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AN OUTLINE ABOUT PROBLEMS OF VOLCANIC CALDERA HYPOTHESIS OF THE
POOS DE CALDAS ALKALINE COMPLEX ROCK BODY, MINAS GERAIS -
SO PAULO, BRAZIL
AKIHISA MOTOKI * * Departamento de Geologia/Geofsica da
Universidade do Estado do Rio de Janeiro (UERJ), Rua So
Francisco Xavier 524, Maracan, Rio de Janeiro, Brazil.
RESUMO Foi realizada a reconsiderao vulcanolgica da hiptese de
caldeira vulcnica do complexo alcalino cretceo de Poos de Caldas,
MG-SP, Brasil. O mtodo de seppmen detecta uma morfologia
incompatvel com o modelo atualizado de caldeira vulcnica. Os corpos
sedimentares no possuem mergulho geral para o centro, mas sim,
atitudes aleatrias. Os dados de campo no comprovaram a real
existncia dos derrames de lava fonoltica. A relao de contato entre
as rochas piroclsticas e as fonolticas circunvizinhas
caracterizaram-nas como de preenchimento de conduto vulcnico. A
investigao geolgica e perfil granulomtrico do suposto dique anelar
indica sua inexistncia. Estes dados sugerem que o nvel de denudao
atual relativamente profundo e o referido complexo alcalino no
corresponde a uma caldeira de colapso, mas sim, um corpo intrusivo
raso de magmatic stoping. ABSTRACT The volcanic caldera hypothesis
of the Poos de Caldas alkaline complex rock body, Cretaceous in
age, States of Minas Gerais and So Paulo, Brazil, have been
re-examined. The summit level map shows an incompatible morphology
with updated caldera models. The sedimentary bodies have no general
dip to the centre, but random dips and strikes. The field evidence
has disapproved real existence of the phonolitic lava flows. The
contact relation of the pyroclastic rocks with surrounding
phonolite clarifies them to be vent-filling rocks. The field study
and granulometric cross-section of supposed ring dyke have revealed
inexistence of this body. These data suggest that the present
denudation level is much deeper than the previous estimations, and
the Poos de Caldas body is not a collapse caldera but a shallow
magmatic stoping. COLLAPSE CALDERA HYPOTHESIS OF THE POOS DE CALDAS
ALKALINE COMPLEX The Poos de Caldas alkaline complex rock body,
Cretaceous in age, intruding into Precambrian gneissic basement, is
situated on the boundary of the States of Minas Geris and So Paulo,
south-eastern Brazil, approximately 22 degrees of the south
latitude and 30 degrees of the west longitude, cropping out in a
sub-circular area about 30 km in diameter. This body consists
mainly of phonolites and nepheline syenites, with subordinate
amount of pyroclastic and sedimentary rocks. In spite of great
number of geological papers referring to the Poos de Caldas body,
only few ones have been published on the periodicals of scientific
associations.
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The collapse caldera hypothesis of this alkaline complex was
proposed by Ellert (1959) and Bjrnberg (1959), suggesting the
following evolution model: 1) Regional domic uplift and echelon
faulting, 2) eruptions of pyroclastic materials and lava flows, 3)
subsidence of central part, 4) intrusions of main tinguaitic
(indeed, phonolitic) body 5) formation of ring dyke 6) intrusions
of lujaurite, chibinite and foyaite. The K-Ar datings (Amaral et
al. 1967; Bushee 1974), ranging from 80 to 62 Ma, confirmed
above-mentioned sequence, and the present exposure of this complex
was considered to be an eroded volcanic caldera edifice. This
hypothesis was highly accepted in Brazil and followed by geologists
of the Nuclebrs, with application of Valles type model (Fraenkel et
al. 1984; Loureiro and Santos 1988): 1) Regional uplift and echelon
faulting, 2) explosive volcanism associated with caldera formation,
3) caldera collapse by partial magma withdrawal, 4) resurgent stage
of uplifting and emplacement of nephelinic rocks 5) ring dyke
formation, 6) intrusions of lujaurite, chibinite and foyaite (Fig.
