THE LAS MERCEDES FORMATION Figure 3: Equal-area, lower hemisphere projection of the mineral lineations in the Tinaquillo Peridotite Complex
and Tinaco Complex. 48
N
N
8 49 layers or veins occur in the peridotite. The folds are often isoclinal but locally they may be quite gentle; fold axes generally plunge toward the southeast (Figure 4). The deformation which formed all these structures is contempo¬ raneous with the second or third metamorphic event
(temperatures of about 650°C) as discussed in the petrologic interpretation.
T>2'- Second Deformation As can be seen on the geologic map of the area (Plate
1) and the cross sections (Plate 2), the penetrative S^- foliation is folded into large wavelength, open to gentle folds, which approximately trend southeasterly. No penetrative structures, nor mesoscopic folds are associated with this event. Intersections of two S^-planes measured in or adjacent outcrops were constructed. The constructed j3~ fold axes (Figure 4) trend generally between S2W and S75E and plunge gently to moderately to the southeast. Axial planes were constructed through the j3-axes and the mapped axial traces; the poles to the axial planes form a steeply
NW dipping girdle (Figure 4).
D3: Third Deformation
Structures assigned to this phase of deformation refold the structures of the second phase of deformation. As with the second phase of deformation the structures in this phase are not penetrative, and no mesoscopic folds were encountered, Figure 4: Equal-area, lower hemisphere projection of fold axes (solid symbols) and poles to axial planes (open symbols); squares, circles, and triangles represent D^, D2, and D3 structures, respectively. 50
N
N 51 but only large-scale folds occur. The fold axes were constructed in a similar fashion as for the second phase of deformation. The fold axes are lying in a steeply dipping EW striking girdle and thé axial planes strike WNW (Figure
4). These folds occur only in the northern part of the complex which may imply that they are related to the
Manrique thrust fault.
D4: Fourth Deformation
Throughout the complex a fracture cleavage has been found (Figure 5). In some cases they are penetrative and have a spacing of about 0.5 cm. The orientation of this cleavage is very constant throughout the complex, even where the second and third phase folds occur. Therefore, these cleavages may have formed very late. These cleavages strike generally between N50W to N80W and dip steeply to the southwest.
D5: Fifth Deformation
The Tinaquillo Complex is thrust northward along the Manrique thrust fault across the low-grade metamorphic rocks of the Las Mercedes Formation. Few kinematic data, however,
could be acquired to indicate the sense of displacement
along the Manrique fault. Only locally it could be established that the fault dips between 30° to 40° to the
south; in these places the foliation in the metasediments
of the Las Mercedes Formation is parallel to the fault. The Figure 5: Equal-area, lower hemisphere projection of fold axes (solid symbols) and poles to axial planes (open symbols); circles and triangles represent respectively second and third phases of deformation in the Las Mercedes
Formation. The data of the Tinaquillo peridotite represent the poles to fracture cleavage. 52
W --
THE LAS MERCEDES FORMATION
W -- --E
THE TINAQUILLO PERIDOTITE COMPLEX 53
Tinaquillo Complex is strongly serpentinized along the thrust fault; a foliation (S5) has developed in the serpentinite and it strikes EW and dips moderately to the south (Figure 2). On the basis of this parallelism, and on the basis of the outcrop pattern of the fault, both based on field mapping and on inspection of aerial photo¬ graphs, it appears that the Manrique fault is a low-angle fault.
Dg: Sixth Deformation
In the northern part of the complex north-south striking faults occur, which offset the Manrique fault
(Plate 1). On the basis of field observation and of aerial photograph, these faults are interpreted to be steep or vertical.
Phases of Deformation in the Tinaco Complex
D^: First Deformation The rocks of the Tinaco Complex are penetratively deformed. The most obvious structure is a well-developed foliation (Sj), which is axial planar to mesoscopic isoclinal folds. These folds are rare and difficult to find. The poles to the foliation form a steep EW striking girdle, but most foliations are subhorizontal (Figure 2). Near the contact with the Tinaquillo Complex a strong mineral lineation has been observed. In general, the lineation plunges about 15° to the southeast (Figure3). 54
Amphibolite layers in the feldspar-quartz gneiss sequence are frequently boudinaged.
Second Deformation The foliation (Si) has been refolded into megascopic open to gentle folds (Plate 2). No mesoscopic structures have been recognized related to these folds. The Si measurements just south of the Tinaquillo Complex indicate that the fold (j^) axis plunges gently to the southeast.
Approximate axial planes constructed fromj3~axes and the mapped traces of the folds are steep and strike approxi¬ mately north-south (Figure 4). These second phase folds are continuous with the second phase folds in the
Tinaquillo peridotite (Plate 2).
Phases of Deformation in the Las Mercedes Formation
Di: First Deformation
Very seldomly mesoscopic isoclinal folds have been found and only in outcrop where some lithologic variation exists of layers of more pelitic material next to layers of calcareous phyllites. An axial-plane foliation (S^) is developed and this foliation is the only penetrative structure that was found. This foliation strikes roughly east-west and the dips are variable (Figure 2). Some marbles layers are strongly boudinaged, but no boudin axes were measured, because of poor outcrop. 55
Ü2: Second Deformation The Si foliation is folded in broad megascopic open folds. The constructed fold east-northeast (Figure 5). The constructed axial planes strike approximately northeast-southwest and have variable dip (Figure 5). On the basis of field mapping and inspection of the aerial photographs some axial traces are plotted (Plate 1) but these are interpretative, because the outcrops are not close enough. Therefore, these planes are not plotted on Figure 5.
Dg: Third Deformation
The D2 folds are arcuate (Plate 1). This arcuation may have formed during the second deformation, or during a later event (D3). The lack of outcrop makes it impossible to choose between these two alternatives. A constructed axial plane strikes N36E and dips about 60° to the south. The fold (j3> axis trends N61E and plunges 36° (Figure 5).
D4: Fourth Deformation
The fourth phase of deformation is assigned to the thrusting of the Tinaquillo Complex over the Las Mercedes
Formation. As described before, the Si foliation in the phyllites is always parallel to the thrust contact; locally
a breccia occurs above the calcareous phyllites and beneath
the Tinaquillo Complex; the breccia consists of fragments of serpentinite in a phillitic matrix. 56
D5: Fifth Deformation The left-lateral faults discussed above in the
Tinaquillo Complex are assigned to this phase.
General Structural Interpretation
The study area can be divided into two tectonic provinces which are very different lithologically and structurally, and which were metamorphosed at different conditions and probably at different times. One province consists of the Tinaco and Tinaquillo Complexes, and the other is the Las Mercedes Formation; the two provinces are separated by the Manrique thrust fault.
The Las Mercedes Formation consists of phyHites, calcareous phyllites, and marbles. These rocks have lithologic counterparts elsewhere in the Caracas Group of the Cordillera de la Costa belt. Elsewhere poorly preserved fossils have been found which indicate that the rocks have been deposited during the Jurassic and/or Cretaceous. These rocks may have been deposited on an outer-slope rise of an
Atlantic-type of margin. The Tinaquillo Complex on the other hand represents a slice of upper mantle and the Tinaco
Complex is a metasedimentary and metavolcanic sequence which has been intruded by felsic igneous rocks. The sediments may have been deposited in an inner shelf environment; the volcanic rocks of basaltic composition may be related to arc magmatism. 57
The metamorphic age of the Tinaco Complex is poorly known; the few dates suggest a Late Paleozoic or Early
Mesozoic event. As the oldest structures in the Tinaco and
Tinaquillo Complexes have the same style, and orientation, they are interpreted here as having been formed contempora¬
neously. The metamorphic age of the Las Mercedes Formation
is unknown, but if these rocks are correlative with the
Caracas Group, the metamorphic event must have taken place
during the Cretaceous or later.
The metamorphic grade of the two provinces is very
different. The main structures in the Tinaquillo and Tinaco
Complexes have formed during the amphibolite-facies meta¬
morphism, while the Las Mercedes Formation has never seen
conditions higher than the high greenschist metamorphism.
Structurally the Tinaquillo Complex underwent two deformational events which cannot be found in the Las
Mercedes Formation. In the next paragraph an
interpretative and sometimes speculative structural history
of the study area is given. The correlation of structures in the Tinaquillo and Tinaco Complexes and the Las Mercedes
Formation is given in Table 4.
The earliest (pre-D^) event that affected the rocks of
the Tinaquillo Complex have been discussed elsewhere (See
Petrologic Interpretation). Any possible older structures
were completely destroyed by the penetrative deformation.
