Petrology and geochemistry of alkali basalts and ultramafic inclusions from the palei-aike volcanic field in Southern Chile and the origin of the patagonian plateau lavas

Journal of Volcanology and Geothermal Research, 6 (1979) 3--25

3

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PETROLOGY A N D GEOCHEMISTRY OF ALKALI BASALTS A N D ULTRAMAFIC INCLUSIONS FROM THE PALEI-AIKE VOLCANIC FIELD IN SOUTHERN CHILE A N D THE ORIGIN OF THE PATAGONIAN PLATEAU LAVAS

MILKA A L E X A N D R A SKEWES and CHARLES R. STERN*

Department of Geology, University of Chile, Santiago (Chile) Lamont-Doherty Geological Observatory, Palisades, N Y 10964 (U.S.A.) (Received February 17, 1978; revised and accepted November 13, 1978)

ABSTRACT Skewes, M.A. and Stem, C.R., 1979. Petrology and geochemistry of alkali basalts and ultramafic inclusions from the Palei-Aike volcanic field in southern Chile and the origin of the Patagonian plateau lavas. J. Volcanol. Geotherm Res., 6: 3--25. The Palei-Aike volcanic field, the southernmost unit of the Patagonian plateau lavas, consists of Pleistocene to Recent alkali olivine basalts petrologically and geochemically similar to alkali basalts from diverse tectonic environments. The Palei-Aike baselts have lower SiO 2 and AI=O~ and higherTiO2and P20, than published analyses of other Patagonian plateau basalts. Garnet, garnet + spinel-, and phlogopite-peridotites, not reported from other Patagonian plateau lavas or from elsewhere in South America, are c o m m o n inclusions within Palei-Aike lavas along with spinel-lherzolite, dunite, granulites, and aluminotm clinopyroxene megacrysts. The inclusion of these high-pressure assemblages indicates a mantle origin for the Palei-Aike lavas. The Patagonian plateau lavas are located in a tectonic position similar to back-arc basins, and their origin may be a consequence of subduction. The origin and distinct chemical features o f th~ Palei-Aike basalts may be due in part to thermal or mechanical perturbations o f the mantle related to changes in plate boundaries and motions in the vicinity of the unstable trench-transform triple junction formed by the South American, Antarctic and Scotia plates. INTRODUCTION

Two distinct volcanic associations of Plio-Pleistocene to Recent age are recognized in South America south of 33°S (Vergara, 1970). The circumPacific series consists of high-A1 basalts, andesites, and dacites erupted from composite volcanoes located within the central Andean Cordillera (triangles, Fig. 1; Vergara, 1970; Moreno, 1974; Lopez et al., 1977; Stern et al., 1976). The second series consists dominantly of alkali olivine basalts, associated with minor hawaiite, trachyandesite, and trachyte, that form lava plateaus capped by numerous spatter cones in Patagonia as far west as the Andean *Present address: Department o f Geological Sciences, Come]] University, Ithaca, N.Y. 14853, U.S.A.

foothills along the Chile-Argentina border (Fig. 1; Tyrrell, 1932; Vergara, 1970; Takamura and Yoshida, 1975; Niemeyer, 1975). Available K-At ages for the Patagonian plateau lavas indicate that the youngest flows capping the lava plateaus are less than 5 m.y. old (Lambert, 1956; Vergara and Munizaga, 1974; Niemeyer, 1975; Chattier et al., 1978). Older underlying flows, exposed by glacial or fluvial erosion, range in age from 5 to 50 m.y. (Charrier et al., 1978)., the oldest flows (~ 16 m.y.) usually being separated from the youngest flows by sedimentary sequences. Interfingering of lavas and glacial tills in Meseta Vizcachas and Meseta de la Muerte (Fig. 1) have provided important information regarding the chronology of glacial advances in the southern hemisphere (Mercer, 1976). This paper presents the results of a petrologic and geochemical study of basalts and associated ultramafic mantle xenoliths collected on the Chilean side of the Palei-Aike volcanic field, the southernmost unit of the Patagonian plateau lavas (Fig. 1). This volcanic field covers an area greater than 3000 km 2 straddling the Chile-Argentina border just north of the eastern entrance of the Strait of Magellan (Figs. 1 and 2). Within the Palei-Aike Cave, located in a partially eroded spatter cone on the Chilean side of the volcanic field, prehistoric artifacts have been found buried beneath volcanic debris indicating that active volcanism occurred in this region as recently as five to ten thousand years before the present (Bird, 1938). The ultramafic inclusions found within the Palei-Aike volcanic field include garnets, garnet + spinel-, and phlogopite-peridotites, petrologic types previously unreported from continental South America, as well as spinellherzolites, eclogites, pyroxenites and dunites, which have been found within other units of the Patagonian plateau lavas (Niemeyer, 1975; Takamura and Yoshida, 1975). The presence within the Palei-Aike lavas of these mantle xenoliths, as well as clinopyroxene megacrysts rich in alumina, indicates a mantle origin for the basalts, possibly due to thermal or mechanical perturbations of the subcontinental mantle related either to subduction or to post~ Miocene changes in plate motions and boundaries. GENERAL TECTONIC AND GEOLOGICALSETTING The Patagonian plateau lavas occur between 34°S and 52 ° S. In these latitudes, the western margin of South America is a convergent plate boundary between the South American and both the Nazca and Antarctic plates that are separated by the Chile Ridge, an active spreading ridge (Fig. 1; Herron, 1972; Klitgord et al., 1973; Forsyth, 1972). Near the region of the triple junction among the Nazca, Antarctic and South American plates, at 46 ° S, the calculated rate of convergence between the Nazca and South American plates is 9 cm/yr, but it is only 2 cm/yr between the Antarctic and South American plates (Minster et ah, 1974). Although seismicity is negligible south of 46 ° S, active subduction of the Antarctic plate beneath the South American plate is suggested by the small but finite rate of convergence

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--,46°

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SOUTH AMERICAN PLATE --50"

[I AIKE "=y)

MALVINAS

SCOTIA

,,,,=,..

