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Rubidium-strontium geochronology of a transect of the Chilean Andes between latitudes 45° and 46° S
60
Earth and Planetary Science Letters, 41 (1978) 60-66 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[5]
RUBIDIUM-STRONTIUM GEOCHRONOLOGY OF A TRANSECT OF THE CHILEAN ANDES BETWEEN LATITUDES 45 ° AND 46 ° S
M. HALPERN Geosciences Program, The University of Texas at Dallas, Richardson, TX 75080 (U.S.A.)
and R. F U E N Z A L I D A Instituto de Investigaciones Geologicas, Santiago (Chile)
Received February 23, 1978 Revised version received May 24, 1978
Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45 ° and 46 ° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44 ° S, plutons of similar composition to the Chilean Andes give K-Ar and Rb-Sr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.
1. Introduction
Geochronological and geochemical studies [ 1,2] have postulated a post-Paleozoic continent-ward migration o f plutonic foci in the central Andes of South America. These epizonal plutons, as well as their extrusive equivalents, have chemical features similar to island arc magmatic rocks which originated near the Benioff zone with little, if any, indication o f crustal anatexis [3,4]. From these and other investigations, the central Andes o f South America has evolved as a type example o f a consumption plate boundary marked by the interaction o f the South America and Nazca plates. It is important to confirm the postulated post-Paleozoic migration o f igneous activity for the central Andes as well as other segments o f the Andean Cordillera as metallogenetic evolution in the Andes appears closely related to magmatic activity. Rubidium-strontium and potassium-argon dates o f
crystalline rocks from north Chile and Argentina [5] suggest that Permian to Cenozoic magmatic rocks can be found within the mobile belt and that a simple unidirectional migration of magmatic foci through time is not a certainty. In the Chilean Andes south o f 50 ° latitude, the plutonic rocks o f the Patagonian batholithic complex range in age from Jurassic to Tertiary. Although probably episodic in terms of the volumes of magma emplaced in the crust over a given time interval there is little evidence that the plutons are progressively younger or older from west to east over distances of about 300 km [6]. This paper presents new Rb-Sr geochronological data for magmatic rocks from the south Chile Andes between latitudes 45 ° and 46 ° S. Published dates for plutonic rocks from the Argentine Andes between about 40 ° and 44 ° S are also included to demonstrate that the available radiometric ages suggest that magmatic activity was an integral part in the geological evolution o f the southern Andes over periods in excess o f 150 m.y.
61
If magmatic activity within the central Andean orogen is indeed migratory, then the question as to whether the central Andean region is representative of the geologic evolution of the Andean Cordillera, or simply an anomaly, should be raised.
tions 2 and 3), granitic plutons contain roof pendants of Neocomian sedimentary rocks.
3. Analytical methods The methods are described by Halpern [5]. The analytical precision for the Rb or Sr concentration is about 2% X-ray fluorescence or about 1% by isotope dilution (Table 1). The normalized 87Sr/86Sr ratio for duplicate dissolution analyses is precise to about 0.1%. The normalized 87Sr/86Sr ratio of the National Bureau of Standards SrCO3 standard No. 987 is 0.7100 -+ 0.0005 (2o). The least squares method of York [8] is used to fit straight lines to the data presented in Fig. 2. Calculated ages and 87Sr/S6Sr initial ratios are given at the 95% confidence level. The decay constant X~ = 1.42 X l0 -1~ yr -1 is used for all Rb-Sr ages.
2. General geology The basement complex of southern Chile is predominantly metasedimentary ranging from greenschist to lower amphibolite facies and containing associated granitic rocks. Between 33 ° and 40 ° S, the metamorphic basement is considered late Paleozoic from the results of Rb-Sr total-rock analyses [7]. South of 50 ° S the available isotopic age data suggest a Paleozoic age for the basement [6]. The geology of the Chilean cordillera between latitudes 45 ° and 46 ° S has been mapped by R. Fuenzalida of the Chilean Instituto de Investigaciones Geologicas. Fig. 1 illustrates the major rock units in the area sampled for Rb-Sr geochronology. These units consist of a metamorphic basement overlain by intermediate to acidic volcanic rocks and intruded by granitic plutons. The plutonic intrusive rocks are hypidiomorphic-granular, undeformed epizonal bodies of predominantly quartz diorite to granodiorite composition. In the Simpson River area (Fig. 1, loca-
75°W 4 5 ° S .4
74 ° I
I
.
o,~'-~ ~,
/
~
~ - ~ , ~ " ~ "%o ~,3 + L.A, ~) ~ ~ '
/ /
21(72) ~
46°5',,[%~ oow
~
,,-~-',-,'~-
v
,+f'~,
I~" +~-~"+~4,~Ir,'Z~'h'l
Radiometric ages from the Andes, including the central Andean orogen, are mostly K-Ar dates. The analyzed minerals are generally from epizonal plutons or volcanic rocks. The majority of the ages are K-Ar and Rb-Sr biotite dates which should be interpreted to represent the final closure of the isotopic systems
75 ° I
I
"~,
4. Radiometric dates
72 ° I
I
~(...~.....y
~
~C
*,i,,20o9)
4.. ~ -P
~U'~-RTO,I08)÷2~3+ AISEN+ +
+ + + ,.p -I- ÷ -P + + + ~ ~'-~+L +'P /
f+,
+
+
"
I
+
+
"
+,
~
~ ~
,
tJ
'~
~
I
~
I
'--.:-J ~ r ~----]MetomorpNcrockI / )
+ +
I
7l°W I 45os
"r
/
/
' I,,=oo '
Fig. 1. Generalized geologic m a p o f southern Chile between latitudes 45 ° and 46 ° S. Sample n u m b e r s are prefixed by H-74 in Table 1 which lists their analytical data. Rb-Sr dates, in million years, are in parentheses. Total-rock isochron dates are underlined; others are biotite and total-rock model ages.
