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Cretaceous fission track dates of apatites from northern new england
Earth and Planetary Science Letters, 28 (1975) 181 188 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
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CRETACEOUS FISSION TRACK DATES OF APATITES FROM NORTHERN NEW ENGLAND R.A. ZIMMERMANN 1 , G.M. REIMER 2 ,*, K.A. FOLAND 1 and H. FAUL 1
t Department of Geology, University of Pennsylvania, Philadelphia, Pa. (USA) 2National Bureau of Standards, Gaithersburg, Md. (USA) Received June 9, 1975 Revised version received August 26, 1975
Twenty-five Cretaceous fission track ages have been obtained for apatite concentrates from metamorphic and igneous rocks of northern New England. Ages in central northern New England range from 64 to 100 m.y. (83 m,y. average) indicating a region of roughly equivalent apatite fission track ages. However, the boundaries of this region (>60,000 knl2) are not yet defined. A significantly higher Cretaceous age of 123 m.y. has been determined in easternmost Maine. The ages, exclusive of the one distinctly older age, do not correlate with isotopic ages for the same rocks and are taken to represent a regional event. They could be attributed to Cretaceous regional heating of wide extent and low temperature or alternatively to regional cooling which accompanied uplift and denudation. The presently preferred explanation is that the data represent cooling ages, indicating an approximate temperature of 100°C about 83 m.y. ago for the crustal level now exposed.
1. Introduction
reported from the area range from Precambrian to Cretaceous. A large area extending southwestward from central Maine shows K - A t ages disturbed in the Permian [9], and, numerous post-orogenic intrusions associated with the White magma series, generally midMesozoic in age, are scattered throughout the region [10,11]. Naeser [12], Christopher [13], and Naeser and Brookins [14] have reported fission track ages from the region.
Spontaneous fission of 238U in apatite crystals produces narrow tubes of intense damage. Below a certain characteristic temperature these tubes remain preserved in the crystal, but when the ambient temperature rises above this limit, the fission damage tends to repair itself and the tracks, as revealed by etching, fade quickly (in geologic time). Other effects may enter the picture [1,2], but temperature is the primary factor affecting quantitative retention of fission tracks [ 3 - 6 ] . Fission track study of apatites represents a means of examining the thermal history of rocks within a relatively low temperature range. We have selected the northern Appalachian region for our study because it has a wide variety of rock types, an age pattern indicative of multiple igneous and metamorphic events [7], and has been studied extensively by many workers [8]. The isotopic ages
2. Analytical methods Apatite was concentrated (grain size 350--100 ~m) using standard separation techniques. Samples were split with one part being retained for later determination of spontaneous track density. The other part was heated in air at 550°C for one hour to erase all spontaneous tracks. These grains were then irradiated in the thermal neutron facility (RT-4) of the National Bureau of Standards Research Reactor at Gaithersburg, Maryland. Individual apatite samples were sealed in poly-
* Present address: U.S. Geological Survey, Denver, Colorado 80225. 181
182 ethylene bags and placed in a second bag for irradiation. The packet was kept small (1 cm × 1 cm X 0.5 cm) to minimize the effect of neutron flux gradient. In all, four irradiations were performed. The total thermal neutron dose was determined from the 63Cu(n,')')64Cu reaction induced in a copper foil monitor attached to the packets. The nuclear counting equipment has been described elsewhere [ 15]. An internal standard, SRM 614, trace elements in glass [16], and some duplicate apatite samples provided additional cross checks for these irradiations. Table 1 lists the standards, duplicate samples, and the thermal neutron dose for each irradiation. Quoted neutron dose errors (-+1o) are the uncertainties for the gamma counting. There is agreement of the internal standards and the Cu foil monitor within the precision of track counting. Therefore, from the standpoint of neutron irradiations, a good relative basis is established for the fission track ages. "Spontaneous" and "induced" splits were then treated identically. They were mounted in epoxy resin, polished, and etched in 5% HNO3 at 20°C for 30 seconds. In each spontaneous and induced split approximately 1000 tracks were counted which corresponds to a Poisson one sigma uncertainty of approximately 4% in the ratio of spontaneous to induced track densi-
ties (Ps/Pi)" The ratio (Ps/Pi) fl)r each sample was determined at least twice, usually independently by two observers. In general, the apatite grains which were counted had reasonably uniform track densities and care was taken 1o avoid grains with zonation or nonuniform densities, e.g., concentration of tracks along fractures within the grain. For each split of each sample, the total number of 1000 counted tracks represented the sampling of approximately 30 100 grains. This reduces the uncertainty due to nonrepresentative sampling, but no quantitative assessment of this factor is made. In one case (sample 22), repeated determinations of the ratio (Ps/Pi) failed to agree within counting error. This variability made it necessary to determine first spontaneous and then induced densities on the same 35 apatite grains. Track lengths were measured projected on the plane of the polished surface, approximately parallel to prism faces. The total number of length measurements for a particular sample varied from as many as ~ 1 5 0 (for determination of length distributions) to ~ 4 0 (to determine track length reductions). Track shortening is expressed as the ratio of the average spontaneous to induced length. The uncertainty in these values, due to measurement error, is +0.05.