PLATE

--54 °

_

Fig. 1. A location map of the major units of the Patagonian plateau laves and the plates and plate boundaries in the vicinity of southernmost South America (Forsyth, 1975). The figure shows the ranges of radiometric ages determined for the Pleistocene to Recent basalts capping the different units of the plateau lavas (Lambert, 1956; Vergara and Munizaga, 1974; Niemeyer, 1975; Mercer, 1976; Chattier et al., 1978). Small triangles indicate the localities o f s o m e o f the major composite volcanoes of the circum-Pacific magma series within the Andean Cordillera. Also shown is the age, in millions of years, for selected linear magnetic anomalies in the southeast Pacific (Herron et al., 1977).

and the presence of active calc-alkaline volcanism between 48°S and 52°S (Stern et al., 1976). Also a negative gravity anomaly is associated with the Peru-Chile trench which is a well-defined structural feature as far south as 56 ° S, although south of 46°S it is morphologically masked by sediments (Hayes, 1966). Magnetic anomalies south of 46°S decrease in age toward the trench suggesting that the southeastern extension of the Chile Ridge was subducted beneath South America during the last 20--25 m.y. (Herron et al., 1977). The low seismicity south o f 46°S may result in part from the slow rate of convergence between the South American and Antarctic plates, and in part from the y o u n g age of the ocean crust being subducted (Forsyth, 1975). Forsyth (1975) has suggested that a small plate, the Scotia plate, occurs between the South American and Antarctic plates southeast of South America. According to Forsyth (1975), the Scotia plate is separated from the Antarctic plate on the west by the Shackleton fracture zone, and from the South American plate on the north by a strike-slip fault zone within the Malvinas Chasm. These fault zones extend northwestward and westward into the Magellanes fault system (Fuenzalida, 1974). With this plate configuration the southeast tip o f South America is apparently part of the Scotia plate, and a trench-transform-transform triple junction between the Antarctic, Scotia, and South American plates must occur along the Chilean Pacific coast near 54°S. Herron et al. (1977) summarize the major morphological provinces and structural lineaments in this region. The Palei-Aike volcanic field is located north of the major strike-slip faults of the Magellan fault zone, within the structural province known as the Magellan Basin. This region has acted as a sedimentary basin since at least the Late Jurassic (Urien and Zambrano, 1973). Seismic refraction and drill hole data (Natland et al., 1974) indicate that the Magellan Basin is floored by preJurassic basement gneisses dated as 300 + 150 m.y. (Halpern, 1973). This basement is overlain by Jurassic silicic volcanic rocks followed upwards by Cretaceous through Tertiary sediments (Urien and Zambrano, 1973). The basalts of the Palei-Aike volcanic field overlie and are interbedded with Quaternary fluvial-glacial sediment deposits. Mercer {1976) determined a K-At age o f 1.24 + 0.3 m.y. for a basalt covered by a glacial till in the GaUegos River valley that cuts through the Palei-Aike volcanic field (Fig. 2). Basalt flows dated as 0.17 + 0.034 m.y. overlie this till and are unaffected by the most recent glacial advances. Mercer (1976) concluded that the last glacial advances to cover the region occupied b y the Palei-Aike volcanic field occurred approximately 1 m.y. ago. Three distinct volcanic episodes which postdate the 1-m.y.-old glaciation have been recognized within the Palei-Aike basalt field (Skewes, "hapreparation). The oldest event involved the formation o f msars (water-filled holes, Fig. 2), associated with lava flows that are n o w overlain by fluvio-glacial sediments and only exposed in river valleys. A younger volcanic episode is represented by eroded spatter cones and soil-covered laves that overlie

7

Fig. 2. A satellite photo (ERTS image No. E-2413-13183-5) of the Palei-Aike volcanic field. Small, circular black areas within the volcanic field are water-filled maars. Irregular black area to the lower right of the center of the photo is the most recent lava flows. The volcanic field is bounded on the north by the Gallegos River, on the south by the Strait of Magellan (lower right-hand comer of photo), and is cut by the Rio Seco (Dry River).

fluvial-glacial sediments. T h e Palei-Aike cave occurs in a dissected spatter c o n e o f this age. Th e m o s t r ecent volcanic event, correlated with t he 5 0 0 0 - 10,000.year-old volcanic debris t h a t covers prehistoric artifacts in the PaleiAike Cave, is represented by well-preserved scoria and spatter cones and fresh lavas t h a t are n o t y e t covered by soils (black lava flows, Fig. 2). A 1 : 5 0 , 0 0 0 scale map o f t h e volcanic units and sample localities are presented elsewhere (Skewes, in preparation). T h e Palei-Aike basalts have been e r u p t e d f r om fissures clearly defined by t h e distribution o f maars and cones (Fig. 2). T he fissures are oriented eastwest and northwest-southeast, subparallel t o t h e ori ent at i on o f faults within t h e Magellan fault system. This or i e nt a t i on m ay reflect reactivation o f older crustal structures ( H e r r o n e t al., 1977), or the compressive stress regime within t h e c o n t i n e n t a l crust.

PETROLOGY AND GEOCHEMISTRY

Methods The major element composition of basalts was determined by microprobe analysis of unfluxed glasses fused from splits of powdered rock samples. Fusion was performed rapidly (~ 30 seconds) at approximately 1700°C on a molybdenum strip, under ~ 40 p.s.i, argon pressure in order to minimize alkali loss. Natural and synthetic minerals were used as primary standards. The accuracy of the method as determined by repeated analyses of fused USGS standard rocks BCR-1, AGV-1, and W-1 is comparable to that reported by Mazzullo and Bence (1976) who employed a nearly identical method. In Table 1, our analysis of BCR-1 is compared to Flanagan's (1973) recommended composition based on conventional ("wet") chemical analyses. Trace elements (Zr, Sr, Rb, Y) were determined by X-ray fluorescence techniques (Norrish and Chappell, 1967), and are considered precise to + 5%. The mineral chemistry of selected basalts, megacrysts, and ultramafic xenoliths was determined by electron microprobe analysis. TABLE1 U.S.G.S. standard BCR-I Microprobe analysis*

SiO 2 TiO 2 AI20 ~ FeO MnO MgO CaO Na20 K20

average

s.d.

54.23 2.27 13.77 12.06 0.20 3.54 7.08 3.24 1.68

0.24 0.07 0.12 0.37 0.02 0.06 0.09 0.20 0.02

Recommended values**

54.50 2.20 13.61 12.11 0.18 3.46 6.92 3.27 1.70

*Twelve analyses. **Data are from Flanagan (1973).

Basalt petrology and mineralogy The Palei-Aike basalts in general have porphyritic textures with up to 20% phenocrysts within an intergranular groundmass. Colorless subhedral olivine and light brown euhedral clinopyroxene are the c o m m o n phenocrysts, with the proportion of clinopyroxene relative to olivine phenocrysts increasing from 0% to 25% as the Mg/(Mg + Fe 2÷) ratio of the basalts decreases. Olivines rarely have fractures or any signs of alteration, and reaction rela-

Total

FeO* = t o t a l F e as FeO.

i~g/(Mg + F e * ) i

0.86

98.85

CaO Na20 L-~20s NiO

MgO

MnO

0.83

98.74

-16.39 0.24 43.15 0.33 0.01 0.04 0.27

-13.33 0.17 45.77 0.22 0.00 0.05 0.30

A1203 FeO*

38.31 --

0.82

98.59

3.46 6.05 0.12 14.94 22.34 0.51 0.22 --

1.96

48.99

SiO 2 TiO 2

39.01

grdmass

grdmass

pheno

Type:

--

clinopyroxene

olivine

Sample PA-21 olivine olivine, c l i n o p y r o x e n e , plagioclase, T i - m a g n e t i t e 0.57

Mineral:

Mg/(Mg + Fe2*):

Phenocrysts: Groundma~:

0.80

100.~6

39.29 --18.51 0.28 41.79 0.36 0.01 0.02 0.10

pheno

olivine

0.73

100.62

37.73 --24.31 0.32 37.65 0.44 0.02 0.02 0.13

grdmass

0.82

99.77

49.50 1.82 3.56 6.13 0.12 15.18 22.91 0.38 0.17 . . .

pheno

.

