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Interpretation of a discordant KAr age pattern (Capo Vaticano, Calabria)
EARTH AND PLANETARY SCIENCE LETTERS 20 (1973) 113-118. NORTH-HOLLANDPUBLISHINGCOMPANY
INTERPRETATION
OF A D I S C O R D A N T K - A r A G E P A T T E R N
(CAPO VATICANO, CALABRIA)* L. CIVETTA, M. CORTINI and P. GASPARINI Istituto di Geologia e Geofisica, University o f Napoli, Italy
Received 16 March 1973 Revised version received 18 June 1973 K-Ar age determination on whole rocks, biotites, quartz-feldspar separates and pegmatitic muscovites from a small quartz dioritic stock give a complex discordant age pattern. K-Ar dates from whole rocks and mineral separates define a single 116 my isochron with positive intercept, whereas muscovites from pegmatites fit a 180 my isochron with a probable negative intercept. Both ages are younger than the probable crystallization age of the stock (around 300 my), indicating a complex post-crystallization history. The fit of different mineral phases and whole rocks to a single isochron with positive intercept suggests that a thermal event caused rehomogenization of Ar among different mineral phases. The southernmost part of Italy consists of a metamorphic belt extending from Calabria to Mts. Peloritani, Sicily. Several "granitic" stocks (Sila, Serre, Capo Vaticano, Aspromonte) outcrop along this metamorphic belt. A few radiometric dates of these stocks have been published by Ferrara et al. (U~Pb, Aspromonte) [1], Ferrara and Longinelli (Rb-Sr, Serre) [2], Borsi and Dubois (Rb-Sr, Serre) [3], Nicoletti (one K - A r data, Capo Vaticano) [4]. Although these data are scattered and include only mineral dates, nevertheless they indicate that all these intrusions are of Hercynian age. A post Devonian intrusion age of the Serre pluton has been demonstrated on paleontological basis [5, 6]. Geological evidence shows that the whole Calabria-Peloritani metamorphic belt was extensively affected by Alpine metamorphism. The object of this paper is to give an interpretation of the discordant age pattern resulting from a detailed K - A r study of the small Capo Vaticano stock. It forms a small peninsula along the western coast of Calabria (fig. 1), being separated from Aspromonte and Serre by Upper Cenozoic to Quaternary sediments. The Capo Vaticano stock is a medium to coarse grained, locally foliated, quartz-dioritic rock composed mainly of quartz, plagioclase, biotite and K-feldspar. It is bordered on the northeast by garnet and sillimanite * Research carried out with the financial support of CNR.
bearing gneisses and it is overlain by Miocene to Quaternary sediments. Petrographic studies of parts of the Capo Vaticano stock were carried out by Nicotera [7], Bonfiglio [8], Morelli-Amodio and Zirpoli [9]. They suggest that this stock originated by anatexis of the surrounding gneisses. Some evidence for post-crystallization metamorphic reactions exists (exsolution of ruffle from biotite, partial recrystallization of quartz, mechanical deformation of minerals, chloritization of biotite). The K - A r study was carried out on biotite separates, quartz-feldspar separates, whole rock aliquots from the quartz-diorite and muscovites from pegmatites. The location of samples is shown in fig. 1 and table 1. 4°Ar was determined by isotope dilution mass spectrometry using a GD-150 Varian Mat mass spectrometer. K was determined by flame atomic absorption with a Mod. 303 Perkin-Elmer spectrophotometer. Details on instrumentation, techniques and calibrations are available [10]. Ar determinations were made in duplicate. The errors quoted for radiogenic Ar concentrations are overall deviations from the arithmetical mean. Ar determinations were monitored by the Bern 4M interlaboratory standard and by periodic measurements of a muscovite standard, prepared in this laboratory and checked at the Department of Geology of Rice University, at the University of Texas and the
114
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
CAPO VATICANO 0 •
5 10 Km : -~----~ 17,18
O
9,11 ~ Briatico
1~15,1
MAR
TIRRENO
PELORITANI
MARE
JONIO
INTE
SICILY
N t
IT~/~, Metamorpbites "Granitic" stocks O
5O
I_
100Kin ~_J
Location of samples
Fig. 1. Sketch map of Capo Vaticano stock area. Laboratorio per le Ricerche Radiometriche Applicate alia Geocronologia e Paleoecologia in Pisa. K was determined on three separate aliquots for each sample: the reported errors are maximum deviations from the mean value. K determinations were checked by U.S. G.S. rock standards G 2, GSP BCR and AGV. The errors reported in K - A r ages allow for uncertainties both in K and Ar determinations; they were evaluated on the basis of the formula given by Cox and Dahymple. K - A r analytical and age data are reported in table 1. A. Del Moro and G. Ferrara (Laboratorio per le Ricerche Radiometriche Applicate alla Geocronologia ed alia Paleoecologia, Pisa) kindly analyzed three pegmatitic muscovites (CV 6, CV 9 and CV 10) for R b - S r . Analytical data are reported in table 2. A R b - S r isochron age of 284 (+- 4) my was obtained. K - A r dates of muscovites from pegmatites range
from 147 to 167 my. Biotite separates give dates in the range 128 to 143 my*, with only one sample (CV 23) having an exceptionally high date of 181 (+- 6) my. Whole rock K - A r dates scatter from 130 to 196 my, being always older than corresponding biotite ages. This quite unusual pattern and the probable Hercynian age of the stock (as supported by R b - S r isochron age measurements of muscovites and by strong petrographic analogy with the nearby Serre stock, according to Hiecke-Merlin and Lorenzoni [12] ) led us to suspect the occurrence of excess radiogenic 4°Ar in a mineral phase other than biotite.