3. Results TABLE 1 Thermal neutron irradiation data Irradiation
Standards
Dose (X1014 n/cm 2)
A (111/27/73)
Cu foil SRM 614 sample 9
9.14 +- 0.14
B (V/18/73)
Cu foil SRM 614 sample 18
9.54 + 0.16
C (11/6/74)
Cu foil SRM 614 sample 9 sample 18
D (X/17/72)
Cu foil SRM 614
12.8 -+0.21
2.96 ± 0.24
The counting data and ages for apatite froin 25 rock samples are given in Table 2, where ages are calculated using the equation of Fleischer et al. [17]. The decay constant Xf = 8.46 X 10 -17 yr -1 [18] is used. The uncertainties quoted in Table 2 represent -+1a which is the counting uncertainty assuming a Poisson distribution. The uncertainty, therefore, includes neither the systematic uncertainties of the neutron dose (approximately 2%) nor the uncertainty in the decay constant. Sample locations are shown on Fig. 1 and exact locations are given in the Appendix. With one exception, our fission track ages range from 64 to 100 m.y., with an arithmetic mean of 83 m.y. (Table 2). The apatite from the Red Beach Pluton (sample 25) gives an age of 123 m.y. These ages are much lower than K - Ar and R b - S r ages of these same rocks reported by previous workers (Table 3). Only 6 apatite fission track ages have been previously reported for these rocks [12,13]. Three of them are
183
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82 MA
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68"W I
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Fig. 1. Map of the geographic region of the present study showing sample locations (crosses) and sample numbers and apatite fission track ages as given in Table 2. Cretaceous intrusives are indicated by the open outlines or by circles where outlines are very small. Approximate extent of region showing "disturbed" K Ar mica ages [11] is also outlined.
40.
40sample 10
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projected length 1 unit = 0 . 7 8 # Fig. 2. Spontaneous and induced track length distributions for sample 10 and 14, as discussed in the text.
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184 TABLE 2 Fission track data and ages for apatites from New England (the standard deviation associated with the age reflects only counting uncertainty)
Sample No.
Unit
1 2 3 4 5
Marble at Port Henry Devonian granite (Cabot, VI.) Cuttingsville syenite Ascutney granite Mt. Tabor gneiss
6 7 8 9
Gneiss of Chester Dome kittleton formation Littleton formation Concord granite
10 11 12 13 14 15 16 17 18
Pauchaug gneiss Monson gneiss Pelham gneiss Peabody granite Concord granite Belknap syenite Conway granite (Merrymeeting) Winnipesaukee quartz diorite Winnipesaukee quartz diorite
19 20 21 22 23 24 25
Agamenticus Complex granite Cape Neddick gabbro Tatnic Hills gabbro Alfred gabbro Hamlin Hill schist Saddleback Mtn. pluton Red Beach pluton
Short.*
0.89 0.93 0.87 1.00
0.96 0.78
0.90
1.01 0.86 0.93 0.97 1.01 0.85
0.96
lrrad.**
Pi( x 105/cm2)
(Ps/Pi)
C C D D A D A C C C A D D D C A A
30.4 6.24 0.91 1.36 5.48 4.82 1.14 7.04 5.28 9.44 7.14 1.15 1.31 0.98 1.55 6.16 3.28
1.32 1.51 5.63 5.59 1.73 5.23 1.75 1.28 1.20 1.08 1.69 6.60 5.57 6.41 1.06 2.00 1.57
84 + 3 96 + 4 83 ,+ 4 82 ,+ 4 79 ,+ 3 77 _+_3 80 ,+ 5 81 ,+ 3 76 ,+ 3 69 -+ 3 77 -+ 3 97 -+ 5 82_+ 3 94 ,+ 5 68 ,+ 4 91 ,+ 3 72 -+ 3
C D C B B A B C B B C
2.82 2.74 19.76 14.79 1.47 1.25 2.47 3.14 5.87 7.32 2.36
1.40 4.30
89 ,+ 5 64 +_3
1.48 1.76 2.21 2.02 1.40 1.43 1.98 1.94
Age (m.y.)
70 84 100 96 89 68 94 123
-+ 3 ,+ 5 ,+ 4 ±4 -+ 4 ,+ 3 ,+ 3 ,+ 5
* Average spontaneous length divided by average induced length. ** Refers to irradiations in Table 1.
roughly e q u i v a l e n t to our data and three are significantly higher. Track s h o r t e n i n g data, w h e r e d e t e r m i n e d , are also given in Table 2. Average s p o n t a n e o u s t r a c k s h o r t e n ing ranges f r o m 1.01 to a high o f 0.78. Fig. 2 s h o w s single m o d e d i s t r i b u t i o n s o f p r o j e c t e d t r a c k l e n g t h s for samples 10 and 14. The lack o f significant bim o d a l i t y in these d i s t r i b u t i o n s is a p p a r e n t .
4. Discussion The fission track apatite ages in this s t u d y are comp a r e d with i s o t o p i c ages and w i t h previous fission track ages in Table 3. There is n o c o r r e l a t i o n b e t w e e n i s o t o p i c age and apatite fission track age in these samples, either taken t o g e t h e r , or even a m o n g the isotopically y o u n g intrusions. A p a t i t e fission track ages
185 TABLE 3 Comparison of apatite fission track ages with previous isotopic and apatite fission track ages for the units studied (all ages in m.y.)
Unit
Previous work isotopic age
3 4 5 6 7,8 11 13 14 15 16 19 20 21 25
99 113 405 740 324 -250 262 350-405 367 304,318 158 110 216 227 116 120 407
This study method*
ref.
1 1 1 3 1 1 1 1 2 3 1 1,2 1 2 1 1 1
[101 [91 [261 [261 [26] [20] [11 l [27] [27] [28] [29] [91 [9] [301 {29] [291 [29]
fission track apatite age**
ref.
