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Marine Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands 

 METAMORPHISM IN THE MID-ATLANTIC RIDGE, 22°N LATITUDE 



WILLIAM G. MELSON AND TJEERD H. VAN ANDEL 



U.S. National Museum, Washington, D.C. (U.S.A.); 

 Scripps Institution of Oceanography, La Jolla, Calif. (U.S.A.) 



With chemical analyses by eugene jarosewich 

 U.S. National Museum, Washington, D.C. (U.S.A.) 



(Received March 7, 1966) 



SUMMARY 



Greenstones derived from basalts, tuffs, and dolerites were dredged at two 

 stations on the eastern slope of the median valley of the Mid-Atlantic Ridge 

 at 22°N latitude. The rocks consist mainly of the typical greenschist assemblage 

 albite, actinolite, chlorite, and epidote. The greenstones reflect increasing meta- 

 morphic grade and intensity of shearing with the depth at which they were recovered. 



Bulk chemical analyses of six greenstones give spilitic compositions character- 

 ized by high soda (maximum 5.4%) and low potash (minimum 0.05%) contents. 

 Rehcs of calcic plagioclase, pseudomorphed olivine phenocrysts, and other textural 

 features indicate derivation from abyssal basalts, which typically have soda around 

 2.7% and potash around 0.2 %. The alkali contents of the greenstones are most likely 

 a result of metasomatism during metamorphism, and do not indicate a primary 

 spilitic magma, or reaction between magma and sea water. 



The significance of the greenstones in relation to the tectonics and structure 

 of the Mid- Atlantic Ridge is discussed. 



INTRODUCTION 



Previous work 



Rocks previously dredged from the Mid-Atlantic Ridge indicate a terrain made up 

 of fresh basaltic flows on a basement of serpentinites, partially serpentized ultra- 

 mafics, and gabbroic intrusions (Table I). Fresh basalts have been dominant in most 

 dredges. These are characteristically tholeiitic (Engel and Engel, 1964; Engel et al., 

 1965) although some have "alkaline affinities" (MuiR et al., 1964). The contrast 

 between dredged abyssal basalts and the common alkali basalts from oceanic islands 



