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Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) and its geotectonic significance
Joumd
of South
Amaicm Ezmh Scimces, Vol. 8. No. 2. pi. 187-194, 1995
Copyright 0 1995 Elsevier Science Ltd & Earth Sciemes & Rwources Institute
Pergamon
Printed in Great Britain. All rights reserved 0895-9811195
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) and its geotectonic significance ‘W. DGRR,
‘J.R. GROSSER, *G.I. RODRIGUEZ and 3U. KRAMM
’ Institut fiir Geowissenschaften und LithospKrenforschung, JLU GieBen, Senckenbergstra8e 3, D-35390 Giessen. 2 INGEOMINAS Bogota, Diag. 53 No. 34-53, A.A. 4865, Bogota, Colombia. 3 Institut fur Mineralogie, Universibit Mtinster, Corrensstra8e 24, D-48149 Miinster. (Received
$9.50 + 0.00
188
W. DGRR. et al.
m
Quaternary Mesozoic (undivided)
@J
Fm. Silgara (pre-Devonian)
m
Bucaramanga Gneiss (3 Precamb.)
aQ
m
uartz-Monzonite
(undivided)
Santa Barbara Quartz-Monzonite
:::::::s ParamoRicoGranodiorite/Tonalite h$ m Orthogneiss /EJJSample locations Fig. 1. (I!&) Published age data for plutonic rocks of the Santander and Floresta Massifs, Cordillera Oriental. 1 Granodiorita de Pamplona (Boinet er al. 1985); 2 Granito de Pescadero (Goldsmith et ul. 1971); 3 Granodiorita de Santa Barbara (Goldsmith et al. 1971); 4 Batolito de Mogotes (Goldsmith et al. 1971); 5 Monzonita de Onzaga (Cordani in Etayoer al. 1983); 6 Granito de Chuscales (Ulloa and Rodriguez 1982); 7 Granodioritflonalita de Paramo Rico. Fig. 2. (Right) Geological map of the Paramo Rico pluton simplified after Ward et al. (1973) with sample points and sample numbers referred to in the text.
The Paramo Rico plutonic body (PRP) covers an area of approximately 50 km* north of the small town of Berlin, between Bucaramanga and Pamplona (Fig. 1). The Paramo Rico pluton intrudes the Bucaramanga Gneiss (Fig. 2) which consists mainly of metasediments with minor amounts of amphibolites and orthogneisses. This metamorphic basement represents a possible Precambrian unit, but has yielded a minimum age of approximately 420 Ma (K-Ar data from Goldsmith et al., 197 1). The Paramo Rico pluton also intrudes the Silgara Formation, which consists of metamorphic siliciclastic rocks with intercalations of orthoamphibolites intruded by
Lower Paleozoic granitoids (Ward et al., 1973; Ulloa and Rodriguez, 1982) and gabbros (Boinet et aE., 1985). This unit is supposed to be of Lower Paleozoic age because it underlies fossiliferous Middle Devonian strata. These metamorphic series are covered by epicontinental to terrestrial sediments of Middle Devonian to Jurassic age. The contact with the Santa Barbara batholith, probably a fault contact, is not exposed. K/Ar biotite ages from Goldsmith ef al. (1971), yielded ages of 192 + 7 Ma and 194 + 7 Ma for this batholith.
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Ala&e-dikes and quartz-monzonitic aplites of the La Corcova quartz-monzonite intrude the Paramo Rico pluton. These dikes have yielded a WAr biotite age of 1 11 + 4 Ma and a K/Ar muscovite age of 195 rt 7 Ma (Goldsmith et al., 197 1). Field evidence shows these intrusives to be younger than the Paramo Rico pluton. Cretaceous sedimentary rocks surround the Santander Massif and are preserved as erosional relicts within the Massif. Therefore, the age of intrusion of the Paramo Rico pluton is bracketed between approximately 420 Ma and 195 Ma (see also Ward et cd., 1973, Etayo et al., 1983). Most authors have favored a Paleozoic intrusion age. The best exposures within the Paramo Rico pluton are found along the road that runs from Berlin to Vetas and near the small town of California in the northern part of the pluton (see Fig. 2).
