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Rb-Sr and K-Ar evidence for the presence of a 1.6 Ga basement underlying the 1.2 Ga Garzón-Santa Marta granulite belt in the Colombian Andes
Precambrian Research, 42 (1989) 315-324 Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
315
Rb-Sr AND K-Ar EVIDENCE FOR THE PRESENCE OF A 1.6 Ga BASEMENT UNDERLYING THE 1.2 Ga GARZON-SANTA MARTA GRANULITE BELT IN THE COLOMBIAN ANDES H.N.A. P R I E M 1, S.B. K R O O N E N B E R G 2, N.A.I.M. B O E L R I J K 1 and E.H. H E B E D A 1 lz w o Laboratorium voor Isotopen-Geologie, De Boelelaan 1085, 1081 HV Amsterdam (The Netherlands) 2Department of Soil Science and Geology, Agricultural University, P.O. Box 37, 6700 AA Wageningen (The Netherlands)
(Received May 27, 1987; revisionacceptedJanuary 11, 1988)
Abstract Priem, H.N.A., Kroonenberg, S.B., Boelrijk, N.A.I.M. and Hebeda, E.H., 1989. Rb-Sr and K-Ar evidence for the presence of a 1.6 Ga basement underlyingthe 1.2 Ga Garzdn-Santa Marta granulite belt in the ColombianAndes. Precambrian Res., 42: 315-324. Rb-Sr and K-Ar investigationsof rocks from the Garz6n Massif in the northern Andes indicate the presence of three chronologicalunits: (1) an ~ 1.6 Ga old basement of augen gneisses showingages and lithologysimilar to the adjacent parts of the Guiana Shield, (2) a sequence of supracrustals metamorphosedto the granulite facies around 1.2 Ga ago, and (3) a set of pegmatite dykes intruded into the other units ~850 Ma ago. It is suggested that the supracrustal protoliths of the granuliteswere deposited in a Cordillera-typecontinental marginof the Guiana Shield, cratonized in the Parguazan tectonomagmaticevent, and that the 1.2 Ga orogenic event in the Andes was of the continental collisiontype. The latter event is tentatively correlatedwith the GrenvilleOrogenyin North America.
Introduction Precambrian basement is exposed in several parts of the Central and Eastern Cordilleras of the Colombian Andes. The largest exposure is the Garzdn Massif, a vertically u p t h r u s t block covering about 10 000 km 2 in the Eastern Cordillera, 300 km south of Bogot~i. It is partly covered by Palaeozoic and Cretaceous sedimentary rocks and locally intruded by Triassic-Jurassic granitic plutons (Fig. 1; Kroonenberg and Diederix, 1982). W i t h i n this block two main lithological units are distinguished (Fig. 2 ), the Garzdn Group consisting of granulite-facies supracrustal rocks, and the Guapotdn and Mancagua augen gneisses. The latter are
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homogeneous orthogneisses which were first interpreted as syntectonic granites (Kroonenberg, 1982a,b, 1985 ), but are now considered by the authors to be older basement underlying the GarzSn Group supracrustals on the basis of the geochronological data presented in this study (see below). The GarzSn Group occupies the major part of the Garzdn Massif. Charnockitic and enderbitic granulites constitute the greater part of the Garz6n Group. Minor components include (semi-)pelitic gneisses and granulites, mafic granulites and amphibolites, calc-silicate rocks and ultramafic rocks. The bulk of the sequence has a quartzofeldspathic composition with calc-alkaline affinities. T h e y are interpreted to have been
© 1989 Elsevier Science Publishers B.V.
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Fig. 1. Preemnbrian of northwestern South America. Crosses: Guiana Shield. Black: Garzdn-Santa Marta granulite belt. (1: Garzdn Massif; 2: Sierra Nevada de Santa Marta; 3: Santandsr Massif; 4: Guajira Peninsula; 5: San Quintin area, AroaMision block, Estado Yaraeuy; 6: Bonaire, Netherlands Antilles). Stippled: Palaeozoic and Mesozoic. Blank: Cenozoic. Box indicates location of Fig. 2.
derived from supracrustal volcanics of the basalt-andesite-rhyolite association, rather similar to the Jurassic and Cenozoic volcanic sequences in the Colombian Andes. This supracrustal nature is clear from the intercalation of unambiguous metasedimentary rocks such as sillimanite gneisses and phlogopite marbles. Metamorphic conditions attained the granulite facies at intermediate pressures but locally amphibolite-facies assemblages have been found, probably reflecting later retrogradation. Alvarez and Cordani (1980; see also A1varez, 1981) reported a Rb-Sr whole-rock isochron age of 1180 Ma for five granulite samples from the Garz6n Group.
The Guapotdn and Mancagua hornblendebiotite augen gneisses occur along the prominent N E - S W reverse fault bordering the Garzdn Massif to the west (Fig. 2). Although they do not contain granulite-facies minerals, the strongly perthitic-antiperthitic nature of the feldspars suggests that they have also undergone granulite-facies metamorphism. Apart from these two major rock units, many cross-cutting pegmatite veins occur in the Garz6n Massif. The veins contain magnetite or biotite as the main mafic mineral. On the basis of the geochronological and lithological similarities between the Garzdn Massif and other Precambrian outcrops in the
317
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318 TABLE I Investigated samplesa Sample no.
Location
Rock type
CIA 1, 3 CIA 2 CIA 4 CIA 5, 13 CIA 6, 8, 15 CIA 7, 11 CIA 9 CIA 10 CIA 12 CIA 14 CIA 16
Quebrada Aguacaliente near Finca La Yunga, ~ 8 km south of Garzdn
Biotite-hornblende granite-gneiss (banded granulites) Clinopyroxene amphibolite (granulites) Phlogopite marble Hornblende granite-gneiss (augen gneisses) Hornblende-biotite granite-gneiss (augen gneisses) Amphibolite (banded granulites) Magnetite pegmatite Gneissose microgranite (augen gneisses) Pegmatite Garnet aplite (banded granulites) Trondhjemitic biotite gneissb Epidote-quartz-albite rock b
CIA 17
Rio Suaza, ~ 1 km north of Guadalupe
Rio Pa~z, along road La Plata-Inz~i, 4 km north of La Plata Rio Pa~z, along road La Plata-Inz~i, ~ 3 km north of La Plata
aBetween brackets, one of the two main units into which the Garzdn Massif is divided: 'augen gneisses' (allegedly basement ) and 'banded granulites' (allegedly the metamorphic volcanic-sedimentary sequence overlying the basement). t~Metamorphic rocks, allegedly Precambrian; not belonging to the Garz6n Massif.