1-A). In this way, the caldera hypothesis has been established and
considered to be indubitable. On the other hand, Ulbrich (1984)
doubted the application of Valles type model, however, left no
definitive conclusion. Recently Motoki and Oliveira J.L.S. (1987)
revealed the sedimentary bodies, which happen in this alkaline
complex, to be megaxenolithes of various scales, included in
neighbour phonolitic rocks. They attributed the present denudation
level to a shallow intrusive rock body, and pointed out that the
caldera hypothesis is unacceptable (Fig. 1-B). Above-mentioned
caldera hypothesis was based mainly on the regional morphology,
dome uplift, circular en-echelon fault, general dip of the
sedimentary bodies, extrusive rocks, and ring dyke. The author have
re-examined these fundamental justifications of the caldera
hypothesis, and have arrived at a negative conclusion. The present
paper reports a summary of this reconsideration. UPDATED MODELS FOR
VOLCANIC CALDERA AND ITS SUBTERRIAN STRUCTURE Prior to the main
discussion, the author would like to note updated caldera models
and their subterranean structure, which seem to be not familiar in
our continent. The term caldera is defined as sub-circular volcanic
collapse morphology in kilometric scale (Williams 1941; Smith
1966). They are classified roughly into those associated with mafic
shield volcanoes (Kilauea type) and differentiated pyroclastic
eruptions (Smith and Bailey 1968). The latter, which can be related
to the Poos de Caldas body, is subdivided into those of chaotic
collapse (Krakatoa type) and of coherent block subsiding along ring
fractures (Valles type; Fig. 2-A). The collapse was attributed to
evacuation of subsurface magma chamber of comparable diameter with
upper morphologic basin (Williams 1941; Kuno 1953). However,
appeared some objections to these traditional interpretations of
Krakatoa type calderas (e.g. Aramaki 1969; Yokoyama 1969). The
drilling data of some Krakatoa type calderas (Taneda 1963;
Matsumoto and Fujimoto 1969; Aramaki 1968; etc., cited in Aramaki
1969) and geological studies of resurgent calderas (Smith and
Bailey 1968) revealed that the collapse structure is much smaller
than upper morphologic basin, suggesting a diameter expansion due
to caldera-wall engulfment by marginal landslide following the
collapse. The gravitational studies for some Krakatoa type calderas
determined inverted open cone-shaped underground structures, which
attribute their formation process not to a collapse but to an
explosion (Yokoyama 1969), in other words, Krakatoa type calderas
are great explosion craters. The geological data of some Cretaceous
and Tertiary sub-volcanic bodies (Kusanagi 1955 cited in Aramaki
1969; Aramaki 1965; Nakada 1978; Motoki 1979) and seismological
study of a Quaternary Krakatoa type caldera (Wada and Nishimura
1981) permit to suppose
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more detailed underground structure: the circular horizontal
cross section in a shallower sites turns into a fissure vent (or
dyke) in deeper sites, as a flattened coffee filter (Fig. 2-B).
PROBLEMS OF MORPHOLOGY THE POOS DE CALDAS BODY Most of the previous
papers interpret the present morphology of the Poos de Caldas body
to be influenced directly or indirectly by supposed domic uplifting
and central subsidence. However, these papers applied no
geomorphological technique, in spite of the utilization of
morphological ones, e.g. aerial photographs, therefore, the results
was highly subjective. For the purpose of more objective
discussions, the present paper introduces summit level technique,
which estimates a rough palaeo-geomorphology by means of annulling
of fluvial erosion effect. The Fig. 4 visualizes the regional
summit level plane of the studied area, based on the topographic
map of the IBGE (1:50000), with mesh of 2km, by the aid of computer
graphic technique. This figure shows a semi-oval low-relief area,
which do not coincides exactly with the Poos de Caldas body, but
with the area underlain by Cretaceous and Precambrian alkaline
rocks, suggesting a close relation of the regional morphology
rather to the underlying rocks than the volcanism. Ellert (1959)
proposed a domic uplift with echelon faults, and this proposal was
followed by Fraenkel et al. (1984) and Loureiro and Santos (1988).
