Mesozoic sediments in the Sea of Hebrides Basin ...

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Sep 11, 2009 - Low-temperature effects of the Skye Tertiary intrusions on Mesozoic ... age and track length distributions from northern Skye, the Isle of.
Geological Society, London, Special Publications Low-temperature effects of the Skye Tertiary intrusions on Mesozoic sediments in the Sea of Hebrides Basin Cherry L. E. Lewis, Andrew Carter and Anthony J. Hurford Geological Society, London, Special Publications 1992; v. 62; p. 175-188 doi:10.1144/GSL.SP.1992.062.01.16

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University of Bristol Library on 11 September 2009

© 1992 Geological Society of London

Low-temperature effects of the Skye Tertiary intrusions on Mesozoic sediments in the Sea of Hebrides Basin CHERRY

L. E. L E W I S 1, A N D R E W

CARTER

& ANTHONY

J. H U R F O R D

University of London Fission Track Research Group, Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK (Present address: Geotrack International Pty Ltd, 30 Upper High St, Thame, Oxfordshire, UK) Abstract: Fission track analysis of samples from the Sea of Hcbrides Basin and surrounding regions demonstratcs two very distinct age groups around 50 Ma and 300 Ma, regardless of the stratigraphic age. Apatite fission track results from sedimcnts within 8 km of the Tertiary igneous complex on Skyc wcrc totally reset by temperatures >110°C during intrusion of the centres, but now yield ages younger than the granites. Zircon fission track results from the granites also demonstrate ages significantly younger than their hosts. Mean track lengths in apatitcs of < 13.5 t~m, and reduced apatite and zircon ages, suggest that temperatures remained elevated throughout the 6 Ma during which intrusive activity occurred, but were hotter within the granite bodies than thc surrounding sedimcnts. Prior to intrusion of the complex, tempcratures in thc basin sediments presently at outcrop were unlikely to have been higher than 50°C, and beyond the effects of the Tertiary intrusions, age and track length distributions from northern Skye, the Isle of Lewis and the west coast of Scotland illustrate that the area is unlikely to have been buried beneath more than 2 km of sediment at any one time since the end of the Devonian. North of thc Highland Boundary Fault, Tertiary uplift and erosion in Scotland is considered to have been between 1- 1.5 km. This is consistent with a rcgional pattern of Tertiary erosion identified across the British Isles, but considerably less than that recognised in Northern England.

The Sea of Hebrides Basin is one of a series of N E - S W orientated, fault bounded basins that lie off the west coast of Scotland (Fig. 1). The western margin of the basin is bounded by the Minch Fault, while to the east the Moine Thrust divides the Caledonian orogenic belt, which suffered extreme deformation during the Lower Palaeozoic, from the Lewisian gneisses of the Hebridean Craton that are locally overlain by thick Torridonian sediments (Upper Proterozoic). This cratonic area remained relatively unaffected by Caledonian disturbances, the last major thermo-tectonic event occurring c. 1800 Ma (Watson 1985). The basin contains a relatively shallow fill of Mesozoic sediments that outcrop on several of the Hebridean islands, and during the Early Tertiary these sediments were intruded by major igneous centres and multiple dyke and sill swarms. Up to 800 m of basalt was erupted onto the surface, possibly covering an area as large as that represented by the dyke swarms (Preston 1982), although little evidence of it now remains. In order to evaluate the lateral extent to which intrusion of the igneous complex thermally affected sediments in the basin, outcrop samples from sediments of various ages were

taken from the islands of Skye and Raasay and subjected to apatite fission track analysis. Fission track ages of zircons extracted from the Tertiary granites and two Jurassic sediments on Skye were also measured. Finally, apatite fission track results from Torridonian sediments and Lewisian gneisses of the Hebridean craton outcropping on the west coast of Scotland and the Isle of Lewis were modelled, in order to constrain the thermal structure of the region prior to Tertiary activity.

Fission track analysis

Apatite Using A F T A it is possible to determine palaeotemperatures up to I I0°C and to identify the time at which maximum temperatures last occurred. The principles of applying the technique to thermal history analysis have been discussed in detail by Green (1989), but interpretation of the data depends on an assessment of the degree of annealing (track shortening) that has occurred since deposition, which is manifested in the sample by a reduction in the length of confined tracks (Green et al. 1986). As tem-

From PARNELL,J. (ed.), 1992, Basins on the Atlantic Seaboard: Petroleum Geology. Sedimentology and Basin Evolution. Geological Society Special Publication No. 62, pp 175-188.