The structures in the Tinaquillo Complex are similar in
style and orientation to the ones in the Tinaco Complex, INTERPRETATIVE STRUCTURAL HISTORY Wmm > > > - o a o LU H H D E O O O CL LU O o o 5 zi •J »- H LU Z < X T— CD CM O in o o 2 CL -J LU X Ui UCD H 3 OT u. Z (0 *>
NW-S O h» Fractu o ® H JO «♦"* 2 u. 2 U. UJ CO LU CO tL O) c 3 CO w CO o c o o CO
The D3 through Dg structures in the Tinaquillo Complex may all be the result of one tectonic event in which the
Tinaquillo and Tinaco Complexes were emplaced on top of the
Las Mercedes Formation along the Manrique thrust fault. In the Tinaquillo Complex the east-west trending D3 folds may be related to early northward juxtaposition of the two provinces; at the same time the isoclinal folding of the
Las Mercedes Formation may have occurred. The Di structures in the Las Mercedes Formation are contemporaneous with the greenschist facies metamorphisms; at the same time serpentinization may have occurred in the Tinaquillo Complex.
The D4 fracture cleavage may be related to D3. During continued northward thrusting the Las Mercedes Formation is refolded and serpentinite breccia is formed along the thrust plane. At a late stage of thrusting north-south striking
tear faults developed (Dg in the Tinaquillo Complex and D5
in the Las Mercedes Formation). t 60 Textures and Microfabrics
Textures and Microfabrics in the Tinaquillo Peridotite
Complex
The Tinaquillo peridotite consists mainly of two litho¬ logies, ultramafic rocks and gabbros, in which the textures are clearly different. However, the texture of the gabbros is similar to that of the gabbroic rocks in the Tinaco
Complex to the south and therefore the texture of the gabbroic rocks will not be discussed here but later (See
Textures and Microfabrics in Rocks from the Tinaco Complex).
The dunites and harzburgites show a well-developed tectonite mylonite foliation (Si) and lineation (L). In pyroxenites layers which are isoclinally folded, the foliation is penetrative but pyroxenites which are only open to gently folded, have a hypidiomorphic texture with medium to fine grain size. The texture of this type of pyroxenite can be interpreted as an igneous one which means that its intrusion occurred very late during the deformation which is responsible for the foliation (Si). Another penetrative foliation occurs in the serpentinites (S5); they show a well-developed foliation and have a lepidoblastic texture; it formed contemporaneously with the thrusting along the
Manrique thrust fault.
The dunites and harzburgites have a bimodal grain size distribution which is reflected in the porphyroclastic texture of the rocks. Large and frequently elongate strained 61 crystals (porphyroclasts) are enclosed in a matrix of small generally polygonal, strain-free grains (neoblasts), the latter representing over 60 percent of the samples and even much more in the samples with equigranular texture.
The matrix of the porphyroclastic and equigranular rocks show a well-developed foliation which is defined by a dimensional preferred orientation (Figure 6 and 9), and which is related to a lattice preferred orientation. The development of preferred lattice orientation in the ultra- mafic rocks is not clearly established in this study.
Syntectonic recrystallization could be responsible for the origin of the preferred orientation, as it has been shown experimentally to operate above 500°C (Avè Lallemant and Carter, 1970) or translation glide which is a contrary view supported by Nicolas et al. (1973). In some rocks a post- tectonic annealing crystallization is responsible for the coarse and granular texture.
Enstatite constitutes the most abundant porphyroclast phase with a minor amount of spinel, diopside, and olivine.
The enstatite crystals are flattened and elongate (the maximum length measured in thin section is 6.5 mm), with variable length/width ratios; the (100) plane lies parallel to the foliation plane. They show serrated grain boundaries and they are surrounded by a mantle of finely recrystallized enstatite. Kink bands are well-developed; more than one
generation of kink bands may be present according to Basu
(1977) (Figure 6). The porphyroclasts are occasionally bent 62
Figure 6: Tuning fork like (primary, secondary, and tertiary) kink bands in orthopyroxene porphyroclast.
Long dimension = 3 mm. 63 (Figure 7) which makes it possible to obtain the sense of the shear by which the mylonites formed. The enstatite porphyroclasts frequently have (100) exsolution lamellae of diopside and in some samples granular exsolution has occurred as well (Figure 8). In few samples exsolution of spinel is present along the (100) planes of the enstatite porphyroclasts.
The spinel porphyroclasts are frequently flattened and elongate (the maximum length measured in thin section is 4.1 mm) with irregularly serrated grain boundaries and they are surrounded by recrustallized silicate minerals.
Diopside porphyroclasts are very rare in the complex; they are xenoblastic and equidimensional (the maximum diameter measured in thin section is 1.5 mm), showing sweeping undulatory extinction; kink bands and exsolution lamellae were not observed.
Olivine porphyroclasts are rare in Tinaquillo (the maximum length measured in thin section is 5.9 mm); their boundaries tend to be straight at the contact with neoblasts of the same mineral (Figure 9). Of all the porphyroclasts only enstatite and spinel often show a strong lineation in the foliation plane.
The matrix of the porphyroclastic and the equigranular
rocks is formed mainly by olivine neoblasts and to a lesser
degree by enstatite, spinel, and diopside. The grain size
of these minerals is less than 0,1 mm and they show two
different textures: 1. straight boundaries between 64
Figure 7: Bent enstatite porphyroclast with recrystallized, areas at the inner hinge area (pressure shadow) and myloni- tic texture at the outer hinge area. Long dimension = 3 mm. 65
Figure 8: Diopside exsolution lamellae along (100) plane of host orthopyroxene. Long dimension = 3 mm. 66
Figure 9: From the left to the right there are an olivine porphyroclast, a zone of recrystallized olivines, a zone with mylonitic texture, and a zone with incipient annealing texture. Long dimension = 3 mm. 67 neoblasts; equigranular, well oriented elongate grains but sometimes the grain size is larger due to annealing crystallization; 2. mylonitic texture consisting of very fine grained neoblasts and occurring along shear bands which locally are not parallel to the foliation (Figures 7 and 9).
Paleopiezometry
Research on metals during the last two decades has shown that during high temperature deformation, when syntectonic recrystallization (hotworking) occurred, a steady-state grain size develops which is directly related to the differential stress. This feature has been investigated in geological materials in the last ten years.
It has indeed been found that during high temperature flow of silicates where syntectonic recrystallization occurs, a grain size develops which is only a function of differential stress. The latest stress/grain size relationship for olivine was established by Ross et al. (1980a). Similarly, such a relationship exists for enstatite (Ross and Nielsen, 1978). These relationships were applied to two samples with olivine and enstatite paleoblasts and neoblasts. In each sample 100 neoblasts were measured and as many paleoblasts as possible, and the results are plotted on Figures 10 and
11. Sample VT0-82-55: The olivine porphyroclasts have a bimodal grain size distribution with the following means and 68
log (Gi -Cr3), Kbar
Figure 10: Recrystallized grain size vs. stress for olivine after Ross et al. (1980). Solid symbols represent both pop¬ ulations of paleoclasts and open symbols neoblasts; square represents a single crystal in sample VT-82-lb, circles are data from sample VT-82-lb, and triangles from sample VTO-82- 55. t 69
log (G~i - 03), Kbar
Figure 11: Recrystallized grain size vs. stress for enstati- te after Ross and Nielsen (1978). Solid symbols represent paleoclasts and open symbols represent both populations of neoblasts; circles are data from sample VT-82-lb and tri¬ angles from sample VT-82-55. 70 standard deviation: 0.51 + 0.17 mm and 2.07 + 0.66 mm. The population of smaller grains tend to be transitional toward the neoblasts grain size (Figure 9). The enstatite porphyroclasts have a mean grain size and standard deviation of 2.61 + 1.85 mm (Figure 11).
The olivine neoblasts have the following mean grain size and standard deviation: 0.053 + 0.032 mm and the enstatite neoblasts show a bimodal grain size distribution with the following means and standard deviations for each population: 0.159 + 0.073 mm and 0.054 + 0.023 mm.
Sample VT-82-lb: The olivine porphyroclasts show a bimodal grain size distribution; the two populations have a mean and standard deviation of 0.89 + 0.41 mm and 1.26 +
0.45 mm, but in rare cases the olivine grains may attain a very large size (up to 5.7 mm). The enstatite porphyro¬ clasts have a grain size of 2.93 + 2.33 mm, which are the mean and standard deviation, respectively.
