219 



{Reprinted from Nature, Vol. 215, No. 5099, pp. 381-382, 

 July 22, 1967) 



Alkali Olivine Basalt dredged near St. Paul's 

 Rocks, Mid-Atlantic Ridge 



Recent studies of deep sea basalts have revealed the 

 great predominance of olivine tholeiites over both norma- 

 tive nepheline and normative quartz basalts^. An alkali 

 basalt recently dredged between 2,950 and 1,975 m near 

 St. Peter and St. Paul Rocks (St. Paul's Rocks) con- 

 sequently seemed worthy of prompt description. Although 

 mentioned in an abstract^, we have not previously pub- 

 lished a chemical analysis of this basalt. This alkali 

 basalt flow is of special interest because it is not part of a 

 large submarine volcano, but rather evidently was erupted 

 directly on a floor of spinel peridotite mylonites similar 

 to and contiguous with those described from St. Paul's 

 Rocks, a probable high temperature intrusion derived 

 from the mantle^. 



Extensive dredging was carried out around St. Paul's 

 Rocks diu'ing cruise 20 of the R.V. Atlantis II of the 

 Woods Hole Oceanographic Institution in an attempt to 

 delineate the outcrop of the ultrabasic mylonites which 

 are exposed on the islets. Nvimerous rock types, in addition 

 to the mylonites, were dredged within sight of the islets; 

 although their study is not yet complete, some of the more 

 interesting have been examined in the laboratory, and a 

 detailed report is in preparation. 



One of the most renaarkable rock types, the subject of 

 this report, is a vesicular basalt containing abundant 

 small olivine nodules and partly "digested" mylonitized 

 spinel peridotite inclusions. The dredge (No. 43) from 

 which this basalt was obtained is located on Fig. 1. 



The ultramafic intrusion exposed at St. Paul's Rocks 

 extends beneath the sea along a ridge elongated in an 

 E.N.E. direction. Dredge samples indicate that the sub- 

 marine exposures of the intrusion are spinel peridotite 

 mylonites and alkaline ultrabasic brown hornblende 

 mylonites, the two major rock types on the islets^. A 

 complex series of rocks which includes fresh and naeta- 

 morphosed basalts, basaltic pyroclastic rocks, basic and 

 ultrabasic plutonic rocks, and carbonate sedimentary 

 rocks outcrop to the north and south of the intrusive mass. 

 Some of the dredges, even those which only covered a 

 short distance of the bottom, yielded a wide ^'ariety of 

 rocks, suggesting that the rocks probably dip steepl^^ 

 around the inargin on the intrusion. The alkali basalt 

 described here probably occurs unconformably on the 



