Petrology and Mineralogy of Mantle Peridotites from the Southern Marianas - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 145

of impregnating melt that interacted with the residual peridotites, and their compo-
sitions would be a reflection of melt composition. In the analyzed samples, although
the amounts of clinopyroxene and plagioclase vary among the samples, they
occupy up to 7% of the peridotite. If Ti-enrichment in spinel and pyroxene is a true
indication of the influence of melt impregnation, the peridotites along the fault at
144°E are among the most strongly affected mantle peridotites within the Philippine
Sea region.
The Cr# and TiO 2 contents in spinels from the analyzed peridotite samples are
among the highest from the Philippine Sea. Similarly, the TiO 2 contents in both
orthopyroxene and clinopyroxene are higher than those in any other samples from
the Philippine Sea, even though they have similar values of Mg#. Because grains
with the lowest TiO 2 contents have similar compositions to those from the Mariana
Trough and the Parece Vela Rift (Fig. 4 ), the differences in Ti content can be
ascribed to differences in the degree of melt impregnation. The degrees of impreg-
nation, as estimated from mineral compositions, are similar to those of the P-type
peridotites from the Parece Vela Rift, rather than the veined harzburgites from the
Mariana Trough.
Ohara ( 2006 ) suggested that channeled melt flow under relatively low tem-
peratures is dominant beneath the Mariana Trough, whereas porous melt flow is
dominant beneath the Parece Vela Rift. If the differences in degrees of melt
impregnation depend on tectonic setting, then the geological setting of the south-
ern Marianas is more similar to the Parece Vela Rift than to the northern Mariana
Trough. Geophysical data suggest that the tectonic features of the central
Mariana Trough, between 17°N and 18°N, are similar to those of the slow-
spreading Mid-Atlantic Ridge, whereas the morphology of the southern Mariana
Trough, between 12°30¢N and 14°N, has an axial relief similar to that of the East
Pacific Rise with fast to superfast spreading rate (Martinez et al. 2000 ).
Therefore, it might be plausible that the peridotites from along the 144°E fault,
in the southern Marianas, formed as residues following relatively high degrees of
melting and melt impregnation beneath a fast-spreading ridge.

6 Conclusions

Petrographic, petrologic, and mineralogical analyses of mantle peridotites from
along the fault at 144°E, in the southern Marianas, revealed that the peridotites are
characterized by relatively large amounts of plagioclase, the result of crystallization
from an impregnating melt. Reaction with this melt enriched the spinel and pyrox-
enes in TiO 2 , and the spinel compositions indicate that the peridotites represent the
residue after as much as 15% partial melting. The degrees of melting and melt
impregnation in these peridotites from the southern Marianas are higher than those
in peridotites from the Mariana Trough. This might reflect the magma genesis
under higher spreading rate in the southern Marianas.