* Our biotite dates are inconsistent with the only K-Ar biotite data reported by Nicoletti [4] : 238 (+- 8) my for a sample collected by Prof. E. Beneo. We have no explanation for this inconsistency.
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
115
TABLE 1 Sample location and K - A r analytical results. Sample
Coordinates
Phase
CV 1
E 15 ° 53' 11" N 38 ° 35' 24"
W.R.
CV 2
CV 3
E 15 ° 52' 34" N 38 ° 35' 33"
E 15 ° 54' 19" N 38 ° 35' 02"
K%
4°Arrad%
4°Arrad (moles/g × 10- 8)
K-Ar date (my)
1.48 + 0.05
65.6
0.0452 +- 0.0012
164 +, 7
B F-Q
5.97 +, 0.12 0.409 ± 0.007
65.5 57.6
0.150 +- 0.007 0.0185 +, 0.0002
136 +- 5 238 +, 8
W.R.
1.34 +- 0.02
76.6
0.0493 +, 0.0009
196 +- 6
B F-Q
5.48 +- 0.05 0.145 +- 0.005
67.6 72.5
0.136 +, 0.009 0.0237 +- 0.0001
134 +, 4 749 +, 33
W.R.
2.22 +, 0.07
69.2
0.0637 +- 0.0005
155 +- 6
B F-Q
6.10 -+ 0.09 0.198 +- 0.017
66.0 65.2
0.150 +- 0.001 0.0212 +- 0.0001
133 +, 4 520 _+45
CV6
E 15° 52' 34" N 38 ° 35' 33"
M
10.0+,0.05
73.0
0.279+_0.007
151_+4
CV 9
E 16 ° 02' 44" N 38 ° 43' 36"
M
8.67 +_0.06
73.1
0,252 +_0.007
157 +, 5
CV 10
E 15 ° 52' 34" N 38 ° 35' 33"
M
9.42 +_ 0.10
63.8
0.293 ± 0.004
167 +- 5
CV 11
E 16 ° 02' 44" N 38 ° 43' 36"
W.R.
2.77 +_0.08
71.6
0.0661 +_ 0.0030
130 +_ 5
B F-Q
6.58 +, 0.15 0.0654 ± 0.0004
86.9 61.2
0.155 +_ 0.001 0.0262 +- 0.001
W.R.
2.02 ± 0.06
65.2
0.0615 +- 0.0009
128 +_ 5 1480 +- 40 ~
CV 13
E 15 ° 58' 42" N 38 ° 42' 48"
B
6.22 +- 0.12
71.6
0.150 +_ 0.002
131 +_4
CV 15
E 15 ° 49' 43" N 38 ° 37' 18"
B
6.57 +_0.10
77.6
0.174 -+ 0.002
143 +_ 5
CV 16
E 15 ° 49' 43" N 38 ° 37' 17"
B
6.57 +_0.05
90.9
0.172 ± 0.003
142 +, 4
CV 17
E 16 ° Off 33" N 38 ° 43' 14"
M
9.01 +_0.15
89.4
0.252 +_ 0.002
151 +_ 5*
CV 18
E 16 ° 00' 33" N 38 ° 43' 14"
B
7.42 ÷ 0.10
96.4
0.171 ± 0.001
125 +- 4*
CV 20
E 15 ° 49' 58" N 38 ° 37' 02"
M
7.65 ± 0.12
75.3
0.208 ± 0.008
147 -+ 4
CV 23
E 15 ° 49' 58" N 38 ° 37' 02"
B
8.04 +, 0.14
90.0
0.272 +, 0.002
181 +_6
* Only one Ar d e t e r m i n a t i o n . Errors on 4°Arrad c o n c e n t r a t i o n s are one standard deviation of the single analysis Xe = 0.585 X 10-1° yr- l ;
h/3 = 4.72 × 10-1° yr-1
W.R. = wh ole rock; B = b i o t i t e ; F - Q = feldspar + quartz; M = muscovite.