Colombian Andes, Kroonenberg (1982a) postulated that they collectively constitute a separate orogenic belt of 1.2-1.0 Ga age, along the western border of the older Guiana Shield. This Garz6n-Santa Marta Belt has been correlated with the Grenville Belt of North America (Kroonenberg, 1982a). The adjacent parts of the Guiana Shield in the Colombian Amazonas consist essentially of granitic rocks and amphibolite-facies quartzofeldspathic migmatitic gneisses, collectively designated as the Mitfi Migmatitic Complex (Huguett et al., 1979). Rb-Sr whole-rock analyses indicate that this complex was formed by granitic plutonism and metamorphic reconstitution of older material 1560-1450 Ma ago (Priem et al., 1982 ). This event was termed the Parguazan tectonomagmatic episode by Priem et al. (1982), after the anorogenic Parguaza rapakivi granite in Venezuela dated at 1500 Ma by Gaudette et al. (1978). It affected much of the western and southern Guiana Shield, essentially the Rio Negro-Juruena Belt as defined by Tassinari (1981; cf. also Cordani and De Brito Neves, 1982). Resetting of K-Ar and Rb-Sr systems in micas, hornblendes and feld-
spars occurred in the whole complex and in much of the Guiana Shield 1350-1250 Ma ago (Priem et al., 1971, 1982; Gaudette et al., 1978 ), and is related to the Nickerie Metamorphic Episode of low-grade metamorphism and mylonitization in western Suriname (Priem et al., 1971). It has been suggested that this episode correlates with the 1.2 Ga orogeny at the western margin of the Guiana Shield (Kroonenberg, 1982a). Rb-Sr and K-Ar investigations on rock samples and separated minerals from the Garz6n Group and the Guapot6n augen gneisses were carried out to clarify the details of the age and orogenic history of the Garz6n Massif, and to establish correlations with the Guiana Shield and with other outcrops of Late Proterozoic high-grade rocks in both South and North America. The investigated samples are listed in Table I.
Experimental procedures and constants The utilized techniques are Rb-Sr whole-rock dating, Rb-Sr and K-Ar mica dating and K-Ar hornblende dating.
319
Rb-Sr contents and Rb/Sr ratios of whole rocks were measured on pressed powder pellets by X-ray fluorescence spectrometry, using a Philips PW 1450/AHP automatic spectrometer. Mass absorption corrections for both sample and external standard are based upon the Compton scattering of the MoK~ primary beam (Verdurmen, 1977). For micas, the Rb and Sr contents were measured by isotope dilution. The isotopic composition was determined directly on unspiked Sr for the whole rocks by means of a Finnigan MAT 261 mass spectrometer. For the micas, the Rb and Sr analyses were made on a Varian MAT CH5 mass spectrometer. The K contents were determined by flame photometry with a Li internal standard and caesium chloride-aluminium nitrate buffer. Ar was extracted in a bakeable glass vacuum apparatus and determined by isotope dilution under static conditions in a Varian GD-150 mass spectrometer. The analytical accuracy (la) is believed to be within 0.5% for XRF Rb/Sr, 1.0% for isotope dilution Rb/Sr, 0.05% and 0.02% for SVSr/ SSSr measured on the Varian MAT CH5 and the Finnigan MAT 261 mass spectrometer, respectively, and 1.0% for K. These estimated overall limits of relative error are the sum of the known sources of possible systematic error and the precision of the total analytical procedures. For radiogenic Ar, the error (la) is calculated on the basis of standard deviations of 1% in atmospheric 4°Ar/3SAr and 1.5 % in the tracer calibration. Best-fit lines through the suites of RbSr whole-rock data were calculated by means of a least-squares regression analysis according to York (1966, 1967). The values of the mean squares of weighted deviation (MSWD) were calculated according to McIntyre et al. (1966). Throughout this paper the age calculations are based upon the constants ;tS7Rb= 1.42×10 -11 a -1, 2e4°K=0.581×10 -1° a -1, ;t~°K = 4.962 × 10- l°a- 1 and abundance 4°K--0.01167 atom % total K.
on samples from all three rock units collected at two different sites: the Quebrada Aguacaliente near Finca La Yunga, ~ 8 km south of Garzdn, and the Rio Suaza, ~ 1 km north of Guadalupe. Also, two samples were analysed from alleged Precambrian basement in the La Plata Massif along the Rio P~iez north of La Plata, in the Central Cordillera west of Garzdn (Fig. 2, Table I). Rb-Sr whole-rock data were obtained from six samples belonging to the Guapotdn augen gneiss (whole rocks), all from the location in the Rio Suaza (CIA 5, 6, 8, 10, 13, 15) and three gneisses belonging to the Garzdn Group, two from the Quebrada Aguacaliente (CIA 1, 3 ) and one from the Rio Suaza (CIA 14). Rb-Sr data
Results
were also obtained of two pegmatites from the Rio Suaza, one K-feldspar (CIA 9) and one whole rock (CIA 12). The analytical data are
Isotopic age measurements were performed
T A B L E IIA Rb-Sr whole-rock data Sample Rb Sr Rb/Sr S7Sr/~Sr 87Rb/~Sr no. (ppmwt.) (ppmwt.) (wt/wt.) Basement rocks CIA 1 162 CIA 3 151 CIA 4 216 CIA 5 99.2 CIA 6 136 CIA 8 154 CIA 10 78.7 CIA12 328 CIA 13 99.4 CIA 14 187 CIA 15 142 CIA i6 111 CIA17 (1.7)
169 175 79.3 252 368 113 309 94.1 334 120 434 591 450
0.9554 0.8620 2.7428 0.3944 0.3696 1.364 0.2544 3.485 0.2980 1.561 0.3276 0.1882 (0.0039)
0.749165 0.745809 0.822126 0.727800 0.723038 0.794575 0.721156 0.832535 0.724545 0.796238 0.720536 0.707745 0.707429
2.775 2.503 8.025 1.143 1.071 3.980 0.7369 10.21 0.8636 4.555 0.9490 0.5445 (0.0112)
TABLE IIB Rb-Sr whole-rock data (Alvarez, 1981) Sample no.
Rb (ppm wt. )
Sr (ppm wt.)
S7Sr/S~Sr
8VRb/seSr
JAA-1174 JAA-1176A JAA-1177 JAA-1179 JAA-1180
116 295 179 130 131
334 277 227 515 370
0.7210 0.7315 0.7428 0.7145 0.7207
1.00 3.10 2.30 0.73 0.94
320
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Fig. 3. Rb-Sr data plot. O: augen gneisses; • and m: banded
granulites (•: this study; ,: Alvarez, 1981); ~: pegmatites (Kf:K-feldspar);O: gneissesfrom the Rio P~ez, outside the Garzdn Massif. Best-fitlines are shown through the augen gneisses (sixsamples),the banded granulites(six samples) and the pegmatites (two samples). T w o other banded granulites (a and []) are not included in the calculation.
listed in Table IIA and plotted in Fig. 3. The data reported by Alvarez (1981) are listed in Table IIB and also plotted in Fig. 3. The six Guapot6n augen gneisses display a crude linear array corresponding to an age of 1596_+ 300 Ma with initial STSr/S6Sr =0.702_+ 0.005 (2a) (both 2a). However, this array is essentially determined by two points, the high datum point CIA 8 on the one side and a group of five low data points on the other side. Of the eight samples of the Garz6n Group granulites (three in this study and five reported by A1varez (1981)), six define a line (errorchron, M S W D = 8 ) corresponding to an age of 1172+_90 Ma with initial STSr/S6Sr= 0.704+0.002 (2a) (both 2a). Two points plot outside this line, one above (CIA 14; this study) and one below (JAA-1176A; Alvarez, 1981 ). A1varez (1981) has reported a best-fit line through four of his data points corresponding to an age of 1180 Ma with initial STSr/S6Sr=0.704. The two pegmatite samples define a line corresponding to an age of 847 Ma with initial 87Sr/ 868r = 0.709.
