Petrology and Mineralogy of Mantle Peridotites from the Southern Marianas - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 130

(Bloomer 1983 ) or “fore-arc ophiolites” (Ishii 1985 ). Although mantle peridotites
with back-arc basin affinities have been sampled in only a few areas of the
Philippine Sea Plate, and information was very limited until the 1990s (Bloomer
and Hawkins 1983 ; Shcheka et al. 1995 ; Ohara et al. 1996 ; Stern et al. 1996, 1997 ),
our knowledge of the mantle beneath the various back-arc basins has increased
substantially over the past decade. For example, Ohara et al. ( 2003 ) provided infor-
mation on the Parece Vela Rift, which is an extinct Miocene spreading center in the
Parece Vela Basin, and Ohara et al. ( 2002 ) discussed the Mariana Trough, which is
an active spreading center in the southern Philippine Sea Plate.
One of the important localities for exposures of mantle peridotites is the
inner wall of the southern Mariana Trench, which has been analyzed in several
studies (Bloomer and Hawkins 1983 ; Fryer 1993 ; Ohara and Ishii 1998 ;
Michibayashi et al. 2007, 2009 ). Mantle peridotites are exposed along the
southern Mariana Trench, at 143°30¢E, 11°30¢N, representing the residues fol-
lowing high degrees of melting related to island-arc volcanism (Bloomer 1983 ;
Ohara and Ishii 1998 ; Michibayashi et al. 2007 ). East of this site, at 144°10¢E,
is a large scarp that trends mainly N-S along a fault known either as the
Southeastern Mariana Fore-Arc Fault (Fryer 1993 ) or the West Santa Rosa
Bank Fault. Mantle peridotites, together with mafic and intermediate rocks,
have been collected from this scarp (Bloomer and Hawkins 1983 ; Fryer 1993 ;
Michibayashi et al. 2009 ), with most of them considered to represent residues
related to back-arc basin magmatism rather than island-arc activity. Therefore,
two distinct types of mantle peridotite are exposed in the southern Mariana
area. Bloomer and Hawkins ( 1983 ) briefly discussed mantle peridotites dredged
from the fault at 144°E, and Michibayashi et al. ( 2009 ) provided mineralogical
and petrologic data, together with an analysis of olivine crystal preferred orien-
tations, for several mantle peridotites recovered during a submersible dive. In
this paper, we document the petrographic, petrologic, and mineralogical char-
acteristics of mantle peridotites from the fault at 144°E in the southern
Marianas, and discuss the significance of these rocks.

2 Description of Dredge Sites and Geological Background

Plagioclase-bearing spinel peridotites were recovered from two dredge sites,
KH-98-1D1 and KH-98-1D2, during the KH-98-1 cruise of the R/V Hakuho-maru
(Table 1 ). These sites are on the scarp of a large fault which trends mainly N-S at
about 144°10¢E along the Mariana Trench, southwest of Guam.
Prior to this cruise, the same fault scarp had already been investigated during
two Shinkai 6500 dives, 6K#158 and 6K#159 (Fryer 1993 ), and one dredge haul,
D27 (Bloomer and Hawkins 1983 ) (Fig. 1 ). During the submersible dives, seven
basalts (plagioclase-clinopyroxene basalt) and three microgabbros were recovered
(Fryer 1993 ). Bloomer and Hawkins ( 1983 ) recovered serpentinized peridotites,
most of which are considered to be lherzolite, and small amounts of mafic rock.