METHODS Major elements were analyzed by X-ray fluorescence spectrometry (XRF) using fused glass discs. Trace elements were analyzed in pressed powder pellets by XRF using a Philips PW 1400 calibrated to the international standards BM, BR, BHVO-1, and BIR-1. Total volatiles were determined by loss on ignition (LOI) at 1000°C on samples dried overnight at 105’ C. U-Pb systematics of zircons were determined from a tonalite (7-l-l-89; 4 kg) and a granodiorite (10-l-4-89; 5 kg). After removal of the weathered surfaces, the samples were crushed to l-2 cm, and then further reduced by repeated treatment in a roller mill. After each treatment in the mill, the fraction smaller than 0.18 mm was separated by sieving, and heavy minerals were further concentrated by flotation. The concentrate was then gravity settled in Bromoform (2.8 g/cm3) and separated magnetically in a isodynamic separator (20” inclination, 15” tilt of the groove, steps of 0.4 amp up to 1.6 amp). This treatment yielded almost pure concentrates of zircon, which were split into size fractions with nylon sieves. Euhedral zircons without inclusions were selected from these sieve fractions. Both tonalite and granodiorite samples contain similar zircons. They are light pink or colorless and belong to the subtypes 53 or S23 in the classification of Pupin (1980). The chemical decomposition of the zircons, the separation of U and Pb, and the mass spectrometric isotope analysis were performed at the Central Laboratory for Geochronology (University of Miinster) and the IGL (University of Giessen) using the standard procedures described by Persson et al. (1983) and Krogh (1973), except that the zircons were washed in cold 3 N HCl. The analyses were carried out with a NBS-Type 12”-90 solid-source mass-spectrometer (mass fractionation 0.12% a.m.u.) and a MAT 261 (mass fractionation 0.076% a.m.u.). The measured isotopic ratios of lead were corrected by the initial lead and blank lead values. The isotope ratios of the blank lead are 37.5 for 20*Pb/204Pb, 15.52 for *“Pb/*“Pb and 17.72 for 206Pb/204Pb. The measured quantity of blank-lead was 90 pg on the average, and 40 pg
Santander Massif (Cordillera
Oriental, Colombia)
189
for the blank-U. The isotope ratio of the initial lead was calculated after Stacey and Kramers (1975) for a crustal model lead with an age of 215 Ma (38.237 for 208Pb/204Pb, 15.612 for 207Pb/204Pb, and 18.372 for 206Pb/204Pb). The calculation and correlation of errors for the 206Pb/238Uand 207Pb/235U-ratios was carried out after Ludwig (1980) for a confidence level of 95%. This also accounts for the errors connected with the measured isotope ratios, the uncertainty of the U/Pb ratio of the mixed spike, the error magnification by the spike/sample ratio, and the uncertainties of the blank- and initial-lead corrections. The relative error for the isotope ratios of the initial lead and the blanklead are set at l%, for the U/Pb-ratio of the spike at 0.15%, and for the concentration of the blank lead at 50%. The estimated correlation factor for initial lead and blank lead is 0.7. The error ellipses thus calculated were plotted for the samples on the concordia diagram (Fig. 6). Element concentrations and ages were calculated using the constants of Steiger and Jager (1977) and Jaffey et al. (197 1). Regression lines were calculated with the least squares method of York (1969).
PETROLOGY Modal analysis (approximately 500 points per analysis) show that the plutonic body consists mainly of granodiorite and tonalite. The main petrographic features are shown in Table 1. Streckeisen’s (1973) classification is followed. The samples from the southern part are a tonalite (7-1-189) and a granodiorite (IGM 37435); rocks of the northern part of the pluton are granodiorites (lo- l-4-89, lo- l-5-89) and a quartz-monzonite (10-l-2-89) (Fig. 2). The hornblende-biotite bearing rocks of the Paramo Rico pluton are shown in the QAPF double triangle, in comparison to other plutonic rocks of approximately the same age of the Santander Massif in Fig. 3. This figure also includes the proposed fields for talc-alkaline granites (CAG) and island-arc granites (IAG) taken from Maniar and Piccoli (1989). The occurrence of hornblende in these rocks, the absence of muscovite and the presence of sphene are typical of I-type granitoids (Chappell and White, 1974). Further indicators of I-type characteristics of the Eastern Cordillera plutonic group are the porphyry copper mineralizations in the Mesozoic plutonics of the Santander Massif and a relatively broad spectrum of felsic to mafic compositions (Chappell and White, 1974). There are no geochemical data available from other Jurassic-Triassic plutons of the Santander Massif. The analyzed samples have Si02 contents between 54 and 59 wt.%. The rocks belong to a talc-alkaline suite with an alkali-lime index around 57 (Table 2). The log CaO/K20+ Na20 - Si02 diagram (Fig. 4A) reveals that our rocks compare well with other talc-alkaline suites, and plot in a field intermediate between typical continental arc and island arc intrusives. The rocks are metaluminous based on Shand’s index and plot in the field of IAG in the A120s/(Na20 + K20) - A1203/(Ca0 + Na20 + K20) diagram (Fig. 4B) (Maniar and Piccoli 1989).
190
W. DORR, et al.
Table 1. Mineralogical composition of the investigated specimen from the Paramo Rico pluton.
SAMPLE
7- l-l-89 Tonalite
10-l-2-89 Qz-Monzonite
10-l-4-89 Granodiorite
10-l-S-89 Granodiorite
IGM 37435 Granodiorite
Q
15.4
9.0
18.2
14.6
22.0
Plag.
55.0
44.4
43.8
47.1
47.6
10.6
5.6
5.8
7.0
12.6
13.0
1.8
-
0.6
0.4
-
0.8
Ortho
4.0
Microcline
tr.
Biotite
12.8
Chl. in Biotite
2.2
Epidote in Biotite
2.0
Hbl.
6.4
12.8
18.6
13.2
10.8
Pyx.
16.2
6.0
-
0.2
-
Apatite
0.6
0.4
0.4
-
0.6
Zircon
0.2
tr.
tr.
0.6
tr.
Sphene
0.4
0.8
0.4
0.4
Pyrite
-
tr.
tr.
tr.
Magn.
0.8
0.4
0.2
1.4
Table 2. Major and trace element geochemistry of samples from the Paramo Rico pluton.
WT%
7-l-l-89
SiO2 TiOz
lo- l-2-89
10-l-4-89
10-l-5-89
57.0
58.8
58.2
54.9
1.01
1.06
1.08
1.12
A1203
17.8
16.8
16.9
17.0
Fe0
6.69
6.93
7.25
7.38
MnO
0.12
0.14
0.15
0.13
MgO
3.17
3.17
3.37
3.56
CaO
5.50
4.66
6.11
6.36
Na20
3.33
2.74
2.90
3.05
K2O
2.92
3.15
2.60
2.40
p205
0.32
0.3 1
0.31
0.33
1.35
LOI
1.49
0.45
0.68
Total
99.35
99.71
99.32
96.91
30
24
33
45
co
I5
n.d
19
n.d.
Ni
12
IY
13
12
CU
n.d.
