AN OUTLINE ABOUT PROBLEMS OF VOLCANIC CALDERA HYPOTHESIS OF
THE POÇOS DE CALDAS ALKALINE COMPLEX ROCK BODY, MINAS GERAIS SÃO PAULO, BRAZIL
AKIHISA MOTOKI *
* Departamento de Geologia/Geofísica da
Universidade do Estado do Rio de Janeiro (UERJ),
Rua São Francisco Xavier 524, Maracanã, Rio de Janeiro, Brazil.
RESUMO
Foi realizada a reconsideração vulcanológica da hipótese de caldeira vulcânica do
complexo alcalino cretáceo de Poços de Caldas, MG-SP, Brasil.
O m’todo de seppômen detecta uma morfologia incompatível com o modelo
atualizado de caldeira vulcânica. Os corpos sedimentares não possuem mergulho geral para o
centro, mas sim, atitudes aleatórias. Os dados de campo não comprovaram a real existência
dos derrames de lava fonolítica. A relação de contato entre as rochas piroclásticas e as
fonolíticas circunvizinhas caracterizaram-nas como de preenchimento de conduto vulcânico.
A investigação geológica e perfil granulométrico do suposto “dique anelar” indica sua
inexistência.
Estes dados sugerem que o nível de denudação atual’ relativamente profundo e o
referido complexo alcalino não corresponde a uma caldeira de colapso, mas sim, um corpo
intrusivo raso de “magmatic stoping”.
ABSTRACT
The volcanic caldera hypothesis of the Poços de Caldas alkaline complex rock body,
Cretaceous in age, States of Minas Gerais and São 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 ventfilling 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 Poços de Caldas body is not a collapse caldera but a shallow magmatic
stoping.
COLLAPSE CALDERA HYPOTHESIS OF THE POÇOS DE CALDAS ALKALINE
COMPLEX
The Poços 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
São 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 Poços de Caldas body, only few ones have been published on the periodicals of scientific
associations.
The collapse caldera hypothesis of this alkaline complex was proposed by Ellert
(1959) and Bjørnberg (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 Nuclebrás,
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 Poços 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
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 POÇOS DE CALDAS BODY
Most of the previous papers interpret the present morphology of the Poços 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 Poços 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 Poços 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 Poços de Caldas body, as members of the Paraná Basin, but their detailed
correlation and mode of emplacement have not been agreed. Bjørnberg (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
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 Tubarão 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 Véu das Nóivas (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 Corumbataí (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 Águas 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 Águas 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 Poços 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
Poços 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 Ellert’s 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 palaeosoil, 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.
TEXTURES OF VENT-FILLING VOLCANOCLASTIC MATERIALS OF OSAMU
UTSUMI MINE
The pyroclastic rocks, distributed in the border and the central part of the Poços 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; Bjørnberg 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, welldeveloped stratification, and fine-grained tuff for evidence of sub-aerial or sub-aquatic
depositions (e.g. Bjørnberg, 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. 8A).
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 Gonçalves
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
VOLCANOCLASTIC BODY
MODE
OF
EMPLACEMENT
OF
QUARTEL
In western border of the Poços de Caldas body, there is the largest pyroclastic body,
20 x 4 km, so called “Faixa Piroclástica do Vale do Quartel” (Ulbrich 1984). This body, in
brief “Quartel body”, was interpreted as sub-aerial and sub-aquatic graven-fill body
(Bjørnberg 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
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 ventfilling ones. However, those of the Poços 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 Poços 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 Poços de Caldas body (Loc. 10). This section confirms a general grainsize 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 subcircular topographic elevation to chilled margin of the phonolitic intrusive rock body.
PRESENT DENUDATION LEVEL
Ellert (1959), Bjørnberg (1959), Fraenkel et al. (1984) and Loureiro and Santos (1988)
considered the present exposure of the Poços 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 Earth’s 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
shallow intrusive rock body of magmatic stoping, and the Águas 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 Poços 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. Corrêa, 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 São Paulo University; Prof. R.A.
Santos, E. Zimbres, M.C. Heilbron, M. Tupinambá, 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.
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Figure caption
Fig. 1. Volcanic evolution of the Poços 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 Poços 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 Poços 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 Poços de Caldas alkaline
complex: A) Andradas area; B) Águas da Prata area.
Fig. 6. Stereographic diagram for the normal poles of the stratification of the sedimentary
bodies of Poços 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 Gonçalves
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
Poços 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 Poços de Caldas alkaline
complex: A) Andradas area; B) Águas da Prata area.
Fig. 6. Stereographic diagram for the normal poles of the stratification of the sedimentary
bodies of Poços 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 Gonçalves
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
Poços de Caldas body, Loc. 10.