Rock type
adamellite adamellite adamellite adameUite adameUite adamellite adameUite granodiorite quartz diorite quartz diorite
quartz diorite quartz diorite
rhyolite tuff
quartz diorite granodiorite porphyritic quartz diorite granodiorite monzonite quartz diorite quartz diorite
quartz diorite porphyritic andesite quartz diorite
Location in Fig. 1 sample No.
H-74-2A H-74-2B H-74-2C H-74-2D H-74-3 H-74-4A H-74-4B H-74-5 H-74-6 H-74-6
H-74-7 H-74-7
H-74-8
H-74-9 H-74-10 H-74-11 H-74-12 H-74-13 H-74-14 H-74-14
H-74-15 H-74-16 H-74-17
total rock total rock total rock
total rock total rock total rock total rock total rock total rock biotite
total rock
total rock biotite
a b
a b a b
0.7048 0.7123 0.7115
0.7210 0.7056 0.7105
0.7044 0.7171 0.7168 0.7040 0.7172 0.7175 0.7359 0.7365
a b
total rock biotite
87Sr 86Sr * -+0.1%
0.7182 0.7263 0.7198 0.7249 0.7244 0.7181 0.7187
Duplicare sets
rock rock rock rock rock rock rock
total total total total total total total
Material analyzed
14.8 11.8 71.8
118 129 96.7 147 78.3 65.3
134
47.9
215 240 221 210 206 181 178 112 56.1
147 80.4 277
348 75.2 169 109 198 414
17.8
329
67.2 47.5 64.2 43.9 45.7 58.6 55.9 114 377
0.101 0.147 0.259
0.339 1.71 0.572 1.35 0.395 0.158
7.53
0.146
3.20 5.05 3.44 4.78 4.51 3.09 3.18 0.982 0.149
0.216 1.110 1.103
0.308
1.238 1.238 0.425
1.418 1.421
0.678 0.766 0.707 0.666 0.651
0.471 0.0298 0.0300
0.190
0.0175 0.0170 0.0204
0.0193 0.0193
0.0766 0.0533 0.0717 0.0486 0.0496
86Sr (um/g) -+1%
STRb (.am/g) +-1%
Rb (ppm) +-2%
Sr (ppm) +-2%
Isotope dilution
X-ray fluorescence Rb/Sr
Rubidium and strontium analytical results for crystalline rocks from the Chilean Andes between 45 ° and 46 °
TABLE 1
0.459 37.2 36.8
1.62
70.7 72.8 20.8
73.5 73.6
8.85 14.4 9.86 13.7 13.2
87Rb 86Sr
87Sr/86Sr
15 13
109
107
13 13 107
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 z 0.7045 2 107 109
12 12
0.7057 +_ 0.0013 1
initial
100+- 6
Age ** (m.y.)
t~
H-74-18 H-74-18 H-74-19
quartz diorite quartz diorite
total rock biotite total rock
H-74-24A
H-74-22 H-74-23 H-74-23
granodiorite quartz diorite quartz diorite
granodiorite granodiorite
adamellite
granodiorite monzonite monzonite
total rock total rock
biotite total rock biotite
total rock total rock
total rock
total rock total rock biotite
total rock total rock biotite
H-74-24B H-74-24C • H-74-24B H-74-25 H-74-25 adamellite adameUite total rock
quartz-sericite-chlorite schist quartz-sericite schist quartz diorite quartz diorite
H-74-26A H-74-26B adameUite
total rock total rock
H-74-20 H-74-21 H-74-21
H-74-26C dacite porphyritic andesite
a b
a b
a b a b
1.1604 1.1653 0.7146
0.7051 0.7550 0.7523 0.7082
0.7639 0.7631 0.7191 0.7291
1.1432 1.2107 0.7169 0.7176 0.7112 0.7150 0.7151 0.7343
a b
0.7302 0.7306
a b
a b
Normalized to 86Sr]88Sr ratio of 0.1194 Using h~ = 1.42 × 10 -11 y r - 1 . Calculated 87Sr/86Sr initial ratio (95% confidence level). Assumed 87Sr/86Sr initial ratio.