10
Zn
80
112
96
Ga
22
n.d.
23
22
Rb
107
168
120
101
Sr
693
594
615
665
Y
37
SO
41
32
Zr
235
266
202
196
Nb
II
12
9
12 11
ppm Cr
70
Pb
12
Rd.
11
Th
11
n.d.
8
5
V
ad.
173
n.d.
n.d.
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) 19 1 large zircons (sieve-fraction 6) are the most discordant. This suggests that they incorporated the highest amount of inherited lead. Since all zircons show a high transparency, no corrosion effects or cores, they might have been newly grown during pluton genesis, incorporating mineral relics with radiogenic lead. The U-PI>data of the three zircon fractions of the granodiorite plot on a discordia, which intersects the concordia curve at 205 +5/-9 Ma and at about 1340 +500/-420 Ma. If we assume no Pb-loss, the lower intercept age of 205 Ma represents the intrusion age of the granodiorite.
Fig. 3. (Q) Modal quartz - (A) alkali feldspar - (P) plagioclase Ternary Plot for the investigated samples from the Paramo Rico pluton. Fields for CAG (continental arc granites) and IAG (island arc granitoids) after Maniar and Piccoli (1989). Numbered fields according to Streckeisen (1973). The numbered data fields represent petrographical data for other Triassic-Jurassic plutonic rocks (2 Granito de Pescadero; 3 Granodiorita de Santa Barbara; 4 Batolito de Mogotes; same numbers as in Fig. 1). They plot mainly
in the Volcanic Arc Granite field on
the Rb-Y+Nb diagram (after Pearce et al., 1984) and in the field for destructive plate margin magmatism in the R-R2 plot after Batchelor and Bowden (1985). The compositions are typical for rocks formed in active margin settings. The major element geochemistry and their low Sr and Rb content, as well as their moderate Zr, Y, and Nb content is comparable to talc-alkaline rock suites intermediate between Island Arc- and Andean type-settings.
~~j
1
Al$-j/lCoO*No~K~0~
GEOCHRONOLOGY The four unabraded zircon fractions of the tonalite (Pig. 5, l-4) show a negative correlation of uranium content with grain sizes and apparent U-Pb-ages. Crystals of sieve fraction >250 pm are close to concordia at 210 Ma, while the smaller grains are more discordant. The four unabraded zircon fractions fit on a chord with a small spread of their apparent U-Pb ages, which can be attributed to a recent Pb-loss. A discordia is not shown because of the high a priori error and a lower meaningless negative intercept. Because of its low discordance, the apparent UPb ages of 208.9 Ma and 211.1 Ma, obtained from zircon fraction 1, approximate the educt age of the tonalite The negative lower intercept and the U-Pb data of zircon fraction No 5, which was ca. 70% abraded (Pig. 5), demonstrate the presence of inherited radiogenic lead. This allows us to interpret - 210 Ma as maximum intrusion age of the tonalite, according to Williams et al. (1983). The U-Pb data of three zircon fractions of the granodiorite also prove the existence of inherited radiogenic lead. The zircon fractions 6-8 show no correlation between uranium content and apparent U-Pb ages. The discordance depends on the amount of inherited mineral relics. The
io
wt.%
502
Fig. 4A. (Top) Shand’s index and fields for IAG (island arc granitoids), CAG (talc alkaline granitoids) and OP (oceanic plagiogranites) according to Maniar and Piccoli (1989). 4B. (Borrom) Plot of log (CaO/(Na20+KzO)) against SiOz for the Paramo Rico rocks in comparison to intrusive suites from Mesozoic and Cenozoic magmatic arcs (NB=New Britain-Salomon Island arc; NC=New Guinea continental arc) and the range for normal calcalkaline andesites (stippled field; from Brown, 1982) after Browner al. (1984).
192
W. DORR, et al.
5
2 G/3
0._3 Fig. 5. Concordia plot of the analysed zircon fractions from the Paramo Rico pluton of the tonalite (7-l-l-89) grkodiorite (lo- l-2-89) (filled ellipses).
(open ellipses) and the
The apparent ages of one zircon fraction of the tonalite and the lower intercept age of the granodiorite are similar, indicating the intrusion of the Paramo Rico pluton at around 210 Ma, close to the Triassic/Jurassic boundary. The 207Pb/206Pb ages of zircon fractions 5 and 6 demonstrate an inherited radiogenic lead component with a minimum age of 3 16 Ma and 373 Ma respectively. The upper intercept age of the granodiorite suggests Precambrian inherited radiogenic lead. It is uncertain whether this inherited lead originated from one or more sources with different primary ages and/or metamorphic histories. Hence it is uncertain if the upper intercept age reflects a real geological event.
DISCUSSION
Fig. 6. Geotectonic scenario for the Jurassiflriassic boundary in Northwestern South America (modified after Aspden et al. 1987). Dashed field = Paleozoic Schist Belt.
The mineral assemblages and fabric of the Paramo Rico pluton do not show any evidence of a thermal overprint higher than very low grade. Therefore it seems unlikely that the Paramo Rico pluton zircons were reset during Triassic/Jurassic times. The U-Pb ages between 205 Ma and 210 Ma therefore cannot be cooling ages of Mesozoic metamorphism as stated by Boinet et al. (1985), but should be interpreted as primary magmatic ages. K-Ar ages from biotite taken from intrusions in this region range from 177 to 210 Ma (e.g. Goldsmith et aE. 1971). These dates are in agreement with our data, but also have to be
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) 193
Table 3. U-Pb analytical data for the investigated specimen of the Paramo Rico pluton.