However, their geological and morphological vindications can also
be explained by engulfment of the sedimentary megaxenolithes in
host phonolitic magma (Motoki and Oliveira J.L.S. 1987), and the
inferred echelon faults have no evidence to justify their real
existence. Moreover, the geologic map of Ellert et al. (1959)
verified no domic deformation of country Precambrian gneiss, and
such a situation have been confirmed by recent studies (Janaci
1988, personal communication). Above-mentioned discussions make the
real occurrence of domic uplift doubtful. Fraenkel et al. (1984)
and Loureiro and Santos (1988) proposed 14 circular structures
inside of this alkaline complex, based on the LANDSAT photograph
and side scanning radar image, and attributed them to plug-like
intrusive bodies. Indeed, the major one, about 8km in diameter
(Fig. 3), is relatively clear in morphological characteristics and
fits roughly to radiometric high-anomaly areas, where two bodies of
uranium-bearing breccia have been found (Lima 1979), and therefore,
can be attributed to interrupted sub-circular configuration of
volcanic conduits. However, the rest 13 are unclear, being
considered to be highly subjective interpretations. Moreover, the
inferred plug-like bodies have no geological evidence to vindicate
their real existence. Above-mentioned deductions make the stocks
and resurgent stage uncertain. As mentioned before, updated caldera
model show its morphologic basin much greater than the geological
structure, due to the marginal engulfment. However, the morphology
of the Poos de Caldas body is almost coincident with its geology,
being unsuitable to a collapse caldera one. Consequently, the
present morphology provides no justification for the domic uplift
and caldera collapse. It is attributed probably to differential
erosion. MODE OF EMPLACEMENT OF THE SEDIMENTARY ROCKS BODIES All of
the previous papers have interpreted the sedimentary bodies,
present in the border of the Poos de Caldas body, as members of the
Paran Basin, but their detailed correlation and mode of emplacement
have not been agreed. Bjrnberg (1959) and Ellert (1959) correlated
them to the Botucatu Formation (Early Cretaceous Elian sandstones)
and suggested simultaneous deposition with the alkaline pyroclastic
rocks. They also mentioned
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general dip of these rocks (minor than 20 degrees) to the centre
of the alkaline complex to justify the caldera collapse, and this
idea was followed by later papers (Fraenkel et al. 1984; Loureiro
and Santos 1988). On the other hand, Ulbrich (1984) considered them
to be former sedimentary cover of the Tubaro Group (Permian glacial
sedimentary rocks), and described random strikes and dips. Motoki
and Oliveira J.L.S. (1987) proposed a model completely different,
based on detailed field works of Andradas (Loc. 1) and Vu das Nivas
(Loc. 2) area: These bodies are surrounded by neighbour phonolitic
rocks with intrusive contact and have random strikes and dips, and
therefore, considered to be megaxenolithes, from meters to
kilometres in scale, of the Corumbata (Permian lacustrine rocks)
and Botucatu (op. cit.) Formations, included in the phonolitic
stoping body (Fig. 5-A). Such large xenoliths are apparently
unbelievable, but the ones in acidic complex bodies have already
been reported (Aramaki 1966; Motoki 1979). The field work in guas
da Prata area (Loc. 3) have confirmed the sedimentary bodies
situated in similar mode to those of Andradas area: The central
body crops out in a area elongated to NE-SW ward, 5 x 2 km in
scale, and minor peripheral ones are distributed on north-eastern
contact zone of the central one. The south-western boundary is
delimited by a narrow phonolitic belt, which remarks the west
margin of this complex (Fig. 5-B). These sedimentary bodies are
constituted by the sandstone of high angle (30 degrees) cross
laminas with random strikes and dips. These data indicate that they
also are megaxenolithes, derived from the Botucatu Formation. The
Fig. 6 presents a stereographic plot of the stratifications
relative to the centre of the alkaline complex, showing inexistence
of the general dip. The form of the central body of guas da Prata
area suggests in situ fragmentation of this body (Fig. 5-B) with
