217 



Table 1, No. 2) were evidently derived isochemically from 

 basalts. 



The greenstones are associated with basalts in Dredge 2 

 and with rare fragments of diabase in Dredge 3. Table 1 

 (No. 3) gives an analysis of a fresh basalt from Dredge 2. 

 This basalt consists of microlites of plagioclase and olivine 

 in a glassy matrix containing rare phenocrysts of plagio- 

 clase and olivine. Detailed petrographic and chemical 

 data on these basalts and diabases will be included in the 

 final report on the greenstones. 



The occurrence of greenstones raises several questions 

 about the nature of the Mid-Atlantic Ridge. These rocks 

 are not hornfelses and cannot be interpreted as the result 

 of contact metamorphism. The mineral assemblages are in 

 the greenschist metamorphic facies, which is often taken 

 to indicate temperatures of 300°-500° C and water pressures 

 between 3 and 8 kbar (ref. 3). Even in the deep sea, such 

 pressures, along with sustained periods of heating, would 

 require that the original basalts and tuffs were buried 

 under a considerable thickness of subsequent flows or 

 sediments. Because there are very few experimental data 

 available on the actual stability ranges of greenschist 

 assemblages, precise estimates of minimum and maximum 

 depths of burial cannot yet be made. Recovery of the 

 rocks by dredging shows no overburden now to be present ; 

 its elimination in recent times by other than tectonic 

 processes presents great difficulties. The burial postulated 

 appears to require a halt in, or a temporary reversal of, 

 the upward movement of material underneath a rift zone, 

 as suggested by Hess*. The subsequent raising of buried 

 material to the sea floor, and its exposure there, seems to 

 us perfectly consistent with the suggested tectonio 

 patterns. 



Matthews et al.^ have attributed origin of chlorite-rich 

 metamorphosed basalts and gabbros recovered from the 



( 1 ) Epidote — actinolite — chlorite — albite . 



(2) Quartz — chlorite — actinolite — albite. 



(3) Rare plagioclase (Ans,) and olivine (Fojs) phenocrysts. 



• Content as percentage of air-dry ground sample. Analysis by standard 

 wet methods for silicate rocks. Analyst Eugene Jarosewich. 



t Calculated assuming all sulphur in pyrite, the only sulphide visible in 

 polished sections. 



6-705 O— 68— pt. 1—15 