The olivine and enstatite neoblasts have the following means and standard deviations: 0.051 + 0.024 mm and 0.049 +
0.025 mm, and these grain sizes tend to be transitional toward the porphyroclasts.
The magnitudes of the differential stresses during the steady-state flow of the Tinaquillo tectonite were calculated using the experimental calibrations of Ross and Nielsen
(1978) and Ross et al. (1980a) who indicate that the re- crystallized-grain size is a function of differential stresses as long as steady-state conditions were reached. 71 The size of the porphyroclasts probably yields only maximum stresses because their size may have been reduced during subsequent deformation, and thus they may be smaller than they were originally. The size of the neoblasts may have increased because of post-tectonic annealing crystallization, and thus the derived stresses are minimum stresses.
The olivine and enstatite porphyroclasts of Sample
VTO-82-55 yield maximum differential stresses as low as +42 +28 118 _23 bar for olivine and 8.8 _ ^ bar for enstatite. The olivine neoblasts may have formed at minimum differential stresses of 2.1+2 3 Kb and the enstatite neoblasts of -0.6 Kb 0.84 ^Q*29 (Figure 10). The isolated and largest olivine crystal is indicative of a maximum differential stress as low as 52.8 bar. The olivine porphyroclasts of Sample + 72 VT-82-lb yield a maximum differential stress of 174.6 _3^ bar and the enstatite porphyroclasts 7.7 ^3*6 ^ar- T^e +1 4 olivine neoblasts yield stresses of 2.2 _Q‘Q Kb and the enstatite neoblasts of 0.94 Kb (Figure 11).
It is obvious that the stresses derived from the grain size of olivine are quite a bit higher than those of orthopyroxene; this applies for both paleoblasts and neo¬ blasts. There may be many reasons for this discrepancy.
In the first place, it is possible that the experimental stress/grain size equation cannot be extrapolated to natural
conditions. An alternative explanation is that ortho¬ pyroxene crystals ceased to recrystallize at conditions of higher temperature and lower stresses, while olivine 72 continued to recrystallize. Furthermore, pressure shadows developed in particular near orthopyroxene porphyroclasts, resulting in stress gradients and lower stresses where orthopyroxene recrystallized. All these uncertainties cannot be evaluated here. One only can suggest that the porphyroclasts were formed at very low differential stresses (5 - 50 bar) typical for the upper mantle (Mercier,
1980), while the mylonitization occurred at shallow crustal levels at high stresses of between 0.8 and 3 Kb.
Petrofabric Analysis
Since the early 1930's it is known that silicates in metamorphic and tectonite rocks have preferred lattice orientations, but until the advent of experimental studies,
these lattice preferred orientation could not be related to
the physical parameters of deformation. Thus, the question whether lattice preferred orientation were related to the
principal stress axes, principal strain axes, or to the movement picture of deformation, has been resolved to some
extent.
It has been shown in experimental studies that
preferred lattice orientations are related to the strain ellipsoid if the mechanism of deformation is translation
glide, and to the stress ellipsoid if syntectonic re¬ crystallization has occurred. In co-axial deformation it has been shown that X = |oioj olivine will be oriented parallel to the major principal compressive stress axis 73
(CJ^) or the major principal compressive strain axis (€^) and that Z = £lOoj becomes oriented parallel to the least principal compressive stress axis (CJ3) or the major extensile strain axis (€3) (Nicolas et al., 1973; Avè
Lallemant, 1975). In non-coaxial experiments olivine deformed by translation glide may follow the strain ellipsoid (Nicolas et al., 1973) but if syntectonic recrystallization occurs the orientation follows the stress ellipsoid (Kunze and Avè Lallemant, 1981). However, complications occur when de¬ formation is very inhomogeneous and in the field of the brittle-ductile transition, such as in mylonites. Ross et al. (1980b) have shown this for the Vourinos ophiolite in
Greece. They proposed that the mylonites of Vourinos were formed during north-northeast directed obduction. Their conclusions were based on the monoclinic symmetry of the olivine fabric and the orientation of major Z = £lOoj maxima with respect to the shear plane. To find out how the Tinaquillo mylonite was formed, or what the sense of shear was by which the mylonite formed, petrofabric analysis was carried out on six samples from the complex. The thin sections were cut almost perpendicular to the foliation and
100 crystals were measured in each sample. The measurements were plotted on an equal-area projection on the lower hemisphere.
Sample VT-82-lb (Figure 12): Sample VT-82-lb is a harzburgite with porphyroclastic texture. The porphyroclasts Figure 12: Equal-area, lower hemisphere projection of 100 X = ^Oioj> Y = ^OOlj» and Z = £lOoj axes of olivine in sample VT-82-lb. Contours at 1, 2, 3, 4, 5, and 6% for X, at 1, 3, 5, 6, 7, and 8% for Y, and at 1, 2, 3, 4, and
5% for Z per 1% area; white areas have less than 1% points, and stippled areas have between 1 and 3% for Y and between
1 and 2% for X and Z. Solid circle (S^) is the foliation, and L is the enstatite lineation. Dashed circle represents the horizontal plane with S = south and W = west. 74
X - OI i v i n e
Y - OI i v î n e
Z-Oli vine t 75 are mainly enstatite and olivine. The last one tends to be transitional toward neoblast size as described in the para¬ graph on paleopiezometry. The sample comes from the main body (Plate 1 and Figure 25) which is the thickest part of
the complex where the foliation is very constant.
The fabric of this sample is heterotactic. The
strongest fabric element is the Y = £oOlj olivine maximum which lies nearly in the foliation plane at about 65° from
the lineation; this fabric has axial symmetry. The X = [oioj
axes form a diffuse girdle around the Y maximum, with a maximum in the foliation plane; this fabric element has approximately orthorhombic symmetry. The Z = £lOoj axes form also a diffuse girdle around the Y maximum; in this
girdle there are two maxima approximately at 45° to the
foliation; one maximum is clearly much better developed than
the other, resulting in a monoclinic symmetry.
Sample VT-82-2 (Figure 13): Sample VT-82-2 is a
porphyroclastic harzburgite in which enstatite porphyroclasts
are present. The sample comes from the main body (Plate 1 and Figure 25). The fabric of this sample is heterotactic.
The Y = ^OOlj olivine maximum is the strongest fabric element which lies nearly in the foliation plane at about 50° from
the lineation; this fabric shows an axial symmetry. The X = ^OloJ axes approach an orthorhombic symmetry with two mutually perpendicular girdles with several point maxima
although these maxima are not symmetrically distributed
about the symmetry planes. A point maximum makes an angle Figure 13: Equal-area, lower hemisphere projection oi 100 X = » Y = [ooi]« and Z = [loo] axes of olivine in sample VT-82-2. Contours at 1, 2, 3, 4, 5 and 6% for X, at 2, 4, 5, 6, 8 and 10% for Y, and 1, 2, 3, 4, 5, 6, 7 and 8% for Z per 1% area; white areas have less than 1% points for X and Z and less than 2% points for Y, and stippled areas have between 1 and 2% for X and Z and between 2 and 4% for Y. Solid circle (S-^) is the foliation, and L is the enstatite lineation. Dashed circle represents the horizontal plane with S = south and
E = east. 76
X-Ollvine
Z-Olivine 77 of 150 with the foliation plane. The Z = j^lOOJ axes resemble a monoclinic subfabric with a maximum at approximately 45° to the foliation.
Sample VTO-82-22 (Figure 14): Sample VTO-82-22 is a porphyroclastic dunite with enstatite and spinel as porphyroclasts. The sample comes from the Cerrito Blanco area (Plate 1 and Figure 25) from the east limb of the approximately north-south trending antiform.
The fabric of this sample is heterotactic with the Y = [ooij axes of olivine defining the strongest fabric element. This fabric element shows an axial symmetry with a maximum in the foliation plane at about 65° from the lineation. The X = £oioJ axes form a girdle with some local maxima irregularly distributed; thus, the symmetry is almost axial as well. The Z = ^10oJ axes form a girdle pattern around the Y maximum with a maximum at high angle to the foliation. Sample VTO-82-40 (Figure 15): Sample VT0-82-40 is an equigranular dunite with only few small enstatite porphyroclasts; the matrix crystals may have undergone some annealing crystallization. This sample comes from one out¬ crop close to the left-lateral strike fault and the thrust fault (Plate 1 and Figure 25). The foliation in this area has been rotated; it is N29E 27SE instead of the roughly E-W strike elsewhere in the complex.
