211 



suggest derivation by regional metamorphism accompanied by considerable tecto- 

 nism^. The normal vertical sequence of metamorphic grades is consistent with this 

 view. On the other hand, minerals typical of basic greenschist facies rocks are common 

 in other than regional metamorphic environments. Thus, based on mineralogy alone, 

 it is difficult to establish the origin of the 22' N greenstones. Epidote is a common 

 mineral in hydrothermally altered basalts, and is abundant in the hydrothermal 

 aureole of Thingmuli Volcano, Iceland (Carmichael, 1964). There, however, it 

 occurs with abundant zeolites. 



Chlorite is common as a late-magmatic or deuteric mineral in many igneous 

 rocks, and is a characteristic alteration product of olivine and pyroxene in the higher 

 level of flows on Mull (Fawcett, 1965). The comm.on uralitic amphibole produced 

 by late magmatic reaction between residual liquids and augite in gabbros and doleri- 

 tes, even where implaced at shallow depth, is typically thought to be actinolite (Deer 

 et al., 1963). Albite similarly may form in veins, amygdules or as replacement of more 

 calcic plagioclase, in many localized environments. 



Bearing of the greenstones on the tectonics of the Mid-Atlantic Ridge 



Most of the 22°N greenstones were derived from basalt flows. Thus, some burial 

 of the rocks under subsequent flows, tuff's, or sediments was required previous to 

 their present exposure on the sea floor. An important question in interpreting the 

 tectonics of the ridge is the amount of erosion or movement along faults which 

 occurred to bring about this exposure. Because of the slow rates of submarine 

 weathering and erosion, exposure due to faulting was probably dominant. 



Exposure along fault scarps is consistent with most interpretations of the ridge, 

 particularly for rocks exposed along the median valley, which is commonly thought 

 to be a rift valley. 



The amount of overburden during metamorphism can be roughly estimated 

 from postulated pressures and temperatures of greenschist facies metamorphism. 

 However, the pressures and temperatures of low grade metamorphism are particu- 

 larly difficult to infer. Difficulty in attaining equilibrium or even nucleation, parti- 

 cularly of epidote, has been bothersome in experimental studies. Turner and Ver- 

 HOOGEN (1960) point out that ". . .estimates of temperatures and pressures of low 

 grade regional metamorphism are little better than a guess". They, nonetheless, on 

 the basis of scant experimental data, and on field evidence, suggest temperatures 

 between 300 and 500^C, and water pressures between 3,000 and 8,000 bar. These 

 numbers are probably the best available. 



Fyfe et al. (1958) point out that experimentally calcic plagioclase reacts readily 

 to zeolites below 300°C at even low water pressures, and that this temperature is 



1 A recent new series of dredges in the 22°N area has recovered more greenstones, and abundant 

 mylonitized dolerites, extending the probable greenstone outcrop to more than 40 km in a north- 

 south direction. 



