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.

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

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

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.

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

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

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 AMARAL, G.; BUSHEE, J.; CORDANI, U.G.; KAWASHITA, K.; REYNOLDS, J.H. 1967.

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ARAMAKI, S. 1969. Some problems of the theory of caldera formation. Bull. Volcanol. Soc. Japan, Ser. 2, 14-2, 55-76.

BJRNBERG, A.J.S. 1959. Rochas clsticas do Planalto de Poos de Caldas. Bol. Fac. Fil. Cinc. Let. Univ. So Paulo, 237, Geologia 18, 65-132.

BUSHEE, J. 1974. Geology and petrography of the lujaurite and nearby rocks, Poos de Caldas, Brazil. Doctor thesis of Dept. Geol. Geophys. Univ. California, Barjekey, 137p. (unpublished).

ELLERT, R. 1959. Contribuio geologia do macio alcalino de Poos de Caldas. Bol. Fac. Filos. Cinc. Let. Univ. So Paulo, 237, Geologia 18, 1-64.

ELLERT, R.; BJORNBERG, A.J.S.; CONTINHO, J.M.V. 1959. Mapa geolgico do macio alcalino de Poos de Caldas, Brasil. Fac. Filos. Cinc. Let. Univ. So Paulo.

FRAENKEL, M.O.; SANTOS, R.C.; LOUREIRO, F.E.V.L.; MUNIZ, W.S. 1984. Jazida de urnio no planalto de Poos de Caldas - Minas Gerais. Principais Depsitos Mineris do Brasil Vol. 1, DNPM, 89-103.

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KAWASHITA, K.; MAHIQUES, M.M.; ULBRICH, H.H.G.J. 1984. Idades Rb/Sr de nefelina sienitos do anel norte do Macio Alcalino de Poos de Caldas, MG-SP. Resumos do XXXIII Cong. Bras. Geol., 244-245.

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ULBRICH, H.H.G.J. 1984. A petrografia, a esturutra e quimismo de nefelina sienitos do Macio Alcalino de Poos de Caldas, MG-SP. Livre Docncia thesis, Inst. Geoc. Univ. So Paulo (unpublished).

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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.