Volcanism and structure of southwestern Bolivia

Journal of Volcanology and Geothermal Research, 2(1977)73--111 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

73

VOLCANISM AND STRUCTURE OF SOUTHWESTERN BOLIVIA

S. KUSSMAUL ~, P.K. H()RMANN2, E. PLOSKONKA3 and T. SUBIETA3 Mineralogisch-Petrographisches Institut der Universitiit Tfibingen, Ti~bingen (Federal Republic o f Germany) 2 Mineralogisch-Petrographisches Institut, Kiel (Federal Republic of Germany) 3Servicio GeolOgico de Bolivia, La Paz (Bolivia)

(Received June 10, 1976; revised and accepted December 23, 1976)

ABSTRACT Kussmaul, S., H~irmann, P.K., Ploskonka, E. and Subieta, T., 1977. Volcanism and structure of southwestern Bolivia. J. Volcanol. Geotherm. Res., 2: 73--111.

Cenozoic volcanic activity started in southwestern Bolivia during the Oligocene. From the Early Miocene up to the Holocene, volcanism produced about 8000 km 3 of lavas and pyroclastic rocks. Five major cycles could be distinguished. North--south-trending fissures in the eastern part of southwestern Bolivia were sources of Miocene ignimbrites. The eruptive centers of Pliocene ignimbrites are scattered throughout the investigated area, but are concentrated in the western part. During Pleistocene times small ignimbrite flows were formed by collapse of composite volcanoes. The oldest lavas (Oligocene) are alkaline and differ considerably from younger rocks, which are rhyodacites and dacites with only a small limited range of compositions. Based on the K~O/Na20 ratios and the mineralogical composition, the lavas can be subdivided into a calc-alkaline and a high-K calc-alkaline (shoshonitic) association. Hornblende is absent from lavas with high K20/Na20 ratios. In southwestern Bolivia a westward migration of volcanic activity is apparent. The potassium content of the lavas decreases from the Miocene to the Holocene, whereas the sodium content increases. The potassium content cannot, therefore, be correlated with the depth of the Benioff zone. It is suggested, that the lavas and ignimbrites were formed by partial melting of material from different crustal levels. INTRODUCTION

This investigation is p a r t o f a research p r o g r a m carried o u t Within t h e volcanic areas o f Bolivia b y the Servicio GeolSgico de Bolivia ( G E O B O L ) a n d t h e Universities o f Tiibingen a n d Kiel (Federal R e p u b l i c o f G e r m a n y ) . T h e m a j o r p a r t o f s o u t h w e s t e r n Bolivia a n d t h e a d j a c e n t areas o f n o r t h e r n Chile a n d n o r t h w e s t e r n A r g e n t i n a consist o f volcanic rocks. This s t u d y is a review o f the volcanic r o c k s o f an area larger t h a n 3 0 , 0 0 0 k m 2. T h e thickness o f the volcanic sequences m a y e x c e e d 1 0 0 0 m. Its v o l u m e in s o u t h w e s t e r n Bolivia is e s t i m a t e d t o be a b o u t 8 0 0 0 k m s. T h e i n t e n t i o n o f this w o r k was t o d e t e r m i n e w h e t h e r the v o l c a n i c r o c k s

74

of the major volcanic episodes show distinct differences with respect to their chemical and mineralogical composition and to the structural features of that area. The results could have a bearing on the origin of the volcanic rocks and the relation of volcanism to tectonics. PREVIOUS INVESTIGATIONS The knowledge of the geology and petrology of the volcanic areas of Bolivia remains scanty. Observations on parts of Lipez were published by Ahlfeld (1946, 1972), Ahlfeld and Branisa (1960), Jacobson et al. (1969), and Avila (1971). In 1968 members of the Servicio Geol6gico de Bolivia began mapping and exploration for copper, silver, lead, and sulfur in the Lipez region (unpublished report, 19~0). More recently geochemical data were published by Fernandez et al. {1973). This paper continues the studies of Fernandez et al. {1973). More is known of the geology and petrology of the adjacent volcanic areas in northern Chile and northwestern Argentina. Several papers were published, which deal with the Cenozoic volcanism of these countries, e.g. Zeil and Pichler (1967), Guest (1969), Pichler and Zeil (1969), Turner (1970), James (1971), Schwab (1971, 1973), Audebaud et al. (1973), HSrmann et al. (1973), Lefevre (1973), Dupuy and Lefevre (1974), Francis et al. (1974), etc. GEOGRAPHICAL AND GEOLOGICAL SETTING The investigated area comprises the province of South Lipez and the southern part of the province of North Lipez, both belonging to the Department of Potosi (Fig.l). This area is situated in the southern part of the Bolivian Andes, which consist of three distinct belts (from east to west): (1) The Paleozoic Andean Block (Cordillera Oriental) in the east is the divide between the Amazonas/E1 Chaco Basin and the Altiplano. In the north, the Cordillera Oriental forms a conspicuously high mountain chain (up to 6500 m), which was intruded by intermediate plutonic rocks. Towards the south, the Paleozoic block splits into different mountain ridges and attains a width of 400 km and heights of a b o u t 4500 m. (2) The Altiplano has an average height of a b o u t 3800 m and a maximum width of 200 kin. It represents a great elevated depression between the Cordilleras Oriental and Occidental. In the southern part of the Altiplano, volcanoes are scattered throughout the area. These volcanoes reach altitudes up to 6000 m (Uturuncu). South of the Salar de Uyuni (3660 m), the Altiplano rises gradually and changes into the Puna of Argentina, which has an average height of a b o u t 4200 m. The Puna is divided into separate basins, most of which have interior drainage systems and numerous salt-lakes. (3) The Cordillera Occidental consists of composite volcanoes of Cenozoic age. The volcanoes have elevations of up to 6550 m (Sajama). The Cordillera

67’

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83 Occidental is the divide between the Altiplano and the Pacific Ocean, and forms the frontier with Chile. The investigated area has a basement of low-grade metamorphic rocks of Ordovician age. These Ordovician rocks are separated from the overlying Cretaceous and Tertiary sediments by a pronounced angular unconformity. These continental sediments are abundant in the eastern and northern part of the studied area. Towards the west and south the sediments are covered by volcanic rocks. The Cretaceous sequence is divided into three formations (Lohmann and Branisa, 1962). In the El Molino Formation the sedimentation started with a conglomerate composed of Paleozoic quartz-pebbles in a calcareous sandstone matrix, overlain by limestones, silty limestones and marls of Campanian to Maastrichtian age (Fig.2). The Santa Lucia Formation is built up by red clays and marls of Senonian to Danian age. The Cayara Formation consists of reddish and yellowish sandstones with intercalations of clay horizons. The total thickness of the three Cretaceous formations is only a b o u t 500 m. In comparison with the Cretaceous Salta Group of northwestern Argentina, which has a thickness of a b o u t 3300 m (Turner, 1970), the Cretaceous in southwestern Bolivia is poorly developed. Its thickness increases from west to east. The Potoco Formation of Eocene to Early Oligocene age overlies the Cretaceous sediments conformably. This formation is composed of a sequence of dark-red and reddish violet sandstones and clays, each bed being only some 40--60 cm thick. The complete sequence has a total thickness of a b o u t 1300 m (Meave, 1972}. Cross-lamination can often be observed and gypsum veinlets are locally abundant. The Lower Quehua Formation, which is of Oligocene to Lower Miocene age, overlies t h e P o t o c o Formation unconformably. The Lower Quehua Formation is composed of thin beds of reddish and yellow-green arcosic sandstones, which often are cross-laminated. Thin claystone horizons and conglomerate beds containing Ordovician rocks axe intercalated. The thickness of the entire series reaches 2900 m (Meave, 1972). The Lower Quehua Formation has a greater areal extension than both the Cretaceous formations and the P o t o c o Formation. Towards the east, the sandstones change into conglomerates (San Vicente Formation), which axe composed of Ordovician and Cretaceous rocks. These conglomerates were deposited in the marginal zones of the sedimentary basin, in the neighbourhood of the Paleozoic block. CENOZOIC VOLCANISM The oldest volcanic rocks in the studied area are the amphibole-rich Potoco Lavas, which are intercalated in the upper part of the Cretaceous and Tertiary red beds (Fig.2). These lavas have a thickness of only 12 m and axe of mugearitic composition. Outcrops of the P o t o c o Lavas have been found along the Uyuni--Alota road and near San Pablo de Lipez. Another manifes-

84

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tation of volcanic activity is represented by thin tuff horizons in the upper part of the Lower Quehua Formation. This explosive activity of the Early Miocene was followed by quiet effusions of pyroxene-rich, latitic lavas and by dome- and sill-like intrusions. The Rondal Lavas and the Julaca Lavas belong to this series. The Rondal Lavas are exposed in the area between San Pablo de Lipez and the frontier of Argentina. They are cut by numerous quartz veinlets, which often contain amethyst and agate. The Julaca Lavas are abundant in the northwestern part of the investigated area. During the Early Miocene, the Lower Quehua For-

85 mation and the Rondal and Julaca Lavas were folded. After a period of erosion, they were buried by horizontal sheets of pyroclastic material. The Upper Quehua Formation, which consists of ash-flows and ignimbrites of mainly rhyodacitic composition, has a thickness of up to 750 m. This formation occupies an area of a b o u t 2600 krn 2 in the eastern and northern part of southwestern Bolivia. The ash-flows and ignimbrites of the Upper Quehua Formation thin o u t towards the central part of Lipez. This sequence consists in its lower part of reddish to greenish volcanic sediments and white block-rich ash-flows. These are overlain by red ignimbrites and grey, lapilli-rich ash-flows. The upper part of the Upper Quehua Formation consists of white pumice flows. Locally, sheets of reworked volcanic material, agglomerates, and lava flows are interbedded. The whole formation consists of at least 23 units, each having a thickness of a b o u t 30 m. The lapiUirich ash-flows and the pumice flows can be subdivided into t w o or three sub-units. Each unit has similar mineralogical and chemical composition and was erupted within a short span of time. The eruptive centers of the volcanics of the Upper Quehua Formation are n o t known for certain. It seems that the eruptions occurred from fissures parallel to the fracture zone at the edge of the Paleozoic block against the Altiplano. In the Upper Quehua Formation, between Escala and San Antonio de Lipez, obsidian-like lavas must exist. Blocks of such glassy lava, which has the same chemical composition as that of the ignimbrites, are scattered all over the surface in that area. Only one outcrop of this obsidian-like lava has been found, some 7 km north of San Antonio de Lipez. It occurs as small lenses within a flow of reddish flaggy lava. The uppermost part of the Upper Quehua Formation is characterized by products of predominantly effusive volcanic activity. Lava-rich composite volcanoes which pierced the ash-flow units, produced the quartz- and biotitebearing rhyodacitic Bonete Lavas. These Miocene lavas occur all over the distribution area of the Upper Quehua Formation, b u t are definitely more abundant in the eastern part. In the western part of the investigated area outcrops of Miocene lavas can be f o u n d only near San Cristobal and Todos Santos. The Bonete Lavas were dated as Miocene by Ahlfeld (1946, 1972). In the new geological maps their stratigraphic position is regarded as PlioPleistocene. Because these lavas differ from the Quaternary composite volcanoes in morphology and stratigraphic position, it is more probable that the Bonete Lavas belong to the Miocene. These old composite volcanoes have been strongly eroded; no remnants of craters exist. The Bonete Lavas have never been f o u n d upon the Pliocene ignimbrites. In the central part of the studied area, near Quetena and Soniquera, lava flows have been f o u n d below the Pliocene ignimbrite plateau. It seems, that these lavas are contemporaneous with the Bonete Lavas. In Lipez, the Miocene composite volcanoes cover an area of a b o u t 500 km 2. More to the north, contemporaneous volcanic rocks occur frequently.