The fabric of this sample is heterotactic with the X = 010 axes distribution showing an axial symmetry; they Figure 14: Equal-area, lower hemisphere projection of 100 X = [OIO], Y = [ooi]> and Z = £lOoj axes of olivine in sample VTO-82-22. Contours at 1, 2, 3, 5, 6 and 7% for X, at 1, 3, 5, 7, 8 and 9% for Y, and 1, 2, 3, 4 and 5% for Z per 1% area; white areas have less than 1% points, and
stippled areas have between 1 and 2% for X and Z and between 1 and 3% for Y. Solid circle (S^) is the foliation,
and L is the enstatite lineation. Dashed circle represents
the horizontal plane with N = north and W = west. 78
X-Ollvlne
Y-OMvlne
Z-Oli vine Figure 15: Equal-area, lower hemisphere projection of 100
X = [OIO], Y = ^OOlJ, and Z = £lOoJ axes of olivine in sample VT0-82-40. Contours at 1, 2, 3, 4, 5 and 6% for X, at 1, 3, 6, 11 and 13% for Y, and at 1, 2, 3, 4, 5, 6 and 7% for Z per 1% area; white areas have less than 1% points, and the stippled areas have between 1 and 2% for X and Z and between 1 and 3% for Y. Solid circle (Sj) is the foliation, and L is the enstatite lineation. Dashed circle represents the horizontal plane with S = south and W =west. 79
X-Olivine
Y-Olivine
Z-Olivine Figure 16: Equal-area, lower hemisphere projection of 100
X = [oioj » Y = [ooi] > arid Z = ^looj axes of olivine in sample VTO-82-55. Contours at 1, 2, 3, 4, 5 and 6% for X, at 1, 2, 3, 4, 5, 6 and 7% for Y, and at 1, 3, 4, 5, 6, 7 and 8% for Z per 1% area; white areas have less than 1% points, and stippled areas have between 1 and 2% for X and Y and between 1 and 3% for Z. Solid circle (S^) is the
foliation, and L is the enstatite lineation. Dashed
circle represents the horizontal plane with S = south and W = west. 80 form a partial girdle around the Y-axes maximum. The Y =
[ooij axes define an axial subfabric with a strong maximum in the foliation plane, almost normal to the lineation. The Z = [loo] axes pattern show a diffuse girdle around the Y maximum with two point maxima at about 45° to the foliation; one of the point maxima is slightly stronger than the other suggesting a monoclinic symmetry.
Sample VT0-82-55 (Figure 16): Sample VTO-82-55 is a porphyroclastic dunite in which enstatite, spinel, diopside and olivine are the porphyroclasts in order of abundance. In this sample there is a gradation from the smallest olivine porphyroclast population toward the neoblast size as described above. The sample comes from La Pica area, at the east limb of a gentle north-south trending synform
(Plate 1 and Figure 25).
The fabric of this sample is heterotactic with the Y =
[ooij axes defining the strongest fabric. This fabric element shows an axial symmetry with a strong maximum in the foliation plane at about 60° from the lineation. The X = [OIO] axes pattern show a monoclinic symmetry with a maximum at about 45° to the foliation, whereas the Z =£lOoj axes display a similar monoclinic symmetry.
Sample VTO-82-123 (Figure 17): Sample VTO-82-123 is a porphyroclastic harzburgite with enstatite porphyroclasts in an equigranular matrix. This sample comes from the main body (Plate 1 and Figure 25).
The fabric of this sample is heterotactic and the X = 81
X-Ollvine
Y-Olivine
Z-Ollvîne Figure 17: Equal-area, lower hemisphere projection of 100
X = [oio] » Y =[oOl] , and Z = ^10oJ axes of olivine in sample VTO-82-123. Contours at 1, 3, 4, 5 and 7% for X, at 1, 4, 8, 10 and 15% for Y, and at 1, 3, 4, 5, 7 and 10% for Z per 1% area; white areas have less than 1% points, and stippled areas have between 1 and 3% for X and Z and between 1 and 4% for Y. Solid circle (S^) is the foliation, and L is the enstatite lineation. Dashed circle represents the horizontal plane with S = south and W= west. 82
X-Olivine
Y - OI i v I n e
Z-Ollvine 83
£oioj axes have an orthorhombic symmetry with the maximum almost at 90° to the foliation, whereas the strongest fabric is defined by the Y = ^OOlj axes. This fabric has an axial symmetry with the maximum in the foliation plane at about
70° from the lineation. The Z = |^10oJ axes show a pattern which resembles an orthorhombic symmetry, but the symmetry axes do not parallel those of the mesoscopic fabric.
Conclusions based on the olivine fabrics Ross eb Al. (1980) used the monoclinicity of the Z-axes subfabric of olivine, and the orientation of the symmetry axes with respect to the mesoscopic fabric to interpret the sense of the shear in the mylonites of the Vourinos complex in Greece. This method was applied to the subfabrics in this study. On this basis shear senses were derived and plotted on the Figure 25. As can be seen four subfabrics indicate a sense toward the northwest and the other two toward the southeast. Therefore, the monoclinicity of the
Z-axes subfabrics of olivine cannot be used to establish the sense of shear.
An important observation is that the two samples with subfabrics indicating a southeasterly transport are texturally very different because almost 50% of these rocks consist of small porphyroclasts. Both neoblasts and small- porphyroclasts have been measured which may have caused this discrepancy.
Kunze and Avè Lallemant (1981) performed deformation 84 experiments of olivine and while they interpret their fabric symmetries in term of principal stresses, these fabrics could be interpreted differently. In fact, the maximum concen¬ tration of Z-axes appears to be oriented parallel to O3 and not in contrast to the results of Ross et al. (1980).
The trend of the maximum compressive stress is parallel to the mineral lineation (northwest-southeast) and the intermediate principal stress axis (O2) is parallel to the orientation of the olivine Y-axes. Thus, the foliation (S^) is approximately the shear plane and the Y-maximum is perpendicular to the transport direction. Therefore, at 90° from the Y-maximum in the foliation plane is the shear direction but the sense cannot be obtained unequivocally from the monoclinicity or the orientation of Z-axes symmetry with respect to the foliation (S^).
Textures suggesting the sense of the shear
Because the sense of shear in the mylonites could not be derived from the olivine fabric, the samples were studied to find textural evidence for slip in one or the other direction. The textures which indicate the sense of the shear were studied in thin sections cut almost perpendicular to the foliation and at variable angles to the mineral lineation. The samples chosen were the same which were analyzed in the microfabric study. Five of these samples have porphyroclasts and so they are useful to determine the sense of shear. The sixth one is equigranular, without 85 porphyroclasts and it is not useful for this purpose. Basically the criteria used to deduce the sense of the shear are those summarized by Simpson and Schmid (1983).
In Figure 18 some examples of the textures are shown.
Figures 18a and 18c represent asymmetric pressure shadows on equidimensional enstatite porphyroclasts which indicate movement to the northwest. Figures 18b, 18d, and 18e represent a composite texture which is formed by folded enstatite porphyroclasts and their associated pressure shadows. The textures of Figures 18a, 18c, 18d, and 18e are examples of movement to the northwest, whereas Figure
18b suggests southeast directed shear.
Of twenty-two textures that were studied, eighteen are indicative of simple shear toward the northwest and four toward the southeast as is shown in the Figure 18. The minerals that form the pressure shadows in all the textures studied have a larger size than the minerals elsewhere in the matrix. With the exception of the crystals that re¬ crystallized from the porphyroclasts, in both regions the most abundant mineral is olivine. From the studied textures it is possible to establish that the majority of the textures support the sense of movement in the mylonitic rocks toward the northwest; such conclusion would support the hypothesis of Ross et al. (1980b). The enstatite porphyroclasts are pretectonic in relationship to the foliation (S^) and their lineation may have formed by rotation at the earliest time of the orogenic event during Figure 18: A. - Enstatite porphyroclasts with associated asymmetrical pressure shadows related to a sinistral simple shear. B. - Folded and fractured elongate enstatite porphyroclast with sinistral simple shear interpreted from the asymmetry of the pressure shadow areas. C. - Enstatite porphyroclast with associated asymmetrical pressure shadows related to a sinistral simple shear. The porphyroclast recrystallized in poorly-developed "tails" of retort shape which are indicative of the same shear sense. D. - Folded elongate enstatite porphyroclast with associated asymmetrical pressure shadows. The fold and the asymmetry of the strain free areas are indicative of a sinistral simple shear.
E. - Gentle folded enstatite porphyroclasts with associated asymmetrical pressure shadows and poorly-developed "tail" of the retor-shape which are indicative of dextral simple shear. All the pressure shadows consist mainly of olivine which is slightly coarser than the olivine in the matrix.
