202 



indicates tiiat rocks had been only rccenliy exposed. The preservation, unbroken, of 

 thin piaty iVagincnts in the dredge 2 we take to indicate that the fresh surfaces of the 

 dredge 3 fragnicnts are unlikely to have been produced in the dredge. 



Zeolites were searched for in dredge 2 greenstones petrographically, and by 

 X-ray didraction. However, none was noted even though light fraction concentrations 

 were also examined. The lack of zeolites is surprising in view of the otherwise very 

 low melamorphic grade features of the dredge 2 greenstones. 



The extent of deformation is roughly proportional to the depth at which the 

 greenstones were dredged. The dredge 3 greenstones arc characteristically sheared; 

 some show a schislosity and many arc brecciated (Fig. 2). On the other hand, dredge 2 

 greenstones are mainly undeformcd and the primary igneous textures are clearly 

 pi'cscrved (Fig. 3). 



There are also diflcrences between the dredges in the types of rocks from which 

 the greenstones were derived. Mctabasalts and mctadolerites arc abundant in both 

 dredges, but dredge 2 greenstones include a number of nontronites and chloritc-rich 

 metatulTs. The metatuflfs are undeformed and consist of lapilli of originally glass-rich 

 basalt in a fine-grained ashy matrix. The glass is, however, completely replaced by 

 ripidolitic chlorite. 



These features suggest that mctamorphic intensity increases with depth in a 

 pile of basalts capped with interbedded basaltic tuffs and flows. 



Mineralogy 



Several features of the mineralogy help to define more clearly the mctamorphic 

 facies of the rocks and are worthy of special comment. Other features of the minera- 

 logy are summarized in Table IV. 



Fig.4 gives a plot of the Al, Fe, and Mg contents of actinolite and chlorite in 

 two greenstones recalculated in terms of FeO, MgO and AI2O3. Thus, all Fc has been 

 assumed as FcO; actinolitcs and chloritcs from previously studied greenstones arc 

 also plotted and total Fe in these was recalculated as FcO. 



Two features arc apparent in Fig.4. First, the actinolitcs show a wide range 

 of composition in a given rock and it is thus evident from microprobc data that 

 equilibrium was not attained. This is further c\ idcnt petrographically in, for example, 

 the presence of relic calcic plagioclase. A second feature is the general similarity in 

 the compositions of the actinolitcs and chloritcs from 22'N to the compositions of 

 actinolite and chlorite from previously studied greenstones. 



The alumina and soda contents of amphiboles in basic metamorpliic rocks 

 have been shown by numerous authors to be sensitive indicators o^ mctamorphic 