SAMPLE
WEIGHT
SIEVE FRACTION (MICROMETER)
mg
CONCENTRATIONS
U
Pb
Pb radiog. (ppm) (ppm)
(ppm) tot.
MEASURED ISDTOPIC RATIOS 208 Pb 206 Pb
207Pb 206Pb
206Pb 204Pb
CALCUL.ATEDISOTOPIC RATIOS 206Pb 238U
207Pb 235 U
207Pb 206Pb
APPARENT AGES (Ma) 206Pb 238U
207Pb 235U
207Pb 206Pb
7-l-l-89 -250 (1)
3.30
253.2
11.1
9.1
0.340376 0.103372 279.7
0.032935 0.231136 0.050899 208.9
211.1
236.3
-200 (2)
5.21
293.9
18.2
10.6
0.593377 0.203255 96.7
0.032180 0.228920 0.051593 204.2
209.3
267.4
-160 (3)
4.39
319.3
11.9
11.0
0.278179 0.079458 533.8
0.031618 0.226800 0.052024 200.7
207.5
286.5
-125 (4) -200 (5)
,
4.56
, 361.7 113.5 112.6
10.269328 10.070912 1753.4 I 0.031653 I 0.224533 I 0.051447 I 200.9 I 205.7 I 260.9 I
1.50
248.1
9.62
9.47
0.225310 0.056910 3489.8 0.034992 0.254333 0.052715 221.7
230.1
316.5
-250 (6)
3.02
163.3 7.33
6.33
0.291564 0.093546 370.1
0.035995 0.268256 0.054051 228.0
241.3
373.2
-160 (7)
2.85
179.6 6.65
6.42
0.215287 0.061652 1398.0 0.033474 0.236142 0.051163 212.3
215.3
248.2
-125 (8)
2.16
213.6
7.77
0.235117 0.070325 834.6
224.1
319.8
lo-1489
8.26
reinterpreted to represent cooling ages after the solidification of the magmatic melts, rather than ages which represent cooling after Mesozoic metamorphism as proposed by Boinet et al. (1985). Field relations, xenoliths of metamorphic rocks and inherited lead component of the zircons demonstrate that the Paramo Rico pluton was intruded into a continental crust. Furthermore, epicontinental and terrestrial sediments were deposited in the region of the Santander Massif in Triassic/Jurassic times - coeval with the Paramo Rico pluton. These relationships, together with the results of the petrographic and geochemical investigations, give clear evidence that the geotectonic setting of the pluton was a continental arc, not an island arc. On the basis of our findings and the paleogeographic considerations of Aspden er al. (1987), we propose an eastward subduction of Pacific ocean crust under the westem margin of the Guayana Shield, possibly beginning during the Triassic (Fig. 6). This is in accordance with geochronological and petrological data from other parts of the Eastern Cordillera and the Central Cordillera of Colombia: initiation of a subduction related magmatism along the western border of the Guayana shield at the Triassic/Jurassic boundary is indicated by plutonic activity around 200 Ma in the Santander Massif (Goldsmith et al., 1971). This plutonic activity also seems to be widespread in the Cordillera Central and in other parts of the Cordillera Oriental, as shown by geochronological and field data: Intrusives with ages between 170 and 200 Ma described by Sillitoe et al. (1982) from the southern part of the Cordillera Central include granodioritic (210 Ma) and younger adamellitic rocks (198 Ma). From the central parts of the Cordillera Central, Barrero et al. (1969), Hall et al. (1972), Botero (1975), Gonzalez er al. (1980), and Brook (1984)
0.033933 0.246990 0.052791 215.0
reported several plutonic bodies with ages around 200 Ma. These bodies are characterized by quartz-diorites, monzonites, and tonalites (e.g. Alvarez 1983). Moreover, Tschanz et al. (1974) reported granodiorites and quartz-monzonites with ages around 180 Ma from the Sierra Nevada de Santa Marta. This widespread magmatic activity at the TriassicJurassic boundary suggests a geotectonic position of the Cordillera Oriental similar to the present collision front of the Nazca and the South American plates. Further geochronological, petrographical, and geochemical work is necessary for a better understanding of the mechanisms and geotectonics of this important northwest comer of South America. Acknowledgements - This study was supported by the German Academic Exchange Service (DAAD) and the German Research Foundation (DFG, grant no. GR 94411-l). We are grateful to R. Emmermann, U. Haack (Giessen) and B. Grauert (Mtlnster), who made available the geochemical analysis and isotopic studies. We would also like to thank J. Leveque and S. Philippe (both from Giessen) for their invaluable help. We are grateful to F. Stibane (Giessen) for fruitful discussions about Colombian geology. H. Mendoza (Ingeominas Regional Bucaramanga) from the Ingeominas Bogota, and the Universidad National Bogota provided important help during field work. We thank D. Tanner and T. Schmidt (both from Giessen) for corrections of the English text. Last but not least, the reviews of H. Duque-Care (Bogota) and D. Kimbrough (San Diego) improved the paper.