0.78

99.31

48.76 2.26 3.89 7.30 0.12 14.27 22.25 0.41 0.05

grdmass

clinopyroxene

Sample PA-14 olivine, c l i n o p y r o x e n e olivine, c l i n o p y r o x e n e , plagioclase, alkali f e l d s p a r , T i - m a g n e t i t e 0.37

C o m p o s i t i o n s o f m a f i c silicates in t w o basalts o f d i s t i n c t Mg/(Mg + F e 2.)

•T A B L E 2

¢O

10 tions between olivine phenocrysts and the groundmass have not been observed. Plagioclase is rarely observed as a phenocryst. The groundmass of the basalts is composed of olivine, clinopyroxene, microlites of plagioclase and alkali feldspar, titanomagnetite, and only rarely orthopyroxene. Dark brown glass (n >> 1.54) is frequently present but only as a small volume of the groundmass. Table 2 lists the mafic silicate chemistry for two samples, PA-21 and PA-14, which span the range of Mg/(Mg + Fe 2÷) observed in the Palei-Aike basalts (Table 3). Olivine is the only phenocryst phase in PA-21, while both olivine and clinopyroxene are phenocrysts in PA-14. Analyses of olivine phenocrysts range in composition from Fos~ in PA-21 to FoB0 in PA-14. Analyses of groundmass olivines in these samples range from Fos3 to Fo~3 (Table 2). Both phenocrystic and groundmass clinopyroxenes are titaniferous augites {Table 2). Analyses of plagioclase microlites in PA-14 are AnssAb43Or2. Both phenocrystic and groundmass olivine have small but measurable CaO contents (Table 2). Olivines within ultramafic xenoliths associated with the basalt have distinctly lower CaO contents (Table 5). CaO content was used as a criterion to distinguish phenocrystic from xenocrystic olivines that are common in many of the basalts and have compositions in the range Fo92 to Foss.

Basalt chemistry Table 3 lists the major-element compositions and Rb, St, Zr and Y contents of seven representative basalts and the average of all analyzed samples from each of the three volcanic units distinguished within the Palei-Aike volcanic field. Also listed is the Mg/(Mg + Fe 2÷) ratio and the normative mineralogy of each composition, calculated with Fe 2+ = 0.75% Fetotal, as suggested for alkali ba~alts by Sun and Hansen (1975}. The representative samples of basalt listed in Table 3 were selected to encompass the complete range of Mg/(Mg + Fe 2÷) ratios encountered in the analyzed Palei-Aike basalts. Table 4 lists the composition of the average of 43 samples from the Palei-Aike basalt field, and for comparison, averages of other published analyses of Patagonian basalts, alkali basalts from distinct tectonic environments, and high-Al olivine basalts of the circum-Pacific series forming composite volcanoes within the Andean Cordillera in the vicinity of 40 ° S. The Palei-Alke basalts are dominantly nepheline normative, and according to the normative mineral classification of Irvine and Baragar {19711, they correspond to alkali basalts, nephelinites and hawaiites. Highly differentiated trachytes, that occur within the Pino Hachado plateau lavas {Fig. 1; Vergara, 1970) have not been encountered within the Palei-Aike volcanic field. On a silica versus total alkali diagram {Fig. 3} the Palei-Aike basalts plot within the field of alkali basalts as defined by Irvine and Baragar {1971}, MacDonald and Katsura {1964}, and Hyndman (1972). On an AFM

11

diagram (Fig. 4) they fall in the field of alkali basalts determined by Schwarzer and Rogers (1974), which overlaps with the fields of both pigeonite.and hypersthene basalts as defined by Kuno (1968) for Japanese volcanic rocks. The Mg/(Mg + Fe 2÷) ratio, which decreases as a result of igneous fractionation involving olivine and pyroxene, ranges from 0.61 to 0.36 for the analyzed samples from the Palei-Aike volcanic field. Decreasing Mg/(Mg + Fe 2÷) is correlated with decreasingMgO without increase in FeO* (FeO* = total Fe as FeO). As Mg/(Mg + Fe 2*) decreases,Al2Os, TiO2, P2Os, K20, Rb, Zr and Y increase (Table 3, Fig. 5). The Y contents of thebasalts with the highest Mg/(Mg + Fe 2+) ratiosare significantlylower than the Y contents of oceanic tholeiites(Y ~ 32 ppm, Erlank and Kable, 1976). This suggests that the Palei-Aikebasaltsmay have formed by partialmelting of a garnetbearing assemblage since the distributioncoefficientof Y between garnet and liquidis greaterthan unity (Lambert and Holland, 1974). As Mg/(Mg + Fe 2+) decreases,Y content increases,indicatingthat garnet was not involved in the processes of crystal-liquidfractionationwhich produced the range of Mg/(Mg + F@*) observed in the Palei-Aikebasalts. F

VOLCANICUNIT5 o o,°e.

6

0 ~

1/

~Pigeonite f/_ X-)~

Series

f~

0,..-.°,.,. f

.3

o t~ v

+4 eN QI

o z

//

2 4O

o ola.~t ,

I

50 Si02

,

I

60

A

wt%

Fig. 3. The Palei-Aike basalts plotted on a silica versus total alkalis diagram. The basalts of the volcanic units of each age fall within the field of alkali basalts as defined by (1) Irvine and Baragar (1971); (2) Mac Donald and Katsura (1964); and (3) H y n d m a n (1972) The data for the solid symbols are listed in Table 3. Fig. 4. The Palei-Aike basalts plotted o n a magnesium--total alkalis--iron (AFM) diagram. The basalts plot in the field of alkali basalts as defined by Schwarzer and Rogers (1974) which, intersects the fields of both the pigeonite and hypersthene series defined by Kuno (1968). Solid symbols represent compositions listed in Table 3.

M

BN-61

Sample No:

0.1

18.6 5.0 7.0 1.7

Hy

Ol Mt n Ap

Or Ab An Ne Cpx

.