H-74-27 H-74-29 * ** 1 2
0.00469 0.00448
0.249 1.159 1.148
48.4 47.2
224
1.345
0.0278 0.0282
234 279 4.73
55.8
1.~32
0.00639 0.00535 0.0970
100 116
146 129 140
33.5
77.2 231
88.1 147 105
259
1.30 0.502
0.129
247 256
.1.497 1.491 0.459
0.163 0.118
11.9
0.675
0.411 0.445
0.0642
270 271 9.98 16.7
115
1.66
0.766
0.00353 0.00353 0.632 0.0345
17.4
77.6
0.878 1.33
0.953 0.957 0.631 0.577
0.0317
0.599 0.313
3.51 5.99
0.551
242 226
56.1 30.4
6.26
145 70.8
197 182
27.8
0.960 0.057
2.52 3.77
174
57.8 436 55.5 24.7
130 127
0.7045 2
0.7045 2
132 128
0.7044 -+ 0.0036 1
0.7045 2
0.7045 2
194 +_ 67
0.7045 2
73 71 70 (9.)
176
0.7045 2
0.7040 -+ 0.0019 I
15 15
106_+9
64 which may or may not represent the time of initial crystallization. In the Chilean Andes south of 50 ° S latitude and in the Argentine Andes at 40040 ' S, RbSr total-rock isochrons were obtained from several suites of plutonic rocks. The isochrons have calculated ages similar to the mineral ages and initial STSr/86Sr ratios of about 0.705 -+ 0.001 [6,9].
Sr87/Sr86
~
0.7300.7 250.720-
B
/
0.71 5-
/
0.7 I0-
A
/
0.7O5- /-----0.7057- 0£)033 0.700"
I
o
I
' 12
4;8,o
~.~:i
O.720.br zbr 0.715-
1'4
, R b s T / S , 8e
16
\ff-~:~"
\oj& .- ' ~ - . - e24A
0 7 I0
/ "~4B
B
/ 0.705-/_._._0.7044+ 0.0036 07[00, RbS?/SrB6
o87,,o86
O.735or
/at
0.730-
~
0.7250.720 0.71 5-
/
~.oJ~/~126 c
"-26A
C_
/
0.710- / 0.705- / - ~ 0 . 7 0 4 0 - + 0.0019 0.700 . . . .
o
26B
;,
5. Conclusions
,R
Sr8s
; s Lo i21416 is bS'/
Fig. 2. Total-rock isochrons for southern Chile plutonic igneous rocks. A. Simpson River (45°29 ' S, 72°15 ' W). B. Victoria Island (45°11P S, 73051 ' W). C. La Paloma Lake (45056 '
S, 72012' W).
Table 1 lists the results of the rubidium and strontium analyses of rocks collected across the Chilean Andean orogen between latitudes 45 ° and 46 ° S (Fig. 1). Most of the plutonic rocks have Rb/Sr ratios less than one and therefore could not be used for precise total-rock Rb-Sr dates with the mass spectrometer used in this study. Total-rock isochrons (Fig. 2) were obtained for the adamellite and granodiorite plutons which crop out along the Simpson River (Fig. 1, locations 2 and 3), along the east shore of Victoria Island (Fig. 1, location 24), and along the south shore of La Paloma Lake (Fig. 1, location 26). In addition, biotite was analyzed from representative plutons across the transect (Fig. 1, Table 1). The RbSr age of 100 + 6 m.y. for the Simpson River adamellite agrees with the local stratigraphy as the pluton contains roof pendants of Neocomian sedimentary rocks. Total-rock samples from Victoria Island give model ages of 190 + 11 m.y., 187 + 20 m.y., and 197 + 12 m.y. assuming an exact initial aTSr/86Sr ratio of 0.7045, no error in the aVRb/86Sr ratio, and 0.1% precision for the present-day 87Sr/S6Srratio. These dates are somewhat similar to the 176 m.y. calculated age for biotite sample H-74-24B which implies that the Rb-Sr biotite dates may approximate the time of crystallization of the magmas. In the Puerto Aisen area, the quartz diorite plutons have a mineral orientation when viewed over distances of tens of meters. The sample collected at location 6 (Fig. 1), when examined in thin section, shows that although the minerals are fresh they have been strongly deformed. The quartz exhibits undulose extinction and the extinction of the plagioclase feldspar and biotite is strained. The Miocene biotite dates for the quartz diorite collected at locations 6, 7, 14 and 25 are considered minimum ages associated with probable local structural deformation. The radiometric dates suggest that the plutonic activity between 45 ° and 46 ° S is of Mesozoic and Cenozoic age.
Fig. 3 is a compilation of radiometric dates of plutonic rocks of the Argentina and Chile Andean Cordillera between latitudes 40 ° and 46 ° S. The Argentine samples are from a north-south traverse and the Chilean samples from an east-west transect.
65
40os
75°W
74'
7:3°
72 °
71°W - 3os
41<
~'2'
43
4z
4,'
6. Speculation Halpern [11] in a review of the geochronologic evolution of the southern half of South America postulated a westward migration of 150 m.y. to 300 m.y. radiometric age provinces throughout Phanerozoic time. It was suggested that the present sialic crust of interior southern South America evolved as a result of igneous and metamorphic processes responding to plate motions involving collision of a Gondwanaland plate and a paleo-Pacific oceanic plate. Granitic plutons related to Paleozoic subduction are presently marked by geological features such as the Pampean Ranges of Argentina. Following fragmentation of Gondwanaland, the South American plate moved westward overthrusting the Pacific (Nazca) plate. Magmatic rocks derived from the partial melting of ocean lithosphere, subcontinental mantle, and/or pelagic sediments were and are presently located along the leading edge of the South American plate. This model postulates that the age and geographic location of the magmatic rocks is controlled by the position and angle o f dip of the subduction zone. The radiometric dates presented in this paper agree, at least to a first approximation with such a model.