little rotation (Fig. 6), therefore, the central body is considered
to be in a initial stage of megaxenolith formation process, just
separated from the upper wall body with a little subsidence into
the phonolitic magma (Fig. 1-B, left side). In the Poos de Caldas
body, large megaxenolithes seem to be dipped in low angle and small
ones, in relative high angle. The megaxenolith hypothesis can
explain the polygenetic origin and variable present altitudes, from
850 to 1500 m, of the sedimentary bodies with block engulfment in
the phonolitic magma (Fig. 1-B). Consequently, these sedimentary
bodies furnish no justification for the caldera collapse
hypothesis. INEXISTENCE OF THE PHONOLITIC LAVA FLOWS Ellert (1959)
proposed the phonolitic lava flows distributed in the south border
of the Poos de Caldas body, of several hundreds of meters thick,
slightly dipped to northward forming morphologic steps. This body
was described to overlie the sandstone, without intercalation of
tuff and breccia, and intruded by tinguaites and ring dykes.
However, later works (e.g. Ulbrich 1984) did not comment the rock
body. The summit level map of valley-fill method (250 m) for this
area, constructed by the author, shows apparent concordance with
Ellerts proposal. However, the fieldwork has revealed that these
phonolites are massive with no block-lave structure or fluidal
texture. The contact outcrop with the sedimentary rock (Loc. 4,
Fig. 7) shows no intercalation of palaeo-soil, organic material,
nor brecciated base of the phonolite. Such a contact mode and the
undulant contact plane indicate that this sedimentary body is a
megaxenolith, about 300 m in dimension. Above-mentioned data
conclude inexistence of the referred lava flows. The phonolites
exposed in this area are considered to constitute a part of the
shallow intrusive rock body.
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TEXTURES OF VENT-FILLING VOLCANOCLASTIC MATERIALS OF OSAMU
UTSUMI MINE The pyroclastic rocks, distributed in the border and
the central part of the Poos de Caldas bodies, were considered to
be older than neighbour phonolites and constituted by extrusive in
situ bodies with lava intercalations and those transported by
surface water (Ellert 1959; Bjrnberg 1959). Afterward, Ulbrich
(1984) mentioned one of the central bodies, Osamu Utsumi Mine (Loc.
5), to be a volcanic conduit younger than the phonolite, however,
still interpreted the border bodies as older extrusive ones, with
additional description of base surge deposits. The previous papers
took the rounded fragments, granulometric sorting, well-developed
stratification, and fine-grained tuff for evidence of sub-aerial or
sub-aquatic depositions (e.g. Bjrnberg, op. cit.), however, similar
textures can be found in vent-filling pyroclastic materials
(epiclastic materials, e.g. Osamu Utsumi Mine; Loc. 5; Oliveira
J.I. 1986). Motoki (1979) referred to the genesis of such
conglomerate-like textures. In volcanic vents, small and light
fragments will be carried upward by ascending eruptive gas, and
large and dense ones will fall down. Therefore, when the gas
velocity is almost constant in certain time, the fragments similar
in dimension, density, and form will be concentrated and fluttered
in a determined space of the vent, and the friction between them
will cause rounding (Fig. 8-A). Oliveira J.I. (op. cit.) also
described secondary-flowed welded tuff-like textures. Motoki (1979)
debated the possibility of the welding and secondary flow of
vent-filling pyroclastic materials in higher grade than those of
sub-aerial deposition: Vent-filling bodies have larger vertical
extension (thickness) and low cooling rate in relation to
sub-aerial ones, and steeply plunged vent wall realize high grade
secondary flowage (Fig. 8-B). Such a high-grade secondary flow is
observed typically in blocks found at Gonalves Farm (Loc. 6). They
have extremely elongated essential fragments (more than 1:100) and
well-developed viscous flow textures (Fig. 9). Such peculiar
textures can be mistaken sometimes for those of sub-aquatic tuff or
base surge deposit, in a first impression. RELATIVE AGE AND MODE OF
EMPLACEMENT OF QUARTEL VOLCANOCLASTIC BODY In western border of the
Poos de Caldas body, there is the largest pyroclastic body, 20 x 4
km, so called Faixa Piroclstica do Vale do Quartel (Ulbrich 1984).