15.0 5.2 7.4 2.0

11.4 13.5 12.6 9.5 23.2

Mg/(Mg + Fe 2÷) 0.48

6.3 23.4 15.0 -22.7

0.42

99.46

Total

C.I.P. W. norms

99.47

46.27 3.68 11.20 12.54 0.21 11.30 9.66 2.76 1.07 0.77

44.36 3.89 12.72 13.12 0.18 9.25 9.44 3.67 1.92 0.92

PA-3

SiO 2 TiO 2 Al20 ~ FeO* MnO MgO CaO Na20 K20 P205

Major elements (wt. %)

oldest

Unit age:

.

23.2 4.5 5.1 1.7

.

7.4 12.2 12.3 8.2 25.8

0.57

100.10

45.23 2.70 11.20 11.20 0.17 14.45 9.91 3.24 1.25 0.77

PA-21

.

9.4 4.5 6.4 1.7

9.8 9.2 12.6 14.3 31.2

0.43

98.72

44.58 3.37 13.32 11.18 0.19 8.19 11.26 4.21 1.66 0.76

BN-31

intermediate

Representative compositions

.

6.3 4.3 6.3 2.2

.

12.2 13.4 13.8 12.7 29.2

0.37

99.84

46.36 3.29 14.42 10.78 0.19 6.13 11.25 4.34 2.06 1.02

PA-14

17.6 4.5 4.9 1.5

.

8.0 21.7 16.6 3.1 21.5

0.49

99.08

47.02 2.57 12.89 11.32 0.14 10.30 9.55 3.25 1.36 0.69

PA-24

youngest

.

11.1 4.5 5.7 1.5

.

9.3 27.6 15.9 1.2 22.0

0.40

98.72

48.41 3.05 13.33 11.28 0.17 7.14 9.52 3.53 1.57 0.70

PA-9

16.5 5.0 6.9 1.6

.

9.0 17.3 13.9 5.9 23.9

0.46

99.57

45.47 3.63 12.21 12.60 0.19 10.18 9.69 3.33 1.52 0.75

5a

oldest

.

17.7 4.6 5.9 1.5

8.3 14.4 14.8 7.1 26.4

0.51

100.43

45.92 3.09 12.30 11.62 0.18 11.50 10.46 3.25 1.41 0.70

intermediate 30 a

Average compositions

14.2 4.5 5.4 1.4

8.2 25.0 16.8 2.0 22.1

0.44

99.24

47.94 2.83 13.23 11.36 0.17 8.67 9.64 3.39 1.39 0.62

8a

youngest

Major and trace element compositions and C.I.P.W. norms o f representative samples from the three distinct volcanic units in the Palei-Aike volcanic field, and of all samples collected from each o f these distinct units

TABLE 3 ~=~ t~

524 0.026

K/Rb Rb/Sr

727 0.030

22 742 233 28

20 768 190 26 692 0.026

13 640 174 16

801 0.020

FeO* ffi total Fe as FeO. a N u m b e r of samples included in average.

17 645 205 25

Rb Sr Zr Y

Se~cted trace elementabundances(ppm) and rat~s

780 0.031

22 700 227 30 596 0.038

19 500 137 24 523 0.040

25 627 203 36 576 0.031

22 717 231 30 534 0.034

22 647 199 26 483 0.042

24 570 178 27

r~

100.34

Total

8.4 16.2 15.0 6.3 25.5 . 17.1 4.7 5.9 1.5

.

14.1 3.9 3.9 1.2

9.4 26.4 24.0 2.1 14.9

.

100.01

48.67 2.03 16.38 11.22 0.19 6.74 9.15 3.57 1.53 0.53

Meseta Buenos Aires a n=9

7.2 24.5 15.3 10.8 21.3 . 11.8 3.2 2.3 0.6

98.75

48.65 1.21 15.55 10.06 0.09 7.68 8.74 5.25 1.22 0.28

Patagonian plateau lavas b n=12

. 16.9 5.4 5.5 .

9.5 7.9 9.5 12.7 30.3

97.30

43.80 2.90 11.30 13.60 -10.90 9.50 3.70 1.60 . .

n=6

.

.

Mongolia (continental) e

.

9.8 5.0 7.2

8.3 4.9 19.9 10.2 33.1

.

98.00

43.20 3.80 13.40 12.50 -8.60 12.30 2.80 1.40 .

Tahiti (ocean island) d n=10

.

5.9 25.4 18.2 -17.9 1.0 18.6 4.9 6.1 .

97.67

46.60 3.20 12.70 12.40 -10.60 8.17 3.00 1.00 .

n=l

A u c k l a n d Is. (island arc) e

4.9 37.4 28.7 -10.0 1.6 11.8 3.7 2.0

99.74

51.76 1.06 18.66 9.18 0.16 5.43 8.25 4.42 0.82

eGreen and Hilberson

Andean Cordilleran ( o l i v i n e high-Al basalt) f n=31

F e O * = t o t a l Fe as FeO. a N i e m e y e r (1975). b T a k a m u r a a n d Y o s h i d a ( 1 9 7 5 ) . CVlodavetz ( 1 9 7 2 ) . d G r e e n a n d P o l d e r v a a r t ( 1 9 5 5 ) . {1970). f M o r e n o {1975).

Or Ab An Ne Cpx Hy Ol Mt I1 Ap

C.I.P. W. n o r m s

46.24 3.10 12.46 11.69 0.18 11.00 10.27 3.29 1.42 0.69

SiO 2 TiO~ AI203 FeO* MnO MgO CaO Na20 K20 P~O 5

n =43

Palei-Aike

Average c o m p o s i t i o n s ( w t . % ) a n d t h e i r C.I.P.W. n o r m s f o r Palei-Aike basalts, o t h e r P a t a g o n i a p l a t e a u lavas, alkali b a s a l t s f r o m diverse t e c t o n i c e n v i r o n m e n t s , a n d olivine high-Al basalts o f t h e c i r c u m - P a c i f i c m a g m a series p r e s e n t w i t h i n t h e A n d e a n Cordillera

TABLE 4

15 VOLCANIC

40

UNITS

[3 £3 o 0

£

Oldest

0

In~rmedia~ Youngest

c~o

•

0

zx

20

I

1

I

I

•3

.4

.5

.6

Mg/Mg+Fe+2 Fig. 5. The Palei-Aikebasalts plotted on a yttrium versus Mg/(Mg+ Fe2'*)ratio diagram. Solid symbols represent compositions listed in Table 3.