86 These volcanics have been described as subvolcanic intrusions. The greatest known massive is that of Karikari-Andacaba which covers an area of about 350 km 2 (Wolf, 1973). During the Late Miocene, the entire volcanic sequence underwent a weak deformation, characterized by undulations and thrusts. Locally, an angular unconformity can be observed between the pumice flows of the Upper Quehua Formation and the overlying Ignimbrite Formation (south of San Antonio de Esmoruco). The Pliocene Ignimbrite Formation corresponds petrologically and stratigraphically to the Rhyolite Formation of northern Chile (Zeil and Pichler, 1967). This formation has a wide distribution, covering more than half of the studied area. It occupies the whole of the southern and western part of Bolivia and continues into adjacent northern Chile and northwestern Argentina. The ignimbrites have rhyodacitic to dacitic composition. They are rich in phenocrysts, lithic fragments, and pumice. The pumice often occurs in the form of "fiammes". In part, the lithic fragments are cumulates, others derive from the walls of the vents, but most of them are fragments of the floor over which the ignimbrite flow passed. Smith (1960) first explained the origin of this type of fragments. Evidence supporting this explanation was found south of Quetena. Here an ignimbrite sheet overlies Paleozoic rocks and contains, near to its base, numerous Ordovician fragments. Most of the ignimbrites are whitish, grey or red. They are characterized by brownish superficial crusts and dense and glassy tops and joints that were probably produced by fumarolic activity and vapor-phase processes. Individual ignimbrite sheets are separated by dark grey sediments, which are composed principally of biotite and of whitish horizons of pumice fragments. Both the biotite and pumice are derivations of older, eroded ignimbrites. The ignimbrite sheets often fill valleys in the older topography. Reticulate joint patterns predominate in southwestern Bolivia. This pattern is made up of small ridges which form a network; a feature caused by the cooling of the sheets. Flow ridges, produced by the pressure of the flowing ignimbrite, are c o m m o n in the young ignimbrites and are best developed between the volcanoes Ollagiie and Cafiapa. Guest (1969) described a similar surface pattern in Chilean ignimbrites. The base of the Ignimbrite Formation is only rarely exposed. Consequently the m a x i m u m thickness of the sequence is unknown. South of San Antonio de Esmoruco, the exposed ignimbrite sequence has a thickness of 200 m, b u t it is certain that a greater thickness occurs towards the Cordillera Occidental, where it could reach as much as 600 m. South of San Antonio de Esmoruco, the ignimbrite plateau consists of at least ten individual units. The sequence begins with four sheets of dark grey ignimbrites which can be subdivided into some sub-units. These sub-units are separated either by ash-fall horizons or by a rectilinear separation line. The grey sheets are overlain by t w o ignimbrite units of reddish colour and whitish "fiammes". Near the Panizo volcanoes, further ignimbrite units are exposed; b u t the region is n o t accessible

87 from the Bolivian side. The thickness o f the individual sheets varies from several meters to some hundred meters b u t in general is a b o u t 30 m. The eruption centers of the ignimbrites are mostly unknown. It seems that the greatest part of the ignimbrites originated from fissures, which were located near to the crest of the Cordillera Occidental. On the other side, however, the ignimbrites in the eastern part of the investigated area do n o t originate from the Cordillera Occidental; images from the ERTS satellite and field observations show clearly that the ignimbrites of the plateau south of San Antonio de Esmoruco were erupted from a great caldera. The oval caldera, situated on the frontier with Argentina, has a diameter of a b o u t 23--17 km (Fig.3). One can also suppose that at least a part of the ignimbrites exposed in the central part of South Lipez was erupted from N-S-striking fissures, which are located between Quetena and San Antonio de Lipez. The field w o r k revealed a further ignimbrite vent: a NE-SW-striking fault located at the southern end of the Laguna Colorada. Here, a pyroclastic section with a thickness of a b o u t 20 m is exposed, resting on an older ignimbrite sheet. The section begins with a layer of round pumice blocks, which are collapsed towards the top. In the upper part of the section the "fiammes" become smaller and more welded. A b o u t 10 km to the north, the " f i a m m e s " within the same sheet are much smaller. The ignimbrite eruptions, which were the dominant form of volcanic activity during a period of a b o u t 20 m.y., ceased at the end of the Pliocene. During the Quaternary, large composite volcanoes were built up (Stratovolcano Formation). None of the lavas were dated radiometricaUy. However, no lava flows were found intercalated between the Pliocene ignimbrite sequences. The most conspicuous volcanoes are situated in the central part of Lipez: the Uturuncu (6008 m), the highest peak of southwestern Bolivia, and the Soniquera (5899 m). As the elevation of the underlying ignimbrite plateau is a b o u t 4300 m, the relative height of these composite volcanoes is only 1600--1700 m. According to their different erosion features, the volcanoes can be subdivided in t w o groups: a Pleistocene (pre-glacial) and Holocene (post-glacial) one. The first group shows clear evidence of glaciation: i.e. cirques, "U"shape8 valleys and lateral and terminal moraines. The second group shows no evidence either of glaciation or of strong erosion; that is, the slopes and craters are well preserved. Some volcanoes were built up in pre-glacial time, and also show post~glacial activity, manifested by lava flows overlying glacial moraines (Uturuncu). There are no reports of historic eruptions from any of the volcanoes on the Bolivian side. In some places, however, there is still fumarolic activity, which has extensively altered the rocks and has formed sulfur deposits. Most of the volcanoes, especially the older ones, are composed of two or more summit craters and show several parasitic cones on their slopes. The volcanoes consist mainly of rhyodacitic to dacitic lavas of the "aa"-type;

88

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volcanics of more basic composition are subordinate. Because of the acidic composition of the rocks, the lavas were very viscous and formed often extremely steep slopes (e.g. Licancabur). Outcrops of pyroclastic deposits are scarse. One of the few volcanoes which produced a great quantity of unwelded scoriae is the Juriques directly at the frontier with Chile. On many composite volcanoes welded scoriae deposits have been observed. During the Pleistocene some of the great composite volcanoes collapsed and calderas were formed. These subsidences normally were accompanied by the effusion of small ignimbrite flows (Utfir~ncfi, San Antonio, Corina). These Pleistocene ignimbrites differ from the underlying Pliocene ignimbrites in their smaller quantity, their morphology and their petrological composition. Lava effusions along fissures (Quetena) and a few endogeneous extrusions (Khara Laguna, Escala) occurred in Holocene time. Most of the Quaternary volcanoes were built up along fractures running approximately N-S, E-W, NW-SE, and NE-SW. Several of the calderas lie at the intersection of fracture zones (San Antonio, Cerro Lipez, Soniquera).

89 Nowhere was it observed that the faults had displaced the Quaternary composite volcanoes. The volume of the Strato-volcano Formation in the studied area can be estimated to be a b o u t 3000 km 3. Most of the volcanoes were formed during the Pleistocene. In the Holocene, the volcanic activity ceased; and the more important eruptive centers were mainly concentrated in the Cordillera Occidental. STRUCTURE

The Altiplano consists of a strongly folded Paleozoic basement, and a cover of sedimentary and volcanic rocks of Cretaceous to Cenozoic age. The overlying rocks are only slightly dislocated. The investigated area has remained under continental conditions since the end of the Ordovician. Only in the Late Cretaceous did a marine transgression of minor importance occur (El Molino Formation). The uplift of the Altiplano occurred during the Andean orogenesis, i.e. in the Upper Miocene and Early Pleistocene {Rutland et al., 1965). The Paleozoic sediments were folded, faulted and got their schistosity as a result of Hercynian movements. By the process of low-grade metamorphosis the rocks were transformed into greenschists (Subieta, 1972). In the studied area, four different Andean movements can be distinguished: the earliest one occurred after the deposition of the Potoco Formation in the Lower Oligocene and produced systems of wide folds, upthrusts, and joints. Later, during the Lower Miocene, the sediments of the Lower Quehua Formation and the Rondal and Julaca Lavas were folded. In the Upper Miocene the ignimbrites and ash-flows of the Upper Quehua Formation were folded and upthrusted. Moreover, a reactivation of the older structures t o o k place. The latest movements occurred a b o u t 3 m.y. ago (Kussmaul et al., 1975), dislocating and thrusting the Pliocene ignimbrite sheets. With the end of these movements, the Andean orogenetic cycle was terminated. The structural elements of the Cenozoic rocks c o m m o n l y follow those of the Paleozoic basement, that is, in N-S and NE-SW directions. The block faulting of the Altiplano during the Andean orogenesis resulted in the formation of tectonic basins. Some of these basins are filled by salt lakes (Salar de Chalviri). The Paleozoic block is separated from the AItiplano by a fault system {Esmoraca--San Vicente fault), which strikes in a N-S direction. Within the investigated area, three large Paleozoic horsts were observed: the horst of San Pablo de Lipez is situated in the eastern part of the area. It strikes in a N30°E direction. The horst of Serrania de las Minas and the horst of Quetena are located in the central part. The former strikes in a N30°E direction, the latter N-S. The horsts consist of Ordovician sediments, folded into a system of anticlines and synclines, the axial directions of which are parallel to the general strike of the horsts. With the exception of the horsts, the whole area

90 is covered by Cretaceous and Cenozoic sedimentary and volcanic rocks. ERTS images (Fig.3) indicate that the dominant direction of faults is N0--10°E; NE-SW, N80°W, and NW-SE fractures are also abundant. The N80°W fractures are especially developed in the western part of the area, whereas NW-SE fractures are prominent in the southeastern part of Lipez. In the southern part of the Cordillera Occidental, a tectonic evaluation of ERTS images could n o t be carried out because the volcanoes were covered by snow. The concentration of the faults is closely correlated with volcanic activity. Some tensional faults provided feeder-channels for Miocene and Pliocene ignimbrite sheets (pp. 85 and 87). Many younger volcanoes ~ o s e along the same fissures. Intersections of fracture systems are characterized by intense volcanic activity and the formation of calderas (Cerro Lipez, Uturuncu, Soniquera). The calderas usually have diameters of 2--3 km. Faults visible on the ERTS images are not always distinctly perceptible in the field. Often they are indicated merely by the alignment of volcanic centers. MINERALOGY AND GEOCHEMISTRY R o c k samples were taken with respect to the large variation of the rock types. It is thought, that the selected material represents the whole mineralogical and petrochemical range occurring in the investigated area.