164 +, 5*
116
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
TABLE 2 Rb-Sr data and age results on some pegmatitic muscovites.* Sample
Rb Common 87Srrad 8 7 S r / ppm Sr ppm ppm S6Sr
87Rb/ Iso86Sr chron age
CV 6 CV 9 CV10
217 206 391
15.57 13.55 284 87.17 -+4my
40.1 44.0 13.0
0.2557 0.2668 0.4669
0.7770 0.7739 1.0793
* Analysis by A. Del Moro and G. Ferrara, Laboratorio per le Ricerche Radiometriche Applicate alia Geocronologia ed alia Paleoecologia, Pisa - T1/2 = 4.7 X 101° yr.
suspect that e x c e s s 4°Ar was transported within the rock during metamorphism as a dissolved phase in hydrous solution. A correlation of excess 4°Ar with structural depth within the nappe was observed: excess 4°Ar of about 2 X 10 -1° moles/g was observed in quartz veins related to phyllites. The complex K - A r age pattern obtained for the Capo Vaticano stock indicates that it underwent a complex thermal history after its crystallization. In order to clarify the meaning of the K - A r age pattern, K - A r dates were dealt with by the isochron method suggested by Harper [14]. The isochron equation is: Arm : (Xe/X)K(e x t - 1) + Aro
Quartz-feldspar separates with differing K contents (reflecting differing quartz/feldspar ratios) were prepared from samples CV 1, CV 2, CV 3, CV 11. Mineral dates ranging from 238 to 1480 my were obtained. These dates were always older than corresponding whole rock and biotite dates and three of them are even older than the muscovite R b - S r isochron age. If the quartz-feldspar data are plotted on a radiogenic 4°Ar vs. K diagram (fig. 2), radiogenic 4°Ar appears to be inversely related to K content, i.e., to feldspar concentration in the separate. This suggests that an excess 4°Ar is located within quartz: the extrapolated excess 4°Ar for K = 0 is 2.7 X 10 -1° moles/g. Excess 4°Ar of this order of magnitude was reported by Harper and Schamel [13] in syntectonic quartz veins from Monte Antola, Ligurian Appennines. They
4°At r~l (moleslgr $.x 10"1° )
I 3 -I ..,,1.Aro• 2.68 (±0.57 ) moles/gr s. x 10"~°
|"'<..... ===~--..
where Arm and K are the measured radiogenic 4°Ar a n d 4 ° K concentrations, Xe and X are the electron capture and total decay constants, Aro is the radiogenic 4°Ar initially present in the system minus the 4°Ar subsequently lost. Samples having the same age and the same initial 4°Ar excess (Aro > 0) will define a single isochron with positive intercept. Samples of the same age and having undergone the same subsequent loss of 4°Ar will define isochron with negative intercept. Strictly speaking, 36Ar as well as 4°Ar may also be trapped in mineral phases and the 4°Ar/36Ar ratio o f the Ar trapped phase is not necessarily 295.5. Therefore in the general case, the intercept is equal to 36Aro [(4°Ar/36Ar)o = 295.5] and a negative intercept may also arise from a 36Ar excess in the initial Ar [15]. When the isochron method is applied to all the samples, two different isochrons appear to be defined (fig. 3). The five muscovites, with more than desirable scatter, define an isochron of 181 (+ 13) my with a negative intercept. The error of the intercept is so large that the negative value of the intercept may not be real. However, if real, it might suggest that muscovites suffered a loss of Ar after they cooled below the "Ar blocking temperature" 181 my ago. Whole rock samples, most of the biotites and t h e quartz-feldspar separates fit a single isochron of 116 (+ 3) my with a significantly positive intercept*. Three biotite samples do not fit the isochron; they come from the Capo Vaticano Point area.
I(%' Rb-Sr mineral data determined by A. Del Moro and G. Ferrara on CV 1 biotite gave 103 -+ 20 my.
* A
Fig. 2. 4°Ar vs. K plot for quartz-feldspar separates.
117
L. Civetta et aL, Interpretation o f a discordant K - A r age pattern
J 4°Ar rad (rnoles/gr s.x 10 -10 )
(.~)
2O
t =181 "- 13 m.y
\
lO
5
7
~'1~--4°Ar0 = -4.6 (*-6.81 moles/gr s.xl0 -1°
9
K%
m
4°At rad {moles/gr s. x 10 -10 )
20
~
10
~
~
J
f
t = 116 -*3 m.y
~ Ar o : 1.73 ( "-015}. moleslgr s . xl0 -lO 0
1
I
f
3
5
I
7 K%
Fig. 3. (a) K - A r isochron plot for muscovites. (b) K - A r isochron plot for whole rocks, biotites and quartz-feldspar separates. Black rectangles represent samples from Capo Vaticano Point area.