Comparing either the average of all samples, or representative samples with similar Mg/(Mg + Fe 2+) ratios, indicates that A1203 increases and TiO2, P2Os, Zr and Sr decrease from the oldest through the youngest units of the Palei-Alke volcanics (Table 3). SiO2 in the most recent lava flows is higher than in the older lavas. The average Palei-Aike basalt has a lower SiO2, A1203, and Na20; and higher TiO2 and P2Os than other Patagonian basalts for which compositions have been published (Table 4). Differences in MgO and FeO* may only reflect the number of differentiated samples included within the averages. Since SiO2 and Al203 increase and TiO2 and P2Os decrease from the oldest through the youngest Palei-Alke lavas, the composition of the most recent Palei-Aike basalts most closely resembles the published composition of other Patagonian basalts (Tables 3 and 4). This suggests that the Palei-Aike volcanic field may be less geochemically evolved than the more northern basalt fields, and is consistent with the suggestion of Takamura and Yoshida (1975) that volcanic activity in Patagonia was initiated in the north and migrated southward. Alternatively, in view of the inconclusiveness of the small amount of widely scattered data, the chemical differences between Palei-Aike and other Patagonian basalts may reflect spatial rather than temporal regional north~south or east~west variations. The average composition of Palei-Aike basalts resembles the composition of alkali basalts from diverse tectonic environments (Table 4). They have distinctly lower SiO2, A1203 and Na20 contents and higher FeO*, MgO, CaO, TiO2 and K20 contents than the high-A1 olivine basalts which occur within the composite volcanoes of the circum-Pacific volcanic series in the Andean Cordillera between 37°S and 46°S (Moreno, 1975; Stem et al., 1976).

16

Petrology of ultramafic xenoliths Ultramafic and mafic inclusions have been found within volcanic units of all ages at many localities within the Palei-Aike volcanic field. The ultramafic xenoliths include phlogopite-, garnet-, garnet + spinel-, and spinel-peridotite and dunite. Garnet-peridotite inclusions have not been reported from other units of the Patagonian plateau. Mafic xenoliths include eclogite, pyroxenite, and garnet- and pyroxene-granulite. The granulites have probably been derived from the lower continental crust, and their petrology and geochemistry will be discussed elsewhere. Peridotite inclusions, which range in their largest dimension from 2 to 40 cm, are composed dominantly of olivine, with lesser amounts of orthopyroxene, clinopyroxene, _+garnet, -+ spinel, -+ phlogopite mica. Modal proportions of these minerals are highly variable, even within hand size samples. Phlogopite + garnet peridotite has not been encountered, but garnet + spinellherzolites, such as Ferguson et al. (1977) reported within kimberlites from Australia, are common. Garnet-free spinel-lherzolite and garnet + spinellherzolite have predominantly granoblastic textures, while spinel-free garnet peridotites have porphyroclastic textures (Mercier and Nicolas, 1975}. Many of the inclusions display textures indicative of strain and recrystallization. Olivine and pyroxene grains commonly display patchy or undulose extinction. Mineral modal proportions and compositions for a garnet-peridotite (sample LS-2}, and a garnet + spinel- (sample BN-4) and garnet-free spinel lherzolite (sample SN-3-B) are listed in Table 5. In general the minerals are compositionally homogeneous, with some slight zoning occurring along the margins of spinel and garnet in contact with each other in the garnet + spinel-lherz~lite sample BN-4. In all the ultramafic inclusions for which mineral chemistry was determined, olivines and orthopyroxenes are highly magnesian. The forsterite component of the olivines lies in the range 92--88 mol.%, and the enstatite component of the orthopyroxenes lies in the range 93--88 tool.%, assuming that the iron of these minerals is in ferrous state. A1203 in orthopyroxenes ranges from 2.57 to 4.23 wt.%. A1203 in clinopyroxenes ranges from 3.40 to 5.80 wt.%, and Cr203 ranges from 0.87 to 1.64 wt.%. Garnets are chromepyropes and spinels are aluminous with appreciable amounts of chrome, magnesium and iron. No noticeable compositional differences occur in garnets within spinel-free and spinel-bearing garnet-peridotites. In contrast, spinel compositions change significantly, with spinels coexisting with garnets having lower alumina and higher chrome than spinels in garnet-free lherzolites (Table 5). Comparison of the mineral compositions reported here (Table 5) with compositions reported for spinel.and garnet-peridotite inclusions in basaltic hosts from other parts of the world (Ross et al., 1954) show that Palei-Aike inclusions are not unusual mineralogically. Varne (1977) has suggested on petrologic grounds that spinel-lherzolite

70

40.64 --9.88 0.09 49.00 0.02 -0.01 0.32

99.96

0.89 --

Modal vol%:

SiO= TiO= AI=O 3 FeO* MnO MgO CaO Na~O Cr20 s NiO=

Total

Mg/)Mg + F e e) Cr/(Cr + Ai)

F e O * = t o t a l Fe as FeO.

OI

Mineral:

0.904 0.047

99.98

55.24 0.05 3.52 6.49 0.16 33.64 0.43 -0.35 --

20

Opx

S a m p l e LS-2

52.20 0.25 4.75 2.38 0.08 16.33 22.66 0.80 0.87 0.02

0.93 0.10

100.34

5

Cpx

0.698 0.15

98.46

0.03 0.06 52.25 13.84 0.13 17.94 --13.95 0.24

5

Sp

--

0.892

100.35

40.85 --10.48 0.06 48.57 0.04 -0.01 0.34

45

Ol

0.902 0.064

100.02

55.41 0.20 3.62 6.41 0.14 32.94 0.76 0.07 0.47 --

20

Opx

S a m p l e BN-4

0.90 0.09

99.69

52.53 0.65 5.47 3.13 0.09 15.96 18.87 1.96 0.99 0.04

15

Cpx

0.66 0.32

98.25

0.14 0.75 37.58 16.11 0.16 17.28 0.01 -25.67 0.25

5

Sp

0.83 0.032

99.79

43.48 0.23 22.18 7.52 0.18 20.21 4.82 0.04 1.08 0.05

15

Gt

0.897 --

98.75

40.85 --9.84 0.05 47.60 0.04 -0.02 0.35

50

Ol

0.887 0.07

98.86

55.15 0.17 3.46 6.29 0.11 32.30 0.83 0.10 0.43 0.02

20

Opx

S a m p l e SN-3-B

Mineral c o m p o s i t i o n s d e t e r m i n e d b y m i c r o p r o b e a n a l y s i s o f t h r e e r e p r e s e n t a t i v e u l t r a m a f i c i n c l u s i o n s w i t h i n P a l e i - - A i k e b u a l t s