Three hornblendes from amphibolites of the
T A B L E III
Rb-Sr mica/whole-rock ages Sample no.
Rb" (ppm wt.)
Sr a (ppm wt.)
CIA 4 P h l
398 398 217 497 497 136 453 456 587 588 111
79.5 79.6 79.3 30.7 31.0 368 197 197 8.41 8.42 591
WR CIA 6 Bio WR CIA 9 Kfsp CIA 16 Bio WR
Rb/Sr a
2.7428
0.3696
0.1882
87Sr/S6Srb
87Rb/S6Sr
0.91009 0.91083 0.822126 0.98446 0.98187 0.723038 0.79063 0.79069 1.1157 1.1112 0.707745
14.76 14.76 8.025 48.09 47.62 1.071 6.716 6.763 210.0 209.8 0.5445
Calculated age (Ma) c 918_+27
390 + 12 895 _+16 d 136_+4
"X-Ray fluorescence spectrometry for the whole rocks and isotope dilution for the micas. bDirect measurement on unspiked sample for the whole rocks (Finnigan MAT 261 ) and calculated from the isotope dilution run for the micas (Varian MAT CH5)~ tErrors based upon estimated errors of 1.0% in isotope dilution Rb and Sr, 1.0% in XRF Rb/Sr, 0.04% in 87Sr/86Sr micas and 0.005% in 87Sr/86Sr whole-rocks. dAssumed initial 87Sr/S~Sr of 0.705.
321 T A B L E IV K-Ar mineral data Sample no.
Mineral
K (ppm wt. )
Radiogenic 4 ° A r (ppb wt. )
Atmospheric 4 ° A r (% total 4°At)
CIA 2
Hornblende
CIA 4
Phlogopite
CIA 6
Biotite
CIA 7
Hornblende
CIA 11
Hornblende
0.436 0.438 8.32 8.34 4.57 4.56 1.00 1.00 1.02 1.03
CIA 16
Biotite
39.20 38.75 709.4 661.7 72.30 72.87 87.33 87.56 97.40 90.28 94.93 96.94 78.19 80.31
4 13 2 2 13 13 2 8 1 7 7 4 9 17
7.92 7.92
Calculated age (Ma) a 971+- 19 912 +-35 216+_ 5 955 +_19 1000 +_25
139 +_ 4
aFor the errors (la) in the ages, a standard deviation of 1% is taken for K, while the standard deviation of radiogenic Ar is based upon standard deviations of 1% in atmospheric 4°Ar/~6Ar and 1.5% in the tracer calibration.
Garzdn Group (CIA 2, 7, 11) have K-Ar dates between 1000 and 955 Ma, averaging 975 + 12 Ma (errors in all mineral ages are la). Phlogopite from a marble (CIA 4) has Rb-Sr (mineral/whole rock) and K-Ar dates concordant at 915+21 Ma (Tables III, IV). Biotite from (Guapotdn) granite-gneiss CIA 6 yields discordant Rb-Sr (mineral/whole rock) and K Ar dates of 390 + 12 and 216_+ 5 Ma, respectively (Tables III, IV). The Rb-Sr whole-rock data of two samples from Precambrian basement in the Rio P~iez west of the Garzdn Massif (CIA 16, 17) are inconclusive. Biotite from one of the samples, trondhjemitic gneiss CIA 16, yields Rb-Sr (mineral/whole rock) and K-Ar dates concordant at 138_+ 3 Ma.
Discussion The six Rb-Sr whole-rock data of the Guapotdn augen gneisses do not allow for an unequivocal conclusion regarding the age, owing to the scatter of the data points and the unfavourable spread in R b / S r of the samples. How-
ever, the best-fit line of 1596 + 300 Ma could very well represent a geologically feasible age in view of the abundance of 1560-1450 Ma old augen gneisses of similar lithology in the adjacent part of the Guiana Shield (Huguett et al., 1979; Priem et al., 1982) and the widespread occurrence of basement rocks of this age range (related to the Parguazan tectonomagmatic episode) in the Guiana Shield of Colombia, Venezuela and Brazil (Tassinari, 1981; Priem et al., 1982; Gaudette and Olszewski, 1985). The Guapotdn augen gneisses are therefore interpreted to represent a Parguazan basement underlying the supracrustals of the Garzdn Group, contrary to the earlier assumption by Kroonenberg ( 1982a, 1982b, 1985 ) that they would have been derived from younger, syntectonic granites. The age of 1172 + 90 Ma defined by six of the eight data points from the Garzdn Group granulites is taken to date the granulite-facies metamorphism. This event has also been demonstrated in granulitic rocks in the Sierra Nevada de Santa Marta in northern Colombia (MacDonald and Hurley, 1969 ). The volcanic-sedimentary precursors of the
322 granulites in the Garz6n Massif should thus have been deposited between the formation of the Parguazan basement and the event of highgrade metamorphism. However, the low initial STSr/S6Sr ratio suggests that the metamorphism has taken place shortly after the deposition. The presence of the Parguazan basement, the calc-alkaline nature of the granulites and the scarcity of mafic volcanic rocks all concur in suggesting a Cordillera-type continental margin setting for the deposition of the protoliths of the GarzSn Group granulites. The orogenic event itself that gave rise to the metamorphism, however, is probably of the continental collision type (Kroonenberg, 1982a) since its effects are reflected to a lesser extent throughout the whole western part of the Guiana Shield by partial mica rejuvenation (down to 1200 Ma), prominent shearing and low-grade metamorphism (Nickerie Metamorphic Episode; Priem et al., 1971, 1982). The 975 + 12 Ma hornblende and 915 + 21 Ma phlogopite dates probably record 'cooling ages' following the 1172+_90 Ma old high-grade metamorphism. The hornblendes, having the highest 'closure temperatures' ( ~ 550-490 ° C, Hart et al., 1968; Andriessen, 1978), display ages most nearly approaching the peak of the metamorphism. Hornblende K - A t dates of 955 +_40 Ma from the Bucaramanga gneiss in the Santander Massif (Goldsmith et al., 1971) and of 949 +_30 Ma from the Santa Marta granulites in the Sierra Nevada de Santa Marta (Tschanz et al., 1974 ) may also be interpreted as cooling ages, as was the 920 +_50 Ma hornblende K-Ar date from the Garz6n Massif reported by A1varez and Linares (1985). The pegmatites yield the youngest dates, in harmony with their cross-cutting relationship with the other two rock units. The concordancy of the Rb-Sr and K-Ar dates of biotite CIA 16 in the La Plata Massif suggests that the age of 138 +_3 Ma (Early Cretaceous) has a geological significance, possibly reflecting an event related to Andean moun-