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gia de 10s cuadrangulos H-12 Bucaramanga y H-13 Pamplona, Departamento de Santander. Boletin Geologico 21, 1-3. Williams I.S., Compston, W. and Chappell, B.W., 1983. Zircon and monazite U-Pb systems and the histories of I-type magmas, Berridale batholith, Australia. Journal of Petrology 24.76-97. York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth and Planerary Science Letters 5.320-324.
of South
Amaicm Ezmh Scimces, Vol. 8. No. 2. pi. 187-194, 1995
Copyright 0 1995 Elsevier Science Ltd & Earth Sciemes & Rwources Institute
Pergamon
Printed in Great Britain. All rights reserved 0895-9811195
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) and its geotectonic significance ‘W. DGRR,
‘J.R. GROSSER, *G.I. RODRIGUEZ and 3U. KRAMM
’ Institut fiir Geowissenschaften und LithospKrenforschung, JLU GieBen, Senckenbergstra8e 3, D-35390 Giessen. 2 INGEOMINAS Bogota, Diag. 53 No. 34-53, A.A. 4865, Bogota, Colombia. 3 Institut fur Mineralogie, Universibit Mtinster, Corrensstra8e 24, D-48149 Miinster. (Received
$9.50 + 0.00
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m
Quaternary Mesozoic (undivided)
@J
Fm. Silgara (pre-Devonian)
m
Bucaramanga Gneiss (3 Precamb.)
aQ
m
uartz-Monzonite
(undivided)
Santa Barbara Quartz-Monzonite
:::::::s ParamoRicoGranodiorite/Tonalite h$ m Orthogneiss /EJJSample locations Fig. 1. (I!&) Published age data for plutonic rocks of the Santander and Floresta Massifs, Cordillera Oriental. 1 Granodiorita de Pamplona (Boinet er al. 1985); 2 Granito de Pescadero (Goldsmith et ul. 1971); 3 Granodiorita de Santa Barbara (Goldsmith et al. 1971); 4 Batolito de Mogotes (Goldsmith et al. 1971); 5 Monzonita de Onzaga (Cordani in Etayoer al. 1983); 6 Granito de Chuscales (Ulloa and Rodriguez 1982); 7 Granodioritflonalita de Paramo Rico. Fig. 2. (Right) Geological map of the Paramo Rico pluton simplified after Ward et al. (1973) with sample points and sample numbers referred to in the text.
The Paramo Rico plutonic body (PRP) covers an area of approximately 50 km* north of the small town of Berlin, between Bucaramanga and Pamplona (Fig. 1). The Paramo Rico pluton intrudes the Bucaramanga Gneiss (Fig. 2) which consists mainly of metasediments with minor amounts of amphibolites and orthogneisses. This metamorphic basement represents a possible Precambrian unit, but has yielded a minimum age of approximately 420 Ma (K-Ar data from Goldsmith et al., 197 1). The Paramo Rico pluton also intrudes the Silgara Formation, which consists of metamorphic siliciclastic rocks with intercalations of orthoamphibolites intruded by
Lower Paleozoic granitoids (Ward et al., 1973; Ulloa and Rodriguez, 1982) and gabbros (Boinet et aE., 1985). This unit is supposed to be of Lower Paleozoic age because it underlies fossiliferous Middle Devonian strata. These metamorphic series are covered by epicontinental to terrestrial sediments of Middle Devonian to Jurassic age. The contact with the Santa Barbara batholith, probably a fault contact, is not exposed. K/Ar biotite ages from Goldsmith ef al. (1971), yielded ages of 192 + 7 Ma and 194 + 7 Ma for this batholith.
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Ala&e-dikes and quartz-monzonitic aplites of the La Corcova quartz-monzonite intrude the Paramo Rico pluton. These dikes have yielded a WAr biotite age of 1 11 + 4 Ma and a K/Ar muscovite age of 195 rt 7 Ma (Goldsmith et al., 197 1). Field evidence shows these intrusives to be younger than the Paramo Rico pluton. Cretaceous sedimentary rocks surround the Santander Massif and are preserved as erosional relicts within the Massif. Therefore, the age of intrusion of the Paramo Rico pluton is bracketed between approximately 420 Ma and 195 Ma (see also Ward et cd., 1973, Etayo et al., 1983). Most authors have favored a Paleozoic intrusion age. The best exposures within the Paramo Rico pluton are found along the road that runs from Berlin to Vetas and near the small town of California in the northern part of the pluton (see Fig. 2).
METHODS Major elements were analyzed by X-ray fluorescence spectrometry (XRF) using fused glass discs. Trace elements were analyzed in pressed powder pellets by XRF using a Philips PW 1400 calibrated to the international standards BM, BR, BHVO-1, and BIR-1. Total volatiles were determined by loss on ignition (LOI) at 1000°C on samples dried overnight at 105’ C. U-Pb systematics of zircons were determined from a tonalite (7-l-l-89; 4 kg) and a granodiorite (10-l-4-89; 5 kg). After removal of the weathered surfaces, the samples were crushed to l-2 cm, and then further reduced by repeated treatment in a roller mill. After each treatment in the mill, the fraction smaller than 0.18 mm was separated by sieving, and heavy minerals were further concentrated by flotation. The concentrate was then gravity settled in Bromoform (2.8 g/cm3) and separated magnetically in a isodynamic separator (20” inclination, 15” tilt of the groove, steps of 0.4 amp up to 1.6 amp). This treatment yielded almost pure concentrates of zircon, which were split into size fractions with nylon sieves. Euhedral zircons without inclusions were selected from these sieve fractions. Both tonalite and granodiorite samples contain similar zircons. They are light pink or colorless and belong to the subtypes 53 or S23 in the classification of Pupin (1980). The chemical decomposition of the zircons, the separation of U and Pb, and the mass spectrometric isotope analysis were performed at the Central Laboratory for Geochronology (University of Miinster) and the IGL (University of Giessen) using the standard procedures described by Persson et al. (1983) and Krogh (1973), except that the zircons were washed in cold 3 N HCl. The analyses were carried out with a NBS-Type 12”-90 solid-source mass-spectrometer (mass fractionation 0.12% a.m.u.) and a MAT 261 (mass fractionation 0.076% a.m.u.). The measured isotopic ratios of lead were corrected by the initial lead and blank lead values. The isotope ratios of the blank lead are 37.5 for 20*Pb/204Pb, 15.52 for *“Pb/*“Pb and 17.72 for 206Pb/204Pb. The measured quantity of blank-lead was 90 pg on the average, and 40 pg
Santander Massif (Cordillera
Oriental, Colombia)
189
for the blank-U. The isotope ratio of the initial lead was calculated after Stacey and Kramers (1975) for a crustal model lead with an age of 215 Ma (38.237 for 208Pb/204Pb, 15.612 for 207Pb/204Pb, and 18.372 for 206Pb/204Pb). The calculation and correlation of errors for the 206Pb/238Uand 207Pb/235U-ratios was carried out after Ludwig (1980) for a confidence level of 95%. This also accounts for the errors connected with the measured isotope ratios, the uncertainty of the U/Pb ratio of the mixed spike, the error magnification by the spike/sample ratio, and the uncertainties of the blank- and initial-lead corrections. The relative error for the isotope ratios of the initial lead and the blanklead are set at l%, for the U/Pb-ratio of the spike at 0.15%, and for the concentration of the blank lead at 50%. The estimated correlation factor for initial lead and blank lead is 0.7. The error ellipses thus calculated were plotted for the samples on the concordia diagram (Fig. 6). Element concentrations and ages were calculated using the constants of Steiger and Jager (1977) and Jaffey et al. (197 1). Regression lines were calculated with the least squares method of York (1969).
