Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting c } u-s- Ge^al Me"!°Park' CaUfor"'a 94025 RICHARD W. WHITE U.S. Geological Survey, Denver, Colorado 80225 ABSTRACT (Behrendt and Wotorson, 1970) indicates that they also occur offshore to the edge of the continental shelf. Behrendt and Wotor- Tholeiitic diabase dikes that trend northwest-southeast, parallel son (1970) associated these dikes with the separation of Africa to the coastline, are common in northwestern Liberia. K-Ar from North and South America. whole-rock and mineral ages determined from dikes that intrude In December 1968, samples were collected for K-Ar dating and Precambrian crystalline rocks are discordant and range from 186 paleomagnetic study. The objectives of the investigation were to to 1,213 m.y. Incremental heating experiments on three neutron- provide geochronologic and paleomagnetic data for geologic cor- irradiated samples of these rocks give "saddle-shaped" 40Ar/39Ar re- relation, to test the hypothesis that the dikes are related to the sep- lease diagrams that reach minima of less than 300 m.y. at inter- aration of Africa from North and South America during Mesozoic mediate temperatures and that do not fit a 40Ar/36Ar versus time, and to obtain an early Mesozoic pole position for West 39Ar/36Ar isochron. K-Ar ages determined from diabase dikes and Africa. sills that intrude Paleozoic sedimentary rocks near the coast are all 40 39 within the range 173 to 192 m.y. Ar/ Ar incremental heating GEOLOGIC SETTING data for one of these samples gives a plateau age and a 40Ar/36Ar 39 36 versus Ar/ Ar isochron age that are concordant with the conven- The Precambrian crystalline rocks of Liberia have been sub- 40 39 tional K-Ar age. The conventional and Ar/ Ar K-Ar data show divided into three provinces of contrasting age, lithology, and that the dikes intruding Precambrian basement rocks contain large structure (Hurley and others, 1971; White and Leo, 1970). The 40 and variable amounts of excess Ar, whereas the diabase intruding Liberian age province (about 2,700 m.y.) occupies much of the in- Paleozoic sandstone does not. All of the intrusions are probably terior of western Liberia (Fig. 1) and consists mostly of granitic earliest Jurassic in age. gneisses ranging in composition from granite to quartz diorite. Mean paleomagnetic directions in six dikes and sills that intrude Northeast-trending belts of sedimentary rocks, including iron- sedimentary rocks are nearly parallel to mean paleomagnetic direc- formation, have been folded into the granitic gneisses and tions in 19 dikes that intrude Precambrian rock, further evidence metamorphosed to amphibolite facies. The Pan-African age prov- for contemporaneity. The paleomagnetic pole derived from all 25 ince (about 550 m.y.) comprises a northwest-trending coastal belt diabase units is at lat 68° N., long 242° E., with a95 = 5°, in close of metasedimentary and mafic rocks that have been refoliated and agreement with other Mesozoic paleomagnetic poles from the Afri- remetamorphosed to graniilite and amphibolite facies and have can continent. A mean paleomagnetic pole for northwest Africa has been intruded by granitic rocks. The Eburnean age province (about been calculated using these data and published paleomagnetic di- 2,000 m.y.) is separated from the Liberian province in eastern rections from 19 other intrusive rock units that have similar Liberia by a belt of refoliated Liberian-type granitic gneisses (not radiometric ages in Morocco and Sierra Leone. This pole is com- shown in Fig. 1). The southeastern part of the province consists of pared with another paleomagnetic pole calculated from published tightly folded paragneisses of amphibolite facies, amphibolites, data from 16 localities in igneous rocks of latest Triassic to earliest migmatites, and granitic intrusive rocks. Jurassic age distributed from Nova Scotia to Pennsylvania. The Unmetamorphosed sedimentary rocks crop out along the coast comparison shows that, with the African and North American con- of Liberia between Buchanan and the capital city of Monrovia. tinents in their present positions, the two poles differ by 44° of arc, They include from older to younger, the Paynesville Sandstone, the but when the continents are restored to the predrift configuration Farmington River Formation, and the Edina Sandstone (White, proposed by Bullard and others (1965), the angular difference di- 1972). The Paynesville Sandstone, an arenite with a thickness of minishes to 3°. This coincidence of paleomagnetic poles provides about 1,000 m on the coast east of Monrovia, lies unconformably an earliest limit of 180 ± 10 m.y. for the separation of Africa from on the crystalline basement. Its age is uncertain because fossils are North America. Key words: continental drift, sea-floor spreading, absent, but it is thought to be early or middle Paleozoic because of opening of Atlantic Ocean, paleomagnetism, Africa, African pole its stratigraphic position and from comparison with other sedimen- 40 9 position, Liberia', diabase dikes, potassium-argon, K-Ar, ArP