tain-building processes. The Precambrian outcrops in this area are small fragments amidst large bodies of Jurassic intrusive rocks, for which K-Ar and Rb-Sr dates around 180 Ma are available (Alvarez and Linares, 1983; Priem et al., in preparation). Small Mesozoic intrusions also occur in the Garzdn Massif. The discordant Rb-Sr and K-Ar dates of CIA 6, in the GarzSn Massif, could reflect varying degrees of incomplete isotopic resetting (but weaker than in the La Plata Massif), during heating by nearby Mesozoic intrusions. Conclusions Following the above interpretations, orogeny in the upthrust Precambrian Garzdn Massif in the Colombian Andes should have started in the Mid-Proterozoic. After the cratonization of the Guiana Shield during the Parguazan tectonomagmatic event ~ 1.6 Ga ago, subduction occurred along its western margin. This gave rise to a calc-alkaline volcanic series and deposition of pelitic and carbonate sediments in a Cordillera-type setting, not unlike the present one. A continental collision ~ 1.2 Ga ago produced not only deformation and high-grade metamorphism, but also appreciable thermal and tectonic effects in the Guiana Shield hinterland. The presence of Parguazan basement in the Garzdn Massif suggests that not much continental accretion, if any, took place during this orogeny. The high-grade metamorphism was followed by uplift and cooling between 975 and 915 Ma, followed by pegmatitic intrusion ~ 850 Ma ago. Most other Precambrian fragments distinguished so far in the Colombian Andes (see review by Kroonenberg, 1982a) seem to have shared the same orogenic history. Another possible element of the 1.2 Ga orogenic belt is the Aroa-MisiSn block of Case et al. (1984) in the Venezuelan coastal ranges (Fig. 1). The undated San Quintin anorthosites with ilmenitemagnetite-hematite mineralization in this area, described by Rodriguez and Afiez (1978), are
323
lithologically remarkably similar to the anorthosites described by Tschanz et al. (1974) in the Sierra Nevada de Santa Marta in Colombia. Furthermore, pebbles of ll50-Ma-old granulites found in Early Tertiary fluviatile conglomerates on the island of Bonaire, Netherlands Antilles, also testify to the presence of coeval rocks on the northern South American mainland, possibly in the Guajira Peninsula (Priem et al., 1986). Further south the coeval Rondonian/Suns~s belt follows the western margin of the Brazilian Shield, down to eastern Bolivia (Leal et al., 1978; Cordani and De Brito Neves, 1982; Teixeira and Tassinari, 1984; Litherland et al., 1985). Hence, the whole western margin of the Amazonian Craton appears to have been affected by the 1.2 Ga orogeny, showing trends which roughly parallel those of the present Andes (Litherland et al., 1985). Further north, rocks of similar lithology and age are found in the Oaxaca metamorphic complex in Mexico (Patchett and Ruiz, 1987) and in the Grenville Province of North America (Moore et al., 1986). Kroonenberg (1982a) speculates, on the basis of the strong lithological and geochronological similarity of the Garzdn-Santa Marta belt to the Grenville Province, and their occurrence at opposite margins of older basement cores, that a continental collision occurred between the western margin of the Guiana Shield and the eastern margin of the Canadian Shield around 1.2 Ga. Paleomagnetic research on the Colombian Andean Precambrian could shed more light upon this question.
Acknowledgements This paper is dedicated to the memory of Allan K. Gibbs, an outstanding Precambrian geologist and a good friend. This study would not have been possible without the efforts of the entire staff of the ZWO laboratory of Isotope Geology, Amsterdam. The authors are particularly indebted to the staff members P.A.M. Andriessen, E.A.Th. Verdurmen and R.H.
Verschure for discussion and comments. E.A.Th. Verdurmen also supervised the X-ray fluorescence spectrometric and flame photometric analysis. Thanks are due to Dr. Hern~n Rivera Hermida of the Centro InterAmericano de Fotointerpretacidn, for field support, to Dr. Alfonso Ldpez Reina and Dra. Gloria Inds Rodriguez of INGEOMINAS and Dr. Rubdn Llin~s of the National University, all in Bogot~i, for permission to use thin-section preparation facilities. Dr. H. de Boorder and Drs. H. Diederix are thanked for their continuous support and their assistance in the field. Mr. S. Frans is thanked for drawing the figures. This study forms part of the research programme of the 'Stichting voor Isotopen-Geologisch Onderzoek', supported by the Netherlands Organization for the Advancement of Pure Research (ZWO).
References Alvarez, J., 1981. Determinacidn de la edad Rb-Sr en rocas del Macizo de Garzdn, Cordillera Oriental, Colombia. Geol. norandina (BogotA), 4: 31-38. Alvarez, J. and Cordani, U.G., 1980. Precambrian basement within the septentrionalAndes: age and geological evolution. 26th Congr. Geol. Int.,Paris, Abstr., 1: 10. Alvarez, J. and Linares, E., 1983. Edad K-At del pluton granitoide de La Plata, Departamento del Huila (Colombia). Geol. norandina (BogotA), 7: 35-38. Alvarez, J. and Linares, E., 1985. Una edad K/At del Macizo de Garz6n, Departamento del Huila, Colombia. Geol. norandina (BogotA), 9: 31-33. Andriessen, P.A.M., 1978. Isotopic age relations within the polymetamorphic complex of the island of Naxos (Cyclades, Greece). Verh. No. 3, ZWO Laboratorium voor Isotopen-Geologie, Amsterdam, 71 pp. Case, J.E., Holcombe, T.L. and Martin, R.G., 1984. Map of geologicprovinces in the Caribbean region. Mere. Geol. Soc. Am., 162: 1-30. Cordani, U.G. and De Brito Neves, B.B., 1982. The geologic evolution of South America during the Archaean and Early Proterozoic. Rev. Bras. Geoc., 12: 78-88. Gaudette, H.E. and Olszewski, W.J., 1985. Geochronology of the basement rocks, Amazonas Territory, Venezuela, and the tectonic evolution of the western Guiana Shield. Geol. Mijnbouw, 64: 131-143. Gaudette, H.E., Mendoza, V., Hurley P.M. and Fairbairn, H.W., 1978. Geology and age of the Parguaza rapakivi granite, Venezuela. Bull. Geol. Soc. Am., 89: 1335-1340. Goldsmith, R., Marvin, R.F. and Mehnert, H.H., 1971. Ra-