Ignim brites For petrochemical investigations, welded rocks were preferred, because they are probably less altered by leaching or hydrothermal processes than unwelded types. Eight new major element analyses of ignimbrites are presented (Table 1). The calculated norms of these analyses and of eleven analyses from Fernandez et al. (1973) are plotted in the QAPF (Streckeisen) diagram (Fig.4). Rhyodacite (14 samples) is by far the most abundant rock type. Three samples are dacites, lying very close to the rhyodacite field. Two analyses represent rhyolite and latite, respectively. The silica content of all samples ranges from 59.8% to 73.1%, b u t is greater than 64% in 17 of the 19 analyses. Normative free quartz varies between 13.6 and 40.4 vol.%, being mostly > 20. The FeO/Fe~O3 ratio generally is smaller than 1. In the "Puripicar" Ignimbrite of northern Chile the silica content was found to be higher in the densely welded than in the poorly welded zones (Guest, 1969). In the " S i f o n " Ignimbrite of the same area more silica is concentrated in the vitric zones than in the welded and crystallized parts of the same sheet. The relative great variation of the silica content in our samples is probably due to the different stage of welding and/or vapor-phase crystallization. As no groundmass analysis was carried out, it can only be supposed that the petrochemical variation depends mainly on the different stages of

91

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crystallization. Sample I 7, for example, has a silica content of 72.6% (rhyolite), and is composed nearly completely of unaltered glass. The available chemical analyses of five ignimbrite samples of the Upper Quehua Formation and of twelve samples of the Ignimbrite Formation may not be representative of such a large area. However, the data show that the Upper Quehua Formation has higher K20/Na20 ratios (mostly greater than 1.7) and higher contents of normative cordierite (up to 15.6 vol.%) and of P2Os. The K20/Na20 ratios of the rocks of the Ignimbrite Formation vary between 1.0 and 1.5 and their normative cordierite content only reaches a maximum of 3.0 vol.%. The ignimbrite sample 1 8 does not belong either to the Upper Quehua or to the Ignimbrite Formation, but is related to the Quaternary composite volcanoes. This ignimbrite forms a small flow with an extension of only a few hundred meters. Its chemical composition is very similar to that of the Miocene and Pliocene ignimbrites. The comparative sample of the Los Frailes ignimbrite (I 2) resembles petrochemically the ignimbrites of the Upper Quehua Formation. The sample shows 10.6 vol.% of normative cordierite, but this mineral is also modally existing in the form of small, rounded, and twinned crystals.

92

TABLE1 I g n i m b r i t e s - - s o u t h w e s t e r n Bolivia I1

I2

I3

I4

I5

I6

17

I8

CO: H:O

69.40 14.13 2.90 0.77 0.03 1.21 2.63 2.20 3.86 0.66 0.16 0.17 1.57

65.58 15.92 1.26 2.18 0.05 1.37 3.16 1.93 4.06 1.03 0.41 0.21 2.71

64.02 15.96 3.02 1.90 0.10 2.10 4.88 2.90 3.15 1.01 0.15 0.14 0.32

65.83 15.54 1.93 1.07 0.06 1.46 3.78 2.84 3.43 0.62 0.11 0.24 2.87

67.09 15.73 3.52 0.36 0.05 1.25 3.75 2.53 3.79 0.93 0.19 0.10 0.85

67.09 15.62 3.32 0.04 0.07 1.18 3.09 3.54 4.39 0.58 0.13 0.16 0.58

72.61 13.63 0.94 0.52 0.05 0.33 1.26 3.28 4.56 0.30 0.03 0.12 2.15

66.32 14.89 2.67 0.85 0.06 1.54 3.00 3.09 4.44 0.70 0.15 0.09 2.27

Total

99.69

99.87

99.65

99.78

100.14

99.79

99.78

100.07

Ru Ap C Cc

35.30 22.81 18.62 10.93 0.00 3.01 0.67 2.44 1.25 0.00 0.37 2.33 0.39

31.11 23.99 16.33 11.67 0.00 4.77 1.83 0.00 1.96 0.00 0.95 4.07 0.48

22.55 18.62 24.54 21.23 0.87 4.83 3.52 0.59 1.92 0.00 0.35 0.00 0.32

26.87 20.27 24.03 16.52 0.00 3.64 1.85 0.66 1.18 0.00 0.25 1.10 0.55

28.78 22.40 21.41 16.73 0.00 3.11 0.00 3.52 0.87 0.47 0.44 1.33 0.23

22.12 25.94 29.96 13.47 0.00 2.94 0.00 3.32 0.23 0.46 0.30 0.11 0.36

33.30 26.95 27.76 5.30 0.00 0.82 0.97 0.27 0.57 0.00 0.07 1.36 0.27

23.30 26.24 26.15 13.34 0.00 3.84 0.91 2.04 1.33 0.00 0.35 0.11 0.20

Rittmann norm Quartz Sanidine Plagioclase Clinopyroxene Hypersthene Calcite Biotite Cordierite Magnetite Ilmenite Apatite

35.6 29.2 21.2 0.0 0.0 0.4 6.8 5.6 0.4 0.4 0.3

29.8 31.6 21.8 0.0 0.0 0.0 4.1 10.6 0.4 0.9 0.8

21.2 24.8 42.9 1.5 7.4 0.0 0.0 0.0 0.7 1.1 0.3

29.7 20.4 39.2 0.0 0.0 0.0 9.8 0.0 0.4 0.3 0.2

30.6 23.9 33.7 0.0 0.0 0.0 9.5 0.8 0.4 0.6 0.4

23.0 34.1 30.8 0.8 3.8 0.0 6.1 0.0 0.7 0.3 0.2

32.5 49.2 13.3 0.0 0.0 0.0 1.5 3.0 0.2 0.3 0.1

25.0 32.5 27.7 0.6 4.9 0.0 7.9 0.0 0.7 0.4 0.3

Colour index

13.2

16.0

10.7

10.4

11.3

11.8

4.9

14.5

C o o r d i n a t e s S t r e c k e i s e n double-triangle Q 41.4 35.8 23.9 A 34.0 38.0 27.9 P 24.7 26.2 48.2 F 0.0 0.0 0.0

33.3 22.9 43.9 0.0

34.7 27.1 38.2 0.0

26.2 38.8 35.0 0.0

34.2 51.8 14.0 0.0

29.4 38.1 32.5 0.0

SiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 TiO2 P205

CIPWnorm

Q Or Ab An Di Hy Mt Hm

93 TABLE 1 (continued) I1

I2

I3

I4

I5

I6

I7

I8

Coordinates A F M A F M

triangle 56.7 55.9 31.9 31.4 11.3 12.8

47.0 36.7 16.3

59.2 27.1 13.8

56.7 32.1 11.2

65.0 25.4 9.7

81.8 14.8 3.4

60.8 26.8 12.4

Sigma Rittmann Tau Rittmann

1.39 14.55

1.74 11.26

1.72 17.40

1.66 11.79

2.61 17.01

2.08 31.36

2.43 13.88

I 1: I 2: I 3: 14: I 5: I 6: I 7: 1 8:

1.59 9.72

rhyodacite, Upper Quehua Formation; northern slope of Cerro Lipez, 4900 m; 66°53'W, 21°54'S. rhyodaeite, Los Frailes Formation; road Potosi--Challapata, km 110, 4100 m (comparative sample ). rhyodaeite, Ignimbrite Formation; road Quetena--Laguna Colorada, 22 km from Quetena, 4700 m; 67°32'W, 22°11'S. daeite, Ignimbrite Formation; Laguna Curutu, 4500 m; 67°06'W, 22°26'S. rhyodacite, Ignimbrite Formation; road Laguna Verde--Laguna Curutu, 80 km from Laguna Verde, 4650 m; 67°23 W, 22°29'S. rhyodacite, Ignimbrite Formation; Khara Laguna, 4400 m; 67°48'W, 21°55'S. rhyolite, Ignimbrite Formation; south of Laguna Busch, 4650 m; 6 7 ° 1 5 ~ , 22°41'S. rhyodaeite, Strato-voleano Formation; Cerro Corina, 4250 m; 67°57'W, 21°34'S.

The trace element abundances of Rb, St, Cu, Co, Ni, Zn, and Zr are presented in Table 3. Compared with the data for calc-alkaline volcanic rocks (Taylor, 1969), our Rb values are higher and the Sr contents lower. Our data and the data given by Fernandez et al. (1973) evidence a distinct difference in the trace element abundances of the Upper Quehua and the Ignimbrite Formations. As compared to the rocks of the Ignimbrite Formation, those of the Upper Quehua Formation have higher Sr and Ni contents and differ also with respect to their Rb/Sr and Ni/Co ratios. The Pleistocene ignimbrite (I 8) has a very low Zr content. There is no difference in mineralogical respect between the ash-flows and ignimbrites of the Upper Quehua and the Ignimbrite Formations. The phenotypical mineralogical composition depends neither on the chemical composition, the stage of welding, nor on the locality of the sample. Most of the rocks are conspicuously rich in phenocrysts, which typically constitute about 30--40% of the total volume. With regard to their mineralogical composition, the ignimbrites can be subdivided into two different groups: The first group, comprising about a third of all pyroclastic rocks, is characterized by the presence of sanidine. The phenocrysts consist of plagioclase, quartz, biotite, sanidine, and smaller amounts of hornblende, which also may be absent. Plagioclase, mostly of andesinic composition, is the most abundant mineral. Often the feldspar crystals are fragmented, showing corroded and turbid cores with surrounding rims of clearer material. Most of the plagioclase has pronounced zoning and twinning. Most quartz crystals are fractured, rounded and have numerous deep embayments, although nearly complete crystals can be found. In many rocks, biotite is the only dark mineral. It is

norm

Quartz San]dine Plagioclase Clinopyroxene Hypersthene Olivine Calcite Biotite

Rittmann

Or Ab An Di Hy O1 Mt Hm I1 Ap C Cc

Q

CIPW norm

To t a l

CO2 H20

P~O5

K20 TiO2

Na20

AI20~ Fe:O~ FeO MnO MgO CaO

SiO:

0.0 15.5 53.4 10.9 5.4 9.5 0.6 0.0

15.0 21.0 46.8 0.2 14.0 0.0 0.5 0.0

14.56 16.02 25. 4 7 22.25 0.19 15 . 42 0.00 2.22 0.00 2.13 0.49 0.0O 0.43

100,25

99,55

0.00 12.06 27.16 21.51 9.57 11 . 59 2.09 7.92 0.00 2.96 1.23 0.00 0.57

60.66 16 . 04 1,53 4.38 0.08 4,19 5.05 3.01 2,71 1.12 0.21 0.19 1.08

L2

48.47 15.37 5,46 5.40 0.22 6.35 7.78 3.21 2.04 1.56 0.53 0.25 2.91

L1

Lavas -- southwestern Bolivia

TABLE 2

25.1 36,7 27.0 0.0 0.0 0.0 0.4 3.8

24.97 23.05 32. 7 5 8.50 0.00 5.52 0.00 0.49 0.00 0.85 0.30 2.31 0.39

99.44

68.93 16.01 0.34 2.23 0.03 0.96 2.10 3.87 3.90 0.45 0.13 0.17 0.32

L3

23.0 19.9 44.3 0.0 8.5 0.0 0.0 0.0

25.72 16.13 19.38 24.17 0.00 5.33 0.00 2.32 2.70 1.84 0.00 1.41 0.32

99.66

63.12 16.99 4.30 1.51 0.08 2.14 5.05 2.29 2.73 0.97 0.00 0.14 0.34

L4

28.5 23.7 32.5 0.0 0.0 0.0 0.0 11.7

26.57 23.46 19.38 16.96 0.00 4.06 0.00 3.67 0.07 1.63 0.51 2,00 0.27

100.00

64.84 16.28 2.60 1.86 0.05 1.63 3,86 2.29 3.97 0.86 0.22 0.12 1.42

L5

L6

24.0 30.6 33.7 O.0 7.8 0.0 0.0 0.0

24.30 22.22 18.87 19.17 0.00 8.80 0.00 2.07 0.00 2.51 0.46 0.99 0.11

99.79

64.70 15.75 1,43 3.68 0.08 2.11 4.19 2,23 3.76 1.32 0.20 0.05 0.29

L7

28.7 36.1 26.6 0.0 0.0 0.0 0.0 7.6

26.09 27.24 24,62 12.06 0,00 4.65 0.00 1.33 0.00 1.10 0.32 0.81 0.30

99.74

68.29 15.01 0.92 2.28 0.05 0.84 2.78 2.91 4.61 0.58 0.14 0.13 1.20

23.3 28.0 38.6 0.0 6.6 0.0 0.0 0.0

23.06 20.57 20.73 20,63 0.00 8.21 0.00 1.30 0.00 1.98 0.37 1.04 0.18

99.93

63.95 16.40 0.90 3.20 0,06 1.88 4.47 2.45 3.48 1.04 0.16 0.08 1.86

L8

20.0 27.0 41.9 0.3 8.2 0.0 0.0 0.0

22.25 20.27 20,99 20.64 0.00 4.76 0.00 2.92 1.67 2.18 0.60 0.86 0.98

99.64

61.57 16.22 3.69 1.86 0.08 1.91 5.05 2.48 3.43 1.15 0.26 0.43 1.51

L9

8.9

0.0

1.61 0.32 0.00 0.32

1.16 0.44 0.79 0.23

21.0 21.6 39.4 2.0 5.5

3.13

2.42

22.7 24.4 39.5 0.0 10.4

18.64 21.81 27.16 19.28 1.03 5.26

99.97

63.32 16.34 2.16 2.29 0.08 1.84 4.51 3.21 3.69 0.85 0.14 0.14 1.40

Lll

22.87 19.03 17.60 22.88 0.00 11.52

100.02

63,31 16.08 1.67 4.09 0.09 2.50 4.99 2.08 3,22 0.61 0.19 0.10 1,09

L10

0.0

18.6 17.5 46.7 0.0 10.8

2.24 0.63 3.36 1.82

2.28

21.07 14.48 19.80 19.62 0.00 12.71

99.62

59.40 17.05 1.57 4.49 0.09 3.03 5.33 2.34 2.45 1.18 0.27 0.80 1.62

L12

0.0

17.9 15.7 50.4 3.1 10.3

2.28 0.37 0.00 0.23

3.00

17.80 13.42 24.62 23.92 2.54 10.54

99.73

60.88 16.01 2.07 4.14 0.10 3.08 5.79 2.91 2.27 1.20 0.16 0.10 1.02

L13

0.0

15.2 25.6 48.7 3.8 4.4

1.84 0.42 0.00 0.34

3.57

]5.90 17.91 36.73 16,13 2.98 3.62

99.59

63.45 16.33 2.46 2.23 0.07 1.78 4.44 4.34 3.03 0.97 0.18 0.15 0.16

L14

0.0

16.9 26.3 46.1 2.7 5.7

1.61 0.42 0.00 0.25

2.71

16.82 17.91 33.93 16.46 2.13 6.02

99.48

63.38 15.91 1.87 2.55 0.07 2.08 4.23 4.01 3.03 0.85 0.18 0.11 L21

L15

0.9

20.2 16.4 49.4 4.7 7.3

1.60 0 16 0.01) 0.27

2.45

!9,16 13.06 29.53 19.45 4.04 8.57

99.5(~

63.49 15.26 1.69 3.33 0.09 2,82 5.17 3.49 2.21 0.84 0.07 0.12 !).9~

L16

0.0

12.0 5.7 65.2 4.8 9.4

2.62 0.44 0.00 0.55

3.67

12.7~ 8.39 25.64 32.36 3.41 9.31

i 00,0~,

56.67 18.38 2.53 4.58 0.14 2.68 7.92 3.03 1.42 1.38 0.19 0.24 ~),89

L17

().(i

~i2 ~.8 :,1.5 7 i ?.3:~

21 0.4,1 0.0(~ 2.64

I i~

6.25 9.1(i 18.79 26.9] 0.60 .V).7~!

~9.59

53.85 15.19 0.77 7.39 0.13 7.90 7.3(I 2.22 ! .55 ].11 0.i9 ll~; ~.~:~

L18

29.3

0.0 1.8 1.6 1.2

16.3

0.0 0.9 1.2 0.4 10.7

6.2 0.4 0.3 0.2

O.0 22.5 77.5 0.O

1.85 9.80

36.3 37.1 26.6 2.33 20.93

68.8 22.7 8.5

28.3 41.3 30.4 0.0

1.25 15.15

39.8 43.3 17.0

26.4 22.8 50.8 0.0

12.8

3.6 0.6 0.0 0.0

1.79 12.95

51.6 35.0 13.4

33.6 28.0 38.4 O.0

14.9

2.2 0.5 0.5 0.4

1.65 8.89

45.6 38.4 16.0

27.2 34.7 38.1 0.0

11.2

2.9 0.6 0.0 0.4

2.24 16.81

65.3 27.4 7.3

31.4 39.5 29.1 O.O

1.68 11.62

49.9 34.3 15.8

25.9 31.2 42.9 0.0

9.9

0.O 0.3

0.3 0.3 8.3

2.8 0.5

O.O 0.4

1.88 9.74

45.2 40.2 14.6

22.5 30.4 47.1 O.O

10.5

1.3 0.5

0.0 0.8

1.38 17.50

39.3 42.1 18.5

26.2 28.2 45.6 O.O

13.0

0.0 0.4

1.8 0.7

2.34 13.26

52.9 33.0 14.1

25.6 26.4 48.1 0.0

17.8

0.5 0.3

O.0 0.9

1.40 10.14

34.7 43.4 21.9

22.4 21.1 56.4 0.0

16.7

0.0 0.6

5.2 0.7

olivine-mugearite, Potoco Lavas; road San Cristobal--Alota northwest of Ecia. Culpina, 3800 m; 67°14'W, 21°15'S. quartz-latiandesite,Bonete Lavas (?); Rio San Antonio north of Quetena, 4150 m; 67 ° 18'W, 22°10'S. rhyodacite, Upper Quehua Formation, glassy lava north of San Antonio de Lipez, 4300 m; 66°51'W, 21°48'S. dacite,Strato-volcano Formation; caldera wall of Cerro Lipez, 5100 m; 66°53~, 21°54'S. rhyodacite, Strato-volcano Formation; Cerro Lipez, 5000 m; 66°53'W, 21°54'S. rhyodacite, Soniquera volcano, 5100 m; 67°15'W, 22°02'S. rhyodacite, Corina volcano, 4250 m; 67°5TW, 21°34'S. rhyodacite, Quetena volcano, 4350 m; 67°23~, 22°17'S. rhyodacite, Galera volcano at the road Atocha--Tatasi, 3950 m; 65°58~/, 20°59'S (comparative sample). rhyodacite, Strato-volcano Formation; Laguna Curutu, 4500 m; 67°06'W, 22°26'S. rhyodacite, lava d o m e of Khara Laguna, 4450 m; 67°48'W, 21°55'S. dacite,Strato-volcano Formation; lava between Laguna Collpa and Laguna Busch, 4600 m; 67°18~V, 22°35'S. dacite, Strato-volcano Formation; Campamento Rio Blanco, 4700 m; 67°49'W, 22°40'8. quartz-latiandesite,Cafiapa volcano, 4900 m; 68°05~/, 21°29'8. quartz-latite,Cafiapa volcano, 4400 m; 68°05'W, 21°26'S. dacite,Licancabur volcano, northern slope, 4300 m; 67°52'W, 22°48'S. quartz-andesite, Juriques volcano, escoria, 4400 m; 67°50'W. 22°51'S. quartz-latiandesite,lava d o m e near Escala, 3950 m; 66°53q~/, 21°36'S.

5.04 5.82

23.7 47.6 28.7

18.1 25.4 56.6 0.O

Analyst: P.K. H6rmann, 1973.