The crystals around the porphyroclast are neoblasts of the same mineral. The broken lines represent (Si). 86
N7W S7E
A. SAMPLE VT-82- 1 b
B. SAMPLE VTO-82-22
C. SAMPLE VTO-82-40 D. SAMPLE VT0-82-55
E. SAMPLE VTO-82-123 88 which the foliation (S^) developed, whereas the pressure shadows and the recrystallization of the porphyroclasts occurred subsequently, but during the same orogenic event.
Textures and microfabrics in rocks from the Tinaco Complex
Gabbroic Rocks
The gabbroic rocks directly to the south of the
Tinaquillo peridotite have the same mineralogy and texture as the gabbroic rocks in the Tinaquillo peridotite. The texture is mainly gneissic, nematoblastic with augite relicts and plagioclase, and rarely garnet porphyroclasts; hornblende occurs as porphyroblasts; the matrix is finer grained and consists of the same minerals except for garnet.
Figure 19 shows a porphyroclastic gabbroic rock from the outer part of the so-called "contact aureole" of Mackenzie
(1960) and Green (1967); this sample has garnet porphyroclasts and the last authors explain the formation of such minerals by the following equation: 2 Hypersthene +
Anorthite in Plagioclase = Garnet + Quartz, in which the left hand side of the equation is favored by higher temperature or lower pressure and free quartz is unlikely to appear as it would be expected to react with the hornblende present.
The mineral assemblage representing the first meta- morphic event in the gabbroic rocks (See Local Metamorphism) is hypersthene, augite, plagioclase (andesine, probably transformed from labradorite), and garnet. The interpréta- 89
Figure 19: Garnet porphyroclasts in gabbroic rock at the southern contact with the Tinaquillo peridotite. The garnet has poikiloblastic texture and the inclusions are mainly rounded plagioclase and clinopyroxene. Long dimension 3 mm. 90 tion of Mackenzie (1960) and Green (1967) is not in agreement with this association. This association is based on the relationship between the minerals and the textures of the few rocks with garnet (Example Figure 19).
The maximum amount of hypersthene observed in the samples without garnet is 5% which is not sufficient to form the 20% by volume or more of garnet which occurs in the garnet amphibolites which indicates a lack of mass balance; about 50% of hypersthene is needed in the original rock to get the observed amount of garnet. Most norites (presumably the original rock type) have much less, as shown by Le
Maitre (1976) (26.4% hypersthene; 211 analysis).
The texture is in agreement with the crystaloblastic series (Spry, 1969). The garnet tends to be the most idioblastic mineral and also has the largest grain size.
The garnet is poikiloblastic with rounded inclusions of plagioclase and augite. If the garnet would have formed from the plagioclase and hypersthene, a disequilibrium simplectitic textures should be present at the contact of the inclusins or at the contact of garnet and plagioclase crystals.
Another argument that was mentioned by Mackenzie
(1960) and Green (1967) to support the contact metamorphism is related to the brown hornblende in these rocks. On the basis of their texture it can be shown that the brown
hornblende and the green hornblende were formed during the
'second regional metamorphic event (the first of the green 91 hornblende bearing rocks and the second of the brown hornblende bearing rocks); the change in color only reflects a temperature gradient from north to south during this event or a different original composition.
There are other textures from which it is possible to establish the conditions of crystallization of the brown hornblende. Mackenzie (1960) and Green (1967) proposed that there existed pressure and temperature gradients, character¬ ized by garnet in the outer part of the aureole (higher pressure) and by brown hornblende in the inner part of the aureole (higher temperature). However, in some samples both minerals occur in which the brown hornblende shows charac¬ teristic textures of regional metamorphism and it substitutes pyroxene in the matrix and also pyroxene inclusions in the garnet (Figure 19). Therefore, the garnet and brown hornblende were not formed during the same metamorphic event as proposed by Mackenzie (1960) and Green (1967); instead they represent a polymetamorphic history.
Crystallization of the brown hornblende probably occurred in two stages, based on two textural types. In one type coarse, hypidioblastic, elongate hornblende crystals give the rock a foliated appearance (Figure 20). In other rocks the hornblende is xenoblastic and fined-grained re¬ crystallized zones occur throughout the rock; these zones appear to be parallel to the foliation (Figures 21 and 22). 92
Figure 20: Partial or total substitution (crown texture) of pyroxene by brown hornblende in gabbroic rock from the Tinaquillo peridotite. Long dimension = 3 mm. 93
Figure 21: Gabbroic rock from the Tinaquillo peridotite with mortar texture. Long dimension = 3 mm. 94
Figure 22: Porphyroblastic gabbroic rock from the southern contact of the Tinaquillo peridotite. The pyroxene porphyroclasts were replaced almost totally by hornblende. The mortar texture is well developed. Long dimension =
3 mm. 95 The Tinaco Complex The rocks from the Tinaco Complex south of the pyroxene bearing gabbroic rocks have a well developed gneissic texture and the schistosity is mainly nematoblastic. The amphibole gneisses show a well defined plagioclase and amphibole banding; sometimes the plagioclase is large enough as to define a porphyroblastic gneissic texture in the plagioclase quartz gneisses.
Another common texture in these rocks is the mortar texture. This texture is better developed in the plagioclase quartz gneisses in which the plagioclase and quartz have re¬ crystallized to a finer grain size at the boundaries. The mortar texture is similar to the mortar texture developed during the same orogenic event in the gabbroic rocks to the north.
Petrofabric Analysis
The mineral chosen for petrofabric analysis was quartz.
Many experimental studies have been carried out (Green et al.,
1970; Tullis, 1970; Tullis et al., 1973; Tullis, 1977) showing how the c-axes of quartz are oriented with respect
to the principal stress or strain axes. There are also
several theoretical studies (computer simulation; e.g.
Lister et al. , 1978) which relate the preferred lattice
orientation of quartz to the physical factors which caused
it. Sylvester and Christie (1968) and Hara et^ an (1973)
were able to relate natural quartz fabrics to mesoscopic 96 structures from which they could deduce the strain field, and thus they were able to relate the quartz fabric to the strain axes.
Because the mesoscopic structure of the Tinaco Complex, at least near the Tinaquillo peridotite, indicates that both complexes were deformed at the same time, petrofabric analysis of quartz was performed to compare the mechanism of deformation in the two complexes. The Tinaco Complex south of the pyroxene bearing gabbroic rocks is not well exposed and generally the rocks have a low percentage of quartz.
Therefore, only one sample of plagioclase quartz gneiss from the area was studied. Another one was collected about
15 km to the south near the Tinaco River, where the Tinaco
Complex is well exposed. Sample VT-82-lla (Figure 23): Sample VT-82-lla is a plagioclase quartz gneiss with nematoblastic texture. The sample comes from the Tinaco River (Figure 25) and it shows a mesoscopic open fold in which the foliation (S^) is
folded. The fold axis (p) is plotted on the diagram in
Figure 23. The fabric of the 200 c-axes of quartz from this sample
has orthorhombic symmetry; it consists of two major maxima
at about 45° to the foliation (S^). There is a faint maximum near the p~a.xis.
Sample VTO-82-83 (Figure 23): Sample VTO-82-83 is a
plagioclase quartz gneiss with nematoblastic texture. This
sample comes from the east limb of an approximately north- Figure 23: Equal-area, lower hemisphere projection of 200 c-axes of quartz in samples from the Tinaco Complex.
Contours at 1, 2, 3, 4, 5, 7, 8 and 10% for the sample
VT-82-lla and at 1, 2, 3, 4, 5, 6, 7 and 8% for the sample VTO-82-83 per 1% area in each diagram; white areas have less than 1%. Solid circle (S^) is the foliation and is the fold axis. Dashed circle represents the horizontal plane; N = north, E = east, and W = west. 97 98 south trending open antiform south of Las Mesas (Plate 1 and
Figure 25). The sample is strongly altered which makes it easier to distinguish between feldspar and quartz.
The fabric of the 200 c-axes of quartz from this sample has a monoclinic symmetry; two maxima occur symmetrically displaced at about 45° to Si, but one maximum is much stronger. A maximum near the center is very weak.
Conclusions based on the quartz fabrics
None of these fabrics are completely matched by experimentally derived fabrics although some resemblance exists. These fabrics may be best compared with those obtained by Hara et al. (1973) for quartzes in myIonite zones in Japan. On that basis, the Tinaco fabrics can be explained as follows: the major compressive strain (Z or
G^) is approximately normal to Si and the major extensile strain (X or €3) is in the center of the diagrams. This means that in VT-82-lla, X is oriented N64W 35NW and in
VTO-82-83, EW 10E. The maximum in the center of both
diagrams cannot be explained.