PETROLOGY Modal analysis (approximately 500 points per analysis) show that the plutonic body consists mainly of granodiorite and tonalite. The main petrographic features are shown in Table 1. Streckeisen’s (1973) classification is followed. The samples from the southern part are a tonalite (7-1-189) and a granodiorite (IGM 37435); rocks of the northern part of the pluton are granodiorites (lo- l-4-89, lo- l-5-89) and a quartz-monzonite (10-l-2-89) (Fig. 2). The hornblende-biotite bearing rocks of the Paramo Rico pluton are shown in the QAPF double triangle, in comparison to other plutonic rocks of approximately the same age of the Santander Massif in Fig. 3. This figure also includes the proposed fields for talc-alkaline granites (CAG) and island-arc granites (IAG) taken from Maniar and Piccoli (1989). The occurrence of hornblende in these rocks, the absence of muscovite and the presence of sphene are typical of I-type granitoids (Chappell and White, 1974). Further indicators of I-type characteristics of the Eastern Cordillera plutonic group are the porphyry copper mineralizations in the Mesozoic plutonics of the Santander Massif and a relatively broad spectrum of felsic to mafic compositions (Chappell and White, 1974). There are no geochemical data available from other Jurassic-Triassic plutons of the Santander Massif. The analyzed samples have Si02 contents between 54 and 59 wt.%. The rocks belong to a talc-alkaline suite with an alkali-lime index around 57 (Table 2). The log CaO/K20+ Na20 - Si02 diagram (Fig. 4A) reveals that our rocks compare well with other talc-alkaline suites, and plot in a field intermediate between typical continental arc and island arc intrusives. The rocks are metaluminous based on Shand’s index and plot in the field of IAG in the A120s/(Na20 + K20) - A1203/(Ca0 + Na20 + K20) diagram (Fig. 4B) (Maniar and Piccoli 1989).
190
W. DORR, et al.
Table 1. Mineralogical composition of the investigated specimen from the Paramo Rico pluton.
SAMPLE
7- l-l-89 Tonalite
10-l-2-89 Qz-Monzonite
10-l-4-89 Granodiorite
10-l-S-89 Granodiorite
IGM 37435 Granodiorite
Q
15.4
9.0
18.2
14.6
22.0
Plag.
55.0
44.4
43.8
47.1
47.6
10.6
5.6
5.8
7.0
12.6
13.0
1.8
-
0.6
0.4
-
0.8
Ortho
4.0
Microcline
tr.
Biotite
12.8
Chl. in Biotite
2.2
Epidote in Biotite
2.0
Hbl.
6.4
12.8
18.6
13.2
10.8
Pyx.
16.2
6.0
-
0.2
-
Apatite
0.6
0.4
0.4
-
0.6
Zircon
0.2
tr.
tr.
0.6
tr.
Sphene
0.4
0.8
0.4
0.4
Pyrite
-
tr.
tr.
tr.
Magn.
0.8
0.4
0.2
1.4
Table 2. Major and trace element geochemistry of samples from the Paramo Rico pluton.
WT%
7-l-l-89
SiO2 TiOz
lo- l-2-89
10-l-4-89
10-l-5-89
57.0
58.8
58.2
54.9
1.01
1.06
1.08
1.12
A1203
17.8
16.8
16.9
17.0
Fe0
6.69
6.93
7.25
7.38
MnO
0.12
0.14
0.15
0.13
MgO
3.17
3.17
3.37
3.56
CaO
5.50
4.66
6.11
6.36
Na20
3.33
2.74
2.90
3.05
K2O
2.92
3.15
2.60
2.40
p205
0.32
0.3 1
0.31
0.33
1.35
LOI
1.49
0.45
0.68
Total
99.35
99.71
99.32
96.91
30
24
33
45
co
I5
n.d
19
n.d.
Ni
12
IY
13
12
CU
n.d.
10
Zn
80
112
96
Ga
22
n.d.
23
22
Rb
107
168
120
101
Sr
693
594
615
665
Y
37
SO
41
32
Zr
235
266
202
196
Nb
II
12
9
12 11
ppm Cr
70
Pb
12
Rd.