L i: L 2: L 3: L 4: L 5: L 6: L 7: L 8: L 9: L 10: L 11: L 12: L 13: L 14: L 15: L 16: L 17: L 18:

Sigma R i t t m a n n Tau R i t t m a n n

A F M

Coordinates A F M triangle

Q A P F

Coordinates Streckeisen double-triangle

Colour index

Cordierite Magnetite l]menite Apatite

1.50 9.63

36.1 42.5 21.4

21.3 18.7 60.0 0.0

15.6

1.3 0,3

0.0 0.9

2.66 10.43

53.9 33.0 13.0

17.0 28.6 54.4 0.0

10.3

1.0 0.4

0.O 1.6

2.43 11.55

52.4 32.1 15.5

18.9 29.4 51.7 0.0

10.3

0.9 0.4

0.0 1.0

1.59 12.93

42.3 36.7 20.9

23.4 19.1 57.5 0.0

13.9

0.9 0.1

0.0 1.0

1.45 9.78

31.5 49.5 19.0

14.5 6.8 78.7 O.O

16.7

1.5 0.4

0.0 1.0

e~ ¢jI

1.31 9.98

19.0 41.3 39.7

9.3 13.2 77.5 0.0

32.8

1.2 0.4

0.0 1.1

96

p r e s e n t in t h e f o r m o f t h i c k , o f t e n b e n t f l a k e s , t h e m a r g i n s o f w h i c h have b e e n a l t e r e d t o a d u s t y a g g r e g a t e of i r o n ores. T h e b i o t i t e is s t r o n g l y p l e o c h r o i c , g e n e r a l l y w i t h X = l i g h t b r o w n o r l i g h t green a n d Z = d a r k b r o w n t o b l a c k . H o r n b l e n d e , o c c a s i o n a l l y r o u n d e d a n d w i t h o p a q u e o x i d i z e d rims, is u s u a l l y an o l i v e - b r o w n or d a r k r e d v a r i e t y a n d f o r m s e u h e d r a l c r y s t a l s . T h e s e c o n d g r o u p is c h a r a c t e r i z e d b y t h e a b s e n c e o f s a n i d i n e . T h e m i n e r a l a s s e m b l a g e p l a g i o c l a s e , q u a r t z , b i o t i t e , a n d h o r n b l e n d e is t y p i c a l ( a b o u t 40% o f all p y r o c l a s t i c s ) . S o m e o f t h e s a m p l e s h a v e small a m o u n t s o f o r t h o - a n d / o r c l i n o p y r o x e n e s . A n o t h e r f r e q u e n t m i n e r a l a s s e m b l a g e is p l a g i o c l a s e , q u a r t z , a n d b i o t i t e ( a b o u t 20% o f t h e i g n i m b r i t e s ) . P h e n o m e n o l o g i c a l l y t h e p h e n o c r y s t s are v e r y s i m i l a r in overall a p p e a r a n c e t o t h o s e o f the first group.

Lavas M a j o r e l e m e n t d a t a a n d c o r r e s p o n d i n g CIPW a n d R i t t m a n n n o r m s o f 18 n e w a n a l y s e s o f lavas a r e given in T a b l e 2. C o r r e s p o n d i n g t r a c e e l e m e n t a b u n d a n c e s a r e l i s t e d in T a b l e 3. A c c o r d i n g t o t h e R i t t m a n n n o r m s (Rittmann, 1973) and the Streckeisen classification (Streckeisen, 1968), the

TABLE

3

Trace element abundances (in ppm), K/Rb, Rb/Sr and Ni/Co ratios of ignimbrites and lavas of southwestern Bolivia. Analytical procedures of major and trace element analyses and standard deviations are described in H S r m a n n et al. (1973, pp. 405--407)

Rb

Sr

K/Rb

Rb/Sr

Zr

Cu

Co

424 307 304 292 135 349

188 197 173 188 205 209

0.60 0.63 0.81 0.96 2.02 0.74

163 175 133 127 126 29

7 12 5 14 11 19

7 11 8 6 2 12

251 168 230

330 278 272

186 198 211

0.76 0.60 0.85

182 221 171

21 26

113

553

403

0.20

235

4

279 173 145 146 311 130 95

259 302 428 564 257 461 420

201 246 308 247 117 205 197

1.08 0.57 0.34 0.26 1.21 0.28 0.23

197 234 171 222 212 217 153

25 12 11 23 19 11 38

Ignim b~tes I1 253 I3 193 I4 245 I6 281 I7 273 I8 260

Ni

Zn

Ni/Co

16 10 5 3 2 7

75 103 76 66 60 107

2.29 0.91 0.63 0.50 1.00 0.58

7 13 7

5 7 12

93 99 75

0.71 0.54 1.71

9 7

5 7

126 88

0.56 1.00

6

3

8

2

9 13 16 33

7 7 24 123

113 99 125 108 94 94

0.50 0.25 0.78 0.54 1.50 3.73

Lav~

L 3 L 4 L L L

5 6 7

L 8 L 11

L 14 L 15 L 16 L 18

A n ~ y s t : P.K. HSrmann, 1973.

97 18 lavas are rhyodacites (8 of the samples), dacites (4), latiandesites (3), latite (1), andesite (1), and mugearite (1). Their silica content ranges from 48.5 to 68.9 wt.%. All rocks except lava L 1 (Table 2) have normative quartz. Our data, together with those on 25 lavas investigated by Ahlfeld and Branisa (1960), Fernandez et al. (1973), and HSrmann et al. (1973), indicate that there is a trimodal frequency distribution of the silica content. The frequency distribution has a maximum at 63.4% SiO2 and two minor maxima at 65.1 and 67.7% SiO2, respectively. Lavas with silica contents less than 60% are rare (9 of 43), those with more than 63% occur frequently. The Rb, St, Zr, Cu, Co, Ni, and Zn contents (Table 3) vary sympathetically with the major element composition. Thus, the Cu, Co, and Ni contents decrease with increasing silica and increase with rising Fe and Mg levels. Rb is correlated with the K content; Sr with the Ca content. The values of Sr, Cu, Co, and Ni are similar to those reported by Taylor (1969) for dacitic and andesitic lavas of the calc-alkaline series. Our Rb levels are higher than those of Taylor (1969). In the Rb(Sr) versus SiO2 diagram (Fig.5) the Rb curve for the lavas indicates that Rb increases nearly linearly up to 62% SiO2. At higher silica percentages, the Rb content increases more rapidly. This tendency is consistent with the increase of K, which depends on the increase of modal alkali-feldspar and/or biotite. Rb is taken into the lattice of the K-feldspars and biotite. The Rb content of the ignimbrites is variable and shows no distinct tendency to increase with increasing silica. Quite similarly, as in the lavas, the i

:pprn) 5r

Rb SW BOI~V [ A

1000

D

[gnimbrites

,c Rb

+Sr

Lavas

.Rb

oSr

]gnrnbrites

~ Rb

+St

Lavas

,Rb

~Sr

N-CHILE

800 o

n

13

a

D

o D D

600

d'

r~

o

0

0

o

D ° °o

ca

0

400

0

+ o

o .+.

+

o

ol

+

o

o

o4.

o o

o ] ~ : 1

•

+o

+

x

x

200 +

x

+.+

• _~-

~b "

--~

"

• .....

5'5 "

" "v" ,.

" " sb

x

.

.

" " " d5

~" .~+

*xx

÷ +

.I-

+

. .

.

.

+ .

?b

75

"

% SiO2

Fig.5. Diagram of the Rb and Sr contents o f the Cenozoic volcanic rocks o f southwestern Bolivia and northern Chile as a function o f the SiO2 content.

98 Rb content increases with rising normative sanidine and biotite values. This tendency cannot be observed in samples rich in glass. The Sr content of lavas and ignimbrites is highly variable. It is obvious, however, that rocks with silica levels higher than about 65% usually have only low Sr contents. The variable Sr c o n t e n t of the lavas cannot be explained w i t h o u t knowledge of the distribution coefficients of Sr between the various Ca-bearing minerals, e.g. orthopyroxenes, ctinopyroxenes, and plagioclases, and the exact modal composition of the rock. With respect to the chemical and mineralogical composition, the lavas of southwestern Bolivia can be subdivided into three associations: the alkaline, the high-K calc-alkaline ("shoshonite"), and the typical calc-alkaline association. This division corresponds closely to that of Jakes and White (1972).

Alkaline association (Table 2, lava L 1). The Potoco Lavas, the oldest volcanic rocks of the investigated area, are obviously different in petrochemical respect from the other Cenozoic lavas. They have an olivine-mugearitic composition and a low silica content. The contents of Fe, alkalis, and P2Os are high, the FeO/Fe203 and K20/Na20 ratios are notably low. In the norm there appears an a m o u n t of 10.9% olivine. Mineralogically these rocks are composed of hornblende and plagioclase with smaller amounts of auglte and some biotite. Modal olivine is absent. High-K calc-alkaline association (Table 2, lavas L 4--L 10). According to the data of Jakes and White (1972), this group can be attributed to the shoshonite association. The lavas differ from those of the alkaline series by the presence of hypersthene, tridymite or cristobalite (Lefevre, 1973). All lavas which have a K20/Na20 ratio greater than 1.0 and no modal hornblende were attributed to this association. These rocks are characterized by high silica contents (61.8--68.3%) and by K20/Na20 ratios up to 1.9 (Fig.6). Thin-section examination shows that these lavas are highly porphyritic and contain phenocrysts of plagioclase, biotite, quartz, hypersthene, and sometimes a few crystals of sanidine. About 35% of our lavas have this mineral assemblage. In some rocks biotite, quartz, or hypersthene may be absent. Hypersthene is never found in the lavas of the Early Pleistocene volcanoes San Antonio and Morijon, whereas it is a typical mineral in the neighbouring and somewhat younger volcanoes Uturuncu and Soniquera. Plagioclase is quantitively the most abundant mineral. Two types, which differ in composition and appearance are present. The first one, mostly labradoritic to bytownitic (Anss to An65), consists of fragmented to strongly corroded turbid crystals showing abundant inclusions and a very pronounced zoning. The second type is less calcic (mostly andesine to labradorite with An40 to An6o), and generally forms euhedral and clear crystals of smaller size. The acid plagioclase is younger and often borders the corroded cores of the calcic type. Most of the "shoshonite" lavas contain quartz, which generally is present in fractured, rounded and embayed crystals, often with a reaction rim

99

t

K 0

1.8t

1.6

1A

1.2

1.0

•elll

&

,',

0.8

. •

. /..

•

/

/

mB

•

o~I /

Main distribution of /1~

0

i

O.E i &~11 i

f~l

0.Z

./

•

•

I

n I / , "/

• II

•

d$ N-ch,e

SW-BOLIVIA • Composite voJcanoes o Old Tertiary lavas

I

/"

NW-ARGENTINA • Composite volcanoes A Ordovician lavas N-CHILE • Composite volcanoes

0.2 50

55

60

65

70 % SiO 2

Fig.6. Diagram o f the I ~ O / N a 2 0 ratio o f the lavas o f s o u t h w e s t e r n Bolivia, northern Chile, and n o r t h w e s t e r n A r g e n t i n a as a f u n c t i o n o f the SiO 2 c o n t e n t s (samples f r o m latitudes 20--24°S).

of fine pyroxenes. The biotite is generally a brown variety with strong pleoehroism (X = light brown and Z = red-brown to black). The biotite crystals are often oxidized at the margins. Orthopyroxene is present in the form of well-shaped, faintly pleoehroic prismatic crystals with 2 V x = 6 0 - - 8 0 ° (bronzite to hypersthene). Most probably the amphibole content was controled by the K20/Na20 ratio; at low ratios amphibole crystallized instead of biotite. In southwestern Bolivia, the Miocene and Early Pleistocene lavas belong to the high-K tale-alkaline association. They are concentrated in the central and eastern part of the studied area.

