Abstract
The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using λ(87Rb)=1.42 × 10−11y−1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.
New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous 40Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous 40Ar. However, dikes from all three age provinces of Liberia have elevated initial 87Sr/86Sr ratios. These results indicate that contamination with radiogenic 87Sr occurred primarily before intrusion of the magma whereas the addition of extraneous 40Ar occurred after emplacement and reflects the age and mineral composition of the country rock.
The δ 18O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial 87Sr/86Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in 18O and enriched in 87Sr by interactions with groundwater at high temperature.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Behrendt JC, Wotorson CS (1970) Aeromagnetic and gravity investigations of the coastal area and continental shelf of Liberia, West Africa, and their relation to continental drift. Geol Soc Am Bull 81:3563–3574
Brooks CK (1976) The Fe2O3/FeO ratio of basalt analyses; an appeal for a standardized procedure. Bull Geol Soc Den 25:117–120
Compston W, McDougall I, Heier KS (1968) Geochemical comparison of the Mesozoic basaltic rocks of Antarctica, South Africa, South America and Tasmania. Geochim Cosmochim Acta 32:129–150
Dalrymple GB, Gromme CS, White RW (1975) Potassium-argon age and paleomagnetism of diabase dikes in Liberia: initiation of central Atlantic rifting. Geol Soc Amer Bull 86:399–411
Erlank AJ (ed) (1984) Petrogenesis of the volcanic rocks of the Karoo Province. Geol Soc S Africa, Spec Pub No 13, p 395
Faure G, Bowman JR, Elliot DH, Jones LM (1974) Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contrib Mineral Petrol 48:153–169
Faure G, Pace KE, Elliot DH (1982) Systematic variation of 87Sr/ 86Sr ratios and major element concentrations in the Kirkpatrick Basalt of Mt. Falla, Queen Alexandra Range, Transantarctic Mountains. In: Craddock C(ed) Antarctic Geoscience, 715-723. University of Wisconsin Press, Madison, p 1172
Faure G (1986) Principles of isotope geology, 2nd edn. Wiley, New York, p 589
Fodor RV, Corwin C, Sial AN (1985) Crustal signatures in the Serra Geral flood basalt province, southern Brazil: O- and Srisotope evidence. Geology 13:763–765
Hedge CE, Marvin RF, Naeser CW (1975) Age provinces in the basement rocks of Liberia. J Res Geol Surv 3(4):425–429
Heier KS, Compston W, McDougall I (1965) Thorium and uranium concentrations, and the isotopic composition of strontium in the differentiated Tasmanian dolerites. Geochim Cosmochim Acta 29:643–659
Hergt JM, Chappell BW, McCulloch MT, McDougall I (1986) Geochemical and isotopic constraints on the genesis of the Jurassic dolerites of Tasmania (Abstract). Terra Cognita 6:183
Hoefs J (1987) Stable isotope geochemistry. 3rd edn. Springer, Berlin Heidelberg New York Tokyo, p 241
Hoefs J, Faure G, Elliot DH (1980) Correlation of δ 18O and initial 87Sr/86Sr ratios in Kirkpatrick Basalt on Mt. Falla, Transantarctic Mountains. Contrib Mineral Petrol 75:199–203
Hurley PM, Leo GW, White RW, Fairbairn HW (1971) Liberian age province (about 2700 m.y.) and adjacent provinces in Liberia and Sierra Leone. Geol Soc Am Bull 82:3483–3490
Kistler RW, Peterman ZE (1978) Reconstruction of crustal blocks of California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks. US Geol Surv Prof Paper 1071, p 17
Kyle PR (1980) Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica. Contrib Mineral Petrol 73:89–104
Mantovani SM, Marques LS, DeSousa MA, Civetta L, Atalla L, Innocenti F (1985) Trace element and strontium isotope constraints on the origin and evolution of Parana continental flood basalts of Santa Catarina state (southern Brazil). J Petrol 26:187–209
Mauche RS (1985) Petrogenesis of diabase dikes in Liberia, West Africa, based on the isotopic compositions of oxygen and strontium. Unpublished M.Sc. thesis, Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio, p 153
Mensing TM, Faure G, Jones LM, Bowman JR, Hoefs J (1984) Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land, Antarctica, based on isotopic compositions of strontium, oxygen and sulfur. Contrib Mineral Petrol 87:101–108
Onstott TC, Dorbor J (1987) 40Ar/39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield. J Afr Earth Sci 6:537–552
Pankhurst RJ, Millar IL, McGibbon F, Menzies M, Hawkesworth CJ, Kyle PR (1986) Evidence for the enriched source of the Jurassic Ferrar magmas of Antarctica. Terra Cognita 6:204
Prinz M (1967) Geochemistry of basaltic rocks: trace elements, 271-323. In: Hess HH, Poldervaart A (eds) Basalts. Wiley, New York, p 482
Steiger RH, Jäger E (1977) Subcommission on geochronology: convention on the use of the decay constants in geo and cosmochronology. Earth Planet Sci Lett 36:359–362
Taylor HP (1980) The effects of assimilation of country rocks by magmas on 18O/16O and 87Sr/86Sr systematics in igneous rocks. Earth Planet Sci Lett 47:243–254
White RW (1972) Stratigraphy and structure of basins on the coast of Liberia. Liberian Geol Surv Spec Paper l, p 18
Yoder HS, Tilley CE (1962) Origin of basalt magmas: an experimental study of natural and synthetic rock systems. J Petrol 3:342–532
Author information
Authors and Affiliations
Additional information
Laboratory for Isotope Geology and Geochemistry Contribution No. 76
Rights and permissions
About this article
Cite this article
Mauche, R., Faure, G., Jones, L.M. et al. Anomalous isotopic compositions of Sr, Ar and O in the Mesozoic diabase dikes of Liberia, West Africa. Contr. Mineral. and Petrol. 101, 12–18 (1989). https://doi.org/10.1007/BF00387197
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00387197