This body, in brief Quartel body, was interpreted as sub-aerial and
sub-aquatic graven-fill body (Bjrnberg 1959) or a roof pendant
(Ulbrich 1984), older than the neighbour intrusive phonolite.
However, no geological evidence for this relative age has been
presented. Moreover, Ulbrich (op. cit.) found the nepheline syenite
xenoliths, included in this body, which must be younger than the
phonolite. The author, fortunately, have a opportunity to observe
the road cut newly opened, which shows the contact relation between
the Quartel body and neighbour phonolite. At the Loc. 7, an outcrop
of sub-vertical contact has been observed. The pyroclastic rock
consists predominantly of matrix with semi-rounded fragments, in
centimetric scale, of phonolitic and syenitic rocks. Neither
intercalation of palaeo-soil nor chilled margin of the phonolite
has been observed. Another contact, exposed along the same road
(Loc. 8), also has no palaeo-soil intercalation. The pyroclastic
rock exposed on this outcrop is a welded tuff with abundant
centimetric pseudoleucite. Near the contact, a remarkable secondary
flow texture have been observed: the essential lenses are highly
elongated (1:15) and oriented in parallel to the high angle contact
plane, and finally the texture grades into the one similar to
lavas, with vitric
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chilled margin of 40 cm wide (Fig. 10). Such a chilled margin of
acidic sub-aerial tuff (Ono and Watababe 1974) and vent-filling one
(Motoki 1979) have already been reported. Therefore, these outcrops
are considered to be vent walls, and the pyroclastic rocks are
younger than the neighbour phonolitic ones. Welded tuffs are
generally originated from sub-aerial pyroclastic flows, which have
very high mobility, and then, these deposits are distributed in a
large area, except for vent-filling ones. However, those of the
Poos de Caldas body are very limited in distribution area, in spite
of ample potential areas. The fact makes it difficult to believe
the pyroclastic bodies to be extrusive ones. Consequently, the
pyroclastic bodies, represented by the Quartel body, are considered
to be epiclastic ones, which is, volcanic conduits and fissures,
younger than the country intrusive phonolites. INEXISTENCE OF THE
RING DYKE On the sub-circular topographic elevation along the
margin of the Poos de Caldas body, Ellert (1959) supposed presence
of ring dykes. Indeed, ring complex bodies are considered generally
to be the roots of a Valles type caldera (Smith and Bailey 1968).
The later papers accepted this proposal as important evidence of
the caldera hypothesis (e.g. Fraenkel et al 1984; Loureiro and
Santos 1988). However, as a matter of fact, no geological evidence
for real existence of this body has been presented. On the other
hand, Motoki and Oliveira J.L.S. (1987) observed two sedimentary
megaxenolith occurring under the supposed ring dyke, in southern
margin of this complex, which are in continuation to the inside
without interruption (Loc. 1, 4; Fig. 5-A), doubting real existence
of this body. A similar example has been observed at Loc. 9, near
the Cascata das Antas. The author has preliminarily applied the
granulometric cross-section method proposed by Motoki (1979):
Shallow intrusive bodies were cooled by the wall rocks, and this
effect must appear in grain-size distribution of the ground mass.
The Fig. 10 shows one of the examples of granulometric cross
sections in photomicrography for supposed ring dyke at northern
margin of the Poos de Caldas body (Loc. 10). This section confirms
a general grain-size reduction from the inside to the outside,
verifying absence of inner chilled margin of supposed ring dyke.