The most obvious interpretation of these patterns is that the two K - A t isochrons date thermal events with different temperatures. The 181 my isochron dates the last time muscovites cooled below the "Ar blocking temperature". This indicates the time when the stock crossed such an isotherm. This may be due either to an extremely low cooling rate or to a quite intensive metamorphic event. The fitting of different mineral phases to a single 116 my isochron suggests that this isochron dates an
early Alpine metamorphic event. Furthermore, the positive value of the intercept indicates that all these phases contain the same initial excess 4°Ar (about 2 × 10 -m moles/g). This implies that the metamorphic event caused rehomogenization of Ar among the mineral phases. It probably reset R b - S r biotite mineral ages and may have caused some loss of Ar from pegmatitic muscovites. Similar early Alpine mineral R b - S r ages were reported by Borsi and Dubois [3] for metamorphic
118
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
rocks f r o m Calabria. E v i d e n c e for early A l p i n e m e t a m o r p h i s m o f this age was f o u n d also in t h e Alps (see
e.g. [16]). In c o n c l u s i o n , t h e i s o c h r o n i n t e r p r e t a t i o n o f K - A t dates h a s p r o v e d very h e l p f u l for u n r a v e l l i n g the complex age p a t t e r n resulting f r o m t h e c o m p l i c a t e d geological h i s t o r y o f t h e small igneous s t o c k o f C a p o Vat i c a n o , Calabria.
Acknowledgements The a u t h o r s wish to t h a n k A. Del M o r o a n d Prof. G. Ferrara, L a b o r a t o r i o per le R i c e r c h e R a d i o m e t r i c h e A p p l i c a t e alia G e o c r o n o l o g i a e Paleoecologia, Pisa, for Rb-Sr determinations. They are i n d e b t e d for h e l p to: Profs. P. B e a r t h , E. J~iger a n d P. S c a n d o n e .
References [1 ] G. Ferrara, F. Ippolito, H. Stauffer and E. Tongiorgi, Determinazione con metodi del piombo dell'et~ di un filone pegmatitico presso Delianuova (Aspromonte, Calabria), Boll. Soc. Geol. It. 78 (1959) 215. [2] G. Ferrara and A. Longinelli, Eth di due rocce granitiche della zona delle Serre in Calabria, Boll. Soc. Geol. It. 80 (1961) 25. [3] S. Borsi and R. Dubois, Donndes geocronologiques sur l'histoire hercynienne et alpine de la Calabre Centrale, Compt. Rend. Acad. Sci. Paris D266 (1968) 72. [4] M. Nicoletti, Eth di una roccia granitica di Capo Vaticano (Calabria) ottenuta con il metodo K - A r , Per. Miner. 39 (1970) 159.
[5] C. Afchain, Presence de tentaculitidae d~monstrant l'age d~vonien des niveaux calcaires intercal~s dans les "phyllades" du substratum du chainon calcaire de Stilo-Pazzano (Calabre m~ridionale, Italie), Compt. Rend. Soc. Geol. France 5 (1969) 150. [6] P. De Capoa Bonardi, Segnalazione di una fauna a conodonti del Devonico Superiore nei calcari intercalati a filladi di Stilo-Pazzano (Calabria meridionale), Rend. Acc. Sc. Fis. Mat., Napoli 37 (1970) 126. [7] P. Nicotera, Rilevamento geologico del versante settentrionale del Monte Poro (Calabria), Mem. Ist. Geol. Appl. Univ. Napoli 7 (1959) 92. [8] L. Bonfiglio, Sulla manifestazione di un processo di assimilazione magmatica nella granodiorite delle pendici meridionali di M. Poro (Calabria), Atti Soc. Peloritana Sc. Fis. Mat. Nat., Messina 10 (1964) 1. [9] L. Morelli-Amodio and G. Zirpoli, Le plutoniti del versante meridionale del Monte Poro (Calabria), Rend. Soc. It. Min. Petrol. 25 (1969) 3. [10] L. Civetta, M. Cortini, P. Gasparini and R. Scandone, I1 laboratorio K - A r dell'Istituto di Fisica Terrestre dell'Universith di Napoli, Rend. Soc. It. Mineral. Petrol. 27 (1970) 309. [ 11 ] A. Cox and G.B. Dalrymple, Statistical analysis of geomagnetic reversal data and the precision of the potassium-argon dating method, J. Geophys. Res. 72 (1967) 2603. [12] O. Hieke-Merlin and S. Lorenzoni, I1 massiccio "granitico delle Serre (Calabria), Mem. Ist. Geol. Paleont. Univ. Padova 29 (1972) 41. [13] C.T. Harper and S.S. Schamel, Note on the isotopic composition of argon in quartz veins, Earth Planet. Sci. Letters 12 (1971) 129. [14] C.T. Harper, Graphical solution to the problem of radiogenic argon-40 loss from metamorphic minerals, Ecl. Geol. Helv. 63 (1970) 119. [15] J.C. Roddick and E. Farrar, High initial argon ratios in Hornblendes, Earth Planet. Sci. Letters 12 (1971) 208. [16] J.C. Hunziker, Polymetamorphism in the Monte Rosa, Western Alps, Ecl. Geol. Helv. 63 (1970) 151.