137
112]
203
[121
95,140
[13]
93 84
[13] [13]
83 82 79 80 81,76 82 68 68 91 72 89 84 100 96 123
* Notation is: 1 = K-Ar, mica; 2 = Rb-Sr, whole rock; 3 = Rb-Sr, mica. ** Fission track ages recalculated using: Xf = 8.46 X 10 -17 yr -1 .
in the New England region do not appear to date times of petrogenesis. This is contrary to the conclusions of other studies of apatite fission track ages in New England [12,13]. The apatite fission track ages determined by Christopher [13], when recalculated using the decay constant Xf = 8.46 X 10 -~7 yr -~ , show reasonable agreement with those determined in this study (see Table 3). In addition, preliminary apatite fission track ages for some of the White Mountain magma series intrusions recently determined by J. Doherty (personal communication, 1974) are consistent with the data presented herein. Some of the apatite ages determined by Naeser [12] (also see Table 3) are distinctly older even when recalculated with our decay constant. It is likely that these dates [12] are systematically in error because of early difficulties in precise monitoring of the neutron dose (C.W. Naeser, oral communication, 1975). The apatite data must represent an event which was
regional in nature. In an area of some 60,000 km 2 the ages appear to be distributed within a narrow range ( 6 5 - 1 0 0 m.y.) without any significant regional pattern. While there may be real differences among the ages in Table 2, the present uncertainties in tile fission track technique do not allow their complete resolution. Our present sampling network does not suggest any geographic boundary to this region of low apatite ages except in easternmost Maine (Red Beach Pluton), where we observe a higher age (123 m.y.). While the trend of apatite fission track ages determined by Naeser and Brookins [14] in eastern Maine substantiates this eastern boundary, our age determinations over corresponding areas are lower. The reasons for these discrepancies are not clear at this time but it is hoped that our continuing work in this region will lead to their recognition. It is known that fission tracks in apatite are particularly temperature-sensitive. By extrapolating highertemperature data for track fading in apatite and by
186 considering absolute track production rates, the relationship between various cooling rates and the temperature o f an apatite sample at the time represented by its age may be calculated [5,19]. For an extreme range of monotonic cooling rates, from 1°C/103 yr to I°C/107 yr, a temperature range of 155 -+ 20°C to 100 -+ 20°C respectively would be indicated for the time represented by the apatite age. It would seem reasonable that the conditions in the New England area were near the lower rate of cooling, implying that our ages date the passage through a temperature of roughly 100°C. Three samples from Mr. Monadnock (samples 7, 8, and 9), collected to test for a local "altitude effect" similar to that shown in the Alps [5], were taken at 960, 580, and 330 m above sealevel and give ages of 81 -+ 5, 76 .+ 5, and 69 + 4 m.y. respectively. Taking the data at face value and assuming a constant rate of uplift, constant geothermal gradient, and identical track-fading characteristics, one obtains an uplift rate of about 50 m/m.y, over this interval. Considering the large uncertainty of the data, however, this result must be viewed as tentative. Alternatively, assuming constant and continuous uplift and accepting the average 83 m.y. time as a cooling age which indicates the temperature of approximately 100°C, one arrives at a cooling rate of roughly 1°C/m.y. A geothermal gradient of 25°C/kin then translates to an uplift rate of 40 m/m.y. Based on K--Ar biotite ages in the same area, Krueger and Reesman [20] calculate an uplift rate of 40 m/m.y, during the Permian. (Our attempt to make a similar profile over the 1500-m relief of Mt. Washington was unsuccessful because of poor apatite recovery from the samples collected there.) We interpret the single mode distributions of samples 10 and 14 to exclude the likelihood of any partial annealing event producing mixed apatite fission track ages. Due to unresolved differences between independently determined correction factors [21,22] and recent questioning of the validity of such corrections [23], the ages listed in Table 2 have not been corrected for track annealing. On a regional scale there appears to be no relationship between elevation, geographic location, or isotopic age and track shortening. As most of the variability observed here can be ascribed to the -+0.05 uncertainty in measurement, it would appear that the thermal histories of these samples as recorded by
apatite ages and track shortening are the same. They all cooled to and through the partial stability field at approximately the same rate and approximately the same time. These ages nmst reflect an event, regional in extent, at the end of which the present topographic surface cooled to temperatures below approxinrately 100°C. A discrete low-temperature heating event or regional uplift and denudation individually or in tandem provide two possible mechanisnrs for this regional event. There is no recognized widespread metanlorphic event during the Cretaceous in New England. Fairbairn et al. [24] show evidence for a late Cretaceous disturbance of the Sr isotope systematics in some Monteregian Hills rocks. However, the Monteregian Hills intrusives are about 200 km north from the present area and there is no evidence for any event further south. The last magmatic episode of real significance was the formation of the White Mountain magma series and similar intrusives approximately 170 190 m.y., and roughly 120 m.y. ago ([10,11] and our unpublished K - Ar data). The isotopic ages of the younger Mesozoic intrusives (samples 3, 4, 16, 20, 21) are only somewhat greater than the apatite ages. The possibility that the K - A r ages have also been lowered by the same thermal event, is excluded by the concordance of biotite K - A r and R b - S r wholerock ages [ 10]. It is possible that emplacement of the White Mountain rocks, at a depth of 3 - 5 knr [25] was accompanied by regional heating which was sufficient to fade most of the pre-existing apatite fission tracks. It would seem unreasonable to conclude that heat transfer from the crystallizing stocks could have caused track fading because their areal extent is small and the distance of some of the samples (samples 7, 13, 24, 25) from any Mesozoic intrusive activity is large. If this intrusive interval was accompanied by a regional increase in the geothermal gradient (above 30°C/kin) annealing conditions could have prevailed. The alternative mechanism, preferred at this time, is that all the fission track ages represent a datum in the uplift and denudation history of the region. The somewhat older sample from easternmost Maine may indicate a degree of differential uplift but this relationship is tenuous. A large part of our study area corresponds with a well-defined region of"disturbed" K Ar mica ages which have been interpreted as de-
187 fining a region o f Permian uplift [26,9]. We can only speculate as to the persistence of this uplift from the Permian through the Cretaceous. It will be necessary to determine the geographic e x t e n t and g e o m e t r y of the y o u n g apatite fission track age region before such continuity could be conclusively demonstrated. The regional uplift interpretation would thus far apply to a large area extending at least from the eastern Adirondacks into Maine. This is not to say that the entire region was uplifted as a single block but rather that the rate o f uplift, to a first a p p r o x i m a t i o n , was the same over this large area,
5.
