196 



Although not yet determined, it is likely that the U, Th, and rare earth con- 

 tents of the brown hornblende mylonites are unusually high compared to, for 

 example, "oceanic tholeiites." This is suggested by the presence of accessory 

 allanite (14) and zircon, as well as by the fact that igneous rocks compositionally 

 similar to the brown hornblende mylonites are commonly enriched in these 

 elements. 



On the continents, igneous rocks compositionally similar to the brown horn- 

 blende mylonites (Table 1) occur (i) among the ultrabasic members of alkalic 

 igneous rock provinces (26), (ii) in association with carbonatites (27), and 

 (ill) as alkalic ultrabasic porphyritic dike rocks (lamprophyres). The high 

 contents of volatiles, (Mg-f-Fe), (K-fNa), and Sr especially suggest kinship 

 to the lamprophyres (28). Continental igneous rocks compositionally like the St. 

 Paul's brown hornblende mylonites may contain a few of the following minerals 

 (29) : brown hornblende, titan-augite, plagioclase, nepheline, melilite, apatite, 

 hauyne, and sodalite. Scapolite, an abundant mineral of these mylonites, char- 

 acteristically occurs in metamorphic rocks and, except for some pegmatites, does 

 not occur as a primary mineral in igneous rocks (30). 



We have recognized few clues which suggest that the brown hornblende and 

 spinel peridotites are genetically related by a single process acting on an 

 originally homogeneous body, such as gravity differentiation prior to mylonitiza- 

 tion, a suggested origin for the spinel peridotite mylonites (31), or metamorphic 

 differentiation during mylonitization. On the contrary, the pervasive mylonitiza-' 

 tion, the juxtaposition of assemblages which may have equilibrated to markedly 

 different pressure-temperature environments, and the diversity of rock types are 

 more consistent with movement of a relatively hot (but solid) plastic rock 

 mass through the suboceanic mantle, and incorporation and shearing out of a 

 variety of unrelated rock types during ascent. The amount of hydrous and chlo- 

 rine-rich phases in the recrystallization assemblages suggests that each of these 

 rock types contained, during the intrusion, an abundant interstitial fluid phase. 



We have noted above similarities between the mylonites of St. Paul's and 

 such continental high-temperature, peridotite-rich intrusions as those of the 

 Lizard complex (England) and of Tinaquillo, Venezuela. Like the St. Paul's 

 intrusion, these latter also contain diverse mineral assemblages ; in their cases, 

 however, some of this diversity is attributed to inclusion of crustal contact 

 rocks (11, 12). The thinness of the suboceanic crust makes such inclusion a less 

 likely explanation for the heterogeneity of St. Paul's. A more attractive hypo- 

 thesis is that the diverse rock types found here were derived in fact from the 

 suboceanic upper mantle, and thus bear directly on its heterogeneity, mineralogy, 

 and composition. 



Previous geochemical studies of samples from St. Paul's Rocks, such as those 

 on rare earth elements {32), and strontium isotopes {S3), evidently were done 

 on the spinel peridotite mylonites. Taken alone, neither the spinel peridotite 

 mylonites nor the brown hornblende mylonites have compositions thought to be 

 appropriate for the average composition of the mantle. The former, as a whole, 

 are much too low and the latter much too high in alkalies and probably in U 

 and Th to be appropriate for parental basalt materials, or to be consistent with 

 oceanic heat flow. The pargasite-enriched mylonite (18-900, Table 1) and the 

 estimated average of the mylonites are both more in accord with suggested aver- 

 age mantle compositions. 



Clearly, additional analytical studies are needed to evalute the idea that, 

 as a whole, St. Paul's Rocks material is acceptable as compositionally repre- 

 sentative of the mantle. We are continuing our study of this interestsing intru- 

 sion, both on the suites of samples from the islets and on the extensive suites of 

 samples obtained by dredging about the intrusion. 



U.S. National Museum, 

 Washington, D.C. 



Woods Hole Oceanographic Institution, 

 Woods Hole, Mass. 



William G. Melson. 

 Eugene Jarosewich. 



Vaughan T. Bowen. 

 Geoffrey Thompson. 