100

Typical calc-alkaline association (Table 2, lava L 14--L18). The major element composition of the acid members of the calc-alkaline series and of the silicapoor members of the "shoshonite" series is identical, with the exception of the Na and K levels. The Na20 c o n t e n t of the calc-alkaline lavas is about 1.5% higher than in the " s h o s h o n i t e " lavas, whereas the K20 content is about 0.7% lower. The total alkali content of the typical calc-alkaline association is somewhat higher. The silica content of the calc-alkaline lavas varies between 53.8 and 63.5%; the K20/Na20 ratio between 0.6 and 0.76. Rocks with calcalkaline tendency are younger than those belonging to the high-K series. Many are of Holocene age~ Mineralogical!y, the lavas of the calc-alkaline series can be subdivided into three different groups. The first one, to which about 20% of all lavas belong, is characterized by the presence of hornblende and biotite. Phenocrysts of quartz are very scarce. Both ortho- and clinopyroxenes are present. Orthopyroxene occurs frequently. Augite is found in greater amounts only in the silica-poor lavas. The plagioclase phenocrysts are mostly andesine (An3s to Anss}. Amphibole generally forms euhedral crystals with X = light greenbrown and Z = olive-brown and red-brown to black. Often the crystals show incipient transformation into iron ores at the margins. Most of the amphibole is a oxi-hornblende. The second group is characterized by the absence of hornblende and biotite. Only phenocrysts of plagioclase, ortho- and clinopyroxenes are present. About 20% of all lavas have this mineral assemblage. The An content and the appearance of the plagioclase is very similar to t h a t in the high-K calcalkaline lavas. Orthopyroxene is present as well-shaped, faintly pleochroic, prismatic crystals. The orthopyroxenes are generally smaller than the augites, but occur in greater amounts. The third type is the least frequent (about 5%), and restricted to the silicapoor lavas. It is composed of olivine, augite, plagioclase, and sometimes small amounts of hypersthene. Olivine is present both as xenomorphic, often skeleton-like phenocrysts and as euhedral crystals. The lavas of the typical calc-alkaline series occur mainly in the western part of Bolivia (west of the Rio Quetena) and are concentrated in the Cordillera Occidental. Within this group no systematic change of mineralogical composition with respect to age and locality of the samples was found. In the Cordillera Occidental for example, there exist Holocene volcanoes composed of either biotite- and hornblende-bearing lavas or of pyroxene-bearing ones. However, both types of lava also occur at one individual volcano. Lavas L 10--L 13 (Table 2) represent transition types between the "shoshonite" and calc-alkaline association. Their K20/Na20 ratio is greater than 1.0 or near to 1.0. Their mineralogical composition (with plagioclase, hypersthene, augite, and sometimes hornblende), however, is similar to that of the calc-alkaline series.

102

'FABLE 4 Averages of s o u t h w e s t e r n Bolivia

I

II

Ill

IV

V

VI

VII

VIII

IX

X

CO: H~O

50.30 16.60 5.30 4.40 0.24 5.20 5.50 5.10 1.90 1.31 0.55 0.67 2.90

65.90 15,20 2.70 1.00 0.05 1.60 3.30 2.70 3.90 0.65 0.21 0.41 2,30

66.80 15,60 2.70 1.00 0.07 1.30 3.50 3.00 3.70 0.63 0.13 0,12 i 60

63.60 17.70 0.90 2.80 0.05 0.90 3.50 3.00 5.00 0.54 0.27 0,00 1.60

66.40 15.60 3.10 1.10 0.04 1.10 2.30 2,40 5.90 0,75 0.24 0.20 0,90

67.00 15.40 1.80 2.40 0.05 1.10 2.50 2.90 4.10 0,63 0,18 0.30 1.70

64.00 16.20 2.30 2.80 0.08 2.10 4,40 2.30 3.60 0.96 0.22 0.10 1.00

63.50 15,80 1.90 2.80 0.09 2.30 4.80 3.40 3.00 0.85 0.16 0,10 I 20

56.70 18.40 2.50 4.60 0.14 2.7[) 7,90 3,00 1,40 1,38 0.19 0.20 0,90

53.40 15.30 1.70 6.80 0.15 8.20 7.10 2.60 1.70 1.05 0.23 0.80 0,90

Total

99.97

99.92

100.05

99.86

100.03

100.06

100.06

99.90

100.01

99.93

Cc

0.00 11.23 43.16 16.79 2.11 3.16 7.65 7.68 0.00 2.49 1.27 0.00 1,52

27.52 23.05 22.85 12,41 0.00 3.98 0.00 1,50 1,66 1.23 0,49 1,99 0.93

26.44 21.87 25.39 15.76 0.00 3,24 0.00 1.63 1,58 !.20 0,30 0.89 0.27

17.29 29.55 25.39 15.60 0.00 5.84 0.00 1.30 0.00 1.03 0,63 1.64 0.00

24.51 34.87 20.31 8.58 0.00 2.74 0,00 1.50 2.06 1.42 0,56 2.12 0.45

27.87 24,23 24.54 9.33 0.00 4,71 0.00 2.61 0,00 1.20 0,42 2.77 0.68

24,35 21.28 19.46 19.76 0.00 7.03 0.00 3.33 0.00 1.82 0.51 1.28 0,23

18.87 17,73 28.77 18.99 2.53 6,84 0.00 2,75 0,00 1.61 0,37 0.00 023

12.91 8~27 25.39 32.61 3.33 9,46 0.00 3,62 0,00 2,62 0.44 0.00 0.45

3.56 10.05 22.00 25.06 2.92 28.64 0,00 2.46 0.00 1.99 0.53 0.00 1,82

Rittmann norm Quartz Sanidine Plagioclase Nepheline Clinopyroxene Hypersthene Olivine Calcite Biotite Cordierite Magnetite Ilmenite Apatite

0.0 10.1 67.8 1. l 2.3 0.0 12,5 17 0,0 0.0 1.9 1,5 ]2

29.1 28.1 27A 0D 0.0 0.0 0 0 1.0 9.6 37 05 0.3 04

29.0 23.6 35.8 0.0 0~0 0,0 0,0 03 10 4 0.0 0.5 03 0 3

19.2 38.5 30.6 O0 0.0 0.0 00 0.0 7.1 3.2 06 0,3 0.5

24.3 48.9 13.0 0.0 0,0 0,0 0.0 0.5 7.1 4.7 0.6 0.5 0.5

27.8 34.7 21.9 0.0 0.0 0.0 0,0 0,7 6.4 7.1 0.6 0,4 0.4

22.9 29.3 34.7 0.0 0.0 7.5 0~0 0.2 0.0 4,2 0.6 0,0 0,4

19.1 24.8 44.4 0.0 2.6 6.7 00 0.2 0,0 0.0 1.0 0,9 0 3

12.5 5.5 65.2 0.0 3.4 10.2 0.0 0,5 0.0 00 0,9 1.5 04

3.8 10.1 52.3 0.0 3,1 25.8 0.0 2.0 0.0 0.0 1,2 1.2 0,5

Colour index

18.2

14.l

l 1.1

11.2

12.9

14.5

12.4

11 2

160

31.3

Coordinates Streckeisen double-triangle Q 0.0 34.4 32.8 A 12.7 33.2 26.7 P 85.9 32.4 40.5 F 1.4 0.0 0.0

21.7 43.6 34.7 0.0

28.1 56.8 15.1 0.0

32.9 41.1 26.0 0.0

26.3 33.7 39.9 0.0

21.7 28.0 50.3 0.0

[5.0 6.6 78.4 0.0

5.7 15.3 79.0 0.0

SiO2 Al:O~ Fe203 FeO MnO MgO CaO Na20 K20 TiO~

P205

CIPW n o r m Q Or Ab An Di Hy Ol Mt Hm ]l Ap ('

103 TABLE 4 (continued) I

III

IV

V

VI

VII

VIII

IX

X

Coordinates A FM triangle A 32.4 56.5 F 43.5 29.8 M 24.1 13.7

58.3 30.4 11.3

63.7 29.1 7.2

62.3 29.5 8.3

57.5 33.4 9.0

45.6 38.2 16.2

48.1 34.6 17.3

31.2 49.6 19.2

20.5 40.4 39.1

Sigma R i t t m a n n Tau R i t t m a n n

1.89 16.58

3.11 18.15

2.94 13.33

2.04 15.43

1.66 11.78

2.00 12.28

I II III IV V VI VII VIII IX X

= = = = = = = = = =

II

6.71 6.18

1.90 14.53

1.41 9.81

1.78 9.92

average of 2 0 l i g o c e n e lavas (Potoco F o r m a t i o n ) . average of 5 Miocene i g n i m b r i t e s (Upper Q u e h u a F o r m a t i o n ) . average of 12 Pliocene ignimbrites ( I g n i m b r i t e F o r m a t i o n ) . average of 12 garnet-bearing quartz-latites from the Karikari massiv (Wolf, 1973). average of 3 Miocene lavas (Bonete Lavas~ average of 4 Early Pleistocene lavas (volcanoes San A n t o n i o and Morijon). average of 6 Pleistocene lavas (volcanoes U t u r u n c u , S o n i q u e r a and Cerro Lipez). average of 2 H o l o e e n e lavas (volcano L i c a n c a b u r and extrusive d o m e of Khara Laguna). H o l o c e n e scoria from J u r i q u e s volcano. average o f 2 H o l o c e n e lavas (Chiguana volcano and d o m e of Escala).

this volume is in the range of 8000 km s. It can be divided into the individual formations as follows: Rondal and Julaca Lavas (Oligocene/Miocene) Upper Quehua Formation (Miocene) Bonete Lavas (Middle Miocene) Ignimbrite Formation (Pliocene) Strato-volcano Formation (Quaternary)