Conclusions
(1) Apatite fission track ages from the central northern New England area o f the N o r t h e r n Appalachians do not, in general, represent times of petrogenesis. The apatite ages o f some small Cretaceous intrusions are an e x c e p t i o n as they do not differ greatly from isotopic ages where the latter are available. However, these fission track ages are similar to others in the region and thus do not in themselves determine e m p l a c e m e n t times. (2) Geologic units of different isotopic age within an area over m o r e than 60,000 km z yield apatite dates within 20 m.y. of the average 83-m.y. age. The b o u n d a r y of this area of rather uniformly low ages has not been delineated. Analysis o f one sample from easternmost Maine yields 123 m.y. and suggests that, while generally low ages may be e x p e c t e d over a very large area, apalite fission track ages in all o f the N o r t h e r n Appalachians may not be u n i f o r m l y low. (3) The Cretaceous apatite ages are interpreted as cooling ages corresponding to the time when the area now exposed last cooled from temperatures above about 100°C. We prefer to ascribe this cooling to regional uplift and denudation.
Acknowledgments The authors express their thanks to S. Carpenter for assistance in the irradiation work, and to A.W. Quinn, w h o kindly donated some rock samples. Financial assistance was provided by the National Science
F o u n d a t i o n , Grant Nos, GA-35192 and DES 7418644.
Appendix Exact locations and altitudes for the samples studied (numbering as in Table 2 and Fig. 1) are: 1 44°03.2'N, 73°27.6'W; 34 m 2 44°21.8'N, 72°18.3'W; 380 m 3 - 43°28.6'N, 72°53.TW; 555 m 4 43°26.3'N, 72°25.9'W; 490 m 5 43°20.TN, 72°45.0'W; 610 m 6 43°18.2'N, 72°36.3'W; 230 m 7 42°51.6'N, 72°06.TW; 960 m 8 42°51.2'N. 72°05.TW; 580 m 9 42°47.9'N, 72°09.1'W; 330 m 10 - 42°37.YN, 72°22.1'W; 245 m 11 42°33.5'N, 72°18.2'W; 185 m 12 - 42°36.0'N, 72°22.0'W; 200 m 13 - 42°33.3'N, 70°52.2'W;20 m 14 43°14.2'N, 71°34.3'W; 170 m 15 43°33.3'N, 71°22.1'W; 275 m 16 - 43°28.0'N, 71°II.0'W;215m 17 - 43°42.8'N, 71°27.9'W; 185 m 18 - 43°37.6'N, 71°lI.I'W; 220 m 19 43°12.9'N, 70°40.4'W; 60 m 20 43°10.0'N, 70°35.TW; 0 m 21 - 43°17.0'N, 70°42.8'W; 70 m 22 -- 43°29.3'N, 70°43.TW; 115 m 23 44°24.2'N, 70°16.5'W; 150 m 24 44°55.5'N, 70°32.4'W; 755 m 25 - 45°03.2'N, 67°06.4'W; 0 m
References 1 S. Eakatos and D.S. Miller, Water-pressure effect on fission track annealing in an Alpine muscovite, Earth Planet. Sci. Lett. 9 (1970) 77. 2 J. Burchart and G.M. Reimer, Effect of ionic solutions on fission-track stability in apatite (abstract), Trans. Am. Nuc. Soc. 15 (1972) 129. 3 C.W. Naeser and tl. Faul, Fission track annealing in apatite and spheric, J. Geophys. Res. 74 (1969) 705. 4 G.A. Wagner, Fission track dating of apatites, Earth Planet. Sci. Lett. 14 (1972)411. 5 G.A. Wagner and G.M. Reimer, Fission track tectonics: the tectonic interpretation of fission track ages, Earth Planet. Sci. Lett. 14 (1972) 263. 6 J. Burchart, Fission-track age determinations of accessory apatite from the Tatra Mountains, Poland, Earth Planet. Sci. Lett. 15 (1972) 418. 7 J. Lyons and H. Faul, Isotope geochronology of the Northern Appalachians, in: Zen, E-an et al., eds., Studies
188
8
9
10
11
12
13 14
15 16 17 18
19
of Appalachian Geology, Northern and Maritime (J. Wiley and Sons, New York, 1968) 305. E-an Zen, W.tt. White, J.B. ltadley and J.B. Thompson, Jr., eds., Studies of Appalachian Geology, Northern and Maritime (J. Wiley and Sons, New York, 1968) 475 pp. R.E. Zartman, P.M. Hurley, tt.W. Krueger and B.J. Giletti, A Permian disturbance of K Ar radiometric ages in New England: its occurrence and cause, Geol. Soc. Am. Bull. 81 (1970) 3359. K.A. Foland, A.W. Quinn and B.J. Giletti, K Ar and Rb -St Jurassic and Cretaceous ages for intrusives of the White Mountain magma series, northern New England, Am. J. Sci. 270 (1971) 321. R.L. Armstrong and E. Stump, Additional K Ar dates, White Mountain magma series, New England, Am. J. Sci. 270 (1971) 331. C.W. Naeser, The use of apatite and sphene for fission track age determination, Geol. Soc. Am. Bull. 78 (1967) 1523. P.A. Christopher, Fission track ages of younger intrusions in southern Maine, Geol. Soc. Am. Bull. 80 (1969) 1809. C.W. Naeser and D.G. Brookins, Comparison of fissiontrack, K Ar, and Rb Sr radiometric age determinations from some granite plutons in Maine, J. Res., U.S. Geol. Surv. 3 (1975) 229. P.D. LaFleur and D.A. Becker, eds., NBS Tech. Note 548 (1970). Certificate of Analysis, National Bureau of Standards, Standard Reference Material 614, Track Elements in Glass. R.L. Fleischer, P.B. Price and R.M. Walker, Tracks of charged particles in solids, Science 149 (1965) 383. D. Galliker, E. ltugentobler and B. Hahn, Spontane Kernspaltung von U-238 und Am-241, Helv. Phys. Acta 43 (1970) 593. G.M. Reimer, Fission track geochronology: method for tectonic interpretation of apatite studies with examples from the central and southern Alps, Unpublished Ph.D. Thesis, Univ. of Pennsylvania, Philadelphia, Pa. (1972).