Textures and microfabrics in the Las Mercedes Formation
The rocks from the Las Mercedes Formation consist mostly of fine-grained calcite and dolomite. In these
carbonates a few mm wide bands occur consisting of quartz,
muscovite, and pyrite; this banding may be a primary
sedimentary bedding. In rocks which contain more than 50% 99 carbonates the texture is granoblastic. If they constitute
less than 50%, and mostly of muscovite and quartz, the texture is lepidoblastic.
In one sample pyrite crystals were observed in the carbonate bands; the pyrite cyrstals have a poorly-developed pressure fringe of ellipsoidal shape composed of fibrous quartz. This supports the sedimentary origin of the pyrite, probably in a reducing environment. This interpretation is based on the likely sedimentary origin of pyrite in
carbonate rocks and its development prior to in the Las
Mercedes Formation.
Petrofabric Analysis
Fabric analysis was carried out for quartz from two
samples of calcareous phyllites from the Las Mercedes Formation. The thin section were cut almost perpendicular
to the foliation and 200 c-axes of quartz were measured in
each sample.
Sample VTO-82-5 (Figure 24): Sample VTO-82-5 is a
calcareous quartz muscovite phyllite with lepidoblastic
texture. The sample comes from the north limb of the synform north of Belen (Plate 1 and Figure 25). The sample
shows a well developed quartz lineation which is defined by
recrystallized quartz lenses. These lenses probably
represent the original fabric of the sedimentary rock.
The fabric shown in the Figure 24 has monoclinic sym¬ metry with a girdle pattern and asymmetrical peripheral Figure 24: Equal-area, lower hemisphere projection of 200 c-axes of quartz in samples from the Las Mercedes Formation. Contours at 1, 2, 3, 4, 5, 6 and 7% for the sample VTO-82-5 and at 1, 2, 3 and 4% for the sample
VTO-82-17 per 1% area in each diagram; white areas have less than 1%, and stippled areas have between 1 and 2% for the sample VTO-82-17. Solid circle (S^) is the foliation and L is the mineral lineation. Dashed circle represents the horizontal plane; N = north and E = east. 100 101 maxima. There is also a maximum parallel to the lineation.
Sample VT0-82-17 (Figure 24): Sample VTO-82-17 is a calcareous quartz muscovite phyllite with lepidoblastic texture. The sample comes from the south limb of the synform that crops out along the road to Las Mercedes
(Plate 1 and Figure 25).
The c-axes orientation of this sample is shown in
Figure 24; it is not well defined but it resembles in a general way the pattern of the Sample VTO-82-5. It is probably necessary to measure many more c-axes to get a more distinct pattern.
Conclusions based on the quartz fabrics
As with the Tinaco Complex fabrics, the central maximum in both Las Mercedes samples is unexplained. Both fabrics are rather weak and conclusions have to be tentative. The fabrics may be interpreted as having formed in relationship to strain as follows: the major principal compressive strain is normal to Sj and the major principal extension is oriented within S]_. In geographic terms this means that X is oriented N45E and subhorizontal in VTO-82-5 and N82W in
VTO-82-17. Because of the poor exposure of the Las Mercedes
Formation, the paucity of mesoscopic structural data and the weakness of the quartz fabrics, no conclusion will be given here. It is clear that much more work is needed before a kinematic model can be proposed for the deformation of the Las Mercedes Formation. 68°27* 30* 68°25' 68°22'30" 68°20' «8°I7’30 «H •H •H "p -p £1 •H •H •H -H -P •P +-> -p CSJ in •P ■P ft •H P P d c CO ft d d CD CO o Q) O K ft P a d o O CD p CD a d o P ft P > CD a CD p G CD bD G 0) ellipsoid (x»y»z) derived from the quartz fabrics in the Tinaco Complex and the Las Mercedes Formation. Thus as the principal axes of the stresiCO ellipsoid ) derived from the olivine fabrics for the Tinaquillo 102 peridotite. 103
TECTONIC MODELS
Plate Tectonic Models of Venezuela and the Caribbean
Several plate tectonic models have been proposed to explain the origin and evolution of the Caribbean and its borderlands (Ladd, 1976; Burke and Dewey, 1980; Pindell and Dewey, 1982; Anderson and Schmidt, 1983). These models are based mainly on paleomagnetic data, and to some extent on regional structural geology and petrology and they try to explain the evolution of the Caribbean basin, using the reconstruction of western Pangea from the Permo-Triassic period until the present time. All the models are to large extent speculative because they are based on rotations of small blocks which are necessary to explain the gaps or overlaps that occur between
Africa and America during the Permian-Triassic period according to the Bullard fit (Bullard et al., 1965). One of these gaps is between North and South America. Pindell and
Dewey (1982) show that the most significant aspect of the Upper Paleozoic geology of northern Venezuela and of Yucatan
is the presence of Pennsylvanian-Permian granitic intrusions and metamorphic rocks. They suggested that the Yucatan block was located between Venezuela and the Ouchita belt of
Oklahoma. The southern portion of this block and the north¬ ern part of Venezuela formed an Andean-type volcanic arc
during the late Paleozoic. They proposed a Hercynian suture 104 separating the zone of foreland thrusting (Ouchitas) from zones of related magmatic activity. This model requires that an orogenic event occurred during the Paleozoic in northern Venezuela; however, they were not specific about these granitic intrusions and metamorphic rocks of Venezuela.
Another model which is based on the same approach, was proposed by Anderson and Schmidt (1983). Their paper deals only with the Triassic and younger history of the Caribbean basin. Their Pangea, however, is quite different from Pindell and Dewey (1982). They did not put Yucatan between
North and South America. They proposed that during the
Middle Jurassic, Cuba was in direct contact with northern South America implying a correlation between the distinctive quartzose San Cayetano Formation of Cuba and the Caracas and Juan Griego Groups of northern Venezuela and Margarita
Island, respectively. Anderson and Schmidt (1983) correlated also crystalline rocks considered to be of Paleozoic and/or
Precambrian age on Cuba with similar rocks of the South
America craton which were separated during Late Jurassic rifting with accompanying rotation. The different conclusions of the two models based mainly on paleomagnetic data and on the rotation of small blocks are unsubstantiated because the lack of knowledge of the geologic history of the many regions which are included in the models.
Several models have been proposed which relate directly to northern Venezuela (Bell, 1972; Santamaria and Schubert,
1974; Maresch, 1974; Mascle et al., 1979; Talukdar et al., 105
1981; Talukdar and Loureiro, 1982; Navarro, 1983). These models are based mainly on petrologic and structural data of the Caribbean Mountains System, the Dutch Leeward Islands, and the Venezuelan Caribbean Islands, but all the models to some degree are interpretations and simplifications because of the lack of a geological knowledge of these areas and the complex geologic history.
Almost all the models that have been proposed to explain the evolution of northern Venezuela are, with the exception of some details, very similar. The models of
Talukdar et al.(1981) and Navarro (1983) show the largest differences. All the models proposed that the evolution of northern Venezuela began with the rifting of North and South
America in a general way as proposed by Dewey and Bird
(1970) to explain the evolution of Atlantic-type oceans. This rifting was probably the consequence of new plate boundaries which were formed when the central Atlantic began opening in the Early Jurassic and Africa separated from
North America as proposed by Pitman and Talwani (1972).
In almost all models the origin of the different belts of the Caribbean Mountains System and of the islands are thought to have formed after the rifting episode. Instead, Talukdar et al. (1981) and Navarro (1983) proposed that some of these belts were formed during a pre-rifting orogenic event which occurred during the Late Paleozoic.
The configuration that they proposed for the Paleozoic
(pre-rifting) is from north to south; a microcontinent, a 106 marginal oceanic basin, a volcanic island arc with north facing subduction zone, an oceanic basin and the Guiana shield. Collision of all these terranes was proposed to have occurred during Late Paleozoic times. At this time the high P/T Villa de Cura Group was assumed to have been emplaced. They suggested that later on during the Late
Cretaceous collision of another volcanic island arc occurred, but this arc is not exposed, but may be located somewhere between the Venezuelan and the Dutch Leeward Islands. In a later paper Talukdar and Loureiro (1982) suggested a similar model, but they proposed that the Villa de Cura Group represents the Cretaceous volcanic island arc displaced southward along synthetic thrusts.