Above-mentioned data indicate inexistence of the ring dyke, and
attribute the sub-circular topographic elevation to chilled margin
of the phonolitic intrusive rock body. PRESENT DENUDATION LEVEL
Ellert (1959), Bjrnberg (1959), Fraenkel et al. (1984) and Loureiro
and Santos (1988) considered the present exposure of the Poos de
Caldas body to be an eroded volcanic caldera edifice, but not
denuded. Ulbrich (1984) indicated the denudation level deeper than
the model of Williams (1941). Motoki and Oliveira, J.L.S. (1987),
and the present paper have proved complete elimination of the
original volcanic edifice and extrusive rock bodies, and the fact
attributes the present denudation level to be much deeper than the
previous estimations. Amaral et al. (1967) and Buchee (1974)
considered the volcanic activity during 20 Ma based on K-Ar dating,
but this estimation is too long for Earths volcanoes. On the other
hand, Kawashita et al. (1984) revealed the Rb-Sr ages raging only
within experimental errors, from 85.0 to 89.2 Ma, considering the
K-Ar ages due to the later hydrothermal events. In this sense, the
coexistence of the nepheline syenite body with extrusive ones,
referred by most of the previous papers, is unacceptable.
Therefore, the present denudation level corresponds to a
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shallow intrusive rock body of magmatic stoping, and the guas da
Prata structure, defined by Ulbrich (1984), is attributed to a
block engulfment of wall body, during the main phonolitic magma
intrusion (Fig. 1-B, left side). CONCLUSION According to the former
discussions, all of the previous justifications for the caldera
collapse hypothesis have revealed to be inefficient. The present
exposure of the Poos de Caldas alkaline complex rock body is
considered not to be a volcano nor an eroded volcanic edifice, but
a denuded sub-volcanic intrusive body of magmatic stoping, and
almost no information about surface volcanic activities has been
preserved. Consequently, the author concludes that the collapse
caldera hypothesis is unacceptable, and therefore, the evolution
model in six stages (Ellert 1959; Fraenkel et al. 1984; Loureiro
and Santos 1988, etc.) must be replaced by a new one: 1) Main
phonolitic magma intrusion in stoping mode; 2) nepheline syenite
magma intrusions; 3) explosive pyroclastic eruptions; 4)
hydrothermal events and denudation. The only information about
surface activity of this complex, in spite of indirect ones, is
reflected in roughly circular configuration of the volcanic vents
along the margin of this alkaline complex. However, this circle is
only partial and interrupted, and far from the ring fracture common
in Valles type calderas. Such a configuration suggests occurrence
of main eruptions from the crescent-formed frank fissure of western
border and subordinate ones from the central conduits. In active
volcanoes, the Katmai Volcano, Alaska, which has shallow (minor
than 10 km in depth) and deep (20 to 30 km) magma chambers, about
20 km in diameter (Matusmoto 1971; confirmed by seismological
observations), has a similar volcanic activities. ACKNOWLEDGEMENT
The author is especially grateful to his co-workers, Prof. T.
Vargas, Mr. E. Chianello, F.J.G. Corra, J.L.S. Oliveira, and M.
Klotz of Rio de Janeiro State University, for their excellent field
and laboratory works to accomplish this work. The author wish to
thank Prof. H.H.J.G. Ulbrich, M.C. Ulbrich, and Mr. V.A. Janasi of
So Paulo University; Prof. R.A. Santos, E. Zimbres, M.C. Heilbron,
M. Tupinamb, and M.A. Rodrigues of Rio de Janeiro State University;
Prof. Y. Tokonami of the University of Tokyo; and Prof. A. Aikawa
of Osaka City University, for their helpful advice. The author is
indebted to the CEPUERJ for partial financial support. REFERENCE
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Soc. Japan, Ser. 2, 14-2, 77-83 (in Japanese).
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YOSHIDA, T. 1970. Ishizuchi collapse caldera and Tengudake
pyroclastic flow, Shikoku, Japan. Bull. Japan. Assoc. Mineral.
