214 



A more detailed survey of the greenstone area in 1965 on R. V. "Thomas Wash- 

 ington" indicates that it consists of a lozenge shaped block, 40 km long by 10 km 

 wide in an east-west direction. This block is bounded on all sides by steep slopes of 

 2-45^, which are probably fault scarps, and rises 500 m or more above the ridge 

 crest elsewhere in this area. It is tempting to stress the analogy of this block with the 

 probable fault block of which St. Pauls Rock is a part in the equatorial Atlantic, 

 where uplift of mantle rocks has been postulated. Perhaps the 22°N greenstones 

 represent a section of the lower oceanic crust. This would imply formation of the 

 oceanic crust by accumulation of sea floor basaltic rocks. Seismic velocity data on 

 the 22°N area would be useful in testing this latter hypothesis, for high velocity 

 rocks should occur at shallow depths. 



Ultramafic rocks were not recovered in dredges in the 22°N area and are evi- 

 dently not abundant there^. If the oceanic crust is largely serpentinite, as suggested 

 by Hess (1962), the serpentinite must occur under a considerable thickness of basalts 

 in the 22°N area. 



CONCLUSIONS 



(/) Greenstones composed of epidote, chlorite, actinolite, and albite were first 

 noted from the Mid-Atlantic Ridge in dredges taken at 22°N. Here, they occur at 

 depths between 2,000 and 3,500 m, in a steep eastern slope of the median valley. 



(2) Fresh dolerites and abyssal basalts occurred as minor constituents of the 

 dredges which recovered the greenstones. 



(3) The greenstones from the middle slope have typical greenschist facies 

 mineralogy. Higher on the slope, the greenstones are less deformed and the presence 

 of nontronite and common absence of actinolite and epidote suggest even lower 

 metamorphic grade. 



(4) The greenstones were derived mainly from basalt flows, although many 

 were derived from dolerites and basaltic tuffs. 



(5) The greenstones are characterized by a range of actinolite compositions in 

 the same specimen, showing that equilibrium was not attained. 



(6) The 22°N greenstones are most likely a product of regional metamorphism 

 rather than of localized hydrothermal alteration, or of "autometamorphism". 



(7) The greenstones have spilitic (high soda) compositions, but these are due 

 to post-consolidation metasomatism. Petrographic evidence, such as relicts of calcic 

 plagioclase, indicate derivation from abyssal basalts ("oceanic tholeiites"). 



(8) The low KoO content gives rise to mineralogic peculiarities in the 22°N 

 greenstones when compared to continental greenstones. This is evident in the lack of 

 such phases as stilpnomelane; and in the occurrence of abundant nontronite rather 

 than celadonite. 



^ Subsequent dredges in the area did not recover ultramafic rocks although there were eight successful 

 dredge hauls, and about 1000 lb. of rock were recovered. 



