194 



type" corresponds to the assemblages of most olivine nodules of alkali olivine 

 basalts, and is thought to be a stable assemblage at depths between about 30 

 and about 140 km in the oceanic mantle (9). At greater depths, pyrope probably 

 appears (5) ; this mineral has not so far been noted in samples from St. Paul's, 

 although searched for in every petrographic examination. 



The olivine + pargasite + enstatite + chromian spinel primary assemblage of 

 the "amphibode type" and the paragasite + olivine recrystallized assemblage are 

 stable at much shallower mantle depths, according to Clark and Ringwood(iO), 

 who argue that such pargasite + olivine ("ampholite") assemblages occur only 

 down to about 30 km in the suljoceanic mantle. It appears, then, that the pri- 

 mary assemblages of the "pyroxene type" and "amphibole type" spinel periodotite 

 mylonites equilibrated at, and thus were derived from, different mantle levels. 

 xVs Ringwood notes (9), should Ph^o in the mantle be inhomogeneous, and local- 

 ly equal the load pressure, this could significantly increase the depth of the sta- 

 bility field of the hydrous assemblage, "amphibole-type" minerals. At suflSciently 

 high partial pressures of H-O, this assemblage might even have equilibrated 

 at the same temperatures and total pressures as the "pyroxene type" ; the two 

 assemblages could, in this case, both have originated from the same depths, con- 

 siderably greater than 30 km. Viewed in either of these ways, the mineralogy 

 suggests that St. Paul's Rocks represent an intrusion of mantle materials from 

 below 30 km, and one which was intruded too rapidly to permit reequilibration 

 at any shallower depth. 



The pargasite augen of the spinel periodotite mylonites are similar to the 

 dominant amphiboles of the banded parts of the spinel periodotite bodies near 

 Lizard, England (11), and near Tinaquillo, Venezuela (12), which are postu- 

 lated to be high-temperature, mantle-deriVed intrusions (13). The pargasite 

 augen from St. Paul's Rocks typically have Si02=43 to 47 percent, ALOa^lO to 

 12 percent, MgO=17 to 20 percent, Fe=2.8 to 6.0 percent, CaO=ll to 13 percent, 

 Na2O=2.0 to 3.3 percent, K2O=0.4 to 0.9 percent, and TiO2=0.2 to 0.5 percent. 



The brown hornblende mylonites are characterized by abundant large augen 

 (up to 1 cm across) of brown hornblende. Less abundant and smaller augen of 

 plagioclase, titan-biotite, clinopyroxene, olivine, scapolite, zircon, and allanite 

 (1^) occur. Although CaO (10 to 12 percent) and Fe (7.0 to 10.4 percent) are 

 in the range of many hornblendes, the brown hornblende augen are uncommonly 

 high in TiOa (1.3 to 2.0 percent). AUO3 (15 to 17 percent), Xa^O (2.5 to 3.1 per- 

 cent), and K2O (1.0 to 1.8 percent) and low in SiOi (37 to 40 percent). Com- 

 positionally similar hornblendes have been reported from a garnet-hornblende 

 nodule postulated to be of upper mantle origin (15) and in alkalic igneous rocks 

 which crystallized in continental crust (16). Optically similar brown hornblende 

 has been reported in the "pseudogabbro" bands of the Tinaquillo periodotite 

 (12). Such hornblendes evidently are stable over a wide range of pressure-tem- 

 perature conditions. 



In the St. Paul's intrusion recrystallized assemblages suggestive of high tem- 

 peratures occur in the matrix of some mylonites and in veinlets. In the spinel 

 periodotite mylonites, such assemblages commonly contain olivine, pargasite, 

 phlogopite, carbonate, and sulfide (mainly pyrite). In the brown hornblende 

 mylonites, a much more complex suite, including some rare minerals, charac- 

 terizes the recrystallized assemblages; scapolite (17), magnetite-ilmenite, anal- 

 cite, titan-biotite, chloroapatite, chloro-hornblende (18) (like dashkesanite), car- 

 bonates, and sulfides (including chalcopyrite) have so far been recognized. The 

 stability of olivine in association with hydrous minerals suggests that these 

 assemblages recrystallized in excess of 430 °C (19) ; this temperature is reached 

 in the crust at about 15 km, based on the commonly postulated average crustal 

 geothermal gradient (30°CAni)- This depth is below the M-discontinuity in even 

 the areas of thick oceanic crust beneath the mid-Atlantic Ridge (20). 



Hess (21) has drawn attention to the compositional similarity of the spinel 

 periodotite mylonites to the olivine nodules common in alkali olivine basalts. 

 Because of their low content of basaltic constituents, Tilley (7) and Hess (21) 

 rejected previously analyzed spinel peridotite mylonites from St. Paul's Rocks 

 as representative of rocks likely to yield basaltic magma on partial fusion. 

 Tilley (7) in fact suggested that they are perhaps residua of partial fusion. Our 

 two new analyses of low-pargasite mylonites. (7-327 "proxene type," and 7-479 

 "amphibole type," Table 1) are in accord with these views, and, like the analyses 

 of Tilley (7), show that the "amphibole" and "proxene" types may have essen- 

 tially identical compositions. 