20 ll.W. Krueger and R.It. Reesman, Radiometric determination of the late Paleozoic erosion rate in New tlampshire and the age of metamorphism of the kittleton lk~rmation, (abstract), Geol. Soc. Am., Abstracts with Programs 3 (1971)627. 21 G.A. Wagner and D. Storzer, Die Interpretation yon Spaltspuren-Altern (fission track ages) am Beispiel yon natiirlichen Gl~sern, Apatitcn und Zirkonen, Eclogae Geol. Helv. 63 (1970) 335. 22 E. M/irk, M. Pahl, F. Purtscheller and T.D. M/irk, Thcrmische Ausheilung von Uran-Spaltspuren ill Apatiten, Alterskorrekturen und Beitr/ige zur Geothermochronologie, Tsehermaks Min. Petrol. Mitt. 20 (1973) 131. 23 C.W. Naeser and R.L. Fleischer, Age of the apatite at Cerro de Mercado, Mexico: a problem for fission track annealing corrections, Geophys. Res. Lett. 2 (1975) 67. 24 H.W. Fairbairn, G. Faure, W.H. Pinson, P.M. Hurley and J.L. Powell, Initial ratio of strontium 87 to strontium 86, whole rock age, and discordant biotite in the Monteregian igneous province, Quebec, J. Geophys. Res. 68 (1968) 6515. 25 M.P. Billings, The geology of New Hampshire, 2. Bedrock geology: Concord, N.H. (New Hampshire State Planning and Development Commission, 1956) 203 pp, 26 tt. Faul, T.W. Stern, H.H. Thomas and P.L.D. Ehnore, Ages of intrusion and metamorphism in the northern Appalachians, Am. J. Sci. 261 (1963) 1. 27 R.E. Zartman and R.F. Marvin, Radiometric age (late Ordovician) of the Quincy, Cape Ann, and Peabody granites from eastern Massachusetts, Geol. Soc. Am. Bull. 82 (1971) 937. 28 H.W. Fairbairn, W.H. Pinson, P.M. Hurley and R.F. Cotrater, Comparison of ages of some coexisting biotite and muscovite in some Paleozoic granitic rocks, Geochim. Cosmochim. Acta 19 (1960) 7. 29 K.A. Foland and H. Faul, Unpublished data. 30 J. Hoefs, A Rb Sr investigation in the southern York County area, Maine, M.I.T. Fifteenth Ann. Prog. Rep. to U.S.A.E.C. (1967) 127.
[-~ LL3
CRETACEOUS FISSION TRACK DATES OF APATITES FROM NORTHERN NEW ENGLAND R.A. ZIMMERMANN 1 , G.M. REIMER 2 ,*, K.A. FOLAND 1 and H. FAUL 1
t Department of Geology, University of Pennsylvania, Philadelphia, Pa. (USA) 2National Bureau of Standards, Gaithersburg, Md. (USA) Received June 9, 1975 Revised version received August 26, 1975
Twenty-five Cretaceous fission track ages have been obtained for apatite concentrates from metamorphic and igneous rocks of northern New England. Ages in central northern New England range from 64 to 100 m.y. (83 m,y. average) indicating a region of roughly equivalent apatite fission track ages. However, the boundaries of this region (>60,000 knl2) are not yet defined. A significantly higher Cretaceous age of 123 m.y. has been determined in easternmost Maine. The ages, exclusive of the one distinctly older age, do not correlate with isotopic ages for the same rocks and are taken to represent a regional event. They could be attributed to Cretaceous regional heating of wide extent and low temperature or alternatively to regional cooling which accompanied uplift and denudation. The presently preferred explanation is that the data represent cooling ages, indicating an approximate temperature of 100°C about 83 m.y. ago for the crustal level now exposed.
1. Introduction
reported from the area range from Precambrian to Cretaceous. A large area extending southwestward from central Maine shows K - A t ages disturbed in the Permian [9], and, numerous post-orogenic intrusions associated with the White magma series, generally midMesozoic in age, are scattered throughout the region [10,11]. Naeser [12], Christopher [13], and Naeser and Brookins [14] have reported fission track ages from the region.
Spontaneous fission of 238U in apatite crystals produces narrow tubes of intense damage. Below a certain characteristic temperature these tubes remain preserved in the crystal, but when the ambient temperature rises above this limit, the fission damage tends to repair itself and the tracks, as revealed by etching, fade quickly (in geologic time). Other effects may enter the picture [1,2], but temperature is the primary factor affecting quantitative retention of fission tracks [ 3 - 6 ] . Fission track study of apatites represents a means of examining the thermal history of rocks within a relatively low temperature range. We have selected the northern Appalachian region for our study because it has a wide variety of rock types, an age pattern indicative of multiple igneous and metamorphic events [7], and has been studied extensively by many workers [8]. The isotopic ages
2. Analytical methods Apatite was concentrated (grain size 350--100 ~m) using standard separation techniques. Samples were split with one part being retained for later determination of spontaneous track density. The other part was heated in air at 550°C for one hour to erase all spontaneous tracks. These grains were then irradiated in the thermal neutron facility (RT-4) of the National Bureau of Standards Research Reactor at Gaithersburg, Maryland. Individual apatite samples were sealed in poly-
* Present address: U.S. Geological Survey, Denver, Colorado 80225. 181
182 ethylene bags and placed in a second bag for irradiation. The packet was kept small (1 cm × 1 cm X 0.5 cm) to minimize the effect of neutron flux gradient. In all, four irradiations were performed. The total thermal neutron dose was determined from the 63Cu(n,')')64Cu reaction induced in a copper foil monitor attached to the packets. The nuclear counting equipment has been described elsewhere [ 15]. An internal standard, SRM 614, trace elements in glass [16], and some duplicate apatite samples provided additional cross checks for these irradiations. Table 1 lists the standards, duplicate samples, and the thermal neutron dose for each irradiation. Quoted neutron dose errors (-+1o) are the uncertainties for the gamma counting. There is agreement of the internal standards and the Cu foil monitor within the precision of track counting. Therefore, from the standpoint of neutron irradiations, a good relative basis is established for the fission track ages. "Spontaneous" and "induced" splits were then treated identically. They were mounted in epoxy resin, polished, and etched in 5% HNO3 at 20°C for 30 seconds. In each spontaneous and induced split approximately 1000 tracks were counted which corresponds to a Poisson one sigma uncertainty of approximately 4% in the ratio of spontaneous to induced track densi-
ties (Ps/Pi)" The ratio (Ps/Pi) fl)r each sample was determined at least twice, usually independently by two observers. In general, the apatite grains which were counted had reasonably uniform track densities and care was taken 1o avoid grains with zonation or nonuniform densities, e.g., concentration of tracks along fractures within the grain. For each split of each sample, the total number of 1000 counted tracks represented the sampling of approximately 30 100 grains. This reduces the uncertainty due to nonrepresentative sampling, but no quantitative assessment of this factor is made. In one case (sample 22), repeated determinations of the ratio (Ps/Pi) failed to agree within counting error. This variability made it necessary to determine first spontaneous and then induced densities on the same 35 apatite grains. Track lengths were measured projected on the plane of the polished surface, approximately parallel to prism faces. The total number of length measurements for a particular sample varied from as many as ~ 1 5 0 (for determination of length distributions) to ~ 4 0 (to determine track length reductions). Track shortening is expressed as the ratio of the average spontaneous to induced length. The uncertainty in these values, due to measurement error, is +0.05.