175

176

C. L. E. LEWIS ET AL.

peratures increase, track lengths shorten and this results in a reduction in the density of tracks occurring at the surface of a crystal, thus the measured fission track age also becomes reduced. Gleadow et al. (1986) have shown that all apatite fission track 'ages' are only apparent ages unless the mean track length associated with that age is greater than 14.5 gm and has a standard deviation of less than one. When temperatures exceed 110°C over timescales of l - 1 0 Ma, tracks are not retained in the crystal lattice until it cools again below this temperature. If cooling is rapid then the mean confined track length will be long (>14.5 /~m) and the standard deviation less than one, but if cooling is protracted and temperatures remain elevated, tracks will be annealed during cooling and the resulting distribution will be shorter and wider. Little track annealing occurs below temperatures of 50°C. Zircons Recent laboratory experiments (Tagami et al. 1990) demonstrate that, like apatites, track shortening in zircons is predominantly controlled by temperature, but due to the higher temperatures at which zircons anneal a suitable geological situation with which the laboratory experiments can be compared has not yet been located. As a result of this the annealing kinetics of zircons are still poorly understood and track lengths are not usually measured. At the present time, the 'closure' temperature for fission tracks in zircons has been qualitatively assessed by interpolating a temperature between 30WC for the R b - S r closure temperature in biotite, and 100°C for fission tracks in apatite, from the age given by the zircon. Using this method Hurford (1986) obtained a closure temperature for zircon of 240 _+ 50°C, but Harrison et al. (1979) placed it much lower at 175 _+ 25°C. These differences may reflect a difference in cooling rates or the fact that, like apatites, the annealing rates in zircons are affected by their chemical composition as suggested by Lewis et al. (in press). In this study, a temperature of 200°C was taken to be a reasonable estimate of the temperature at which tracks in zircons are totally annealed over geological timescales, thus partial annealing probably starts to occur between temperatures of 160-170°C. Data presentation At the time of deposition, each apatite grain in a sediment will contain an inherited track distribution which records the thermal history pre-

viously experienced by that grain. The result is that a range of single grain ages can be found within any one sample. If the sediment is then subjected to elevated temperatures subsequent to deposition, differential annealing rates in grains with varying CI contents (Green et al. 1989a) further enhances this spread of single ages. 'Central' ages (Lewis et al. in press) are therefore used for estimating the modal fission track age in a sample which displays a large spread of single grain ages, and the dispersion of single grain ages about the modal age is expressed as a percentage variation which is quoted in addition to the standard error, i.e. 60 + 4 Ma (20%), where +4 indicates the precision (standard error at the 1 s level) of the Central age estimate and (20%) describes the variation (relative standard deviation) about that Central age. Single grain ages within any one sample are presented on histograms as well as radial plots (Galbraith 1990), which are a graphical method of comparing several estimates that have differing precisions. Arranged radially around the edge of the plot are fission track ages in millions of years, and the approximate age of any one grain can be read by drawing a line from the origin on the y axis, through that point to the edge of the plot, as demonstrated in Fig. 4a. For inter-sample comparison, the length of the x axis in all plots throughout this paper is kept constant for both apatites and zircons, and grains with the highest precision plot furthest from the y axis. The display of data in this manner allows instant assessment of both age variation and precision, and provides considerable information regarding the samples' thermal history. Those grains with a higher C1 content that are more resistant to annealing represent the minimum source age of a sample, while grains that are less Cl-rich and more sensitive to annealing constrain the maximum age of any heating event. M o d e l l i n g apatite fission track data Since the degree of annealing is primarily determined by temperature, comparison of the Central age with the stratigraphic age of a sample provides a qualitative assessment as to whether that sample has experienced postdepositional annealing. Sediments with a Central fission track age younger than the stratigraphic age have clearly been annealed, and must therefore have experienced elevated temperatures since deposition, but a more quantitative evaluation of maximum palaeotemperatures can be determined by comparing

L O W - T E M P E R A T U R E EFFECTS OF THE SKYE IGNEOUS COMPLEX m e a s u r e d age a n d track l e n g t h d a t a with m o d e l l e d distributions. T h e p r o g r a m m e involves t h e f o r w a r d m o d e l l i n g of p a r a m e t e r s resulting f r o m likely t h e r m a l histories using t h e q u a n t i t a t i v e d e s c r i p t i o n of a n n e a l i n g o u t l i n e d by Laslett et al. (1987), D u d d y et al. (1988) a n d G r e e n et al. (1989b). T h e m o d e l l i n g p r o g r a m m e p r e d i c t s p a r a m e t e r s in apatites with an a v e r a g e C1 c o n t e n t of a b o u t 0.4 w t % ( D u r a n g o apatite).