11
Th
11
n.d.
8
5
V
ad.
173
n.d.
n.d.
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) 19 1 large zircons (sieve-fraction 6) are the most discordant. This suggests that they incorporated the highest amount of inherited lead. Since all zircons show a high transparency, no corrosion effects or cores, they might have been newly grown during pluton genesis, incorporating mineral relics with radiogenic lead. The U-PI>data of the three zircon fractions of the granodiorite plot on a discordia, which intersects the concordia curve at 205 +5/-9 Ma and at about 1340 +500/-420 Ma. If we assume no Pb-loss, the lower intercept age of 205 Ma represents the intrusion age of the granodiorite.
Fig. 3. (Q) Modal quartz - (A) alkali feldspar - (P) plagioclase Ternary Plot for the investigated samples from the Paramo Rico pluton. Fields for CAG (continental arc granites) and IAG (island arc granitoids) after Maniar and Piccoli (1989). Numbered fields according to Streckeisen (1973). The numbered data fields represent petrographical data for other Triassic-Jurassic plutonic rocks (2 Granito de Pescadero; 3 Granodiorita de Santa Barbara; 4 Batolito de Mogotes; same numbers as in Fig. 1). They plot mainly
in the Volcanic Arc Granite field on
the Rb-Y+Nb diagram (after Pearce et al., 1984) and in the field for destructive plate margin magmatism in the R-R2 plot after Batchelor and Bowden (1985). The compositions are typical for rocks formed in active margin settings. The major element geochemistry and their low Sr and Rb content, as well as their moderate Zr, Y, and Nb content is comparable to talc-alkaline rock suites intermediate between Island Arc- and Andean type-settings.
~~j
1
Al$-j/lCoO*No~K~0~
GEOCHRONOLOGY The four unabraded zircon fractions of the tonalite (Pig. 5, l-4) show a negative correlation of uranium content with grain sizes and apparent U-Pb-ages. Crystals of sieve fraction >250 pm are close to concordia at 210 Ma, while the smaller grains are more discordant. The four unabraded zircon fractions fit on a chord with a small spread of their apparent U-Pb ages, which can be attributed to a recent Pb-loss. A discordia is not shown because of the high a priori error and a lower meaningless negative intercept. Because of its low discordance, the apparent UPb ages of 208.9 Ma and 211.1 Ma, obtained from zircon fraction 1, approximate the educt age of the tonalite The negative lower intercept and the U-Pb data of zircon fraction No 5, which was ca. 70% abraded (Pig. 5), demonstrate the presence of inherited radiogenic lead. This allows us to interpret - 210 Ma as maximum intrusion age of the tonalite, according to Williams et al. (1983). The U-Pb data of three zircon fractions of the granodiorite also prove the existence of inherited radiogenic lead. The zircon fractions 6-8 show no correlation between uranium content and apparent U-Pb ages. The discordance depends on the amount of inherited mineral relics. The
io
wt.%
502
Fig. 4A. (Top) Shand’s index and fields for IAG (island arc granitoids), CAG (talc alkaline granitoids) and OP (oceanic plagiogranites) according to Maniar and Piccoli (1989). 4B. (Borrom) Plot of log (CaO/(Na20+KzO)) against SiOz for the Paramo Rico rocks in comparison to intrusive suites from Mesozoic and Cenozoic magmatic arcs (NB=New Britain-Salomon Island arc; NC=New Guinea continental arc) and the range for normal calcalkaline andesites (stippled field; from Brown, 1982) after Browner al. (1984).
192
W. DORR, et al.
5
2 G/3
0._3 Fig. 5. Concordia plot of the analysed zircon fractions from the Paramo Rico pluton of the tonalite (7-l-l-89) grkodiorite (lo- l-2-89) (filled ellipses).
(open ellipses) and the
The apparent ages of one zircon fraction of the tonalite and the lower intercept age of the granodiorite are similar, indicating the intrusion of the Paramo Rico pluton at around 210 Ma, close to the Triassic/Jurassic boundary. The 207Pb/206Pb ages of zircon fractions 5 and 6 demonstrate an inherited radiogenic lead component with a minimum age of 3 16 Ma and 373 Ma respectively. The upper intercept age of the granodiorite suggests Precambrian inherited radiogenic lead. It is uncertain whether this inherited lead originated from one or more sources with different primary ages and/or metamorphic histories. Hence it is uncertain if the upper intercept age reflects a real geological event.
DISCUSSION
Fig. 6. Geotectonic scenario for the Jurassiflriassic boundary in Northwestern South America (modified after Aspden et al. 1987). Dashed field = Paleozoic Schist Belt.
The mineral assemblages and fabric of the Paramo Rico pluton do not show any evidence of a thermal overprint higher than very low grade. Therefore it seems unlikely that the Paramo Rico pluton zircons were reset during Triassic/Jurassic times. The U-Pb ages between 205 Ma and 210 Ma therefore cannot be cooling ages of Mesozoic metamorphism as stated by Boinet et al. (1985), but should be interpreted as primary magmatic ages. K-Ar ages from biotite taken from intrusions in this region range from 177 to 210 Ma (e.g. Goldsmith et aE. 1971). These dates are in agreement with our data, but also have to be
Zircon U-Pb age of the Paramo Rico tonalite-granodiorite, Santander Massif (Cordillera Oriental, Colombia) 193
Table 3. U-Pb analytical data for the investigated specimen of the Paramo Rico pluton.
SAMPLE
WEIGHT
SIEVE FRACTION (MICROMETER)
mg
CONCENTRATIONS
U
Pb
Pb radiog. (ppm) (ppm)
(ppm) tot.