3. Results TABLE 1 Thermal neutron irradiation data Irradiation
Standards
Dose (X1014 n/cm 2)
A (111/27/73)
Cu foil SRM 614 sample 9
9.14 +- 0.14
B (V/18/73)
Cu foil SRM 614 sample 18
9.54 + 0.16
C (11/6/74)
Cu foil SRM 614 sample 9 sample 18
D (X/17/72)
Cu foil SRM 614
12.8 -+0.21
2.96 ± 0.24
The counting data and ages for apatite froin 25 rock samples are given in Table 2, where ages are calculated using the equation of Fleischer et al. [17]. The decay constant Xf = 8.46 X 10 -17 yr -1 [18] is used. The uncertainties quoted in Table 2 represent -+1a which is the counting uncertainty assuming a Poisson distribution. The uncertainty, therefore, includes neither the systematic uncertainties of the neutron dose (approximately 2%) nor the uncertainty in the decay constant. Sample locations are shown on Fig. 1 and exact locations are given in the Appendix. With one exception, our fission track ages range from 64 to 100 m.y., with an arithmetic mean of 83 m.y. (Table 2). The apatite from the Red Beach Pluton (sample 25) gives an age of 123 m.y. These ages are much lower than K - Ar and R b - S r ages of these same rocks reported by previous workers (Table 3). Only 6 apatite fission track ages have been previously reported for these rocks [12,13]. Three of them are
183
I
1
,;w
72"W
74°W
'
..22+
96 NH VT
£+0 I~
- "Y ~t
,oo 14 +
I
/. /
o ~
Z 'f6
I
,~%~i'1'9 (~20
9-T
somple number apatite F-T age (my.)
rl~
0 100-120rny. intrusions
A
73
extent of Permian reset K-At mica oges
82 MA
42"N
68"W I
t
Fig. 1. Map of the geographic region of the present study showing sample locations (crosses) and sample numbers and apatite fission track ages as given in Table 2. Cretaceous intrusives are indicated by the open outlines or by circles where outlines are very small. Approximate extent of region showing "disturbed" K Ar mica ages [11] is also outlined.
40.
40sample 10
L--!__]
C
L-
sample 14 C
spon. - - - -
G
ind.
L__~
10
i I'---- I~, •
|
•
g
i
1o
•
•
10
I.----4-------! I
15
I
I
I
5
|
I
I
I
|
!
I
!
I
10
projected length 1 unit = 0 . 7 8 # Fig. 2. Spontaneous and induced track length distributions for sample 10 and 14, as discussed in the text.
I
15
I
184 TABLE 2 Fission track data and ages for apatites from New England (the standard deviation associated with the age reflects only counting uncertainty)
Sample No.
Unit
1 2 3 4 5
Marble at Port Henry Devonian granite (Cabot, VI.) Cuttingsville syenite Ascutney granite Mt. Tabor gneiss
6 7 8 9
Gneiss of Chester Dome kittleton formation Littleton formation Concord granite
10 11 12 13 14 15 16 17 18
Pauchaug gneiss Monson gneiss Pelham gneiss Peabody granite Concord granite Belknap syenite Conway granite (Merrymeeting) Winnipesaukee quartz diorite Winnipesaukee quartz diorite
19 20 21 22 23 24 25
Agamenticus Complex granite Cape Neddick gabbro Tatnic Hills gabbro Alfred gabbro Hamlin Hill schist Saddleback Mtn. pluton Red Beach pluton
Short.*
0.89 0.93 0.87 1.00
0.96 0.78
0.90
1.01 0.86 0.93 0.97 1.01 0.85
0.96
lrrad.**
Pi( x 105/cm2)
(Ps/Pi)
C C D D A D A C C C A D D D C A A
30.4 6.24 0.91 1.36 5.48 4.82 1.14 7.04 5.28 9.44 7.14 1.15 1.31 0.98 1.55 6.16 3.28
1.32 1.51 5.63 5.59 1.73 5.23 1.75 1.28 1.20 1.08 1.69 6.60 5.57 6.41 1.06 2.00 1.57
84 + 3 96 + 4 83 ,+ 4 82 ,+ 4 79 ,+ 3 77 _+_3 80 ,+ 5 81 ,+ 3 76 ,+ 3 69 -+ 3 77 -+ 3 97 -+ 5 82_+ 3 94 ,+ 5 68 ,+ 4 91 ,+ 3 72 -+ 3
C D C B B A B C B B C
2.82 2.74 19.76 14.79 1.47 1.25 2.47 3.14 5.87 7.32 2.36
1.40 4.30
89 ,+ 5 64 +_3
1.48 1.76 2.21 2.02 1.40 1.43 1.98 1.94
Age (m.y.)