177

Results and interpretation T a b l e 1 r e c o r d s t h e localities f r o m which o u t c r o p s a m p l e s w e r e collected, a n d T a b l e s 2 a n d 3 p r e s e n t full analytical d a t a for apatites a n d zircons respectively. T h e zircons w e r e e x t r a c t e d f r o m the T e r t i a r y granites on Skye (which did n o t yield apatite) as well as f r o m t w o s a m p l e s of Jurassic s e d i m e n t s , while the apatites w e r e de-

Table 1. Sample type and location details Sample No.

Stratigraphic/ Formation Age

Locality

Formation

Carn Brcac Milton Cruarg Inverbain Rhutoin Kinlochbervie Gualin House Durness

Torridonian Torridonian Torridonian Lewisian Torridonian Torridonian Torridonian

Precambrian Precambrian Precambrian Precambrian Precambrian Precambrian Precambrian Ordovician

582 462 593 375 599 382 570 361 575 342 574 341

North Raasay Bcinn na Lcac Rubha na Lcac Fcarns Road Eyrc Eyre

Torridonian Bcarreraig Stornoway Scalpa Stornoway Torridonian

Precambrian Jurassic Triassic Jurassic Triassic Precambrian

520 540 702 248 671 122 672 160 700 150 639 196 639 178 516 188 528 113

Bearrcraig Bay Ob Lusa Skulamus Drumfearn Loch na Dal Loch Buidhe Beinn nan Charn Camasunary Strathaird

Bcarreraig Torridonian Torridonian Torridonian Torridonian Torridonian Stornoway Torridonian Bcarrcraig

Jurassic Precambrian Precambrian Prccambrian Precambrian Precambrian Triassic Precambrian Jurassic

NF 994 907 NG 182 062 NB 488 327

Northton, Harris N of Tarbert, Harris Eye Peninsula

Lewisian Stornoway

Precambrian Precambrian Triassic

NG NG NG NG NG NG NG NG NG NG NG NG NG

Western Red Hills Western Red Hills Western Red Hills Western Red Hills Wcstern Red Hills Western Red Hills Eastern Red Hills Eastern Red Hills Eastern Red Hills Eastern Red Hills Eastern Red Hills Crcag Strollamus Elgol

Glamaig Maol na Gainmhich Bcinn Dearg Mhor Loch Ainort Loch Ainort Northern Feisite Glas Bheinn Mhor Allt Fearna Beinn an Dubhaich Beinn na Caillich Creag Strollamus Bearreraig Bearreraig

Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Tertiary Jurassic Jurassic

Rock type

Grid ref

Sandstone Sandstone Sandstone Gneiss Sandstone Sandstone Grit Grit

NG NG NG NG NG NC NC NC

753 707 705 775 798 198 300 410

Sandstone Sandstone Sandstone Sandstone Conglomerate Sandstone

NG NG NG NG NG NG

Sandstone Sandstone Sandstone Sandstone Sandstone Conglomerate Sandstone Sandstone Sandstone

NG NG NG NG NG NG NG NG NG

Tonalite Gneiss Conglomerate Granite Granite Granite Granite Granite Felsite Granite Granite Granite Granite Granite Sandstone Sandstone

Apatites Mainland SKY 166 SKY 167 SKY 171 SKY 175 SKY 178 SCOT 300 SCOT 302 SCOT 304 Isle of Raasay SKY 161 SKY 315 SKY 317 SKY 318 SKY 319 SKY 320 Skye SKY 308 SKY 327 SKY 329 SKY 330 SKY 331 SKY 337 SKY 341 SKY 350 SKY 358 Lewis/Harris LEW 292 LEW 294 LEW 298

440 438 457 560 540 601 562 675

Zircons GE MG BDME LAE1 LAE2 NPF GBMDE AFE BADE BNC CSE BERR/AS BERR/ELG

538 300 561 316 534 288 565 298 534 268 562 307 585 281 614 234 606 198 610 232 603 266 590 257 513 131

308 327 329 3311 331 337 341 350 358

161 315 317 318 319 320

Northton, Harris Tarbcrt, Harris Eye Peninsula, Lewis

Bearrcraig Bay Ob Lusa Skulamus Drumfearn Loch na Dal Loch Buidhe Beinn nan Charn Camasunary Strathaird

North Raasay Beinn na Leac Rubha na Leac Fearns Rd Eyre Lyre

C a m Brcac Milton Cruarg Invcrbain Rhutoin Kinlochbcrvie Gualin House Durness

Locality

2tI 20 21)