Acknowledgements 4E,
75 ° 74° 73 ° 72 ° 7~VV Fig. 3. Rb-Sr and K-Ar radiometric ages in million years of plutonic igneous rocks from the southern Andes Mountains of Argentina and Chile. Open circles are K-Ar dates, solid circles are Rb-Sr dates, a = amphibole, b = biotite, t = total rock. The Argentine dates are taken from papers by Toubes and Spikermann [10] and Halpern et al. [9]. Samples A and B were analyzed at the Instituto de Investigacions Geologicas (Chile) by F. Munizaga. Biotite A, from a diorite, has a 87Sr/ 86Sr ratio of 0.9006 and a STRb/86Sr ratio of 294;biotite B, from a granodiorite, has a 87Sr/86Sr ratio of 0.7673 and a 87Rb/86Sr ratio of 37.7.
Admittedly the sampling density is sparse and only a few total rock Rb-Sr isochron ages are available;however, there is no indication of simple unidirectional migration of intrusive foci. The plutons constitute a batholithic complex in which granitic rocks of predominantly diorite to granodiorite composition have been emplaced over periods of about 200 m.y.
We thank K. Kawashita and U.G. Cordani of the Centro de Pesquisas Geocronologicas (University of S~'o Paulo, Brasil) for providing the X-ray fluorescence analyses, M. Ruiz (University of Texas at Dallas) for the STSr/S6Sr analysis of samples H-74-4A, 4B, 10, 12, 19, and 22, J.F. Reilly II (University of Texas at Dallas) for assistance in the preparation of the samples. The lnstituto de Investigaciones Geologicas (Chile) supported the field work and the National Science Foundation (U.S.A.) provided M.H.'s airfare and miscellaneous expenses to Chile under grant No. GV28757 A1. This paper was written while M.H. was a Visiting Professor at the Department o f Geophysics and Planetary Sciences, Tel-Aviv University; the assistance of the faculty and staff is gratefully acknowledged. This research is part of Project No. 120, Magmatic Evolution of the Andes, of the International Geological Correlation Program. Geosciences Program, The University o f Texas at Dallas, Contribution No. 350.
66 References 1 E. Farrar, A.H. Clark, S.J. Haynes, G.S. Quirt, H. Conn and M. Zentilli, K-Ar evidence for the post-Paleozoic migration of granitic intrusion foci in the Andes of northern Chile, Earth Planet. Sci. Lett. 10 (1970) 6 0 - 6 6 . 2 A.H. Clark, E. Farrar, J.C. Caelles, S.J. Haynes, R.B. Lortie, S.L. McBride, G.S. Quirt, R.C.R. Robertson and M. Zentilli, Longitudional variations in the metallogenetic evolution of the central Andes: a progress report, Geol. Assoc. Can., Spec. Paper No. 14 (1976) 2 3 - 5 8 . 3 D.E. James, C. Brooks and A. Cuyubamba, Strontium isotopic composition and K, Rb, Sr geochemistry of Mesozoic volcanic rocks of the central Andes, Annu. Rep. Director Dep. Terrestr. Magnet., Carnegie Inst. Washington (1974) 9 7 0 - 9 8 3 . 4 D.E. James, C. Brooks and A. Cuyubamba, Andean Cenozoic volcanism: magma genesis in the light of strontium isotopic composition and trace-element geochemistry, Geol. Soc. Am. Bull. 87 (1976) 5 9 2 - 6 0 0 . 5 M. Halpern, Geological significance of Rb-Sr isotopic data
6 7
8 9
10
11
of north Chile crystalline rocks of the Andean orogen between latitudes 23 and 27 degrees south, Geol. Soc. Am. Bull. 89 (1978) 5 2 2 - 5 3 2 . M. Halpern, Regional geochronology of Chile south of 50 ° latitude, Geol. Soc. Am. Bull. 84 (1973) 2407-2422. F. Munizaga, L. Aguirre and F. Herv6, Rb/Sr ages of rocks from the Chilean metamorphic basement, Earth Planet. Sci. Lett. 18 (1973) 8 7 - 9 2 . D. York, Least-squares fitting of a straight line, Can. J. Phys. 44 (1966) 1079-1086. M. Halpern, P.N. Stipanicic and R.O. Toubes, Geocronologia (Rb/Sr) en los Andes Australes Argentinos, Rev. Asoc. Geol. Argentina, 30 (1975) 180-192. R.O. Toubes and J.P. Spikermann, Algunas edades K/Ar y Rb/Sr de plutonitas de la Cordillera Patagonica entre los paralelos 40 ° y 44 ° de latitud sur, Rev. Asoc. Geol. Argentina 28 (1973) 382-396. M. Halpern, Geochronologic evolution of southern South America, in: Proc. Int. Symp. on the Carboniferous and Permian Systems in South America, An. Acad. Bras. Cienc. 44, Supplement (1972) 149-160.