70 84 100 96 89 68 94 123
-+ 3 ,+ 5 ,+ 4 ±4 -+ 4 ,+ 3 ,+ 3 ,+ 5
* Average spontaneous length divided by average induced length. ** Refers to irradiations in Table 1.
roughly e q u i v a l e n t to our data and three are significantly higher. Track s h o r t e n i n g data, w h e r e d e t e r m i n e d , are also given in Table 2. Average s p o n t a n e o u s t r a c k s h o r t e n ing ranges f r o m 1.01 to a high o f 0.78. Fig. 2 s h o w s single m o d e d i s t r i b u t i o n s o f p r o j e c t e d t r a c k l e n g t h s for samples 10 and 14. The lack o f significant bim o d a l i t y in these d i s t r i b u t i o n s is a p p a r e n t .
4. Discussion The fission track apatite ages in this s t u d y are comp a r e d with i s o t o p i c ages and w i t h previous fission track ages in Table 3. There is n o c o r r e l a t i o n b e t w e e n i s o t o p i c age and apatite fission track age in these samples, either taken t o g e t h e r , or even a m o n g the isotopically y o u n g intrusions. A p a t i t e fission track ages
185 TABLE 3 Comparison of apatite fission track ages with previous isotopic and apatite fission track ages for the units studied (all ages in m.y.)
Unit
Previous work isotopic age
3 4 5 6 7,8 11 13 14 15 16 19 20 21 25
99 113 405 740 324 -250 262 350-405 367 304,318 158 110 216 227 116 120 407
This study method*
ref.
1 1 1 3 1 1 1 1 2 3 1 1,2 1 2 1 1 1
[101 [91 [261 [261 [26] [20] [11 l [27] [27] [28] [29] [91 [9] [301 {29] [291 [29]
fission track apatite age**
ref.
137
112]
203
[121
95,140
[13]
93 84
[13] [13]
83 82 79 80 81,76 82 68 68 91 72 89 84 100 96 123
* Notation is: 1 = K-Ar, mica; 2 = Rb-Sr, whole rock; 3 = Rb-Sr, mica. ** Fission track ages recalculated using: Xf = 8.46 X 10 -17 yr -1 .
in the New England region do not appear to date times of petrogenesis. This is contrary to the conclusions of other studies of apatite fission track ages in New England [12,13]. The apatite fission track ages determined by Christopher [13], when recalculated using the decay constant Xf = 8.46 X 10 -~7 yr -~ , show reasonable agreement with those determined in this study (see Table 3). In addition, preliminary apatite fission track ages for some of the White Mountain magma series intrusions recently determined by J. Doherty (personal communication, 1974) are consistent with the data presented herein. Some of the apatite ages determined by Naeser [12] (also see Table 3) are distinctly older even when recalculated with our decay constant. It is likely that these dates [12] are systematically in error because of early difficulties in precise monitoring of the neutron dose (C.W. Naeser, oral communication, 1975). The apatite data must represent an event which was
regional in nature. In an area of some 60,000 km 2 the ages appear to be distributed within a narrow range ( 6 5 - 1 0 0 m.y.) without any significant regional pattern. While there may be real differences among the ages in Table 2, the present uncertainties in tile fission track technique do not allow their complete resolution. Our present sampling network does not suggest any geographic boundary to this region of low apatite ages except in easternmost Maine (Red Beach Pluton), where we observe a higher age (123 m.y.). While the trend of apatite fission track ages determined by Naeser and Brookins [14] in eastern Maine substantiates this eastern boundary, our age determinations over corresponding areas are lower. The reasons for these discrepancies are not clear at this time but it is hoped that our continuing work in this region will lead to their recognition. It is known that fission tracks in apatite are particularly temperature-sensitive. By extrapolating highertemperature data for track fading in apatite and by
186 considering absolute track production rates, the relationship between various cooling rates and the temperature o f an apatite sample at the time represented by its age may be calculated [5,19]. For an extreme range of monotonic cooling rates, from 1°C/103 yr to I°C/107 yr, a temperature range of 155 -+ 20°C to 100 -+ 20°C respectively would be indicated for the time represented by the apatite age. It would seem reasonable that the conditions in the New England area were near the lower rate of cooling, implying that our ages date the passage through a temperature of roughly 100°C. Three samples from Mr. Monadnock (samples 7, 8, and 9), collected to test for a local "altitude effect" similar to that shown in the Alps [5], were taken at 960, 580, and 330 m above sealevel and give ages of 81 -+ 5, 76 .+ 5, and 69 + 4 m.y. respectively. Taking the data at face value and assuming a constant rate of uplift, constant geothermal gradient, and identical track-fading characteristics, one obtains an uplift rate of about 50 m/m.y, over this interval. Considering the large uncertainty of the data, however, this result must be viewed as tentative. Alternatively, assuming constant and continuous uplift and accepting the average 83 m.y. time as a cooling age which indicates the temperature of approximately 100°C, one arrives at a cooling rate of roughly 1°C/m.y. A geothermal gradient of 25°C/kin then translates to an uplift rate of 40 m/m.y. Based on K--Ar biotite ages in the same area, Krueger and Reesman [20] calculate an uplift rate of 40 m/m.y, during the Permian. (Our attempt to make a similar profile over the 1500-m relief of Mt. Washington was unsuccessful because of poor apatite recovery from the samples collected there.) We interpret the single mode distributions of samples 10 and 14 to exclude the likelihood of any partial annealing event producing mixed apatite fission track ages. Due to unresolved differences between independently determined correction factors [21,22] and recent questioning of the validity of such corrections [23], the ages listed in Table 2 have not been corrected for track annealing. On a regional scale there appears to be no relationship between elevation, geographic location, or isotopic age and track shortening. As most of the variability observed here can be ascribed to the -+0.05 uncertainty in measurement, it would appear that the thermal histories of these samples as recorded by