TABLE 5

0.83 0.03

100.29 99.49 0.898 0.11

43.91 0.17 22.22 7.48 0.17 20.46 4.71 0.04 1.10 0.03

15 15 52.42 0.48 5.48 3.27 0.09 16.20 18.60 1.88 1.02 0.05

Gt

Cpx

18

inclusions from Tasmania and Hawaii, that are similar mineralogically to the ultramafic inclusions from Palei-Aike, are subsolidus mineral assemblages and not igneous precipitates or simple residue of partial melting. Trace element and isotope studies of ultramafic inclusions and their host basalt in many cases indicate that equilibrium between them was not established and the inclusions are accidental and not cognate (see Frey and Green, 1974, for review). The occurrence within a single lava flow of the Palei-Aike volcanic field of diverse mineralogical types of ultramafic inclusions that probably equilibrated under different pressure-temperature conditions within the mantle supports the hypothesis of an accidental origin for the inclusions. Experimental studies of solid solutions and reactions in synthetic peridotite assemblages may serve as a guide to the relative temperature and pressure conditions of equilibration of the mineral assemblages of ultramafic xenoliths, although experimental calibration may not yet be sufficient to establish precisely these conditions (Varne, 1977). Table 6 lists the temperatures of equilibration of the three xenoliths, BN-4, LS-2, and SN-3-B, whose mineral chemistry is listed in Table 5. The temperatures were determined using the clinopyroxene-orthopyroxene geothermometer of Wells (1977), and are 100--150°C lower than the temperatures determined by the solubility of Ca in orthopyroxene according to Boyd and Nixon (1973). The full range of temperatures for all the ultramafic inclusions from the Palei-Aike basalts which were analyzed for mineral chemistry is 840--1270 ° C. Also shown in Table 6 are the pressure of equilibration of the three inclusions, determined by the solubility of Al2Os in orthopyroxene (Akella, 1976). The full range of pressures for all the analyzed inclusions from Palei-Alkali basalts is 20--42.5 kbar. The pressure and temperature of equilibration of the inclusions defines a geotherm which lies between normal oceanic and continental geotherms as calculated from heat data (Fig. 6; Mercier and Carter, 1975). TABLE 6 Temperature and pressure of equilibration of three representative ultrarnafic inclusions within Palei-Aike basaltsa Sample

Temperature

Pressure (kbar)

(°c) LS-2 BN-4 SN-3-B

892 1022 1036

A

B

24 28 27.6

(16.8) 19.5 (19.3)

aTemperatures determined by the clinopyroxene-orthopyroxene geothermometer of Wells (1977). Pressures determined by (A) the solubility of AI~O3 in orthopyroxene according to Akella (1976), and (B) the spinel-garnet phase boundary in peridotite systems (Green and Ringwood, 1967, 1970; O'Hara et al., 1971) which determines a pressure for BN-4, and the other pressures are reduced proportionally.

19

The pressures of equilibration of ultramafic inclusions encountered within the Palei-Aike volcanic field, when calculated using the solubility of A1203 in orthopyroxene as a geobarometer, are all greater than 20 kbar, and suggest a depth of origin greater than 60 kin. Granulite inclusions within the basalts probably originated in the lower continental crust at depths less than 30 kin. The apparent gap, between 30 and 60 kin, in the crest-mantle section represented by the mafic and ultramafic inclusions in the Palei-Aike basalts has no simple physical explanation and suggests that the pressure of equilibration of ultramafic inclusions determined by the solubility of A1203 in orthopyroxenes may be too high. An independent estimate of the pressure of equilibration for the garnet + spinel peridotite inclusions (BN-4, Table 6) may be determined from the quasi-invariant boundary separating the spineland gamet-lherzolite fields (Green and Ringwood, 1967, 1970; O'Hara et al., 1971). At 1020 ° C, this boundary is at approximately 19.5 kbar which is a 30% reduction in the pressure of equilibration of BN-4 compared to the pressure estimated by the solubility of A1203 in orthopyroxene. Ferguson et al. (1977) have also noted the significantly lower pressure of equilibration estimated for garnet + spinel lherzolites using the experimentally determined garnet-spinel phase boundary rather than the solubility of A1203 in orthopyroxene. Reducing the estimated pressure of equilibration of all the 1300

T,L

,2oo

//

/

ot

,,,,,oo tw

'""" ../"

Io° ' oi J ..,.."

1000

MERCIE ( 1976 ) 9OO D [

10

"

L """

20

[

I

30

40

50

PRESSURE Kb Fig. 6. A temperature (o C) versus pressure (kbar) diagram illustrating the conditions of equilibration of ultramafic inclusions within the Palei-Aike basalts and the geotherm dei'med for these conditions. Geotherm A was constructed based on the pressures estimated by the solubility of Al20 ~ in orthopyroxene (squares; Akella, 1976). Geotherm B was constructed based on pressures estimated for garnet + spinel peridotite according to the position of the garnet lherzolite-spinel lherzolite boundary (circles; Green and Ringwood, 1967, 1970; O'I-lara et al., 1971) and reduction of the pressure estimates for other inclusions by similar proportions. Typical oceanic and continental geotherms are from Mercier and Carter (1975). Mercier's (1976) geotherm wss based o n ultramafic inclusions from France. The solid symbols represent the three rocks listed in Tables 5 and 6.

20

analyzed inclusions an equal proportion (Table 6) significantly narrows the apparent gap in the crust-mantle cross section sampled by the inclusions in the basalts. A geotherm constructed using the reduced pressure estimates (Fig. 6) is similar to a geotherm calculated by Mercier (1976) based on ultramafic inclusions in basalts in France. This geotherm is higher than the normal oceanic geotherm, but Irving (1975) has pointed out that since inclusions are genetically related to processes of magma generation they probably represent a region of mantle with a perturbed geothermal gradient.

Clinopyroxene megacrysts Clinopyroxene megacrysts are encountered in many of the Palei-Aike lavas, and are abundant within the spatter comprising and the soils on top of some basaltic cones. The megacrysts vary from prismatic to elongated, and range up to 14 cm in their largest dimension. They are typically black, and display conchoidal fractures and glassy luster on freshly broken surfaces. When found within basalts they have light colored reaction rims. Some megacrysts contain opaque inclusions regularly distributed along curved planes, similar to the Ni- and Cr-bearing iron sulfide inclusions reported from clinopyroxene megacrysts in other parts of the world (Irving, 1974). The chemical analysis of a typical megacryst is listed in Table 7. The megacryst is a titaniferous augite, rich in A1, poor in Cr, and with a small amount of jadeite component in solid solution. The megacryst is chemically similar to clinopyroxene megacrysts occurring in alkali basalts in Australia (Irving, 1974}, but distinct from the clinopyroxenes within ultramafic inclusions (Table 5). Trace element and isotope studies of clinopyroxene megacrysts and their host basalts in many cases suggest that, unlike ultraTABLE 7 Composition (wt.%) of a clinopyroxene megacryst included within a Palei-Aike basalt Molecular proportion on the basis o f 6 oxygens SiO 2 TiO 2 AI20 s FeO* MnO MgO CaO Na20 Cr~O s Total

51.89 0.84 7.92 8.62 0.14 14.52 14.45 2.56 0.03 100.97

F e O * = total F e as F e O .