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diometric ages in the Santander Massif, Eastern Cordillera, Colombian Andes, U.S. Geol. Surv. Prof. Paper, 750-D: 44-49. Hart, S.R., Davis, G.L., Steiger, R.H. and Tilton, G.R., 1968. A comparison of the isotopic mineral age variations and petrological changes induced by contact metamorphism. In: E.I. Hamilton and R.M. Farquhar (Editors), Radiometric Dating for Geologists. Interscience, New York, pp. 73-110. Huguett, A., Galvis, J. and Ruge, P., 1979. Geolog/a de la Amazonia colombiana. In: PRORADAM, La Amazonia Colombiana y sus recursos, Bogotel, 1: 29-92. Kroonenberg, S.B., 1982a. A Grenvillian granulite belt in the Colombian Andes and its relation to the Guiana Shield. Geol. Mijnbouw, 61: 325-333. Kroonenberg, S.B., 1982b. Litologla, metamorfismo y origen de las granulitas del Macizo de Garz6n, Cordillera Oriental (Colombia). Geol. norandina (Bogot~i), 6: 3946. Kroonenberg, S.B., 1985. E1 borde occidental del Escudo de Guayana en Colombia. Memoria Symposium Amaz6nico I, Puerto Ayacucho, Venezuela (1981). Boletin de Geologfa (Caracas) Publ. Especial 10: 55-64. Kroonenberg, S.B. and Diederix, H., 1982. Geology of South-Central Huila, Uppermost Magdalena Valley, Colombia. Guide Book Ann. Field Trip Col. Soc. Petrol. Geol. Geophys., 1-39. Leal, J.W.L., Silva, G.H., Santos, D.B., Teixeira, W., Lima, M.I.C., Fernandes, C.A.C. and Pinto, A.C., 1978. Geolog/a. In: Projeto Radambrasil Folha SC 20 Porto Velho, Rio de Janeiro. Levantamento de Recursos Naturais, 16: 1-161. Litherland, M., Klinck, B.A., O'Connor, E.A. and Pitfield, P.E.J., 1985. Andean-trending mobile belts in the Brazilian Shield. Nature, 314: 345-348. Macdonald, W.D. and Hurley, P.M., 1969. Precambrian gneisses from northern Colombia. Bull. Geol. Soc. Am., 80: 1867-1872. McIntyre, G.A., Brooks, C., Compston, W. and Turek, A., 1966. The statistical assessment of Rb-Sr isochrons. J. Geophys. Res., 71: 5459-5468. Moore, J.M., Davidson, A. and Baer, A.J. (Editors), 1986, The Grenville Province. Geol. Ass. Canada Spec. Paper, 31:358 pp. Patchett, P.J. and Ruiz, J., 1987. Nd isotopic ages of crust
formation and metamorphism in the Precambrian of eastern and southern Mexico. Contrib. Mineral. Petrol., 96: 523-528. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.Th. and Verschure R.H., 1971. Isotopic ages of the Trans°Amazonian acidic magmatism and the Nickerie Metamorphic Episode in the Precambrian basement of Suriname, South America. Bull. Geol. Soc. Am., 82: 1667-1680. Priem, H.N.A., Andriessen, P.A.M, Boelrijk, N.A.I.M., de Boorder, H., Hebeda, E.H., Huguett, A., Verdurmen, E.A.Th. and Verschure, R.H., 1982. Geochronology of the Precambrian in the Amazonas Region of southeastern Colombia (western Guiana Shield). Geol. Mijnbouw, 61: 229-242. Priem, H.N.A., Beets, D.J. and Verdurmen, E.A.Th., 1986. Precambrian rocks in an Early Tertiary conglomerate on Bonaire, Netherlands Antilles (southern Caribbean borderland): evidence for a 300 km eastward displacement relative to the South American mainland? Geol. Mijnbouw, 65: 35-40. Rodriguez, S.E. and Afiez, G., 1978. Los dep6sitos de mena titanffera de San Quintln Central, Estado Yaracuy; g~nesis, caracteres geol6gicos y estimaci6n de reservas. Min. Energ/a y Minas, Dir. General Sect. Minas y Geol., 13: 83-181. Tassinari, C.C.G., 1981. Evolu¢~o Geotect6nica da Provincia Rio Negro-Juruena na Regiho Amaz6nica. Diss. de mestrado, Inst. Geoci6ncias, Univ. Sho Paulo, 99 pp. Teixeira, W. and Tassinari, C.C.G., 1984. Caracterizaqho geocronol6gica da Provincia Rondoniana e suas implicaqSes geotect6nicas. Anais II Symposium Amazonico, Manaus, Brazil, pp. 87-102. Tschanz, C.M., Marvin, R.F., Cruz, J.B., Mehnert, H.H. and Cebula, G.T., 1974. Geologic evolution of the Sierra Nevada de Santa Marta. Bull. Geol. Soc. Am., 85: 273284. Verdurmen, E.A.Th., 1977. Accuracy of X-ray fluorescence spectrometric determinations of Rb and Sr concentrations in rock samples. X-Ray Spectr., 6: 117-122. York, D., 1966. Least-squares fitting of a straight line. Can. J. Phys., 44: 1079-1086. York, D., 1967. The best isochron. Earth Planet. Sci. Lett., 2: 479-482.
315
Rb-Sr AND K-Ar EVIDENCE FOR THE PRESENCE OF A 1.6 Ga BASEMENT UNDERLYING THE 1.2 Ga GARZON-SANTA MARTA GRANULITE BELT IN THE COLOMBIAN ANDES H.N.A. P R I E M 1, S.B. K R O O N E N B E R G 2, N.A.I.M. B O E L R I J K 1 and E.H. H E B E D A 1 lz w o Laboratorium voor Isotopen-Geologie, De Boelelaan 1085, 1081 HV Amsterdam (The Netherlands) 2Department of Soil Science and Geology, Agricultural University, P.O. Box 37, 6700 AA Wageningen (The Netherlands)
(Received May 27, 1987; revisionacceptedJanuary 11, 1988)
Abstract Priem, H.N.A., Kroonenberg, S.B., Boelrijk, N.A.I.M. and Hebeda, E.H., 1989. Rb-Sr and K-Ar evidence for the presence of a 1.6 Ga basement underlyingthe 1.2 Ga Garzdn-Santa Marta granulite belt in the ColombianAndes. Precambrian Res., 42: 315-324. Rb-Sr and K-Ar investigationsof rocks from the Garz6n Massif in the northern Andes indicate the presence of three chronologicalunits: (1) an ~ 1.6 Ga old basement of augen gneisses showingages and lithologysimilar to the adjacent parts of the Guiana Shield, (2) a sequence of supracrustals metamorphosedto the granulite facies around 1.2 Ga ago, and (3) a set of pegmatite dykes intruded into the other units ~850 Ma ago. It is suggested that the supracrustal protoliths of the granuliteswere deposited in a Cordillera-typecontinental marginof the Guiana Shield, cratonized in the Parguazan tectonomagmaticevent, and that the 1.2 Ga orogenic event in the Andes was of the continental collisiontype. The latter event is tentatively correlatedwith the GrenvilleOrogenyin North America.
Introduction Precambrian basement is exposed in several parts of the Central and Eastern Cordilleras of the Colombian Andes. The largest exposure is the Garzdn Massif, a vertically u p t h r u s t block covering about 10 000 km 2 in the Eastern Cordillera, 300 km south of Bogot~i. It is partly covered by Palaeozoic and Cretaceous sedimentary rocks and locally intruded by Triassic-Jurassic granitic plutons (Fig. 1; Kroonenberg and Diederix, 1982). W i t h i n this block two main lithological units are distinguished (Fig. 2 ), the Garzdn Group consisting of granulite-facies supracrustal rocks, and the Guapotdn and Mancagua augen gneisses. The latter are
0301-9268/89/$03.50
homogeneous orthogneisses which were first interpreted as syntectonic granites (Kroonenberg, 1982a,b, 1985 ), but are now considered by the authors to be older basement underlying the GarzSn Group supracrustals on the basis of the geochronological data presented in this study (see below). The GarzSn Group occupies the major part of the Garzdn Massif. Charnockitic and enderbitic granulites constitute the greater part of the Garz6n Group. Minor components include (semi-)pelitic gneisses and granulites, mafic granulites and amphibolites, calc-silicate rocks and ultramafic rocks. The bulk of the sequence has a quartzofeldspathic composition with calc-alkaline affinities. T h e y are interpreted to have been
© 1989 Elsevier Science Publishers B.V.