Earth and Planetary Science Letters, 41 (1978) 60-66 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[5]
RUBIDIUM-STRONTIUM GEOCHRONOLOGY OF A TRANSECT OF THE CHILEAN ANDES BETWEEN LATITUDES 45 ° AND 46 ° S
M. HALPERN Geosciences Program, The University of Texas at Dallas, Richardson, TX 75080 (U.S.A.)
and R. F U E N Z A L I D A Instituto de Investigaciones Geologicas, Santiago (Chile)
Received February 23, 1978 Revised version received May 24, 1978
Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45 ° and 46 ° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44 ° S, plutons of similar composition to the Chilean Andes give K-Ar and Rb-Sr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.
1. Introduction
Geochronological and geochemical studies [ 1,2] have postulated a post-Paleozoic continent-ward migration o f plutonic foci in the central Andes of South America. These epizonal plutons, as well as their extrusive equivalents, have chemical features similar to island arc magmatic rocks which originated near the Benioff zone with little, if any, indication o f crustal anatexis [3,4]. From these and other investigations, the central Andes o f South America has evolved as a type example o f a consumption plate boundary marked by the interaction o f the South America and Nazca plates. It is important to confirm the postulated post-Paleozoic migration o f igneous activity for the central Andes as well as other segments o f the Andean Cordillera as metallogenetic evolution in the Andes appears closely related to magmatic activity. Rubidium-strontium and potassium-argon dates o f
crystalline rocks from north Chile and Argentina [5] suggest that Permian to Cenozoic magmatic rocks can be found within the mobile belt and that a simple unidirectional migration of magmatic foci through time is not a certainty. In the Chilean Andes south o f 50 ° latitude, the plutonic rocks o f the Patagonian batholithic complex range in age from Jurassic to Tertiary. Although probably episodic in terms of the volumes of magma emplaced in the crust over a given time interval there is little evidence that the plutons are progressively younger or older from west to east over distances of about 300 km [6]. This paper presents new Rb-Sr geochronological data for magmatic rocks from the south Chile Andes between latitudes 45 ° and 46 ° S. Published dates for plutonic rocks from the Argentine Andes between about 40 ° and 44 ° S are also included to demonstrate that the available radiometric ages suggest that magmatic activity was an integral part in the geological evolution o f the southern Andes over periods in excess o f 150 m.y.
61
If magmatic activity within the central Andean orogen is indeed migratory, then the question as to whether the central Andean region is representative of the geologic evolution of the Andean Cordillera, or simply an anomaly, should be raised.
tions 2 and 3), granitic plutons contain roof pendants of Neocomian sedimentary rocks.
3. Analytical methods The methods are described by Halpern [5]. The analytical precision for the Rb or Sr concentration is about 2% X-ray fluorescence or about 1% by isotope dilution (Table 1). The normalized 87Sr/86Sr ratio for duplicate dissolution analyses is precise to about 0.1%. The normalized 87Sr/86Sr ratio of the National Bureau of Standards SrCO3 standard No. 987 is 0.7100 -+ 0.0005 (2o). The least squares method of York [8] is used to fit straight lines to the data presented in Fig. 2. Calculated ages and 87Sr/S6Sr initial ratios are given at the 95% confidence level. The decay constant X~ = 1.42 X l0 -1~ yr -1 is used for all Rb-Sr ages.
2. General geology The basement complex of southern Chile is predominantly metasedimentary ranging from greenschist to lower amphibolite facies and containing associated granitic rocks. Between 33 ° and 40 ° S, the metamorphic basement is considered late Paleozoic from the results of Rb-Sr total-rock analyses [7]. South of 50 ° S the available isotopic age data suggest a Paleozoic age for the basement [6]. The geology of the Chilean cordillera between latitudes 45 ° and 46 ° S has been mapped by R. Fuenzalida of the Chilean Instituto de Investigaciones Geologicas. Fig. 1 illustrates the major rock units in the area sampled for Rb-Sr geochronology. These units consist of a metamorphic basement overlain by intermediate to acidic volcanic rocks and intruded by granitic plutons. The plutonic intrusive rocks are hypidiomorphic-granular, undeformed epizonal bodies of predominantly quartz diorite to granodiorite composition. In the Simpson River area (Fig. 1, loca-
75°W 4 5 ° S .4
74 ° I
I
.
o,~'-~ ~,
/
~
~ - ~ , ~ " ~ "%o ~,3 + L.A, ~) ~ ~ '
/ /
21(72) ~
46°5',,[%~ oow
~
,,-~-',-,'~-
v
,+f'~,
I~" +~-~"+~4,~Ir,'Z~'h'l
Radiometric ages from the Andes, including the central Andean orogen, are mostly K-Ar dates. The analyzed minerals are generally from epizonal plutons or volcanic rocks. The majority of the ages are K-Ar and Rb-Sr biotite dates which should be interpreted to represent the final closure of the isotopic systems
75 ° I
I
"~,
4. Radiometric dates
72 ° I
I
~(...~.....y
~
~C
*,i,,20o9)
4.. ~ -P
~U'~-RTO,I08)÷2~3+ AISEN+ +
+ + + ,.p -I- ÷ -P + + + ~ ~'-~+L +'P /
f+,
+
+
"
I
+
+
"
+,
~
~ ~
,
tJ
'~
~
I
~
I
'--.:-J ~ r ~----]MetomorpNcrockI / )
+ +
I
7l°W I 45os
"r
/
/
' I,,=oo '
Fig. 1. Generalized geologic m a p o f southern Chile between latitudes 45 ° and 46 ° S. Sample n u m b e r s are prefixed by H-74 in Table 1 which lists their analytical data. Rb-Sr dates, in million years, are in parentheses. Total-rock isochron dates are underlined; others are biotite and total-rock model ages.