Ar, tary sequences in West Africa. The Paynesville Sandstone is over- excess argon. lain unconformably by wackes and conglomerates of the Farming- ton River Formation, whose thickness may exceed 1 km (Behrendt INTRODUCTION and Wotorson, 1970). Pollen and spores indicate that the Farming- ton River Formation is Cretaceous, probably Albian. The Edina Parallel dikes of tholeiitic diabase are common in Liberia where Sandstone (not shown in Fig. 1), of Tertiary(?) age, disconformably they intrude Precambrian basement rocks and, locally near the overlies the Farmington River Formaton and is only a few meters coast, Paleozoic sedimentary rocks. Their strike is northwest- thick. Unconsolidated Quaternary sediments form a thin veneer southeast parallel to the African coast, and geophysical evidence along the entire coast of Liberia. Geological Society of America Bulletin, v. 86, p. 399-411, 8 figs., March 1975, Doc. no. 50316. 399 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/399/3433744/i0016-7606-86-3-399.pdf by guest on 24 September 2021 2 DALRYMPLE AND OTHERS ¡PkH wMk Formington River Formotion l/y l V<) Diabase dikes and sills including to IE minor gabbro and basalt Paynesville Sandstone p€z (Pan-African) Parogneiss, mafic and metasedi- mentary granulate, omphibolite, ond granitic rocks p-6w (Liberiani Granitic gneiss containing infolded belts of metasedimentary rocks ° L22 Sample site 5°JO'L 11° 30 IfOO' 10° 30 I0°00 9°30 9°00 Figure 1. Geologic map of part of western Liberia showing sample locations. Geology after White and Leo (1969) and White (1972), modified from interpretation of aeromagnetic data. Dotted line is approximate boundary between Pan-African age province (ca. 550 m.y.) and Liberian age province (ca. 2,700 m.y.) of Hurley and others (1971). Dikes of diabase intrude the Precambrian crystalline rocks tectonic rotation since emplacement has been minimal or absent. throughout western Liberia. Three broad geographic zones have Several sills are found within the Paynesville Sandstone, and two been recognized: coastal, central, and northern (White and Leo, small flows of tholeiitic basalt overlying the sandstone are thought 1969; White, 1970; Behrendt and Wotorson, 1972). The coastal to be the extrusive equivalent of the diabase (White, 1972). Except and central zones trend northwest-southeast parallel to the coast. for the sills, the trend of the dikes is unrelated to any primary struc- Some dikes and sills in the coastal zone intrude the Paynesville tures in either the sediments or the crystalline basement rocks, and Sandstone and are unconformably overlain by the Farmington the parallelism of the dikes (Fig. 1) suggests that they are controlled River Formation; other dikes intrude Precambrian rocks of the by a through-going fracture system. Pan-African age province (Fig. 1). An aeromagnetic survey The diabase is composed primarily of labradorite and titanifer- (Behrendt and Wotorson, 1970, 1972) indicates that the dikes also ous augite with minor magnetite, ilmenite, and apatite and, in some occur beneath sediments on the continental shelf. The central zone, dikes, olivine, pigeonite, sulfides, and traces of native copper. about 90 km inland, consists of abundant dikes that intrude Pre- Primary graphic intergrowths of accessory quartz and alkali cambrian rocks of the Liberian age province; it continues north- feldspar are common. The diabase is generally fine to medium west into Sierra Leone (Wilson, 1965) but apparently dies out to- grained with uniform ophitic to subophitic texture but contains oc- ward the southeast near the border of Liberia and Ivory Coast casional phenocrysts of plagioclase or pyroxene. The thicker dikes (Tagini, 1965). The northern zone, which occurs far inland (north are coarse grained. The diabase is generally unaltered where it is of the area of Fig. 1), consists of widely spaced dikes that trend unweathered, except for some rocks that have incipient develop- east-west. Dikes of the northern zone not only differ in structural ment of sericite in the plagioclase. There is no petrologic or field trend from dikes of the coastal and central zones but are mildly evidence to suggest that the diabase dikes have been reheated since metamorphosed and unsuitable for these studies. These dikes emplacement. Dikes and sills are petrographically indistinguish- probably differ in age and origin from those of the coastal and cen- able. Chemical analyses of samples from sites L24 (White, 1972), tral zones and will not be discussed further. Except for rough geo- and L29, and two other localities (unpublished) indicate that the graphic distribution, there is no structural or petrographic evidence diabase is tholeiitic. that the dikes of the central and coastal zones are of different age or origin; they are considered as a single group throughout most of SAMPLING AND LABORATORY TECHNIQUES the following discussion. Individual dikes vary in length; some are as much as 50 km long A total of 316 oriented cores 2.50 cm in diameter was collected but most are less than 10 km long.