All the models proposed a change of the polarity of subduction after an island arc-continent collision and the development of a Cordillera-type mountain belt of Dewey and Bird (1970). The post-Coniacian volcanic and magmatic rocks in the upper part of the Caracas Group and on the islands north of Venezuela would be related to this volcanic arc.
The collision was responsible for the first period of deformation in the Caracas Group. When the arc-continent
collision occurred and the subduction was inverted, the magmatism was mainly felsic and it is represented by the
youngest undeformed volcanic rocks in the Caucagua-El
Tinaco belt. Similar rocks in the north have undergone a second period of deformation probably related to antithetic
thrusting which affected almost all the rocks of the 107
Caribbean Mountains System and the island to the north, following the classical evolution of a Cordillera-type mountain belt. This event represents the third heating event of the basement of the Cordillera de la Costa belt.
All the authors postulated that the end of the second subduction stage occurred at approximately 30 m.y. ago
(Oligocène), whence the largest faults started to be formed which separate each belt and a strike-slip boundary formed south of the Caribbean plate.
Analysis of Age Dates in Venezuela
The Paleozoic history of northern Venezuela is not well- known due to the lack of a systematic study of the Caribbean
Mountains System and because rocks of this age are covered by younger sedimentary basins.
Cordillera de la Costa belt: Kovach et al. (1977)
acquired a whole-rock Rb/Sr age of 220+ 20 m.y. (Permian-
Triassic) from gneisses of the Pena de Mora formation, which
Vfehrmann (1972) interpreted at least in part as lateral
equivalent of the Las Brisas Formation. Urbani (1978)
suggested that the Pena de Mora Formation is part of the
basement of the Caracas Group and he considered the Guaremal Granite as a lateral equivalent of this formation. Urbani (1982, personal communication) had two samples of the
Guaremal Granite dated by Teledyne Isotopes Inc., USA, using
the Rb/Sr whole-rock method. A 396 m.y. isochron was 108 calculated but because it was based on only two samples, no standard deviation was calculated. The initial 87/86 Sr ratio is 0.70569.
Kovach et al. (1977) did a whole-rock Rb/Sr determina¬ tion on a gneiss of the Las Brisas Formation which yielded an age of 270 m.y. The Sebastopol Complex, which is the metamorphosed basement of the La Cordillera de la Costa belt, has yielded a whole-rock Rb/Sr age of 420 m.y. (Hurley and Hess, 1963). Olmeta (in: Martin, 1968) did a K/Ar determination on muscovite of this complex which yielded an age of 41+ 2 m.y., which probably indicates a Tertiary heating event or uplift.
The Caucagua-El Tinaco belt : The rocks of the Caucagua- E1 Tinaco belt are generally considered Paleozoic or even
Precambrian (Urbani, 1982) but no pre-Triassic ages have been found (See Regional Geology). The Triassic ages were
determined using the K/Ar method. These K/Ar ages could
be interpreted in several ways. First, the metamorphic
event recorded by the rocks, is Triassic. This is unlikely
as no Triassic event has been recognized elsewhere. These
ages may be uplift ages, and the original metamorphic event
must have occurred earlier.
The Villa de Cura belt: Piburn (1968) described the
Tiara Volcanics as composed of coarse volcanic conglomer¬
ates with few layers of lithic tuff in their lower part and in the upper part as mainly lithic tuff. He interpreted
some metavolcanic rocks as volcanic flows. Piburn (1968) 109 did a whole-rock K/Ar determination on a meta-tuff of this formation which yielded an age of 100+ 10 m.y. This age is the only date which constrains the age of the Villa de
Cura Group, because he recognized that this formation overlies the Villa de Cura Group discomformably. This date was widely used in the models to estimate the age of emplacement of the allochthonous Villa de Cura Group. This age can also be used to constrain the latest thrusting related to the arc-microcontinent collision at the marginal ocean basin in the models of Talukdar et al. (1981) and
Navarro (1983). Basement of the southern sedimentary basin: As described in the Regional Geology, south of the Villa de
Cura belt a flysch basin occurs of Paleocene to Eocene age overlain by a non-flysch basin of Miocene to Pliocene age
(Beck, 1978). Bell (1968) defined four more tectonic belts all occurring in this basin which have formed during the
Cretaceous-Tertiary orogeny. The dates of the basement of this basin are limited but they are interesting for the interpretation of the Paleozoic history.
The oldest rocks known are within a meta-sedimentary sequence which crops out to the northwest of the town of El Baul. This sequence is formally named the Barbasco
Group. The lower part of this group consists of phyllitic rocks with fossils of Ordovician age; the middle part consists of siltstones, and the upper part mainly of psammitic rocks (Gonzalez de Juana et al., 1980). The 110
Barbasco Group was intruded by the El Baul Granite which is porphyritic, with local development of rapakivi texture and showing contact metamorphism (Martin Bellizia, 1961). A
Rb/Sr age of biotite from this granite is 287+ 10 m.y.
(Martin Bellizia, 1968). Feo Codecido (1963) acquired an orthoclase K/Ar age of this granite of 270+ 10 m.y. Gonzalez de Juana et al. (1980) investigated a volcanic
sequence named the Guacamaya Volcanics, which he interpreted
to have been deposited subaerially and unconformably on the metasedimentary rocks of the Barbasco Group. These volcanic
rocks are composed of a lower rhyolite and an upper quartz
latite. Martin Bellizia (1961) acquired a whole-rock K/Ar
date of 192+ 3.8 m.y. for a rhyolite of this group.
MacDonald and Opdyke (1974) carried out two K/Ar whole-rock
determination of rhyolites of this formation which yielded
ages of 192+ 3.8 m.y. and 195+ 3.9 m.y.
The basement of the Cordillera de la Costa and of the
Caucagua-El Tinaco belts yielded some ages indicative of a
Paleozoic metamorphic event. The geologic control of the
Villa de Cura Group is poor. Thus, one could speculate
that its metamorphism could have taken place during Lower
Mesozoic and the igneous age could have been Paleozoic. The few data on the basement in the southern basin are indicative
of a Paleozoic orogenic event with magmatic phases which
continued unto the Jurassic.
To get a better understanding of the Paleozoic orogeny,
it would be necessary to carry out a complete study of the Ill
Caucagua-El Tinaco, Paracotos, and Villa de Cura belts and also a study in the few areas where the Paleozoic rocks in the sedimentary basin crop out and to get a compilation of the information from the oil companies which have worked in the area.
Tectonic Setting of the Tinaquillo and Tinaco Complexes
The Tinaquillo peridotite has formed clearly at great depth in the earth's upper mantle. The tectonic setting is unknown, but several environments are possible, such as a mid-oceanic ridge, a spreading back arc basin, the root zone of a volcanic island arc, or the basement of a continental crust. In the study area the foliation (Sj) in the Tinaquillo and Tinaco Complexes were probably formed simultaneously during the same orogenic event. The open megascopic folds trending north-south were formed during the same orogenic event. The age of this event is poorly constrained but the few dates suggest a Late Paleozoic or Early Mesozoic age.
The Tinaquillo peridotite shows younger structures which may be the result of the juxtaposition of the Tinaco and Tinaquillo Complexes across the Las Mercedes Formation along the Manrique thrust fault. During this orogenic event the Las Mercedes Formation was isoclinally folded and meta¬ morphosed in the greenschist facies. The age of this 112 orogenic event is not known but it must be post-Jurassic because the Las Mercedes Formation north of the present study area has fossils of Cretaceous age. The orogenic event may be related to synthetic thrusting related to a Late Cretaceous-Tertiary collision.
Thus, two orogenic events may have occurred in the study area. The older one during which the Caucagua-El
Tinaco belt was metamorphosed took place during the
Paleozoic Era and may be related to the closing of the marginal ocean basin and collision of a micro-continent as proposed by Talukdar et al. (1981) and Navarro (1983).
The olivine fabrics from the Tinaquillo Peridotite
Complex are indicative of a southeast-northwest transport direction and the textures studied are mostly indicative of a sense toward the northwest (See Textures suggesting the sense of the shear). This shear sense can only be interpreted in a speculative way. Three alternative models are proposed:
The thrusting may be related to the Ouchita thrust and fold belt (Anderson and Schmidt, 1983). Briggs and Roeder
(1975) and Wickham et al. (1976) have concluded that the
Ouchitas thrust and fold belt was produced by a collision between another continent or island arc and North America the boundary of which represented a passive margin. If the
Caucagua-El Tinaco belt and more specifically the Tinaquillo
Complex were part of the continent or island arc which
collided with North America, it is possible that the north- 113 westerly thrusting of the Tinaquillo and Tinaco Complexes is synthetic with the proposed subduction (Figure 26a).
