195 



TABLE 1.— BULK ANALYSES OF SPINEL PERIDOTITE AND BROWN HORNBLENDE MYLONITES, ST. PAUL'S ROCKS 

 AND WEIGHTED AVERAGE COMPOSITION OF MYLONITES (90 PERCENT AVERAGE OF 7-327 AND 7-479, 5 PERCENT 

 18-900, AND 5 PERCENT SE 13, RECALCULATED TO 100 PERCENT). 



1 Includes 0.67 percent Sr (34). 



2 Contains less than 0.05 percent F. 



Table 2. Minekalogy and Locality of Analyzed Sampiles (35) 



Sample No. and Mineralogy : 



7-327 : "Pyroxene type" spinel peridotite mylonite. Primary assemblage : 

 olivine, enstatite, diopside, chromian spinel. Recrystallized assemblage : oli- 

 vine, pargasite. Dredge 7, R.V. Chain Cruise 35. 



7— i79 : "AmplWbole type'' spinel peridotite mylonite. Primary assemblage : 

 olivine, enstatite, pargasite, chromian spinel, blue spinel (SpsoHe2o), and 

 phlogopite. Recrystallized assemblage : olivine, pargasite, phlogopite, calcite, 

 and pyrite. Dredge 7, R.V. Chain Cruise 35. 



18-900 : "Amphibole type" spinel peridotite mylonite, enriched in pargasite. 

 Primary assemblage : olivine, enstatite, pargasite, chromian spinel. Recrys- 

 tallized assemblage : olivine and pargasite. Considerable secondary serpen- 

 tine. Dredge 18, R.V. Chain Cruise 35. 



SE-13 : Brown hornblende mylonite. Primary assemblage : brown horn- 

 blende, plagioclase, magnetite-ilmenite, seapolite, apatite, allanite. Recrys- 

 tallized assemblage: seapolite, chloro-hornblende, apatite, titan-biotite, anal- 

 cite, sphene, natrolite. Near north side of Southeast Islet; from band 3 m 

 thick. Atlantis II, Cruise 20. 



Table 1 includes a pargasite-rich spinel peridotite mylonite (18-900) which 

 is markedly high in basaltic constituents, such as Na, which tend to be concen- 

 trated in pargasite. This analysis is similar to the "1 :3 pyrolite" of Green and 

 Ringwood {22), a postulated mantle composition composed of 1 part basalt and 

 3 parts dunite, and thought likely to yield basalt on partial fusion. This parga- 

 site-rich mylonite most closely approaches the postulated average composition 

 of St. Paul's Rocks (Table 1). although we estimate that such pargasite-rich 

 mylonites compose only about 5 percent of the islets as a whole. 



The brown hornblende mylonites (SE 13, Table 1), although ultrabasic, are 

 remarkably high in AUG., TiO^, CaO, the alkalies, P2O5, and CI (2S) . They con- 

 tain abundant normative diopside, anorthite, and nepheline, and are thus com- 

 positionally similar to the alkali olivine basalts of many oceanic islands, which 

 are on the average high in TiO^ (24) and differ markedly from "oceanic thol- 

 eiites," probably the most abundant deep-sea basalts {25). 



