201 



GREENSTONES 



General features 



The principal minerals in the greenstones are chlorite, albite, actinolite, epidote, and 

 nontronite. Chlorite and albite occur in the greenstones from both dredges, but the 

 remaining minerals may or may not be present in a given specimen. The source rock 

 of a given greenstone is generally petrographically obvious. Most were derived from 

 basalt flows and many features, such as relicts of calcic plagioclase in otherwise albi- 

 tized phenocrysts and microlites, preserve the stages in the mineralogic transformation 

 from basalt to greenstone. Dolerites and lapilli tuffs, the latter perhaps being the 

 first recorded tuffs dredged from the Mid-Atlantic Ridge, were the source rocks for 

 some of the greenstones. 



Contrast between dredges 



The greenstones from the two dredges differ in several ways (Table III). The minera- 

 logic differences are largely in the relative amounts of chlorite, nontronite, epidote, 

 actinolite and albite. Nontronite is extremely rare in dredge 3 greenstones. Similarly, 

 epidote and actinolite, which are abundant in dredge 3 greenstones, are rare in those 

 from dredge 2. The scarcity of epidote and actinolite, and abundance of nontronite 

 in dredge 2 greenstones suggest zeolite facies "affinities", whereas most dredge 3 

 greenstones clearly belong in the greenschist facies. There are, however, some metadole- 

 rites in dredge 3 which consist largely of unalbitized plagioclase and abundant 

 chlorite and nontronite. 



The rocks from dredge 2 have a thin manganian coating usually on all sides. 

 This observation and their angularity suggests that they were collected from a slowly 

 accumulating talus slope. The collection from dredge 3 exhibits only very thin and 

 spotty manganian coatings, and the fresh, unweathered aspect of the uncoated faces 



TABLE III 



CONTRAST BETWEEN DREDGE 2 AND DREDGE 3 GREENSTONES 



Dredge 2 (upper slope) Dredge 3 (middle slope) 



essentially undeformed some have incipient schistosity 



abundant metatuffs with nontronite none 



zeolites may be present in small amounts but presence could none noted 



not be confirmed 



abundant quartz veins, and matrix locally replaced by quartz rare 



Epidote rare, chlorite abundant chlorite rare, epidote abundant 



86-705 O — 68— pt. 1 14 



