208 



The compositional variations of the greenstones among themselves and their 

 deviation from the average Mid-Atlantic Ridge abyssal basalt could result from 

 several factors, the maiii ones being: (/) metamorphic differentiation (e.g., CaO and 

 FeaOs enrichment in epidote-bearing varieties); (2) changes in the composition of 

 the rocks as a v/hole, perhaps due to the passage of hydrothermal solutions, or ion 

 exchange under temperature or other gradients; and (i) differences between the conv 

 positions of the original basalts. The latter is unlikely in view of the narrow compo- 

 sition spread in abyssal basalts. 



Metamorphic differcntation, with epidote enrichment on the one hand and 

 chlorite enrichment on the other, can account in part for the inverse relation between 

 the CaO and MgO contents (Fig.7). This may be shown by referring back to Fig.5, 

 and noting that fresh abyssal basalts occupy intermediate positions between green- 

 stones rich in epidote, and those rich in chlorite. Fig. 2 shows a clot of epidote crystals 

 (essentially an epidosite) which is typical in the high CaO greenstones. 



HijO' is also proportional to the amount of chlorite present. Greenstones with 

 high MgO i FeO and low CaO {2-5, 3-2, and 3-7, Tabic VI) have high II2O' . Partial 



^icy 



A 3-2 e 



Ao 2-1 

 A 3-7 • OA2-2 



A 3-3 • 

 A 2-5 O 



2 4 6 8 10 12 14 



FeO (A) ond MgO(o) W6T. % 



Fig.7. FeO and MgO versus CaO of 22'N greenstones (solid) and of two fresh basalts from dredge 2 

 (open). 



replacement of plagioclase phcnocrysts and microlites by chlorite is common in such 

 greenstones (Fig. 6). 



The generally low K2O content, high Na^O, and enrichment of i-(5 in sulfur, and 

 depletion of 3-6 in FeO are difficult to explain in terms of local enrichment of one 

 or more principal minerals of the greenstones. In 3-6, which consists of metabasalt 

 fragments in a fine-grained mylonitized matrix, the NaaO is typically higher in the 

 matrix (around 5.37) than in the fragments (around 3.68), but both values are higher than 

 typical oceanic tholeiites. Specimen 2-5 is the only one of the five analyzed greenstones 

 which contains an NaaO content equivalent to and even slightly lower than that of 

 some abyssal basalts (Engel and Excel, 1964, table I, analysis D2-1 and D2-4). 