100 1000 300 3500 3000

km s km s km 3 km 3 km s

As no sedimentological studies have been carried out on the sediments of the Tertiary formations, we do not k n o w the derivation of this material. A great part of the sediments, however, derive from volcanic rocks. Therefore, the total volume of the igneous rocks produced during the Cenozoic is still greater. Fernandez et al. (1973) emphasized the close chemical similarity between the lavas and the ignimbrites of southwestern Bolivia. This similarity is clearly shown in Table 4, where the average contents of the major elements of the Upper Quehua and the Ignimbrite Formation (II and HI) do not differ greatly from the average composition of the Miocene and Pleistocene lavas (V to VIII). This similarity becomes even more striking if we only regard the silica-richlavas, whlch are more frequent. Similarity is also found with respect to the trace element abundances. The only difference exists in the Zr contents, which are somewhat higher in the lavas. In the Streckeisen diagram (Fig.4), the bulk of the lavas shows the same distribution pattern as the ignimbrites. Lavas corresponding to latiandesites or andesites are scarce. The only chemical

104 difference between the lavas and the ignimbrites is in the FeO/Fe203 ratio, which generally is higher in the lavas. The similar chemical composition of lavas and ignimbrites contrasts with the corresponding rocks of northern Chile. There, the ignimbrites are mainly of rhyolitic to alkali-rhyolitic composition and the lavas are mainly latiandesites (Pichler and Zeil, 1972). The lavas of southwestern Bolivia show a systematic change in their composition with respect to age and space. Compositional progression starts from the basic alkaline rocks, through the K-rich, acidic lavas of the "shoshonite" association to the acidic and intermediate lavas of the calc-alkaline series. This compositional change is connected with a migration of the feederchannels from east to west. We suppose, however, that the lavas of the highK and the typical calc-alkaline association had a similar derivation, and that one series represents the continuation of the other. This is seen in the transitional types (p. 100) and trace element abundances, which do not differ significantly between the two groups. Jakes and White (1971) found an increase of Rb and Sr and a decrease of K/Rb ratios from the calc-alkaline to the shoshonite association in island arc environments. It was further pointed out by these authors (1969) that the rocks of the shoshonite series are generally younger than the calc-alkaline rocks. In southwestern Bolivia, however, the " s h o s h o n i t e " rocks were erupted earlier than the calc-alkaline volcanics and no increase of Rb and Sr and no decrease of the K/Rb ratio could be observed. The close relationship of the high-K and the calc-alkaline associations is also demonstrated in Fig. 6, where a continuous transition from low to high K20/Na20 ratios is obvious. The variation of the K20/Na~O ratio depends on the decrease of the K20 content as well as on the increase of the Na20 content with time. In Table 4 the average chemical compositions of volcanoes of similar age are grouped together. It is evident, that there exists a strong decrease of K c o n t e n t with time. The Miocene to Early Pleistocene lavas (Table 4, V and VI) are characterized by extremely high silica (66--67%), and high K (4--6%) contents, and low Na levels. In the Holocene lavas (VIII to X), the Na20 contents are high with respect to their intermediate silica levels. K contents in the range of 5% are not unusual among the Miocene volcanic rocks of Bolivia. Wolf (1973) presented 45 chemical analyses from the Karikari-Andacaba massive, situated southeast of Potosf. In all the analyses high K20 contents (generally higher than 5%) are conspicuous (Table 4, IV}. Dickinson and Hatherton (1967) and Dickinson (1970) believed that the K20 c o n t e n t of volcanic rocks from island arcs depends on the depth of the Benioff zone (K--h relation), and stated that this K--h relation depends neither on the thickness nor on the composition of the crust. They supposed therefore, that the magma generation only occurs near to or in the Benioff zone. South of latitude 20°S, the Chile-Peru trench runs approximately northsouth. If we assume that the inclination of the Benioff zone does n o t vary

105

within a north-south distance of about 500 km, we can plot, instead of the K--h relation, K20 as a function of longitude. Fig.7 shows the K20 values for different silica levels of lavas from northern Chile, southwestern Bolivia, and northwestern Argentina. Only analyses of samples situated between latitude 20 ° and 24°S were plotted (Pichler and Zeil, 1969; Francis et al., 1974; Ahlfeld and Branisa, 1960; Fernandez et al., 1973; HSrmann et al., 1973; Schwab, 1972, 1973}. Fig.7 indicates that there is no notable increase of the K20 content from longitude 69°W to about 67°30'W, i.e. at a distance of about 150 km. From here to the east, the K20 content increases rapidly, but this increase is caused by the transition from the area of the calc-alkaline to that of the older "shoshonitic" lavas. This is at least valid for southwestern Bolivia. It seems that within the calc-alkaline association, the K:O level does not change from west to east. In the Cordillera Occidental for example, the K20 content at 55% SiO2 is about 1.6%. In the eastern part of the studied area, the Holocene dome-like lava of Escala has a similar potassium content (Table 2, L 18). W

E

K20 %

•

ie

•__ .....

.,,,,-,,-, -

.--".

- :.

~ . . . . .

•

•

p

,

.

.,

•

,

SiO 2 62.5-675

_ _%.._•a_

%

_ SiO 2

57.5-625

%

/ /

/

__

--

-a,- ..A_ - . . . .

-

-~- .......

•'.-

/ -

- -

j

• •

'X

SiO 2

69"

6~"

"

.

.

.

.

.

. 6 7. "

.

.

.

.

52,5-57.5

66 •

%

long. W

Fig.7. Diagram of the K20 content of the Cenozoic lavas from southwestern Bolivia, northern Chile, and northwestern Argentina as a function of longitude (samples from latitudes 20--24°S).

106 A Late Pliocene to Holocene age is attributed to the volcanoes Tuzgle and Cerro Bayo on the Argentinian side (Schwab, 1973). Schwab (1973) described a development of the Tuzgle volcano from rhyodacitic to latiandesitic lavas. The rhyodacitic, older lavas have K20/Na20 ratios greater than 1.0 and probably correspond to the early Pleistocene composite volcanoes of Lipez. The younger, latiandesitic lavas have K20/Na20 ratios less than 1.0 and correspond to the Holocene volcanoes of southwestern Bolivia. If the variation of the K20 c o n t e n t during the Cenozoic depends on a decrease of the inclination of the Benioff zone or on the existence of a Miocene and Quaternary Benioff zone, then the young volcanic rocks in the eastern part of the central Andes should present higher potassium levels than those in the Cordillera Occidental. However, the Benioff zone, with an inclination of 25 ° towards the east, would be some 70 km deeper below the Cerro Bayo volcano than below the volcanoes San Pedro and San Pablo, located in the west of the Cordillera Occidental. According to the K--h diagram of Dickinson (1970), an about 1% greater K20 c o n t e n t of the Cerro Bayo lavas should be expected. However, the lavas with similar silica content in the Cerro Bayo and the San Pedro area (Francis et al., 1974), both have an average K20 content of about 2.0%. All the younger lavas of northwestern Argentina have similar potassium contents to those of the lavas of the Chilean composite volcanoes about 150 km more to the west. It seems therefore, that the increase o f potassium towards the east is controlled only by the age of the rocks and does not depend on the distance from the trench. Average chemical compositions of lavas from northern Chile, southwestern Bolivia, and northwestern Argentina (all between latitude 20 ° and 24°S) have been calculated and are presented in Table 5. There is a conspicuous and uniform difference in the K20 and Na20 contents. Potassium reaches its highest values in the volcanics of Lipez, because here the older, high-K calcalkaline lavas are more frequent. The siliceous types of southwestern Bolivia have about 1% more K20 than the corresponding lavas of northern Chile and northwestern Argentina. On the other hand, the K:O levels of the intermediate lavas are equal. The sodium content of the lavas from northern Chile is significantly high: 1% above the levels from the corresponding rocks of Lipez and about 0.6% above those of northwestern Argentina. The Na20 content as function of the longitude is shown in Fig.8. The Na:O values do n o t vary significantly from the silica-poor to the silica-rich lavas. It is obvious, however, that the volcanic rocks of the western part of the central Andes are richer in Na20 than those located more to the east. Judging from the average chemical compositions (Table 5) and the frequency distribution of the silica contents, it appears probable that the lavas {with exception of those belonging to the alkaline series) originated from a parent magma of rhyodacitic composition with a silica content of about 63%. It is suggested that the magma originated by anatectic melting of crustal rocks. According to Zeil and Pichler (1967), Pichler and Zeil (1969), HSrmann et al. (1973), and Fernandez et al. (1973) the volcanic rocks of the central Andes

107

W

Na20 °/o 5

A

~

~oo

52.5-57.5 "1. SiO2

0

57.5-62.5 "/. SiO2

0

625-6?.5 % SiO2

o

o~

~ }'"'-~-..

D

[] 5D

o

~ o

[]

[]

o

o--

~o- . . . . . . ~. . . . . . . . . .

[]

o

.

6'7 °

o

o

6'9"

'

.

.

.

.

68 °

'

.

.

.

'

.

.

.

.

.

66"

tong.

W

Fig.8. Diagram of the Na20 content of Cenozoic lavas from southwestern Bolivia, northern Chile, and northwestern Argentina as a function of longitude (samples from latitudes 20--24°S).

were derived from the continental crust. Jakes and White (1972) suggested that the volcanics of the Andean continental margin are products of partial melting of the lower crust, and that the major and trace element abundances were controlled by hornblende fractionation. The derivation of the volcanic rocks of southwestern Bolivia from the continental crust is supported by the 87Sr/S6Sr isotopic composition. The ignimbrites show 87Sr/S6Sr ratios varying between 0.7084 and 0.7125, and the lavas between 0.7059 and 0.7133 (Klerkx et al., 1977). Hurley (1968) calculated the mean value of the continental crust as 0.7080 and the mean value of the mantle as 0.7035. When the STSr/S6Sr ratios of lavas and ignimbrites from southwestern Bolivia, northwestern Argentina, and northern and central Chile are plotted as a function of the K20/Na20 ratio, three different fields can be distinguished: one group with 87Sr/S~Sr ratios around 0.7045 and with low K20/Na20 ratios. The S7Sr/S6Sr ratios of this group are identical with those of the "andesites" of the island arcs. Only the volcanics of central Chile have such low ratios. The second group is characterized by STSr/S6Sr ratios varying between 0.705 and 0.708 and K20/Na20 ratios between 0.4 and 1.0. Within this field falls the bulk o f the lavas from the central Andes and some ignimbrites from northern Chile. The third group has 87Sr/S6Sr ratios varying between 0.708 and 0.713 and K20/Na20 ratios between 1.2 and 1.9. To this group belong most of the ignimbrites, the Rondal Lavas and the high-K calc-alkaline lavas. The t w o other analysed lavas from southwestern Bolivia belong to the basic, olivine-bearing members of the calc-alkaline association and fall into the second group. Betweeen the second and third field

108

TABLE 5 Averages of southwestern Bolivia, northern Chile, and northwestern Argentina BO 1

