Hostname: page-component-77c89778f8-vpsfw Total loading time: 0 Render date: 2024-07-18T22:17:52.270Z Has data issue: false hasContentIssue false
  • English
  • Français

The South Harris gravity anomaly

Published online by Cambridge University Press:  01 May 2009

E. M. Durrance ,
P. Grainger  and
P. Kearey
E. M. Durrance
Affiliation:
Department of Geology, University of Exeter, Exeter, U.K.
P. Grainger
Affiliation:
Department of Geology, University of Exeter, Exeter, U.K.
P. Kearey
Affiliation:
Department of Geology, University of Bristol, Bristol, U.K.
Get access Rights & Permissions [Opens in a new window]

Summary

The results of a gravity survey of South Harris show that a residual gravity anomaly of + 220 g.u. occurs towards the southeast of the anorthosite-gabbro-tonalite igneous suite, with a minor low of + 150 g.u. over the anorthosite outcrop and a minor high of + 170 g.u. over the main gabbro outcrop. The distribution of the maximum positive anomaly and the minor anomalies is interpreted as due to interbanding of gabbro and anorthosite. The tonalites do not produce a local anomaly and are interpreted as extending to only limited depth. North of the igneous suite the residual gravity anomaly falls to - 10 g.u. over a hidden granite within migmatitic gneisses. It is suggested that the igneous suite was formed when a large body of gabbroic-anorthositic magma was injected into older continental crust, causing melting and the formation of contemporaneous tonalites, with complex fractionation producing gabbro and anorthosite layering. The sequence of events is interpreted as relating to an igneous episode that was a prelude to continental rifting which predated later phases of metamorphic/orogenic activity.

Type
Articles
Information
Geological Magazine , Volume 120 , Issue 3 , May 1983 , pp. 253 - 266
Copyright
Copyright © Cambridge University Press 1983

