Petrology and K-Ar age of volcanic tuff and ash from the Walvis Seamount province, DSDP Site 359, Leg 39

Online Access: Get full text
doi: 10.2973/dsdp.proc.39.120.1977
Author(s): Fodor, R. V.; Keil, K.; Husler, J. W.; McKee, E. H.
Author Affiliation(s): Primary:
Univ. N.M., Dep. Geol., Albuquerque, N.M., United States
Volume Title: Initial Reports of the Deep Sea Drilling Project
Source: Initial Reports of the Deep Sea Drilling Project, Vol.39, p.525-536. Publisher: Texas A & M University, Ocean Drilling Program, College Station, TX, United States. ISSN: 0080-8334 CODEN: IDSDA6
Note: In English. 30 refs.; illus. incl. tables, sketch map
Summary: An ash-flow tuff of 3.3 m thickness cored from a seamount at the southwest extremity of the Walvis Ridge has trachytic bulk composition and contains abundant alkali feldspar, clinopyroxene, biotite, and Fe-Ti oxides in a dusty, partly devitrified matrix. Moderately compacted pumice lapilli are abundant, but no original shard structures remain. The relative freshness and absence of graded bedding and marine material indicate that the tuff was emplaced subaerially. Fossils in overlying sediments indicate an open-marine environment about 40 m.y. in age. Within those sediments are three ash layers, 3.6, 4.0, and 4.6 m above the tuff. The ash is composed of high-SiO2 glass and contains alkali feldspar, hedenbergite, and Fe-Ti oxides. The feldspar in the trachytic ash flow gives a K-Ar age of 40.1 m.y. 520 [dollar]The ash-flow tuff is compositionally similar to trachytes on the nearby islands of Tristan da Cunha and Gough (moderately alkalic volcanic provinces). The seamount it caps probably represents a volcanic center that originated on or near the Mid-Atlantic Ridge at least 50 m.y. ago, differentiated from basalt to trachyte, and migrated northeastward. The ash layers in overlying sediments indicate that intermediate to silicic volcanism persisted in the region of the seamount until at least late Eocene. (Auth.)
Year of Publication: 1977
Research Program: DSDP Deep Sea Drilling Project
Key Words: 05 Petrology, Igneous and Metamorphic; Absolute age; Atlantic Ocean; Cenozoic; Chemical composition; Composition; Dates; Deep Sea Drilling Project; Igneous rocks; K/Ar; Leg 39; Lower Tertiary; Mineral composition; Petrology; Pyroclastics; Pyroclastics and glasses; Site 359; South Atlantic; Tertiary; Tuff; Volcanic ash; Volcanic rocks; Walvis Ridge
Coordinates: S345900 S345900 W0043000 W0043000
Record ID: 1978003287
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from the Antarctic Bibliography, United States