Rock type
adamellite adamellite adamellite adameUite adameUite adamellite adameUite granodiorite quartz diorite quartz diorite
quartz diorite quartz diorite
rhyolite tuff
quartz diorite granodiorite porphyritic quartz diorite granodiorite monzonite quartz diorite quartz diorite
quartz diorite porphyritic andesite quartz diorite
Location in Fig. 1 sample No.
H-74-2A H-74-2B H-74-2C H-74-2D H-74-3 H-74-4A H-74-4B H-74-5 H-74-6 H-74-6
H-74-7 H-74-7
H-74-8
H-74-9 H-74-10 H-74-11 H-74-12 H-74-13 H-74-14 H-74-14
H-74-15 H-74-16 H-74-17
total rock total rock total rock
total rock total rock total rock total rock total rock total rock biotite
total rock
total rock biotite
a b
a b a b
0.7048 0.7123 0.7115
0.7210 0.7056 0.7105
0.7044 0.7171 0.7168 0.7040 0.7172 0.7175 0.7359 0.7365
a b
total rock biotite
87Sr 86Sr * -+0.1%
0.7182 0.7263 0.7198 0.7249 0.7244 0.7181 0.7187
Duplicare sets
rock rock rock rock rock rock rock
total total total total total total total
Material analyzed
14.8 11.8 71.8
118 129 96.7 147 78.3 65.3
134
47.9
215 240 221 210 206 181 178 112 56.1
147 80.4 277
348 75.2 169 109 198 414
17.8
329
67.2 47.5 64.2 43.9 45.7 58.6 55.9 114 377
0.101 0.147 0.259
0.339 1.71 0.572 1.35 0.395 0.158
7.53
0.146
3.20 5.05 3.44 4.78 4.51 3.09 3.18 0.982 0.149
0.216 1.110 1.103
0.308
1.238 1.238 0.425
1.418 1.421
0.678 0.766 0.707 0.666 0.651
0.471 0.0298 0.0300
0.190
0.0175 0.0170 0.0204
0.0193 0.0193
0.0766 0.0533 0.0717 0.0486 0.0496
86Sr (um/g) -+1%
STRb (.am/g) +-1%
Rb (ppm) +-2%
Sr (ppm) +-2%
Isotope dilution
X-ray fluorescence Rb/Sr
Rubidium and strontium analytical results for crystalline rocks from the Chilean Andes between 45 ° and 46 °
TABLE 1
0.459 37.2 36.8
1.62
70.7 72.8 20.8
73.5 73.6
8.85 14.4 9.86 13.7 13.2
87Rb 86Sr
87Sr/86Sr
15 13
109
107
13 13 107
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 2
0.7045 z 0.7045 2 107 109
12 12
0.7057 +_ 0.0013 1
initial
100+- 6
Age ** (m.y.)
t~
H-74-18 H-74-18 H-74-19
quartz diorite quartz diorite
total rock biotite total rock
H-74-24A
H-74-22 H-74-23 H-74-23
granodiorite quartz diorite quartz diorite
granodiorite granodiorite
adamellite
granodiorite monzonite monzonite
total rock total rock
biotite total rock biotite
total rock total rock
total rock
total rock total rock biotite
total rock total rock biotite
H-74-24B H-74-24C • H-74-24B H-74-25 H-74-25 adamellite adameUite total rock
quartz-sericite-chlorite schist quartz-sericite schist quartz diorite quartz diorite
H-74-26A H-74-26B adameUite
total rock total rock
H-74-20 H-74-21 H-74-21
H-74-26C dacite porphyritic andesite
a b
a b
a b a b
1.1604 1.1653 0.7146
0.7051 0.7550 0.7523 0.7082
0.7639 0.7631 0.7191 0.7291
1.1432 1.2107 0.7169 0.7176 0.7112 0.7150 0.7151 0.7343
a b
0.7302 0.7306
a b
a b
Normalized to 86Sr]88Sr ratio of 0.1194 Using h~ = 1.42 × 10 -11 y r - 1 . Calculated 87Sr/86Sr initial ratio (95% confidence level). Assumed 87Sr/86Sr initial ratio.
