210 



22°N greenstones. Nontronite evidently may occur in lieu of celadonite in the low- 

 grade derivatives of abyssal basalts. 



Relation to spiUtes 



Compositionally, some of the 22°N greenstones are similar to certain spihtes, parti- 

 cularly to those from Anglesey, Wales (Vaugnat, 1949). Petrographically, the 22°N 

 greenstones are not spilites in the sense the term is used by Dewey and Flett (1911) 

 or Battey (1956). The lack of augite, presence of abundant pseudomorphs of chlorite 

 after olivine, and relict calcic plagioclase clearly distinguish them from what these 

 writers have termed spilites. 



Vallance (1960) has, however, discussed the contradictory uses of the term 

 spilite, and perhaps some petrologists would consider the 22°N greenstones spilites. 

 The important point is that the petrographic features of the 22°N greenstones suggest 

 derivation by post-consolidation metamorphism of oceanic tholeiitic flows and 

 tuffs. There is no evidence of derivation of the 22°N greenstones by reaction of 

 molten magma and sea water, or from a primary spilitic magma. If spihtes were 

 readily produced by reaction of molten magma and sea water (Rittmann, 1958), it 

 is surprising that past dredging in deep water in the Pacific and Atlantic has revealed 

 basalts which are essentially unaltered (e.g., Moore, 1965). 



As previously pointed out, the bulk analyses indicate that the high Na20 con- 

 tents of the greenstones are not related to metamorphic differentiation, but to an 

 overall composition change. Sea water derived from associated fragmental rocks is 

 commonly postulated as a source for Na in "spilitic rocks". If specimen 3-6 (Table 

 V) had an original Na20 content of 2.70% (2.00% Na), the average for abyssal 

 basalts, its present Na content (3.31% Na) would require addition of 1.36 g Na/lOOg 

 average abyssal basalt. Thus, all Na in 1.28 g sea water (in which Na is assumed 

 equal 1 .06 %) would be required per gram average abyssal basalt to give the Na content 

 of 3-6. The large quantity of sea water required makes it unlikely that the albitization 

 occurred as a result of reaction with only static interstitial sea water. Only a system 

 in which access to additional sea water, or in which brines were included in associated 

 sediments, would give sufficient Na. Alternatively, "juvenile" hydrothermal solutions 

 may have played an important role. 



Derivation by regional metamorphism 



Fresh basalts and dolerites which are similar to those from which the greenstones 

 were derived occur in the dredges which recovered the greenstones. It is thus arguable 

 that the greenstones were produced in a more localized environment; perhaps the 

 dredges crossed a hydrothermal aureole around a volcanic vent. Hydrothermal aureo- 

 les at Mull, Scotland, and Thingmuli Volcano, Iceland, are perhaps analagous 

 to the possible 22°N aureole. However, greenstone fragments in sediments ca. 160 km 

 southwest of the dredge sites, and the common schistosity in the dredge 3 greenstones 