316
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Fig. 1. Preemnbrian of northwestern South America. Crosses: Guiana Shield. Black: Garzdn-Santa Marta granulite belt. (1: Garzdn Massif; 2: Sierra Nevada de Santa Marta; 3: Santandsr Massif; 4: Guajira Peninsula; 5: San Quintin area, AroaMision block, Estado Yaraeuy; 6: Bonaire, Netherlands Antilles). Stippled: Palaeozoic and Mesozoic. Blank: Cenozoic. Box indicates location of Fig. 2.
derived from supracrustal volcanics of the basalt-andesite-rhyolite association, rather similar to the Jurassic and Cenozoic volcanic sequences in the Colombian Andes. This supracrustal nature is clear from the intercalation of unambiguous metasedimentary rocks such as sillimanite gneisses and phlogopite marbles. Metamorphic conditions attained the granulite facies at intermediate pressures but locally amphibolite-facies assemblages have been found, probably reflecting later retrogradation. Alvarez and Cordani (1980; see also A1varez, 1981) reported a Rb-Sr whole-rock isochron age of 1180 Ma for five granulite samples from the Garz6n Group.
The Guapotdn and Mancagua hornblendebiotite augen gneisses occur along the prominent N E - S W reverse fault bordering the Garzdn Massif to the west (Fig. 2). Although they do not contain granulite-facies minerals, the strongly perthitic-antiperthitic nature of the feldspars suggests that they have also undergone granulite-facies metamorphism. Apart from these two major rock units, many cross-cutting pegmatite veins occur in the Garz6n Massif. The veins contain magnetite or biotite as the main mafic mineral. On the basis of the geochronological and lithological similarities between the Garzdn Massif and other Precambrian outcrops in the
317
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x ~ ~
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318 TABLE I Investigated samplesa Sample no.
Location
Rock type
CIA 1, 3 CIA 2 CIA 4 CIA 5, 13 CIA 6, 8, 15 CIA 7, 11 CIA 9 CIA 10 CIA 12 CIA 14 CIA 16
Quebrada Aguacaliente near Finca La Yunga, ~ 8 km south of Garzdn
Biotite-hornblende granite-gneiss (banded granulites) Clinopyroxene amphibolite (granulites) Phlogopite marble Hornblende granite-gneiss (augen gneisses) Hornblende-biotite granite-gneiss (augen gneisses) Amphibolite (banded granulites) Magnetite pegmatite Gneissose microgranite (augen gneisses) Pegmatite Garnet aplite (banded granulites) Trondhjemitic biotite gneissb Epidote-quartz-albite rock b
CIA 17
Rio Suaza, ~ 1 km north of Guadalupe
Rio Pa~z, along road La Plata-Inz~i, 4 km north of La Plata Rio Pa~z, along road La Plata-Inz~i, ~ 3 km north of La Plata
aBetween brackets, one of the two main units into which the Garzdn Massif is divided: 'augen gneisses' (allegedly basement ) and 'banded granulites' (allegedly the metamorphic volcanic-sedimentary sequence overlying the basement). t~Metamorphic rocks, allegedly Precambrian; not belonging to the Garz6n Massif.
Colombian Andes, Kroonenberg (1982a) postulated that they collectively constitute a separate orogenic belt of 1.2-1.0 Ga age, along the western border of the older Guiana Shield. This Garz6n-Santa Marta Belt has been correlated with the Grenville Belt of North America (Kroonenberg, 1982a). The adjacent parts of the Guiana Shield in the Colombian Amazonas consist essentially of granitic rocks and amphibolite-facies quartzofeldspathic migmatitic gneisses, collectively designated as the Mitfi Migmatitic Complex (Huguett et al., 1979). Rb-Sr whole-rock analyses indicate that this complex was formed by granitic plutonism and metamorphic reconstitution of older material 1560-1450 Ma ago (Priem et al., 1982 ). This event was termed the Parguazan tectonomagmatic episode by Priem et al. (1982), after the anorogenic Parguaza rapakivi granite in Venezuela dated at 1500 Ma by Gaudette et al. (1978). It affected much of the western and southern Guiana Shield, essentially the Rio Negro-Juruena Belt as defined by Tassinari (1981; cf. also Cordani and De Brito Neves, 1982). Resetting of K-Ar and Rb-Sr systems in micas, hornblendes and feld-
spars occurred in the whole complex and in much of the Guiana Shield 1350-1250 Ma ago (Priem et al., 1971, 1982; Gaudette et al., 1978 ), and is related to the Nickerie Metamorphic Episode of low-grade metamorphism and mylonitization in western Suriname (Priem et al., 1971). It has been suggested that this episode correlates with the 1.2 Ga orogeny at the western margin of the Guiana Shield (Kroonenberg, 1982a). Rb-Sr and K-Ar investigations on rock samples and separated minerals from the Garz6n Group and the Guapot6n augen gneisses were carried out to clarify the details of the age and orogenic history of the Garz6n Massif, and to establish correlations with the Guiana Shield and with other outcrops of Late Proterozoic high-grade rocks in both South and North America. The investigated samples are listed in Table I.
Experimental procedures and constants The utilized techniques are Rb-Sr whole-rock dating, Rb-Sr and K-Ar mica dating and K-Ar hornblende dating.
319
Rb-Sr contents and Rb/Sr ratios of whole rocks were measured on pressed powder pellets by X-ray fluorescence spectrometry, using a Philips PW 1450/AHP automatic spectrometer. Mass absorption corrections for both sample and external standard are based upon the Compton scattering of the MoK~ primary beam (Verdurmen, 1977). For micas, the Rb and Sr contents were measured by isotope dilution. The isotopic composition was determined directly on unspiked Sr for the whole rocks by means of a Finnigan MAT 261 mass spectrometer. For the micas, the Rb and Sr analyses were made on a Varian MAT CH5 mass spectrometer. The K contents were determined by flame photometry with a Li internal standard and caesium chloride-aluminium nitrate buffer. Ar was extracted in a bakeable glass vacuum apparatus and determined by isotope dilution under static conditions in a Varian GD-150 mass spectrometer. The analytical accuracy (la) is believed to be within 0.5% for XRF Rb/Sr, 1.0% for isotope dilution Rb/Sr, 0.05% and 0.02% for SVSr/ SSSr measured on the Varian MAT CH5 and the Finnigan MAT 261 mass spectrometer, respectively, and 1.0% for K. These estimated overall limits of relative error are the sum of the known sources of possible systematic error and the precision of the total analytical procedures. For radiogenic Ar, the error (la) is calculated on the basis of standard deviations of 1% in atmospheric 4°Ar/3SAr and 1.5 % in the tracer calibration. Best-fit lines through the suites of RbSr whole-rock data were calculated by means of a least-squares regression analysis according to York (1966, 1967). The values of the mean squares of weighted deviation (MSWD) were calculated according to McIntyre et al. (1966). Throughout this paper the age calculations are based upon the constants ;tS7Rb= 1.42×10 -11 a -1, 2e4°K=0.581×10 -1° a -1, ;t~°K = 4.962 × 10- l°a- 1 and abundance 4°K--0.01167 atom % total K.