Si Ti A1 Cr Fe Mn Mg Ca Na

1.874 0.022 0.337 0.000 0.260 0.004 0.782 0.559 0.179

21

mafic inclusions, the megacrysts are cognate rather than accidental (Irving, 1974). For instance, the low Cr content and significantjadeite component in solid solution within the megacrysts suggests that they crystallizedin equilibrium with a basic composition such as the host magma. The high-Al content of the clinopyroxene megacrysts suggests a high-pressure origin, consistent with the megacrysts being chemically distinct from the groundmass clinopyroxene in the basalts (Table 2) and exhibiting marginal erosion and reaction attesting to their disequilibrium with the erupting host m a g m a (Irving, 1974). DISCUSSION The basalts of the Palei-Aike volcanic field are mineralogically and chemically similar to alkali olivine basalts from diverse tectonic environments. The presence within the basalts of ultramafic inclusions mineralogically similar to mantle xenoliths that commonly occur in alkali basalts in other parts of the world indicates a mantle origin for the Palei-Aike basalts, possibly at depths ~ 100 km as suggested by the maximum estimated pressure of equilibration of the ultramafic inclusions determined according to currently available geobarometers. T h e low Y contents, compared to ocean tholeiites, of the Palei-Aike basalts with the highest Mg/(Mg + Fe 2+) ratio suggest that these basalts formed by partial melting of a garnet-bearing mantle mineral assemblage, consistent with the presence of garnet~bearing ultramafic inclusions in the lavas. Experimental and trace element geochemical Studies indicate that nepheline-normative olivine alkali basalts form by small (3--15%) degrees of partial melting of mantle under conditions of low partial pressure of H20 (XVH~o ~ 0.4; Green, 1971; Mysen and Boettcher, 1975; Kay and Gast, 1973; Sun and Hanson, 1975). The observed range of Mg/(Mg + Fe 2+) ratios of the Palei-Aike basalts, from 0.61 to 0.36, and the increase in incompatible element contents which correlates with decreasing Mg/iMg + Fe2+), may have resulted from crystalliquid fractionation, either at high pressure involving aluminous clinopyroxenes that occur as megacrysts within the lavas, or at low pressures involving olivine and clinopyroxene that occur as both phenocrysts and within the groundmass of the basalts. In order for ultramafic inclusions to have preserved their high-pressure mineralogies the rates of eruption from depth of xenolith-bearing lavas must have been rapid. However, the c o m m o n occurrence in these same lavas of clinopyroxene megacrysts, probably of cognate origin, suggests that despite the rapid eruption rates these lavas may not represent unmodified primary mantle melts. None of the Palei-Aike basalts have Mg/(Mg + Fe 2÷) as high as the value of 0.68, considered representative of primary basalt magma in equilibrium with mantle olivine (Green, 1971). The Patagonian plateau lavas are located in a tectonic position similar to back-arc basins in the western Pacific, and their origin may be a consequence

22 of plate convergence related to thermal or mechanical perturbation of the subcontinental mantle due to subduction. The alkali olivine basalts of the Palei-Aike volcanic fields are compositionally similar to alkali olivine basalts from oceanic islands (Table 4) which suggests that they do not contain a material component derived from subducted oceanic lithosphere as has been suggested for some circum-Pacific type volcanism (Kay, 1977). The Palei-Aike basalts differ compositionally from the other Patagonian basalts and contain garnet-bearing ultramafic inclusions derived from greater depths than the ultramafic inclusions reported to be found in other Patagonian lavas. These differences between the Palei-Aike and other Patagonian plateau lavas may relate to the particular tectonic situation in the southernmost part of South America, where a convergent plate boundary merges into a strike-slip boundary through a complex system of faults related to a trench-transform-transform triple junction between the South American, Antarctic,and Scotia plates (Fig. I). ACKNOWLEDGEMENTS

We wish to thank B. Bergman, C. Castro, and E. Gonzalez of E N A P of Chile, and the Fell family of Punta Arenas for assistancein the field.This study benefited from the ideas of R. Chattier,R. Fuenzalida, L. Lopez, H. Moreno and M. Vergara of the U. of Chile, and D. Elthon. The work was supported financiallyby National Science Foundation grants D E S 75 04076, EAR 76 82456, and EAR 76 03816.

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23 Forsyth, D.W., 1975. Fault plane solutions and tectonics of the South Atlantic and Scotia Sea. J. Geophys. Res., 80: 1492--1443. Frey, F.A. and Green, D.H., 1974. The mineralogy, geochemistry and origin of lherzolite inclusions in Victoria basanites. Geochim. Cosmochim. Acta, 38: 1023--1054. Fuenzalida, R., 1974. The Magellan fault zone. In: O. Gonzalez-Ferran (Editor), Proceedings of the IAVCEI Symposium on Andean and Antarctic Volcanology Problems, Santiago, pp. 373--392. Green, D.H., 1971. Compositions of basaltic magmas as indicators of conditions of origin : application to oceanic volcanism. Philos. Trans. R. Soc. London, 268: 707--725. Green, D.H. and Hibberson, W., 1970. Experimental duplication of conditions of precipitation of high-preasure phenocrysts in basaltic magma. Phys. Earth Planet. Inter., 3: 247--254. Green, D.H. and Ringwood, A.E., 1967. The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structure. Earth Planet. Sci. Lett., 3: 151--160. Green, D.H. and Ringwood, A.E., 1970. Mineralogy of peridotite compositions under upper mantle conditions. Phys. Earth Planet. Inter., 3: 359--371. Green, J. and Poldevaart, A., 1955. Some basaltic provinces, Geochim. Cosmochim. Acta, 7: 177--188. Halpern, M., 1973. Regional geochronology of Chile south of 50 ° latitude. Geol. Soc. Am. Bull., 84: 2407--2421. Hayes, D.E., 1966. A geophysical investigation of the Peru-Chile Trench. Mar. Geol., 4: 309--351. Herron, E.M., 1972. Sea-floor spreading and the Cenozoic history of the east-central Pacific. Geol. Soc. Am. Bull., 83: 1671--1710. Herron, E.M., Bruhn, R., Winslow, M. and Chuaqui, L., 1977. Post-Miocene tectonics of the margin of southern Chile. In: M. Talwani and W.C. Pitman III (Editors), Island Arcs, Back-Arc Basins, and Deep-Sea Trenches. Am. Geophys. Union, Maurice Ewing Ser., 1: 273--284. Hyndman, D.W., 1972. Petrology of Igneous and Metamorphic Rocks. McGraw-Hill, New York, N.Y., 533 pp. Irvine, T.N. and Baragar, W.R.A., 1971. Guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523--548. Irving, A.J., 1974. Megacrysts from newer basalts and other basaltic rocks of southeastern Australia. Geol. Soc. Am. Bull., 85: 1503--1514. Irving, A.J., 1975. On deducing paleogeotherms from xenolith suites in basalts and kimberlites: a Heisenbergian uncertainty? Extended Abstr., Int. Conf. Geothermometry and Geobarometry, State College, Penn. Kay, R., 1977. Geochemical constraints on the origin of Aleutian magmas. In: M. Talwani and W.C. Pitman III (Editors), Island Arcs, Back-Arc Basins, and Deep-Sea Trenches. Am. Geophys. Union, Maurice Ewing Ser., 1: 229--242. Kay, R. and Gast, P., 1973. The rare earth content and origin of alkali rich basalts. J. Geol., 81: 653--682. Klitgord, K.D., Mudie, J.D., Larson, P.A. and Grow, J.A., 1973. Fast sea floor spreading on the Chile Ridge. Earth Planet. Sei. Lett., 20: 93--99. Kuno, H., 1968. Differentiation of basalt magmas. In: H.H. Hess and A. Poldevaart (Editors), Basalts. Interscience, New York, N.Y., 2: 623--688. Lambert, L.R., 1956. Descripcion geologica de la hoja 356, Zapala (T.N. de NenLmen) Dir. Natl. Min., ]3ol., 83. Lambert, R.J.T. and Holland, J.G., 1974. Yttrium geochemistry applied to petrogenesis utilizing calcium- yttrium relations of minerals and rocks. Geochim. Comnochim. Acta, 38: 1393--1414. Lopez, L., Frey, F.A. and Vergara, M., 1977. Andesitas and high-alumina basalts from the central-south Chile high Andes: geochemical evidence bearing on their petrogenesis. Contrib. Mineral. Petrol., 63: 199--228.