BO 2

BO 3

CH 1

CH 2

CH 3

AR 1

AR 2

AR 3

SiO: A1203 Fe~O 3 FeO MnO MgO CaO Na20 K20 TiO~ P205 H~O

62.70 16.00 2.20 3.20 0.08 2.70 4.50 2.80 3.50 0.87 0.22 1.10

64.50 16.00 2.10 2.70 0.06 1.90 3.90 2.80 3.80 0.82 0.20 1.10

56.10 16.10 2.40 5.00 0.13 5.80 6.50 2.80 2.20 1.05 0.29 1.20

60.20 16.60 2.40 3.20 0.09 3.30 5.50 3.90 2.40 0.81 0.23 1.10

62.40 16.40 2.30 2.50 0.07 2.40 4.70 4.00 2.80 0,72 0.22 1.20

56.80 17.00 2.50 4.20 0.10 4.80 6.90 3.80 1.90 0.95 0.25 1.00

58.00 16.30 3.00 3.30 0.15 4.90 6.20 3.20 2.50 0.89 0.18 1.20

63.80 17.20 2.60 1.20 0.10 1.60 4.40 3.50 3.00 0.65 0,07 ] .80

56.00 16.00 3.20 4.10 0.17 6.00 6.90 3.10 2.30 0.97 0.22 1.00

Total

99.87

99.88

99.57

99.73

99.71

100.20

99.82

99.92

99.96

CIPW norm Q Or Ab An Di Hy Mt Hm 11 Ap C

18.74 20.68 23.69 20.75 0.11 9.44 3.19 0.00 1.65 0.51 0.00

22.14 22.46 23.69 18.04 0.00 6.71 3.04 0.00 1.56 0.46 0.67

8.22 13.00 23.69 24.87 4.42 18.03 3.48 0.00 1.99 0.67 0.00

12.13 14.18 33.00 20.70 4.07 8.99 3.48 0.00 1.54 0.53 0.00

15.37 16.55 33.85 18.53 2.68 6.33 3.33 0.00 1.37 0.51 0.00

6.16 11.23 32.16 23.72 7.13 12.80 3.62 0.00 1.80 0.58 0.00

10.13 14.77 27.08 22.73 5.44 12.02 4.35 0.00 1.69 0.42 0.00

20.34 17.73 29.62 21.37 0.00 3.98 2.31 1.01 1.23 0.16 0.36

6.54 13.59 26.23 22.95 7.84 14.82 4.64 0.00 1.84 0.51 0.00

R i t t m a n n norm Quartz Sanidine Piagioclase Clinopyroxene Hypersthene Biotite Cordierite Magnetite Ilmenite Apatite

18.8 28.6 40.5 0.1 9.5 0.0 0.0 1.1 0.9 0.4

21.2 32.8 35.4 0.0 7.4 0.0 2.2 0.6 0.0 0.4

7.9 15.4 51.5 4.7 17.6 0.0 0.0 1.1 1.1 0.6

12.1 17.3 55.0 4.2 8.9 0.0 0.0 1.3 0.8 0.5

15.2 22.8 50.2 2.7 6.9 0.0 0.0 1.1 0.7 0.4

6.0 11.8 60.0 7.5 11.8 0.0 0.0 1.4 1.0 0.5

9.5 18.0 51.4 5.8 12.8 0.0 0.0 1.3 0.9 0.4

23.7 13.8 50.7 0.0 0.0 11.0 0.0 0.5 0.2 0.1

5.8 16.5 51.4 8.4 15.1 0.0 0.0 1.5 1.0 0.5

Colour index

11.7

10.2

24.6

15.2

11.4

21.7

20.8

11.7

26.0

Coordinates Streckeisen double-triangle Q 21.4 23.8 10.5 A 32.5 36.7 20.6 P 46.1 39.6 68.8 F 0.0 0.0 0.0

14.3 20.5 65.2 0.0

17.2 25.8 56.9 0.0

7.8 15.2 77.1 0.0

12.0 22.8 65.2 0.0

26.9 15.6 57.5 0.0

7.8 22.4 69.8 0.0

109

TABLE

5 (continued)

BO 1

BO 2

BO 3

CH 1

CH 2

CH 3

AR 1

AR 2

AR 3

27.6 40.3 32.1

41.9 36.2 21.9

49.1 33.5 17.3

33.4 38.4 28.2

34.0 36.7 29.3

55.4 31.0 13.6

29.1 38.5 32.3

2.31 12.21

2.38 13.19

2.35 11.00

2.17 12.24

2.03 19.03

2.24 10.84

Coordinates A F M triangle A F M

44.2 36.9 18.9

50.2 35.4 14.4

Sigma R i t t m a n n Tau R i t t m a n n

2.01 12.11

2.03 12.94

BO 1

BO 2 BO 3 CH 1 CH 2 CH 3 AR 1

AR 2 AR 3

1.91 9.93

= average of 39 Cenozoic lavas of S W Bolivia (thiswork, Fernandez et al., 1973, H 6 r m a n n et al., 1973). = average of 31 silica-rich lavas of SW Bolivia (SiO 2 greater 60%). = average of 8 silica-poor lavas of SW Bolivia (SiO 2 smaller 60%). = average of 41 Cenozoic lavas of N Chile (between fat. 20 ° and 24°S; Zeil and Pichler, 1967; Pichler and Zeil, 1969; Francis et al.,1974). = average of 25 silica-richlavas of N Chile. ffiaverage of 16 silica-poorlavas of N Chile. = average of 20 Cenozoic lavas of N W Argentina (between lat. 20 ° and 24°S; H S r m a n n et al., 1973; Schwab, 1973). = average of 5 silica-richlavas of N W Argentina. = average of 15 silica-poorlavas of N W Argentina.

there exists a wide gap, which is caused mainly by a gap in the K20/Na20 ratio. From Fig.6, however, it is obvious that there exists a continuous transition from low to high K20/Na20 ratios. It m a y be supposed, therefore, that the gap in the STSr/S6Sr--K20/Na20 diagram will disappear when lavas with a K20/Na20 ratio between 1.0 and 1.2 are analyzed for SVSr and S6Sr. Because of the mostly acidic chemical composition and the high STSr/SeSr ratios of the lavas and ignimbrites from southwestern Bolivia, it appears possible that these volcanic rocks were derived by fusion or partial melting of different crustal zones. The ignimbrite melts and the magmas of the high-K calc-alkaline series could have originated in higher, silica-and potassiumricher levels than the Late Pleistocene to Holocene lavas of the typical calcalkaline association. Crustal derivation would explain not only the great volumetric predominance of rhyodacitic and dacitic volcanics over more basic ones, but also the complete absence of basaltic rocks and ultrabasic accumulations within the Upper Cenozoic volcanic sequences. ACKNOWLEDGEMENTS

This paper was presented at the IAVCEI Symposium on Volcanology in

110

Santiago de Chile, 1974. The authors acknowledge the financial support of the Deutsche Forschungsgemeinschaft, Bonn, and of the Servicio Geol6gico de Bolivia, La Paz. Dr. C.E. Brockmann, director of the Earth Resources Technology Satellite Program of Bolivia kindly provided the ERTS images. We express our thanks to many colleagues of GEOBOL, especially to Ing. L. Jordan and Ing. F. Tavera for assistance in the field. We are greatly indebted to Prof. H. Pichler, Tiibir.gen, for stimulating discussions and suggestions and to Sr. G. Viscarra, administrator of the SAIBOL Uturuncu mine for his hospitality and technical help. We also express our appreciation of St. J. Castillo, an experienced driver and an excellent hunter and cook. Finally we wish to thank Ing. J. GuiUermo Torrez, director of GEOBOL for the permission to publish the paper and Mr. R. Sutherland for critically reading the manuscript. REFERENCES Ahlfeld, F., 1946. Investigaciones geol6gicas en la provincia Sud Lipez. Cuad. Inst. Mineral. Geol., Univ. Nac. Tucum~n, 4(15): 149--162. Ahlfeld, F., 1972. Geologfa de Bolivia. Los Amigos del Libro, La Paz, 190 pp. Ahlfeld, F. and Branisa, L., 1960. Geolog~a de Bolivia. Inst. Petrol. Boliviano, La Paz, 245 pp. Audebaud, E., Capdevila, R., Dalmayrac, B., Debelmas, J., Laubacher, G., Lefevre, C., Marocco, R., Martinez, C., Mattauer, M., M~gard, F., Paredes, J. and Tomasi, P., 1973. Les traits g~ologiques essentiels des Andes Centrales (P~rou-Bolivie). Rev. G~ogr. Phys. G4ol. Dyn., 15: 73--114. Avila, W., 1971. Estudio petrogr~fico de la regi6n de San Antonio de Lipez. Soc. Geol. Boliviana, Bol., 16: 31--54. Dickinson, W.R., 1970. Relation of andesites, granites, and derivative sandstones to arctrench tectonics. Rev. Geophys. Space Phys., 8: 813--860. Dickinson, W.R. and Hatherton, T., 1967. Andesitic volcanism and seismicity around the Pacific. Science, 157: 801--803. Dupuy, C. and Lefevre, C., 1974. Fractionnement des filaments en trace Li, Rb, Ba, Sr dans les s~ries and~sitiques et shoshonitiques du P~rou. Comparaison avec d'autres zones orog~niques. Contrib. Mineral. Petrol., 46: 147--157. Fernandez, A., HSrmann, P.K., Kussmaul, S., Meave, J., Pichler, H. and Subieta, T., 1973. First petrologic data on young volcanic rocks of SW-Bolivia. Tschermaks Mineral. Petrogr. Mitt., 19: 149--172. Francis, P.W., Roobol, M.J., Walker, G.P.L, Cobbold, P.R. and Coward, M., 1974. The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits. Geol. Rundsch., 63: 357--388. Guest, J.E., 1969. Upper Tertiary ignimbrites in the Andean Cordillera of part of the Antofagasta Province, northern Chile. Bull. Geol. Soc. Am., 80: 337--362. HSrmann, P.K., Pichler, I-I.and Zeil, W., 1973. N e w data on young volcanism in the Puna of N W Argentina. Geol. Rundsch., 62: 397--418. Hurley, P.M., 1968. Absolute abundances and distribution of Rb, K, and Sr in the earth. Geochim. Cosmochim. Acta, 32: 273--283. Jacobson, H.S., Murillo, C., Ruiz, L., Tapia, O., Zapata, H., Alcaron, H. and Delgadillo, E., 1969. Geology and mineral deposits of the San Cristobal district,Villa Martfn Province, Potosf, Bolivia. U.S. Geol. Surv. Bull., 1273, 22 pp. Jake~, P. and White, A.J.R., 1969. Structure of the Melanesian arcs and correlation with distribution of m a g m a types. Tectonophysics, 8: 223--236.

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