Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting

Abstract

Research Article| March 01, 1975 Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting G. BRENT DALRYMPLE; G. BRENT DALRYMPLE 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar C. SHERMAN GROMMÉ; C. SHERMAN GROMMÉ 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar RICHARD W. WHITE RICHARD W. WHITE 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information G. BRENT DALRYMPLE 1U.S. Geological Survey, Menlo Park, California 94025 C. SHERMAN GROMMÉ 1U.S. Geological Survey, Menlo Park, California 94025 RICHARD W. WHITE 2U.S. Geological Survey, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (3): 399–411. https://doi.org/10.1130/0016-7606(1975)86<399:PAAPOD>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. BRENT DALRYMPLE, C. SHERMAN GROMMÉ, RICHARD W. WHITE; Potassium-Argon Age and Paleomagnetism of Diabase Dikes in Liberia: Initiation of Central Atlantic Rifting. GSA Bulletin 1975;; 86 (3): 399–411. doi: https://doi.org/10.1130/0016-7606(1975)86<399:PAAPOD>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Tholeiitic diabase dikes that trend northwest-southeast, parallel to the coastline, are common in northwestern Liberia. K-Ar whole-rock and mineral ages determined from dikes that intrude Precambrian crystalline rocks are discordant and range from 186 to 1,213 m.y. Incremental heating experiments on three neutron-irradiated samples of these rocks give “saddle-shaped” 40Ar/39Ar release diagrams that reach minima of less than 300 m.y. at intermediate temperatures and that do not fit a 40Ar/36Ar versus 39Ar/36Ar isochron. K-Ar ages determined from diabase dikes and sills that intrude Paleozoic sedimentary rocks near the coast are all within the range 173 to 192 m.y. 40Ar/39Ar incremental heating data for one of these samples gives a plateau age and a 40Ar/36Ar versus 39Ar/36Ar isochron age that are concordant with the conventional K-Ar age. The conventional and 40Ar/39Ar K-Ar data show that the dikes intruding Precambrian basement rocks contain large and variable amounts of excess 40Ar, whereas the diabase intruding Paleozoic sandstone does not. All of the intrusions are probably earliest Jurassic in age.Mean paleomagnetic directions in six dikes and sills that intrude sedimentary rocks are nearly parallel to mean paleomagnetic directions in 19 dikes that intrude Precambrian rock, further evidence for contemporaneity. The paleomagnetic pole derived from all 25 diabase units is at lat 68° N., long 242° E., with α95 = 5°, in close agreement with other Mesozoic paleomagnetic poles from the African continent. A mean paleomagnetic pole for northwest Africa has been calculated using these data and published paleomagnetic directions from 19 other intrusive rock units that have similar radiometric ages in Morocco and Sierra Leone. This pole is compared with another paleomagnetic pole calculated from published data from 16 localities in igneous rocks of latest Triassic to earliest Jurassic age distributed from Nova Scotia to Pennsylvania. The comparison shows that, with the African and North American continents in their present positions, the two poles differ by 44° of arc, but when the continents are restored to the predrift configuration proposed by Bullard and others (1965), the angular difference diminishes to 3°. This coincidence of paleomagnetic poles provides an earliest limit of 180 ± 10 m.y. for the separation of Africa from North America. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.