INTERPRETATION
OF A D I S C O R D A N T K - A r A G E P A T T E R N
(CAPO VATICANO, CALABRIA)* L. CIVETTA, M. CORTINI and P. GASPARINI Istituto di Geologia e Geofisica, University o f Napoli, Italy
Received 16 March 1973 Revised version received 18 June 1973 K-Ar age determination on whole rocks, biotites, quartz-feldspar separates and pegmatitic muscovites from a small quartz dioritic stock give a complex discordant age pattern. K-Ar dates from whole rocks and mineral separates define a single 116 my isochron with positive intercept, whereas muscovites from pegmatites fit a 180 my isochron with a probable negative intercept. Both ages are younger than the probable crystallization age of the stock (around 300 my), indicating a complex post-crystallization history. The fit of different mineral phases and whole rocks to a single isochron with positive intercept suggests that a thermal event caused rehomogenization of Ar among different mineral phases. The southernmost part of Italy consists of a metamorphic belt extending from Calabria to Mts. Peloritani, Sicily. Several "granitic" stocks (Sila, Serre, Capo Vaticano, Aspromonte) outcrop along this metamorphic belt. A few radiometric dates of these stocks have been published by Ferrara et al. (U~Pb, Aspromonte) [1], Ferrara and Longinelli (Rb-Sr, Serre) [2], Borsi and Dubois (Rb-Sr, Serre) [3], Nicoletti (one K - A r data, Capo Vaticano) [4]. Although these data are scattered and include only mineral dates, nevertheless they indicate that all these intrusions are of Hercynian age. A post Devonian intrusion age of the Serre pluton has been demonstrated on paleontological basis [5, 6]. Geological evidence shows that the whole Calabria-Peloritani metamorphic belt was extensively affected by Alpine metamorphism. The object of this paper is to give an interpretation of the discordant age pattern resulting from a detailed K - A r study of the small Capo Vaticano stock. It forms a small peninsula along the western coast of Calabria (fig. 1), being separated from Aspromonte and Serre by Upper Cenozoic to Quaternary sediments. The Capo Vaticano stock is a medium to coarse grained, locally foliated, quartz-dioritic rock composed mainly of quartz, plagioclase, biotite and K-feldspar. It is bordered on the northeast by garnet and sillimanite * Research carried out with the financial support of CNR.
bearing gneisses and it is overlain by Miocene to Quaternary sediments. Petrographic studies of parts of the Capo Vaticano stock were carried out by Nicotera [7], Bonfiglio [8], Morelli-Amodio and Zirpoli [9]. They suggest that this stock originated by anatexis of the surrounding gneisses. Some evidence for post-crystallization metamorphic reactions exists (exsolution of ruffle from biotite, partial recrystallization of quartz, mechanical deformation of minerals, chloritization of biotite). The K - A r study was carried out on biotite separates, quartz-feldspar separates, whole rock aliquots from the quartz-diorite and muscovites from pegmatites. The location of samples is shown in fig. 1 and table 1. 4°Ar was determined by isotope dilution mass spectrometry using a GD-150 Varian Mat mass spectrometer. K was determined by flame atomic absorption with a Mod. 303 Perkin-Elmer spectrophotometer. Details on instrumentation, techniques and calibrations are available [10]. Ar determinations were made in duplicate. The errors quoted for radiogenic Ar concentrations are overall deviations from the arithmetical mean. Ar determinations were monitored by the Bern 4M interlaboratory standard and by periodic measurements of a muscovite standard, prepared in this laboratory and checked at the Department of Geology of Rice University, at the University of Texas and the
114
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
CAPO VATICANO 0 •
5 10 Km : -~----~ 17,18
O
9,11 ~ Briatico
1~15,1
MAR
TIRRENO
PELORITANI
MARE
JONIO
INTE
SICILY
N t
IT~/~, Metamorpbites "Granitic" stocks O
5O
I_
100Kin ~_J
Location of samples
Fig. 1. Sketch map of Capo Vaticano stock area. Laboratorio per le Ricerche Radiometriche Applicate alia Geocronologia e Paleoecologia in Pisa. K was determined on three separate aliquots for each sample: the reported errors are maximum deviations from the mean value. K determinations were checked by U.S. G.S. rock standards G 2, GSP BCR and AGV. The errors reported in K - A r ages allow for uncertainties both in K and Ar determinations; they were evaluated on the basis of the formula given by Cox and Dahymple. K - A r analytical and age data are reported in table 1. A. Del Moro and G. Ferrara (Laboratorio per le Ricerche Radiometriche Applicate alla Geocronologia ed alia Paleoecologia, Pisa) kindly analyzed three pegmatitic muscovites (CV 6, CV 9 and CV 10) for R b - S r . Analytical data are reported in table 2. A R b - S r isochron age of 284 (+- 4) my was obtained. K - A r dates of muscovites from pegmatites range
from 147 to 167 my. Biotite separates give dates in the range 128 to 143 my*, with only one sample (CV 23) having an exceptionally high date of 181 (+- 6) my. Whole rock K - A r dates scatter from 130 to 196 my, being always older than corresponding biotite ages. This quite unusual pattern and the probable Hercynian age of the stock (as supported by R b - S r isochron age measurements of muscovites and by strong petrographic analogy with the nearby Serre stock, according to Hiecke-Merlin and Lorenzoni [12] ) led us to suspect the occurrence of excess radiogenic 4°Ar in a mineral phase other than biotite.