211 21) 18 2t) 19 20 21) 21) 21

211 5 20 20 21) 20

17 18 20 22 9 11) 21) 4

1.322 1.322 1.322

1.53[} 1.563 1.464 1.464 1.464 1.543 1.464 1.556 1.555

2.407 1.464 1.523 1.464 1.464 1.536

2.416 2.410 2.412 2.416 2.418 1.322 1.322 1.322

pd

9158 9158 9158

6412 6412 2535 2535 2535 6412 2535 6412 6412

10665 2535 6412 2535 2535 6412

10665 11)665 10665 10665 10665 9158 9158 9158

Nd

Dosimeter

[1.942 1.286 1.264 2.099 1.375 1.362

1.796 1.312 1.201 2.886 2.211 0.780 1.498 1.349

pi

1.474 1.330 1.141

992 2137 1328

3132 1794 2782 2129 709 1224 1721 1801 1424

1216 463 1413 1227 1071 1837

1~)5 505 993 2578 781 496 1867 327

Ni

Induced

3167 2.267 376 1.972 493 2.298 418 4.211 211) 2.438 267 1.9911 327 1.129 362 2.010 318 1.397

822 98 271 263 300 357

1269 360 617 1736 55 536 2675 536

Ns

1.778 1197 1.681 2700 1.4[16 1 6 3 6

3.368 0.413 0.407 [).827 0.722 [I.434 0.214 I).244 0.312

0.637 0.272 0.243 0.450 I).385 I).265

1.196 I).935 0.746 1.943 1.581) 0.892 2.146 2.211

ps

Spontaneous

4
I0 9 0 7 9 9 16 I 7 0 0 0 18

Agc variation %

51.90 - 2.70 48.40 -+ 2.50 51 .(X) -+ 3.40 45.80 + 4.311 45.th) -+ 3.50 49.60 -+ 3.00 53.10 - 3.70 49.70 + 2.20 47.50 +- 2.0(I 49.(X) +- 2.60 53.611 -+ 3.60 44.40 +- 3.311 62.70 --+ 3.90

Central Age (Ma) __+1o

t"" X

©

©

180

C.L. E. LEWIS E T A L .

rived from sedimentary and metamorphic rocks ranging in age from Lewisian to Jurassic, and collected over a much wider area than the zircons (Fig. 1). It should be noted however, that all apatite samples were collected from west of the Moine Thrust and are thus representative of either the Hebridean craton or the Mesozoic basin. As can be seen from Fig. 2, which plots fission track age against age variation, results

from both apatites and zircons fall into two very distinct categories: apatites and zircons that gives ages younger than 100 Ma, clustering around 50 Ma, and apatites that are within error of 3(X) Ma. On Skye and Raasay both age groups were observed, but elsewhere only apatite ages in the older group have been found. In Fig. 3 the relationship between the mean track length and the measured fission track age of apatites for all samples in this study is shown,

(::apeW r s ~

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Fig. 1. Sample location map for apatites and zircons showing ages obtained from each locality sampled in this study (filled squares and triangles, respectively). The enlarged area on Skye demonstrates the relationship of the reset apatite ages to Bott & Tuson's (1973) Bouger anomaly map. Also included for comparison are apatite fission track ages of the 'newer and last granites' from Hurford (1977) (open squares). The similarity of all apatite fission track ages, outside the effects of the Tertiary intrusions, suggests a similar post-Devonian history for the whole region north of the Highland Boundary Fault.

LOW-TEMPERATURE EFFECTS OF THE SKYE IGNEOUS COMPLEX l e Zircon

n Skye/Raasay apatites

35 . . . . . . . . . .

//

and representative track length distributions are displayed a r o u n d the edge of this plot. For c o m p a r a t i v e purposes the Late C r e t a c e o u s annealing curve for rocks from N o r t h e r n and Central E n g l a n d is s u p e r i m p o s e d upon the data from this study ( G r e e n 1986 and Lewis et al. in press). This curve presupposes that the source age of the apatites was a r o u n d 3(X) Ma (i.e. that all apatites were totally a n n e a l e d at 300 Ma) and that all samples have been affected to some extent by elevated t e m p e r a t u r e s during the Late C r e t a c e o u s , which was followed by cooling and erosion in the Tertiary. While these Scottish data fit well onto the y o u n g e r end of the trend, there is considerable scatter at the o l d e r e n d suggesting that in this region the source age is less clearly defined and probably s o m e w h a t o l d e r than in N o r t h e r n England. F u r t h e r m o r e , these samples do not a p p e a r to have been significantly affected by Late C r e t a c e o u s burial.

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