The northwesterly thrusting could also be explained in the models of Talukdar elt al^. (1981) and Navarro (1983).
In their models the Tinaquillo peridotite must have formed north of the Permian volcanic island arc which itself was emplaced synthetically southward, forming the Villa de
Cura sheet. The Tinaquillo and Tinaco Complexes were thrust northwestward antithetically across the micro-continent which is now represented by the basement of the Cordillera de la Costa and Caucagua-El Tinaco belts (Figure 26b).
Another possibility which has never been proposed, however, is that the Tinaquillo peridotite was part of an oceanic volcanic island arc lying above a south-dipping
subduction zone. The thrusting of both complexes would be
synthetic with the subduction zone. The felsic rocks
(plutonic and volcanic) are part of the island arc. It is
required in this model that the Villa de Cura Group is
allochthonous and has formed much later in Late Mesozoic
times (Maresch, 1974; Talukdar and Loureiro, 1982). The
problem here is however that the Tinaco Complex shows a low
P/T metamorphism (Figure 26c). 114
NORTH AMERICA SOUTH AMERICA
N THERMAL DOMING > T':!M I I I I I I
TINAQUILLO PERIDOTITE I CAUCAGUA-EL TINACO BELT
A. The thrusting related to the Ouchita thrust and fold belt.
TINAQUILLO PERIDOTITE
VILLA DE CURA
CAUCAGUA-EL TINACO BELT N
TIN AOUILLO—| VILLA DE CURA PERIDOTITE | CAUCAGUA-EL
B. Models of Talukdar et al.(1981) and Navarro (1983).
Figure 26: Plate tectonic models. 115
CAUCAGUAEL TINACO BELT
N
TINAQUILLO P.
C. Model in which the Tinaquillo peridotite was part of an
oceanic island arc lying above a south-dipping subduction
zone.
TINAQUILLO P.
CAUCAGUA:EL
D. Preferred model.
Figure 26 (Continuation): Plate tectonic models. 116
SUMMARY AND CONCLUSIONS
Three major rock units are recognized in the field area: (a) The Tinaquillo Peridotite Complex which consists of approximately 90% ultramafic rocks (75% harzburgite,
20% dunite, and 5% serpentinite) and 10% gabbroic rocks;
(b) The Tinaco Complex which consists mainly of amphibole gneiss, amphibolite, and plagioclase-quartz gneiss; and
(c) The Las Mercedes Formation which consists mainly of calcareous phyllite and marble.
The Tinaquillo Peridotite Complex: The dunites and harzburgites from the Tinaquillo peridotite show a bi¬ modal grain size distribution with porphyroclasts of mostly orthopyroxene but also of clinopyroxene and olivine
in a mylonitic matrix of fined-grained olivine and pyroxene. This texture suggests that the rocks underwent
at least two metamorphic events, during the first of which
the large grains grew, and during the second the fine¬
grained matrix.
The composition of the porphyroclasts were determined to derive equilibrium temperatures using two pyroxenes
paleothermometry methods. The highest temperature has been found in the cores of pyroxene porphyroclasts (1400°C) and
the lowest temperature (650°C) in the matrix and rims of small porphyroclasts. The cores of the small porphyroclasts
and the rims of the large ones yield temperatures of
approximately 1000°C. The 1000°C temperatures could be 117 interpreted in two alternative ways: (1) they represent a separate thermal event; (2) the chemical composition of these pyroxenes has been partly changed by the low- temperature event. The first alternative is preferred here, because the gabbros - both within the Tinaquillo Complex and just south of it, display two metamorphic events: an older granulite facies event (900°C) and an amphibolite facies event (600°C).
Almost all the ultramafic rocks from the Tinaquillo peridotite are metamorphic tectonites reflecting the second metamorphic event and these tectonite fabrics of the
Tinaquillo peridotite argue against its origin as a hot crystal-mush intrusion as proposed by Mackenzie (1960, 1966) and Green (1967). The gabbroic rocks in the Tinaquillo peridotite and immediately south of it which were interpreted as contact metamorphic rocks, show a tectonite fabric related to the second (amphibolite) metamorphic event; they do not show contact metamorphic textures. The assemblages of these rocks reflect a polymetamorphic history as well; the first assemblage is related to a basic granulite and the second one to an amphibolite facies metamorphism.
The magnitude of the differential stresses which caused these deformations were determined by paleopiezometry. During the first metamorphic event the maximum differential stresses were 54 bars (as derived from olivine porphyroclasts)
and 5 bars (as derived from enstatite porphyroclasts). The
second metamorphic event occurred at differential stresses 118 of 2.1 Kb (olivine neoblasts) and 0.84 Kb (enstatite neoblasts). The porphyroclasts were formed at very low differential stresses (5 - 54 bars) typical for the upper mantle, while the mylonitization occurred at shallow
crustal levels at high stresses of between 0.8 and 2.1 Kb. The rocks of the Tinaquillo peridotite have a penetra¬
tive foliation (Si) and mineral lineation (L) which is
parallel to the foliation (Si) and lineation (L) in the
Tinaco Complex. Thus it appears that the two complexes were
juxtaposed during an orogenic event.
From the olivine fabric of the Tinaquillo peridotite
the following conclusions have been drawn: the foliation (Si)
is approximately parallel to the shear plane and the trend of the maximum compressive stress (CTi) is parallel to the
mineral lineation (northwest-southeast) but from the olivine
fabrics the sense of transport could not be obtained un¬
equivocally. The textural study however strongly indicates that the displacement was toward the northwest.
The enstatite porphyroclasts are pre-tectonic in relationship to the foliation (Si). During the orogenic
event in which the foliation (Si) developed, the elongated
orthopyroxene rotated toward the shear direction, creating the strong lineation. The recrystallization of the
porphyroclasts, and the development of the pressure shadows
occurred also during this event.
The Di-structures have been folded into large north-
south trending folds (D2) which may have formed during the 119 same orogenic event. Several younger phases of deformation in the Tinaquillo Complex may be related to a later orogenic event during which it was thrust northward on the Las Mercedes Formation along the Manrique thrust fault. The Tinaco Complex: The Tinaco Complex south of the pyroxene-bearing gabbroic rocks has undergone only one metamorphic event which is probably the same as the second metamorphism that affected the Tinaquillo peridotites and the pyroxene-bearing gabbroic rocks. The metamorphic conditions of this event are approximately 600°C and 4 Kb, indicative of a medium grade metamorphism of the amphibolite facies. There appears to be a metamorphic gradient in the area: the rocks in the north seem to be of higher grade than in the south. However, this feature has not been studied in detail. The rocks of the Tinaco Complex have a penetrative foliation (Si), mineral lineation (L), and folds (jji and jQ2) which have the same style and orientation as those in the Tinaquillo Complex. Thus, they probably formed at the
same time.
Microfabrics of quartz in the Tinaco Complex are not
completely matched by experimentally derived fabrics.
Tentatively the fabrics may be explained by northwest-
southeast to west-east compression.
The bas Mercedes Formation : The characteristic meta¬
morphic assemblage of the Las Mercedes Formation is 120
indicative of the conditions of low to medium grade of regional metamorphism, but on a regional scale it seems that the conditions never were higher than those of the upper part of the greenschist facies.
The contact between the Tinaquillo peridotite and the
Las Mercedes Formation to the north is a thrust fault
(Manrique thrust). This contact was developed during the
second orogenic event recognized in the study area which
caused the formation of several phases of deformation in the Las Mercedes Formation.
The preferred orientations of c-axes of quartz from
two samples of the Las Mercedes Formation are difficult to
explain. The fabrics are weak and are only tentatively
explained by northeast-southwest to east-west compression.
Preferred Model (Figure 26d): The model which best seem
to fit the few age dates in Venezuela is the model of
Talukdar et al. (1981) and Navarro (1983), but slightly
modified. In this model, the Tinaquillo peridotite has formed north of a Permian island arc. The Tinaquillo and
Tinaco Complexes were thrust antithetically northwestward
across a micro-continent which is now the Caucagua-El Tinaco
belt. The difference of this model with the models of Talukdar
et al. (1981) and Navarro (1983) is that the expected high-
pressure belt has been destroyed or not exposed. There is as yet no data to suggest that the Villa de Cura belt is of
Paleozoic age. The Villa de Cura belt is probably related to the Late Cretaceous orogenic event. 122
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