on samples from all three rock units collected at two different sites: the Quebrada Aguacaliente near Finca La Yunga, ~ 8 km south of Garzdn, and the Rio Suaza, ~ 1 km north of Guadalupe. Also, two samples were analysed from alleged Precambrian basement in the La Plata Massif along the Rio P~iez north of La Plata, in the Central Cordillera west of Garzdn (Fig. 2, Table I). Rb-Sr whole-rock data were obtained from six samples belonging to the Guapotdn augen gneiss (whole rocks), all from the location in the Rio Suaza (CIA 5, 6, 8, 10, 13, 15) and three gneisses belonging to the Garzdn Group, two from the Quebrada Aguacaliente (CIA 1, 3 ) and one from the Rio Suaza (CIA 14). Rb-Sr data
Results
were also obtained of two pegmatites from the Rio Suaza, one K-feldspar (CIA 9) and one whole rock (CIA 12). The analytical data are
Isotopic age measurements were performed
T A B L E IIA Rb-Sr whole-rock data Sample Rb Sr Rb/Sr S7Sr/~Sr 87Rb/~Sr no. (ppmwt.) (ppmwt.) (wt/wt.) Basement rocks CIA 1 162 CIA 3 151 CIA 4 216 CIA 5 99.2 CIA 6 136 CIA 8 154 CIA 10 78.7 CIA12 328 CIA 13 99.4 CIA 14 187 CIA 15 142 CIA i6 111 CIA17 (1.7)
169 175 79.3 252 368 113 309 94.1 334 120 434 591 450
0.9554 0.8620 2.7428 0.3944 0.3696 1.364 0.2544 3.485 0.2980 1.561 0.3276 0.1882 (0.0039)
0.749165 0.745809 0.822126 0.727800 0.723038 0.794575 0.721156 0.832535 0.724545 0.796238 0.720536 0.707745 0.707429
2.775 2.503 8.025 1.143 1.071 3.980 0.7369 10.21 0.8636 4.555 0.9490 0.5445 (0.0112)
TABLE IIB Rb-Sr whole-rock data (Alvarez, 1981) Sample no.
Rb (ppm wt. )
Sr (ppm wt.)
S7Sr/S~Sr
8VRb/seSr
JAA-1174 JAA-1176A JAA-1177 JAA-1179 JAA-1180
116 295 179 130 131
334 277 227 515 370
0.7210 0.7315 0.7428 0.7145 0.7207
1.00 3.10 2.30 0.73 0.94
320
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granulites (•: this study; ,: Alvarez, 1981); ~: pegmatites (Kf:K-feldspar);O: gneissesfrom the Rio P~ez, outside the Garzdn Massif. Best-fitlines are shown through the augen gneisses (sixsamples),the banded granulites(six samples) and the pegmatites (two samples). T w o other banded granulites (a and []) are not included in the calculation.
listed in Table IIA and plotted in Fig. 3. The data reported by Alvarez (1981) are listed in Table IIB and also plotted in Fig. 3. The six Guapot6n augen gneisses display a crude linear array corresponding to an age of 1596_+ 300 Ma with initial STSr/S6Sr =0.702_+ 0.005 (2a) (both 2a). However, this array is essentially determined by two points, the high datum point CIA 8 on the one side and a group of five low data points on the other side. Of the eight samples of the Garz6n Group granulites (three in this study and five reported by A1varez (1981)), six define a line (errorchron, M S W D = 8 ) corresponding to an age of 1172+_90 Ma with initial STSr/S6Sr= 0.704+0.002 (2a) (both 2a). Two points plot outside this line, one above (CIA 14; this study) and one below (JAA-1176A; Alvarez, 1981 ). A1varez (1981) has reported a best-fit line through four of his data points corresponding to an age of 1180 Ma with initial STSr/S6Sr=0.704. The two pegmatite samples define a line corresponding to an age of 847 Ma with initial 87Sr/ 868r = 0.709.
Three hornblendes from amphibolites of the
T A B L E III
Rb-Sr mica/whole-rock ages Sample no.
Rb" (ppm wt.)
Sr a (ppm wt.)
CIA 4 P h l
398 398 217 497 497 136 453 456 587 588 111
79.5 79.6 79.3 30.7 31.0 368 197 197 8.41 8.42 591
WR CIA 6 Bio WR CIA 9 Kfsp CIA 16 Bio WR
Rb/Sr a
2.7428
0.3696
0.1882
87Sr/S6Srb
87Rb/S6Sr
0.91009 0.91083 0.822126 0.98446 0.98187 0.723038 0.79063 0.79069 1.1157 1.1112 0.707745
14.76 14.76 8.025 48.09 47.62 1.071 6.716 6.763 210.0 209.8 0.5445
Calculated age (Ma) c 918_+27
390 + 12 895 _+16 d 136_+4
"X-Ray fluorescence spectrometry for the whole rocks and isotope dilution for the micas. bDirect measurement on unspiked sample for the whole rocks (Finnigan MAT 261 ) and calculated from the isotope dilution run for the micas (Varian MAT CH5)~ tErrors based upon estimated errors of 1.0% in isotope dilution Rb and Sr, 1.0% in XRF Rb/Sr, 0.04% in 87Sr/86Sr micas and 0.005% in 87Sr/86Sr whole-rocks. dAssumed initial 87Sr/S~Sr of 0.705.
321 T A B L E IV K-Ar mineral data Sample no.
Mineral
K (ppm wt. )
Radiogenic 4 ° A r (ppb wt. )
Atmospheric 4 ° A r (% total 4°At)
CIA 2
Hornblende
CIA 4
Phlogopite
CIA 6
Biotite
CIA 7
Hornblende
CIA 11
Hornblende
0.436 0.438 8.32 8.34 4.57 4.56 1.00 1.00 1.02 1.03
CIA 16
Biotite
39.20 38.75 709.4 661.7 72.30 72.87 87.33 87.56 97.40 90.28 94.93 96.94 78.19 80.31
4 13 2 2 13 13 2 8 1 7 7 4 9 17
7.92 7.92
Calculated age (Ma) a 971+- 19 912 +-35 216+_ 5 955 +_19 1000 +_25
139 +_ 4
aFor the errors (la) in the ages, a standard deviation of 1% is taken for K, while the standard deviation of radiogenic Ar is based upon standard deviations of 1% in atmospheric 4°Ar/~6Ar and 1.5% in the tracer calibration.
Garzdn Group (CIA 2, 7, 11) have K-Ar dates between 1000 and 955 Ma, averaging 975 + 12 Ma (errors in all mineral ages are la). Phlogopite from a marble (CIA 4) has Rb-Sr (mineral/whole rock) and K-Ar dates concordant at 915+21 Ma (Tables III, IV). Biotite from (Guapotdn) granite-gneiss CIA 6 yields discordant Rb-Sr (mineral/whole rock) and K Ar dates of 390 + 12 and 216_+ 5 Ma, respectively (Tables III, IV). The Rb-Sr whole-rock data of two samples from Precambrian basement in the Rio P~iez west of the Garzdn Massif (CIA 16, 17) are inconclusive. Biotite from one of the samples, trondhjemitic gneiss CIA 16, yields Rb-Sr (mineral/whole rock) and K-Ar dates concordant at 138_+ 3 Ma.