MEASURED ISDTOPIC RATIOS 208 Pb 206 Pb
207Pb 206Pb
206Pb 204Pb
CALCUL.ATEDISOTOPIC RATIOS 206Pb 238U
207Pb 235 U
207Pb 206Pb
APPARENT AGES (Ma) 206Pb 238U
207Pb 235U
207Pb 206Pb
7-l-l-89 -250 (1)
3.30
253.2
11.1
9.1
0.340376 0.103372 279.7
0.032935 0.231136 0.050899 208.9
211.1
236.3
-200 (2)
5.21
293.9
18.2
10.6
0.593377 0.203255 96.7
0.032180 0.228920 0.051593 204.2
209.3
267.4
-160 (3)
4.39
319.3
11.9
11.0
0.278179 0.079458 533.8
0.031618 0.226800 0.052024 200.7
207.5
286.5
-125 (4) -200 (5)
,
4.56
, 361.7 113.5 112.6
10.269328 10.070912 1753.4 I 0.031653 I 0.224533 I 0.051447 I 200.9 I 205.7 I 260.9 I
1.50
248.1
9.62
9.47
0.225310 0.056910 3489.8 0.034992 0.254333 0.052715 221.7
230.1
316.5
-250 (6)
3.02
163.3 7.33
6.33
0.291564 0.093546 370.1
0.035995 0.268256 0.054051 228.0
241.3
373.2
-160 (7)
2.85
179.6 6.65
6.42
0.215287 0.061652 1398.0 0.033474 0.236142 0.051163 212.3
215.3
248.2
-125 (8)
2.16
213.6
7.77
0.235117 0.070325 834.6
224.1
319.8
lo-1489
8.26
reinterpreted to represent cooling ages after the solidification of the magmatic melts, rather than ages which represent cooling after Mesozoic metamorphism as proposed by Boinet et al. (1985). Field relations, xenoliths of metamorphic rocks and inherited lead component of the zircons demonstrate that the Paramo Rico pluton was intruded into a continental crust. Furthermore, epicontinental and terrestrial sediments were deposited in the region of the Santander Massif in Triassic/Jurassic times - coeval with the Paramo Rico pluton. These relationships, together with the results of the petrographic and geochemical investigations, give clear evidence that the geotectonic setting of the pluton was a continental arc, not an island arc. On the basis of our findings and the paleogeographic considerations of Aspden er al. (1987), we propose an eastward subduction of Pacific ocean crust under the westem margin of the Guayana Shield, possibly beginning during the Triassic (Fig. 6). This is in accordance with geochronological and petrological data from other parts of the Eastern Cordillera and the Central Cordillera of Colombia: initiation of a subduction related magmatism along the western border of the Guayana shield at the Triassic/Jurassic boundary is indicated by plutonic activity around 200 Ma in the Santander Massif (Goldsmith et al., 1971). This plutonic activity also seems to be widespread in the Cordillera Central and in other parts of the Cordillera Oriental, as shown by geochronological and field data: Intrusives with ages between 170 and 200 Ma described by Sillitoe et al. (1982) from the southern part of the Cordillera Central include granodioritic (210 Ma) and younger adamellitic rocks (198 Ma). From the central parts of the Cordillera Central, Barrero et al. (1969), Hall et al. (1972), Botero (1975), Gonzalez er al. (1980), and Brook (1984)
0.033933 0.246990 0.052791 215.0
reported several plutonic bodies with ages around 200 Ma. These bodies are characterized by quartz-diorites, monzonites, and tonalites (e.g. Alvarez 1983). Moreover, Tschanz et al. (1974) reported granodiorites and quartz-monzonites with ages around 180 Ma from the Sierra Nevada de Santa Marta. This widespread magmatic activity at the TriassicJurassic boundary suggests a geotectonic position of the Cordillera Oriental similar to the present collision front of the Nazca and the South American plates. Further geochronological, petrographical, and geochemical work is necessary for a better understanding of the mechanisms and geotectonics of this important northwest comer of South America. Acknowledgements - This study was supported by the German Academic Exchange Service (DAAD) and the German Research Foundation (DFG, grant no. GR 94411-l). We are grateful to R. Emmermann, U. Haack (Giessen) and B. Grauert (Mtlnster), who made available the geochemical analysis and isotopic studies. We would also like to thank J. Leveque and S. Philippe (both from Giessen) for their invaluable help. We are grateful to F. Stibane (Giessen) for fruitful discussions about Colombian geology. H. Mendoza (Ingeominas Regional Bucaramanga) from the Ingeominas Bogota, and the Universidad National Bogota provided important help during field work. We thank D. Tanner and T. Schmidt (both from Giessen) for corrections of the English text. Last but not least, the reviews of H. Duque-Care (Bogota) and D. Kimbrough (San Diego) improved the paper.
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Sillitoe, R.H., Jaramillo, L., Damon, I?E., Shtiqullah, M. and Escovar, R., 1982. Setting, characteristics, and age of the Andean porphyry copper belt in Colombia. Economic Geology 77, 1837-1850. Stacey J.S. and Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two stage model. Earth and Planetary Science Letters 26,207-221. Steiger R.H. and Jtiger, E., 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earrh and Planetary Science Letters 36,359-362. Streckeisen, A., 1973. Classification and nomenclature of plutonic rocks. Geologische Rundschau 63,773-786.
Tschanz, C.M., Marvin, R.F., Cruz B., J., Mehnert, H.H. and Cebula, G.T., 1974. Geologic evolution of the Sierra Nevada de Santa Marta, northeastern Colombia. Bullerin of rhe Geological Sociefy of America 85.273-284.
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