* Our biotite dates are inconsistent with the only K-Ar biotite data reported by Nicoletti [4] : 238 (+- 8) my for a sample collected by Prof. E. Beneo. We have no explanation for this inconsistency.
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
115
TABLE 1 Sample location and K - A r analytical results. Sample
Coordinates
Phase
CV 1
E 15 ° 53' 11" N 38 ° 35' 24"
W.R.
CV 2
CV 3
E 15 ° 52' 34" N 38 ° 35' 33"
E 15 ° 54' 19" N 38 ° 35' 02"
K%
4°Arrad%
4°Arrad (moles/g × 10- 8)
K-Ar date (my)
1.48 + 0.05
65.6
0.0452 +- 0.0012
164 +, 7
B F-Q
5.97 +, 0.12 0.409 ± 0.007
65.5 57.6
0.150 +- 0.007 0.0185 +, 0.0002
136 +- 5 238 +, 8
W.R.
1.34 +- 0.02
76.6
0.0493 +, 0.0009
196 +- 6
B F-Q
5.48 +- 0.05 0.145 +- 0.005
67.6 72.5
0.136 +, 0.009 0.0237 +- 0.0001
134 +, 4 749 +, 33
W.R.
2.22 +, 0.07
69.2
0.0637 +- 0.0005
155 +- 6
B F-Q
6.10 -+ 0.09 0.198 +- 0.017
66.0 65.2
0.150 +- 0.001 0.0212 +- 0.0001
133 +, 4 520 _+45
CV6
E 15° 52' 34" N 38 ° 35' 33"
M
10.0+,0.05
73.0
0.279+_0.007
151_+4
CV 9
E 16 ° 02' 44" N 38 ° 43' 36"
M
8.67 +_0.06
73.1
0,252 +_0.007
157 +, 5
CV 10
E 15 ° 52' 34" N 38 ° 35' 33"
M
9.42 +_ 0.10
63.8
0.293 ± 0.004
167 +- 5
CV 11
E 16 ° 02' 44" N 38 ° 43' 36"
W.R.
2.77 +_0.08
71.6
0.0661 +_ 0.0030
130 +_ 5
B F-Q
6.58 +, 0.15 0.0654 ± 0.0004
86.9 61.2
0.155 +_ 0.001 0.0262 +- 0.001
W.R.
2.02 ± 0.06
65.2
0.0615 +- 0.0009
128 +_ 5 1480 +- 40 ~
CV 13
E 15 ° 58' 42" N 38 ° 42' 48"
B
6.22 +- 0.12
71.6
0.150 +_ 0.002
131 +_4
CV 15
E 15 ° 49' 43" N 38 ° 37' 18"
B
6.57 +_0.10
77.6
0.174 -+ 0.002
143 +_ 5
CV 16
E 15 ° 49' 43" N 38 ° 37' 17"
B
6.57 +_0.05
90.9
0.172 ± 0.003
142 +, 4
CV 17
E 16 ° Off 33" N 38 ° 43' 14"
M
9.01 +_0.15
89.4
0.252 +_ 0.002
151 +_ 5*
CV 18
E 16 ° 00' 33" N 38 ° 43' 14"
B
7.42 ÷ 0.10
96.4
0.171 ± 0.001
125 +- 4*
CV 20
E 15 ° 49' 58" N 38 ° 37' 02"
M
7.65 ± 0.12
75.3
0.208 ± 0.008
147 -+ 4
CV 23
E 15 ° 49' 58" N 38 ° 37' 02"
B
8.04 +, 0.14
90.0
0.272 +, 0.002
181 +_6
* Only one Ar d e t e r m i n a t i o n . Errors on 4°Arrad c o n c e n t r a t i o n s are one standard deviation of the single analysis Xe = 0.585 X 10-1° yr- l ;
h/3 = 4.72 × 10-1° yr-1
W.R. = wh ole rock; B = b i o t i t e ; F - Q = feldspar + quartz; M = muscovite.