Discussion The six Rb-Sr whole-rock data of the Guapotdn augen gneisses do not allow for an unequivocal conclusion regarding the age, owing to the scatter of the data points and the unfavourable spread in R b / S r of the samples. How-
ever, the best-fit line of 1596 + 300 Ma could very well represent a geologically feasible age in view of the abundance of 1560-1450 Ma old augen gneisses of similar lithology in the adjacent part of the Guiana Shield (Huguett et al., 1979; Priem et al., 1982) and the widespread occurrence of basement rocks of this age range (related to the Parguazan tectonomagmatic episode) in the Guiana Shield of Colombia, Venezuela and Brazil (Tassinari, 1981; Priem et al., 1982; Gaudette and Olszewski, 1985). The Guapotdn augen gneisses are therefore interpreted to represent a Parguazan basement underlying the supracrustals of the Garzdn Group, contrary to the earlier assumption by Kroonenberg ( 1982a, 1982b, 1985 ) that they would have been derived from younger, syntectonic granites. The age of 1172 + 90 Ma defined by six of the eight data points from the Garzdn Group granulites is taken to date the granulite-facies metamorphism. This event has also been demonstrated in granulitic rocks in the Sierra Nevada de Santa Marta in northern Colombia (MacDonald and Hurley, 1969 ). The volcanic-sedimentary precursors of the
322 granulites in the Garz6n Massif should thus have been deposited between the formation of the Parguazan basement and the event of highgrade metamorphism. However, the low initial STSr/S6Sr ratio suggests that the metamorphism has taken place shortly after the deposition. The presence of the Parguazan basement, the calc-alkaline nature of the granulites and the scarcity of mafic volcanic rocks all concur in suggesting a Cordillera-type continental margin setting for the deposition of the protoliths of the GarzSn Group granulites. The orogenic event itself that gave rise to the metamorphism, however, is probably of the continental collision type (Kroonenberg, 1982a) since its effects are reflected to a lesser extent throughout the whole western part of the Guiana Shield by partial mica rejuvenation (down to 1200 Ma), prominent shearing and low-grade metamorphism (Nickerie Metamorphic Episode; Priem et al., 1971, 1982). The 975 + 12 Ma hornblende and 915 + 21 Ma phlogopite dates probably record 'cooling ages' following the 1172+_90 Ma old high-grade metamorphism. The hornblendes, having the highest 'closure temperatures' ( ~ 550-490 ° C, Hart et al., 1968; Andriessen, 1978), display ages most nearly approaching the peak of the metamorphism. Hornblende K - A t dates of 955 +_40 Ma from the Bucaramanga gneiss in the Santander Massif (Goldsmith et al., 1971) and of 949 +_30 Ma from the Santa Marta granulites in the Sierra Nevada de Santa Marta (Tschanz et al., 1974 ) may also be interpreted as cooling ages, as was the 920 +_50 Ma hornblende K-Ar date from the Garz6n Massif reported by A1varez and Linares (1985). The pegmatites yield the youngest dates, in harmony with their cross-cutting relationship with the other two rock units. The concordancy of the Rb-Sr and K-Ar dates of biotite CIA 16 in the La Plata Massif suggests that the age of 138 +_3 Ma (Early Cretaceous) has a geological significance, possibly reflecting an event related to Andean moun-
tain-building processes. The Precambrian outcrops in this area are small fragments amidst large bodies of Jurassic intrusive rocks, for which K-Ar and Rb-Sr dates around 180 Ma are available (Alvarez and Linares, 1983; Priem et al., in preparation). Small Mesozoic intrusions also occur in the Garzdn Massif. The discordant Rb-Sr and K-Ar dates of CIA 6, in the GarzSn Massif, could reflect varying degrees of incomplete isotopic resetting (but weaker than in the La Plata Massif), during heating by nearby Mesozoic intrusions. Conclusions Following the above interpretations, orogeny in the upthrust Precambrian Garzdn Massif in the Colombian Andes should have started in the Mid-Proterozoic. After the cratonization of the Guiana Shield during the Parguazan tectonomagmatic event ~ 1.6 Ga ago, subduction occurred along its western margin. This gave rise to a calc-alkaline volcanic series and deposition of pelitic and carbonate sediments in a Cordillera-type setting, not unlike the present one. A continental collision ~ 1.2 Ga ago produced not only deformation and high-grade metamorphism, but also appreciable thermal and tectonic effects in the Guiana Shield hinterland. The presence of Parguazan basement in the Garzdn Massif suggests that not much continental accretion, if any, took place during this orogeny. The high-grade metamorphism was followed by uplift and cooling between 975 and 915 Ma, followed by pegmatitic intrusion ~ 850 Ma ago. Most other Precambrian fragments distinguished so far in the Colombian Andes (see review by Kroonenberg, 1982a) seem to have shared the same orogenic history. Another possible element of the 1.2 Ga orogenic belt is the Aroa-MisiSn block of Case et al. (1984) in the Venezuelan coastal ranges (Fig. 1). The undated San Quintin anorthosites with ilmenitemagnetite-hematite mineralization in this area, described by Rodriguez and Afiez (1978), are
323
lithologically remarkably similar to the anorthosites described by Tschanz et al. (1974) in the Sierra Nevada de Santa Marta in Colombia. Furthermore, pebbles of ll50-Ma-old granulites found in Early Tertiary fluviatile conglomerates on the island of Bonaire, Netherlands Antilles, also testify to the presence of coeval rocks on the northern South American mainland, possibly in the Guajira Peninsula (Priem et al., 1986). Further south the coeval Rondonian/Suns~s belt follows the western margin of the Brazilian Shield, down to eastern Bolivia (Leal et al., 1978; Cordani and De Brito Neves, 1982; Teixeira and Tassinari, 1984; Litherland et al., 1985). Hence, the whole western margin of the Amazonian Craton appears to have been affected by the 1.2 Ga orogeny, showing trends which roughly parallel those of the present Andes (Litherland et al., 1985). Further north, rocks of similar lithology and age are found in the Oaxaca metamorphic complex in Mexico (Patchett and Ruiz, 1987) and in the Grenville Province of North America (Moore et al., 1986). Kroonenberg (1982a) speculates, on the basis of the strong lithological and geochronological similarity of the Garzdn-Santa Marta belt to the Grenville Province, and their occurrence at opposite margins of older basement cores, that a continental collision occurred between the western margin of the Guiana Shield and the eastern margin of the Canadian Shield around 1.2 Ga. Paleomagnetic research on the Colombian Andean Precambrian could shed more light upon this question.
Acknowledgements This paper is dedicated to the memory of Allan K. Gibbs, an outstanding Precambrian geologist and a good friend. This study would not have been possible without the efforts of the entire staff of the ZWO laboratory of Isotope Geology, Amsterdam. The authors are particularly indebted to the staff members P.A.M. Andriessen, E.A.Th. Verdurmen and R.H.
Verschure for discussion and comments. E.A.Th. Verdurmen also supervised the X-ray fluorescence spectrometric and flame photometric analysis. Thanks are due to Dr. Hern~n Rivera Hermida of the Centro InterAmericano de Fotointerpretacidn, for field support, to Dr. Alfonso Ldpez Reina and Dra. Gloria Inds Rodriguez of INGEOMINAS and Dr. Rubdn Llin~s of the National University, all in Bogot~i, for permission to use thin-section preparation facilities. Dr. H. de Boorder and Drs. H. Diederix are thanked for their continuous support and their assistance in the field. Mr. S. Frans is thanked for drawing the figures. This study forms part of the research programme of the 'Stichting voor Isotopen-Geologisch Onderzoek', supported by the Netherlands Organization for the Advancement of Pure Research (ZWO).
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