Petrol. Econom. Geol. 64-1, 1-12 (in Japanese).
Figure caption Fig. 1. Volcanic evolution of the Poos de Caldas
body: A) old model, after Fraenkel et al.
(1984), and Loureiro and Santos (1988); B) new model, modified
from Motoki and Oliveira, J.L.S. (1987).
Fig. 2. Supposed underground structures of volcanic calderas,
based on models of various authors about Quaternary calderas and
older intrusive bodies: A) Valles type, compiled from Smith and
Bailey (1968) and Yoshida (1970); B) Krakatoa type, compiled from
Aramaki (1965; 1969), Yokoyama (1969), Nakada (1978), Motoki
(1979), and Wada and Nishimura (1981).
Fig. 3. Locality map superposed on a simplified geologic map of
the Poos de Caldas alkaline complex rock body: Vc - volcanic
conduit or fissure; Ns - nepheline syenite body; Ph - phonolitic
intrusive body; Sd - megaxenoliths of Palaeozoic and Mesozoic
sedimentary rocks; without marking - country Precambrian gneissic
basement body.
Fig. 4. Summit level plane of the Poos de Caldas region,
visualized by the aid of computer graphic techniques. The vertical
pitch represents 100 m and the horizontal one corresponds to 1
km.
Fig. 5. Mode of emplacement of the sedimentary bodies of the
Poos de Caldas alkaline complex: A) Andradas area; B) guas da Prata
area.
Fig. 6. Stereographic diagram for the normal poles of the
stratification of the sedimentary bodies of Poos de Caldas alkaline
complex: If the general dip (e.g. Ellert 1959) to the centre of the
main phonolitic body were present, the plotted points would be
plotted along the dotted line.
Fig. 7. Sketch of the contact outcrop between sedimentary rock
and phonolitic one, along the BR-146, near Andradas (Loc. 4).
Fig. 8. Explanation figure of the A) mechanism of welding and
consequent secondary flow in a volcanic vent and B) granulometric
sorting and rounding of fragments in a volcanic vents, based on the
text of Motoki (1979).
Fig. 9. Sketch of well-developed secondary flow texture of the
blocks found at the Gonalves Farm (Loc. 6).
Fig. 10. Sketch of the contact outcrop between the Quartel body
(welded tuff) and host phonolitic rock observed at the Loc. 8.
Fig. 11. Granulometric cross-section of the supposed ring dyke
at the northern end of the Poos de Caldas body, Loc. 10.
-
Fig. 2. Supposed underground structures of volcanic calderas,
based on models of various authors about Quaternary calderas and
older intrusive bodies: A) Valles type, compiled from Smith and
Bailey (1968) and Yoshida (1970); B) Krakatoa type, compiled from
Aramaki (1965; 1969), Yokoyama (1969), Nakada (1978), Motoki
(1979), and Wada and Nishimura (1981).
-
Fig. 5. Mode of emplacement of the sedimentary bodies of the
Poos de Caldas alkaline complex: A) Andradas area; B) guas da Prata
area.
-
Fig. 6. Stereographic diagram for the normal poles of the
stratification of the sedimentary bodies of Poos de Caldas alkaline
complex: If the general dip (e.g. Ellert 1959) to the centre of the
main phonolitic body were present, the plotted points would be
plotted along the dotted line.
Fig. 8. Explanation figure of the A) mechanism of welding and
consequent secondary flow in a volcanic vent and B) granulometric
sorting and rounding of fragments in a volcanic vents, based on the
text of Motoki (1979).
-
Fig. 9. Sketch of well-developed secondary flow texture of the
blocks found at the Gonalves Farm (Loc. 6).
Fig. 10. Sketch of the contact outcrop between the Quartel body
(welded tuff) and host phonolitic rock observed at the Loc. 8.
Fig. 11. Granulometric cross-section of the supposed ring dyke
at the northern end of the Poos de Caldas body, Loc. 10.