apatite ages and track shortening are the same. They all cooled to and through the partial stability field at approximately the same rate and approximately the same time. These ages nmst reflect an event, regional in extent, at the end of which the present topographic surface cooled to temperatures below approxinrately 100°C. A discrete low-temperature heating event or regional uplift and denudation individually or in tandem provide two possible mechanisnrs for this regional event. There is no recognized widespread metanlorphic event during the Cretaceous in New England. Fairbairn et al. [24] show evidence for a late Cretaceous disturbance of the Sr isotope systematics in some Monteregian Hills rocks. However, the Monteregian Hills intrusives are about 200 km north from the present area and there is no evidence for any event further south. The last magmatic episode of real significance was the formation of the White Mountain magma series and similar intrusives approximately 170 190 m.y., and roughly 120 m.y. ago ([10,11] and our unpublished K - Ar data). The isotopic ages of the younger Mesozoic intrusives (samples 3, 4, 16, 20, 21) are only somewhat greater than the apatite ages. The possibility that the K - A r ages have also been lowered by the same thermal event, is excluded by the concordance of biotite K - A r and R b - S r wholerock ages [ 10]. It is possible that emplacement of the White Mountain rocks, at a depth of 3 - 5 knr [25] was accompanied by regional heating which was sufficient to fade most of the pre-existing apatite fission tracks. It would seem unreasonable to conclude that heat transfer from the crystallizing stocks could have caused track fading because their areal extent is small and the distance of some of the samples (samples 7, 13, 24, 25) from any Mesozoic intrusive activity is large. If this intrusive interval was accompanied by a regional increase in the geothermal gradient (above 30°C/kin) annealing conditions could have prevailed. The alternative mechanism, preferred at this time, is that all the fission track ages represent a datum in the uplift and denudation history of the region. The somewhat older sample from easternmost Maine may indicate a degree of differential uplift but this relationship is tenuous. A large part of our study area corresponds with a well-defined region of"disturbed" K Ar mica ages which have been interpreted as de-
187 fining a region o f Permian uplift [26,9]. We can only speculate as to the persistence of this uplift from the Permian through the Cretaceous. It will be necessary to determine the geographic e x t e n t and g e o m e t r y of the y o u n g apatite fission track age region before such continuity could be conclusively demonstrated. The regional uplift interpretation would thus far apply to a large area extending at least from the eastern Adirondacks into Maine. This is not to say that the entire region was uplifted as a single block but rather that the rate o f uplift, to a first a p p r o x i m a t i o n , was the same over this large area,
5.
Conclusions
(1) Apatite fission track ages from the central northern New England area o f the N o r t h e r n Appalachians do not, in general, represent times of petrogenesis. The apatite ages o f some small Cretaceous intrusions are an e x c e p t i o n as they do not differ greatly from isotopic ages where the latter are available. However, these fission track ages are similar to others in the region and thus do not in themselves determine e m p l a c e m e n t times. (2) Geologic units of different isotopic age within an area over m o r e than 60,000 km z yield apatite dates within 20 m.y. of the average 83-m.y. age. The b o u n d a r y of this area of rather uniformly low ages has not been delineated. Analysis o f one sample from easternmost Maine yields 123 m.y. and suggests that, while generally low ages may be e x p e c t e d over a very large area, apalite fission track ages in all o f the N o r t h e r n Appalachians may not be u n i f o r m l y low. (3) The Cretaceous apatite ages are interpreted as cooling ages corresponding to the time when the area now exposed last cooled from temperatures above about 100°C. We prefer to ascribe this cooling to regional uplift and denudation.
Acknowledgments The authors express their thanks to S. Carpenter for assistance in the irradiation work, and to A.W. Quinn, w h o kindly donated some rock samples. Financial assistance was provided by the National Science
F o u n d a t i o n , Grant Nos, GA-35192 and DES 7418644.
Appendix Exact locations and altitudes for the samples studied (numbering as in Table 2 and Fig. 1) are: 1 44°03.2'N, 73°27.6'W; 34 m 2 44°21.8'N, 72°18.3'W; 380 m 3 - 43°28.6'N, 72°53.TW; 555 m 4 43°26.3'N, 72°25.9'W; 490 m 5 43°20.TN, 72°45.0'W; 610 m 6 43°18.2'N, 72°36.3'W; 230 m 7 42°51.6'N, 72°06.TW; 960 m 8 42°51.2'N. 72°05.TW; 580 m 9 42°47.9'N, 72°09.1'W; 330 m 10 - 42°37.YN, 72°22.1'W; 245 m 11 42°33.5'N, 72°18.2'W; 185 m 12 - 42°36.0'N, 72°22.0'W; 200 m 13 - 42°33.3'N, 70°52.2'W;20 m 14 43°14.2'N, 71°34.3'W; 170 m 15 43°33.3'N, 71°22.1'W; 275 m 16 - 43°28.0'N, 71°II.0'W;215m 17 - 43°42.8'N, 71°27.9'W; 185 m 18 - 43°37.6'N, 71°lI.I'W; 220 m 19 43°12.9'N, 70°40.4'W; 60 m 20 43°10.0'N, 70°35.TW; 0 m 21 - 43°17.0'N, 70°42.8'W; 70 m 22 -- 43°29.3'N, 70°43.TW; 115 m 23 44°24.2'N, 70°16.5'W; 150 m 24 44°55.5'N, 70°32.4'W; 755 m 25 - 45°03.2'N, 67°06.4'W; 0 m
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8
9
10
11
12
13 14
15 16 17 18
19
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