H-74-27 H-74-29 * ** 1 2
0.00469 0.00448
0.249 1.159 1.148
48.4 47.2
224
1.345
0.0278 0.0282
234 279 4.73
55.8
1.~32
0.00639 0.00535 0.0970
100 116
146 129 140
33.5
77.2 231
88.1 147 105
259
1.30 0.502
0.129
247 256
.1.497 1.491 0.459
0.163 0.118
11.9
0.675
0.411 0.445
0.0642
270 271 9.98 16.7
115
1.66
0.766
0.00353 0.00353 0.632 0.0345
17.4
77.6
0.878 1.33
0.953 0.957 0.631 0.577
0.0317
0.599 0.313
3.51 5.99
0.551
242 226
56.1 30.4
6.26
145 70.8
197 182
27.8
0.960 0.057
2.52 3.77
174
57.8 436 55.5 24.7
130 127
0.7045 2
0.7045 2
132 128
0.7044 -+ 0.0036 1
0.7045 2
0.7045 2
194 +_ 67
0.7045 2
73 71 70 (9.)
176
0.7045 2
0.7040 -+ 0.0019 I
15 15
106_+9
64 which may or may not represent the time of initial crystallization. In the Chilean Andes south of 50 ° S latitude and in the Argentine Andes at 40040 ' S, RbSr total-rock isochrons were obtained from several suites of plutonic rocks. The isochrons have calculated ages similar to the mineral ages and initial STSr/86Sr ratios of about 0.705 -+ 0.001 [6,9].
Sr87/Sr86
~
0.7300.7 250.720-
B
/
0.71 5-
/
0.7 I0-
A
/
0.7O5- /-----0.7057- 0£)033 0.700"
I
o
I
' 12
4;8,o
~.~:i
O.720.br zbr 0.715-
1'4
, R b s T / S , 8e
16
\ff-~:~"
\oj& .- ' ~ - . - e24A
0 7 I0
/ "~4B
B
/ 0.705-/_._._0.7044+ 0.0036 07[00, RbS?/SrB6
o87,,o86
O.735or
/at
0.730-
~
0.7250.720 0.71 5-
/
~.oJ~/~126 c
"-26A
C_
/
0.710- / 0.705- / - ~ 0 . 7 0 4 0 - + 0.0019 0.700 . . . .
o
26B
;,
5. Conclusions
,R
Sr8s
; s Lo i21416 is bS'/
Fig. 2. Total-rock isochrons for southern Chile plutonic igneous rocks. A. Simpson River (45°29 ' S, 72°15 ' W). B. Victoria Island (45°11P S, 73051 ' W). C. La Paloma Lake (45056 '
S, 72012' W).
Table 1 lists the results of the rubidium and strontium analyses of rocks collected across the Chilean Andean orogen between latitudes 45 ° and 46 ° S (Fig. 1). Most of the plutonic rocks have Rb/Sr ratios less than one and therefore could not be used for precise total-rock Rb-Sr dates with the mass spectrometer used in this study. Total-rock isochrons (Fig. 2) were obtained for the adamellite and granodiorite plutons which crop out along the Simpson River (Fig. 1, locations 2 and 3), along the east shore of Victoria Island (Fig. 1, location 24), and along the south shore of La Paloma Lake (Fig. 1, location 26). In addition, biotite was analyzed from representative plutons across the transect (Fig. 1, Table 1). The RbSr age of 100 + 6 m.y. for the Simpson River adamellite agrees with the local stratigraphy as the pluton contains roof pendants of Neocomian sedimentary rocks. Total-rock samples from Victoria Island give model ages of 190 + 11 m.y., 187 + 20 m.y., and 197 + 12 m.y. assuming an exact initial aTSr/86Sr ratio of 0.7045, no error in the aVRb/86Sr ratio, and 0.1% precision for the present-day 87Sr/S6Srratio. These dates are somewhat similar to the 176 m.y. calculated age for biotite sample H-74-24B which implies that the Rb-Sr biotite dates may approximate the time of crystallization of the magmas. In the Puerto Aisen area, the quartz diorite plutons have a mineral orientation when viewed over distances of tens of meters. The sample collected at location 6 (Fig. 1), when examined in thin section, shows that although the minerals are fresh they have been strongly deformed. The quartz exhibits undulose extinction and the extinction of the plagioclase feldspar and biotite is strained. The Miocene biotite dates for the quartz diorite collected at locations 6, 7, 14 and 25 are considered minimum ages associated with probable local structural deformation. The radiometric dates suggest that the plutonic activity between 45 ° and 46 ° S is of Mesozoic and Cenozoic age.
Fig. 3 is a compilation of radiometric dates of plutonic rocks of the Argentina and Chile Andean Cordillera between latitudes 40 ° and 46 ° S. The Argentine samples are from a north-south traverse and the Chilean samples from an east-west transect.
65
40os
75°W
74'
7:3°
72 °
71°W - 3os
41<
~'2'
43
4z
4,'
6. Speculation Halpern [11] in a review of the geochronologic evolution of the southern half of South America postulated a westward migration of 150 m.y. to 300 m.y. radiometric age provinces throughout Phanerozoic time. It was suggested that the present sialic crust of interior southern South America evolved as a result of igneous and metamorphic processes responding to plate motions involving collision of a Gondwanaland plate and a paleo-Pacific oceanic plate. Granitic plutons related to Paleozoic subduction are presently marked by geological features such as the Pampean Ranges of Argentina. Following fragmentation of Gondwanaland, the South American plate moved westward overthrusting the Pacific (Nazca) plate. Magmatic rocks derived from the partial melting of ocean lithosphere, subcontinental mantle, and/or pelagic sediments were and are presently located along the leading edge of the South American plate. This model postulates that the age and geographic location of the magmatic rocks is controlled by the position and angle o f dip of the subduction zone. The radiometric dates presented in this paper agree, at least to a first approximation with such a model.