164 +, 5*
116
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
TABLE 2 Rb-Sr data and age results on some pegmatitic muscovites.* Sample
Rb Common 87Srrad 8 7 S r / ppm Sr ppm ppm S6Sr
87Rb/ Iso86Sr chron age
CV 6 CV 9 CV10
217 206 391
15.57 13.55 284 87.17 -+4my
40.1 44.0 13.0
0.2557 0.2668 0.4669
0.7770 0.7739 1.0793
* Analysis by A. Del Moro and G. Ferrara, Laboratorio per le Ricerche Radiometriche Applicate alia Geocronologia ed alia Paleoecologia, Pisa - T1/2 = 4.7 X 101° yr.
suspect that e x c e s s 4°Ar was transported within the rock during metamorphism as a dissolved phase in hydrous solution. A correlation of excess 4°Ar with structural depth within the nappe was observed: excess 4°Ar of about 2 X 10 -1° moles/g was observed in quartz veins related to phyllites. The complex K - A r age pattern obtained for the Capo Vaticano stock indicates that it underwent a complex thermal history after its crystallization. In order to clarify the meaning of the K - A r age pattern, K - A r dates were dealt with by the isochron method suggested by Harper [14]. The isochron equation is: Arm : (Xe/X)K(e x t - 1) + Aro
Quartz-feldspar separates with differing K contents (reflecting differing quartz/feldspar ratios) were prepared from samples CV 1, CV 2, CV 3, CV 11. Mineral dates ranging from 238 to 1480 my were obtained. These dates were always older than corresponding whole rock and biotite dates and three of them are even older than the muscovite R b - S r isochron age. If the quartz-feldspar data are plotted on a radiogenic 4°Ar vs. K diagram (fig. 2), radiogenic 4°Ar appears to be inversely related to K content, i.e., to feldspar concentration in the separate. This suggests that an excess 4°Ar is located within quartz: the extrapolated excess 4°Ar for K = 0 is 2.7 X 10 -1° moles/g. Excess 4°Ar of this order of magnitude was reported by Harper and Schamel [13] in syntectonic quartz veins from Monte Antola, Ligurian Appennines. They
4°At r~l (moleslgr $.x 10"1° )
I 3 -I ..,,1.Aro• 2.68 (±0.57 ) moles/gr s. x 10"~°
|"'<..... ===~--..
where Arm and K are the measured radiogenic 4°Ar a n d 4 ° K concentrations, Xe and X are the electron capture and total decay constants, Aro is the radiogenic 4°Ar initially present in the system minus the 4°Ar subsequently lost. Samples having the same age and the same initial 4°Ar excess (Aro > 0) will define a single isochron with positive intercept. Samples of the same age and having undergone the same subsequent loss of 4°Ar will define isochron with negative intercept. Strictly speaking, 36Ar as well as 4°Ar may also be trapped in mineral phases and the 4°Ar/36Ar ratio o f the Ar trapped phase is not necessarily 295.5. Therefore in the general case, the intercept is equal to 36Aro [(4°Ar/36Ar)o = 295.5] and a negative intercept may also arise from a 36Ar excess in the initial Ar [15]. When the isochron method is applied to all the samples, two different isochrons appear to be defined (fig. 3). The five muscovites, with more than desirable scatter, define an isochron of 181 (+ 13) my with a negative intercept. The error of the intercept is so large that the negative value of the intercept may not be real. However, if real, it might suggest that muscovites suffered a loss of Ar after they cooled below the "Ar blocking temperature" 181 my ago. Whole rock samples, most of the biotites and t h e quartz-feldspar separates fit a single isochron of 116 (+ 3) my with a significantly positive intercept*. Three biotite samples do not fit the isochron; they come from the Capo Vaticano Point area.
I(%' Rb-Sr mineral data determined by A. Del Moro and G. Ferrara on CV 1 biotite gave 103 -+ 20 my.
* A
Fig. 2. 4°Ar vs. K plot for quartz-feldspar separates.
117
L. Civetta et aL, Interpretation o f a discordant K - A r age pattern
J 4°Ar rad (rnoles/gr s.x 10 -10 )
(.~)
2O
t =181 "- 13 m.y
\
lO
5
7
~'1~--4°Ar0 = -4.6 (*-6.81 moles/gr s.xl0 -1°
9
K%
m
4°At rad {moles/gr s. x 10 -10 )
20
~
10
~
~
J
f
t = 116 -*3 m.y
~ Ar o : 1.73 ( "-015}. moleslgr s . xl0 -lO 0
1
I
f
3
5
I
7 K%
Fig. 3. (a) K - A r isochron plot for muscovites. (b) K - A r isochron plot for whole rocks, biotites and quartz-feldspar separates. Black rectangles represent samples from Capo Vaticano Point area.