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

REFERENCES

Anderson, A. T. 1969. Massif-type anorthosite, a widespread Precambrian igneous rock. In Origin of Anorihosite and Related Rocks (ed. Isachsen, Y. W.), pp. 47–56. New York State Mus. Sci. Serv. Mem. 18.Google Scholar
Beach, A., Coward, M. P. & Graham, R. H. 1974. An interpretation of the structural evolution of the Laxford Front, north-west Scotland. Scott. J. Geol. 9, 297–308.CrossRefGoogle Scholar
Bowes, D. R. 1980. The absolute time-scale and the subdivision of Precambrian rocks in North-western Britain. In Principles and Criteria of Subdivision of Precambrian in Mobile Zones (ed. Mitrofanov, F. P.), pp. 32–54. Leningrad: Nauka.Google Scholar
Bridgwater, D. & Windley, B. F. 1973. Anorthosites, post-orogenic granites, acid volcanic rocks and crustal development in the North Atlantic Shield during the mid-Proterozoic. In Symposium on Granites and Related Rocks (ed. Lister, L. A.), pp. 307–17. Geol. Soc. S. Afr. Spec. Publ. 3.Google Scholar
Bullard, E., Everett, J. E. & Smith, A. C. 1965. The fit of the continents around the Atlantic. In Symposium on Continental Drjft (ed. Blackett, P. M. S., E., Bullard & Runcorn, S. K.), pp. 41–51. Phil. Trans. R. Soc. Land. Ser. A 258.Google Scholar
Chapman, H. & Moorbath, S., 1977. Lead isotope measurements from the oldest recognised Lewisian gneisses of north-west Scotland. Nature, Lond. 268, 41–2.Google Scholar
Coward, M. P., Francis, P. W., Graham, R. H. & Watson, J. V. 1970. Large-scale Laxfordian structures of the Outer Hebrides in relation to those of the Scottish Mainland. Tectonophysics 10, 425–35.CrossRefGoogle Scholar
Dearnley, R. 1963. The Lewisian Complex of South Harris; with some observations on the metamorphosed basic intrusions of the Outer Hebrides, Scotland. Q. Jl. geol. Soc. Lond. 119, 243–312.CrossRefGoogle Scholar
Emslie, R. F. 1978. Anorthosite massifs, rapakivi granites and Late Proterozoic rifting of North America. Precambrian Res. 7, 61–98.Google Scholar
Garson, M. S. & Livingstone, A. 1973. Is the South Harris Complex in North Scotland a Precambrian Overthrust Slice of Oceanic Crust and Island Arc? Nature Phys. Sci. 243, 74–6.Google Scholar
Goodacre, A. K. 1973. Some comments on the calculation of the gravitational and magnetic attraction of a homogeneous rectangular prism. Geophys. Prosp. 21, 66–9.CrossRefGoogle Scholar
Goode, A. D. R. 1977. Floatation and remelting of plagioclase in the Kalka Intrusion, central Australia: petrological implications for anorthosite genesis. Earth Planet. Sci. Lett. 34, 375–80.CrossRefGoogle Scholar
Jehu, T. J. & Craig, R. M. 1927. Geology of the Outer Hebrides. Part IV. South Harris. Trans. R. Soc. Edinb. 55, 457–88.Google Scholar
Jehu, T. J. & Craig, R. M. 1934. Geology of the Outer Hebrides. Part V. North Harris and Lewis. Trans. R. Soc. Edinb. 57, 839–74.Google Scholar
Kearey, P. 1978. An interpretation of the gravity field of the Morin Anorthosite Complx, southwest Quebec. Bull. geol. Soc. Am. 89, 467–75.Google Scholar
Kearey, P. & Thomas, M. D. 1979. Interpretation of the gravity field of the Lac Fournier and Romaine River anorthosite massifs, eastern Grenville Province: significance to the origin of anorthosite. J. geol. Soc. Lond. 136, 725–36.Google Scholar
McBirney, R. R. & Noyes, R. M. 1979. Crystallization and layering of the Skaergaard Intrusion. J. Petrology 20, 489–554.CrossRefGoogle Scholar
Moorbath, S., Powell, J. L. & Taylor, P. N. 1975. Isotopic evidnce for the age and origin of the ‘grey gneiss’ complex of the southern Outer Hebrides, Scotland. J. geol. Soc. Lond. 131, 213–22.CrossRefGoogle Scholar
Moorbath, S., Welke, H. & Gale, N. H. 1969. The significance of lead isotope studies in ancient high grade metamorphic basement complexes, as exemplified by the Lewisian rocks of northwest Scotland. Earth Planet. Sci. Lett. 6, 245–56.CrossRefGoogle Scholar
Myers, J. S. 1970. Gneiss types and their significance in the repeatedly deformed and metamorphosed Lewisian Complex of Western Harris, Outer Hebrides. Scott. J. Geol. 6, 186–99.Google Scholar
Myers, J. S. 1971. The late Laxfordian granite-migmatite complex of western Harris, Outer Hebrides. Scott. J. Geol. 7, 234–86.CrossRefGoogle Scholar
Philpotts, A. R. 1966. Origin of the anorthosite-mangerite rocks in southern Quebec. J. Petrology 7, 1–64.Google Scholar
Sabine, P. A. & Watson, J. V. 1965. Isotopic age-determinations of rocks from the British Isles 1955–64. Q. Jl. geol. Soc. Lond. 121, 477–533.Google Scholar
Simmons, G. 1964. Gravity Survey and geological interpretation, northern New York. Bull. geol. Soc. Am. 75, 81–98.CrossRefGoogle Scholar
Van Breeman, O., Aftalion, M. & Pidgeon, R. T. 1971. The age of the granitic injection complex of Harris, Outer Hebrides. Scott. J. Geol. 7, 139–52.CrossRefGoogle Scholar
Von Knorring, O. & Dearnley, R. 1960. The Lewisian pegmatites of South Harris, Outer Hebrides. Mineralog. Mag. 32, 366–378.Google Scholar
Westbrook, G. K. 1974. The South Harris Magnetic Anomaly. Proc. Geol. Assoc. Lond. 85, 1–12.CrossRefGoogle Scholar