24 MacDonald, G.A. and Katsura, T., 1964. Composition of Hawaiian lavas. J. Petrol., 5 : 82--133. Mazzullo, L.J. and Bence, A.E., 1976. Abyssal tholeiitesfrom D S D P Leg 34: the Nazca plate. J. Geophys. Res., 81: 4327---4352. Mercer, J.H., 1976. Glacial history of southernmost South America. Quatern. Res., 6: 125--166. Mercier, J.-C.C., 1976. Single pyroxene geothermometry and geobarometry. Am. Mineral., 61: 603--615. Mercier, J.-C.C. and Carter, N.L,, 1975. Pyroxenes geotherms. J. Geophys. Res., 80: 3349--3362. Mercier, J.-C.C. and Nicolas, A., 1975. Textures and fabrics of upper mantle peridotites as illustratedby xenoliths from basalts. J. Petrol., 16: 454--487. Minster, J.B.,Jordan, T.H., Molnar, P. and Haines, E., 1974. Numerical modeling of instantaneous plate tectonics. Geophys. J. R. Astron. Soc., 36: 541. Moreno, H., 1974. The Upper Cenozoic volcanism in the Andes of southern Chile (from 40 ° 00' to 41 ° 30'S.L.). In: O. Gonzalez-Ferran (Editor), Proceedings of the I A V C E I Symposium on Andean and Antarctic Volcanology Problems, Santiago pp. 143--171. Moreno, H., 1975. Carateristicas petrologicas del volcanismo Cenozoico Superior en Los Andes del sur de Chile (39 ° and 41 ° 30'S). Proc. 6th Geol. Congr. Argentina, Bahia Blanca. Mysen, B.O. and Boettcher, A.L., 1975. Melting of hydrous mantle, II. Geochemistry of crystals and liquids formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen, and carbon dioxide. J. Petrol., 16: 549--593. Natland, M.L., Gonzalez, E., Canon, A. and Ernst, M., 1974. A system o f stages for correlation of Magellanes Basin sediments. Geol. Soc. Am., Mem., 139: 1--126. Niemeyer, H., 1975. Geologia de la Provincia de Aisen entre los lagos General Carrera y el rio Chacabuco. Thesis, University of Chile, Santiago. Norrish, K. and Chapell, B.W., 1967. X-ray fluorescence spectrography. In: J. Zuasman (Editor), Physical Methods in Determinative Mineralogy. Academic Press, New York, N.Y. O'Hara, M.J., Richardson, S.W. and Wilson, G., 1971. Garnet-peridotite stability and occurrence in crust and mantle. Contrib. Mineral. Petrol., 32: 4 8 - 6 8 . Ross, C.S., Foster, M.D. and Myers, A.J., 1954. Origin o f dunites and olivine-rich inclusions in basaltic rocks. Am. Mineral., 39: 693--727. Schwarzer, R. and Rogers, J., 1974. A worldwide comparison of alkali olivine basalts a n d their differentiation trends. Earth Planet. Sci. Lett., 23: 286--296. Skewes, M.A., in preparation. The petrology and geochemistry o f the Palei-Aike volcanic field, Magellan Province, Chile. Thesis, University o f Chile, Santiago. Stern, C.R., Skewes, M.A. and Duran, M., 1976. Volcanismo orogenico en Chile Austral. Proc. 1st Chilean Geol. Congr., Santiago, 2: 195--212. Sun, S.S. and Hanson, G.N., 1975. Origin of Ross Island hasanitoids and limitations upon the heterogeneity of mantle sotlrces for alkali basalts and nephelinites. Contrib. Mineral Petrol., 52: 77--106. Takamura, H. arid Yoshida, H., 1975. The basalts o f Patagonia. J. Sci. Hiroshim'a Univ., 7 : 149--164. Tyrrell, G.W., 1932. The basalts of Patagonia. J. Geol., 40: 374--383. Urien, C.M. and Zambrano, J.J., 1973. The geology o f t h e . b ~ i n s o f the Argentine continental margin and Malvinu Plateau. In: A.E.M. Nalrn and F.O. Stelhi (Editors), The Ocean Basins and Margins. Plenum lh~m, New York, N.Y., pp. 135--161. Varne, R., 1977. On the origin o f spinel lherzolite inehmions in bsmdtic rocks from Tasmania and elsewhere. J. Petrol., 18: 1--23. Vergara, M., 1970. Note on the zonation o f the Upper Cenozoic volcanism of the Andean area o f central-south Chile and Argentina. Conf. Solid Earth Problems, Buenos Aires, Int. Upper Mantle Proj., 2: 381--397.

25 Vergara, M. and Munizaga, F., 1974. Age and evolution of the Upper Cenozoic andesite volcanism in central-south Chile. Bull. Geol. Soc. Am., 85: 603--606. Vlodavetz, I.V., 1972. Trace elements in alkali basalts and their inclusions in the Dariganga area, Mongolia. Bull. Volcanol., 35: 462--469. Wells, P.A., 1977. Pyroxene thermometry in simple and complex systems. Contrib. Mineral. Petrol., 62: 129--139.