The most obvious interpretation of these patterns is that the two K - A t isochrons date thermal events with different temperatures. The 181 my isochron dates the last time muscovites cooled below the "Ar blocking temperature". This indicates the time when the stock crossed such an isotherm. This may be due either to an extremely low cooling rate or to a quite intensive metamorphic event. The fitting of different mineral phases to a single 116 my isochron suggests that this isochron dates an
early Alpine metamorphic event. Furthermore, the positive value of the intercept indicates that all these phases contain the same initial excess 4°Ar (about 2 × 10 -m moles/g). This implies that the metamorphic event caused rehomogenization of Ar among the mineral phases. It probably reset R b - S r biotite mineral ages and may have caused some loss of Ar from pegmatitic muscovites. Similar early Alpine mineral R b - S r ages were reported by Borsi and Dubois [3] for metamorphic
118
L. Civetta et al., Interpretation o f a discordant K - A r age pattern
rocks f r o m Calabria. E v i d e n c e for early A l p i n e m e t a m o r p h i s m o f this age was f o u n d also in t h e Alps (see
e.g. [16]). In c o n c l u s i o n , t h e i s o c h r o n i n t e r p r e t a t i o n o f K - A t dates h a s p r o v e d very h e l p f u l for u n r a v e l l i n g the complex age p a t t e r n resulting f r o m t h e c o m p l i c a t e d geological h i s t o r y o f t h e small igneous s t o c k o f C a p o Vat i c a n o , Calabria.
Acknowledgements The a u t h o r s wish to t h a n k A. Del M o r o a n d Prof. G. Ferrara, L a b o r a t o r i o per le R i c e r c h e R a d i o m e t r i c h e A p p l i c a t e alia G e o c r o n o l o g i a e Paleoecologia, Pisa, for Rb-Sr determinations. They are i n d e b t e d for h e l p to: Profs. P. B e a r t h , E. J~iger a n d P. S c a n d o n e .
References [1 ] G. Ferrara, F. Ippolito, H. Stauffer and E. Tongiorgi, Determinazione con metodi del piombo dell'et~ di un filone pegmatitico presso Delianuova (Aspromonte, Calabria), Boll. Soc. Geol. It. 78 (1959) 215. [2] G. Ferrara and A. Longinelli, Eth di due rocce granitiche della zona delle Serre in Calabria, Boll. Soc. Geol. It. 80 (1961) 25. [3] S. Borsi and R. Dubois, Donndes geocronologiques sur l'histoire hercynienne et alpine de la Calabre Centrale, Compt. Rend. Acad. Sci. Paris D266 (1968) 72. [4] M. Nicoletti, Eth di una roccia granitica di Capo Vaticano (Calabria) ottenuta con il metodo K - A r , Per. Miner. 39 (1970) 159.
[5] C. Afchain, Presence de tentaculitidae d~monstrant l'age d~vonien des niveaux calcaires intercal~s dans les "phyllades" du substratum du chainon calcaire de Stilo-Pazzano (Calabre m~ridionale, Italie), Compt. Rend. Soc. Geol. France 5 (1969) 150. [6] P. De Capoa Bonardi, Segnalazione di una fauna a conodonti del Devonico Superiore nei calcari intercalati a filladi di Stilo-Pazzano (Calabria meridionale), Rend. Acc. Sc. Fis. Mat., Napoli 37 (1970) 126. [7] P. Nicotera, Rilevamento geologico del versante settentrionale del Monte Poro (Calabria), Mem. Ist. Geol. Appl. Univ. Napoli 7 (1959) 92. [8] L. Bonfiglio, Sulla manifestazione di un processo di assimilazione magmatica nella granodiorite delle pendici meridionali di M. Poro (Calabria), Atti Soc. Peloritana Sc. Fis. Mat. Nat., Messina 10 (1964) 1. [9] L. Morelli-Amodio and G. Zirpoli, Le plutoniti del versante meridionale del Monte Poro (Calabria), Rend. Soc. It. Min. Petrol. 25 (1969) 3. [10] L. Civetta, M. Cortini, P. Gasparini and R. Scandone, I1 laboratorio K - A r dell'Istituto di Fisica Terrestre dell'Universith di Napoli, Rend. Soc. It. Mineral. Petrol. 27 (1970) 309. [ 11 ] A. Cox and G.B. Dalrymple, Statistical analysis of geomagnetic reversal data and the precision of the potassium-argon dating method, J. Geophys. Res. 72 (1967) 2603. [12] O. Hieke-Merlin and S. Lorenzoni, I1 massiccio "granitico delle Serre (Calabria), Mem. Ist. Geol. Paleont. Univ. Padova 29 (1972) 41. [13] C.T. Harper and S.S. Schamel, Note on the isotopic composition of argon in quartz veins, Earth Planet. Sci. Letters 12 (1971) 129. [14] C.T. Harper, Graphical solution to the problem of radiogenic argon-40 loss from metamorphic minerals, Ecl. Geol. Helv. 63 (1970) 119. [15] J.C. Roddick and E. Farrar, High initial argon ratios in Hornblendes, Earth Planet. Sci. Letters 12 (1971) 208. [16] J.C. Hunziker, Polymetamorphism in the Monte Rosa, Western Alps, Ecl. Geol. Helv. 63 (1970) 151.