Acknowledgements 4E,
75 ° 74° 73 ° 72 ° 7~VV Fig. 3. Rb-Sr and K-Ar radiometric ages in million years of plutonic igneous rocks from the southern Andes Mountains of Argentina and Chile. Open circles are K-Ar dates, solid circles are Rb-Sr dates, a = amphibole, b = biotite, t = total rock. The Argentine dates are taken from papers by Toubes and Spikermann [10] and Halpern et al. [9]. Samples A and B were analyzed at the Instituto de Investigacions Geologicas (Chile) by F. Munizaga. Biotite A, from a diorite, has a 87Sr/ 86Sr ratio of 0.9006 and a STRb/86Sr ratio of 294;biotite B, from a granodiorite, has a 87Sr/86Sr ratio of 0.7673 and a 87Rb/86Sr ratio of 37.7.
Admittedly the sampling density is sparse and only a few total rock Rb-Sr isochron ages are available;however, there is no indication of simple unidirectional migration of intrusive foci. The plutons constitute a batholithic complex in which granitic rocks of predominantly diorite to granodiorite composition have been emplaced over periods of about 200 m.y.
We thank K. Kawashita and U.G. Cordani of the Centro de Pesquisas Geocronologicas (University of S~'o Paulo, Brasil) for providing the X-ray fluorescence analyses, M. Ruiz (University of Texas at Dallas) for the STSr/S6Sr analysis of samples H-74-4A, 4B, 10, 12, 19, and 22, J.F. Reilly II (University of Texas at Dallas) for assistance in the preparation of the samples. The lnstituto de Investigaciones Geologicas (Chile) supported the field work and the National Science Foundation (U.S.A.) provided M.H.'s airfare and miscellaneous expenses to Chile under grant No. GV28757 A1. This paper was written while M.H. was a Visiting Professor at the Department o f Geophysics and Planetary Sciences, Tel-Aviv University; the assistance of the faculty and staff is gratefully acknowledged. This research is part of Project No. 120, Magmatic Evolution of the Andes, of the International Geological Correlation Program. Geosciences Program, The University o f Texas at Dallas, Contribution No. 350.
66 References 1 E. Farrar, A.H. Clark, S.J. Haynes, G.S. Quirt, H. Conn and M. Zentilli, K-Ar evidence for the post-Paleozoic migration of granitic intrusion foci in the Andes of northern Chile, Earth Planet. Sci. Lett. 10 (1970) 6 0 - 6 6 . 2 A.H. Clark, E. Farrar, J.C. Caelles, S.J. Haynes, R.B. Lortie, S.L. McBride, G.S. Quirt, R.C.R. Robertson and M. Zentilli, Longitudional variations in the metallogenetic evolution of the central Andes: a progress report, Geol. Assoc. Can., Spec. Paper No. 14 (1976) 2 3 - 5 8 . 3 D.E. James, C. Brooks and A. Cuyubamba, Strontium isotopic composition and K, Rb, Sr geochemistry of Mesozoic volcanic rocks of the central Andes, Annu. Rep. Director Dep. Terrestr. Magnet., Carnegie Inst. Washington (1974) 9 7 0 - 9 8 3 . 4 D.E. James, C. Brooks and A. Cuyubamba, Andean Cenozoic volcanism: magma genesis in the light of strontium isotopic composition and trace-element geochemistry, Geol. Soc. Am. Bull. 87 (1976) 5 9 2 - 6 0 0 . 5 M. Halpern, Geological significance of Rb-Sr isotopic data
6 7
8 9
10
11
of north Chile crystalline rocks of the Andean orogen between latitudes 23 and 27 degrees south, Geol. Soc. Am. Bull. 89 (1978) 5 2 2 - 5 3 2 . M. Halpern, Regional geochronology of Chile south of 50 ° latitude, Geol. Soc. Am. Bull. 84 (1973) 2407-2422. F. Munizaga, L. Aguirre and F. Herv6, Rb/Sr ages of rocks from the Chilean metamorphic basement, Earth Planet. Sci. Lett. 18 (1973) 8 7 - 9 2 . D. York, Least-squares fitting of a straight line, Can. J. Phys. 44 (1966) 1079-1086. M. Halpern, P.N. Stipanicic and R.O. Toubes, Geocronologia (Rb/Sr) en los Andes Australes Argentinos, Rev. Asoc. Geol. Argentina, 30 (1975) 180-192. R.O. Toubes and J.P. Spikermann, Algunas edades K/Ar y Rb/Sr de plutonitas de la Cordillera Patagonica entre los paralelos 40 ° y 44 ° de latitud sur, Rev. Asoc. Geol. Argentina 28 (1973) 382-396. M. Halpern, Geochronologic evolution of southern South America, in: Proc. Int. Symp. on the Carboniferous and Permian Systems in South America, An. Acad. Bras. Cienc. 44, Supplement (1972) 149-160.