DEFORMATION HISTORY OF PERIDOTITES FROM THE MARIANA FOREARC , CONICAL SEAMOUNT , LEG 125 1

Abundant serpentinite seamounts are found along the outer high of the Mariana forearc at the top of the inner slope of the trench. One of them, Conical Seamount, was drilled at Sites 778, 779, and 780 during Leg 125. The rocks recovered at Holes 779A and 780C, respectively, on the flanks and at the summit of the seamount, include moderately serpentinized depleted harzburgites and some dunites. These rocks exhibit evidence of resorption of the orthopyroxene, when present, and the local presence of very calcic-rich diopside in veins oblique to the main high-temperature foliation of the rock. The peridotites, initially well-foliated with locally poikiloblastic textures, show overprints of a two-stage deformation history: (1) a high-temperature (>1000°C), low-stress (0.02 GPa), homogeneous deformation that has led to the present Porphyroclastic textures displayed by the rocks and (2) heterogeneous ductile shearing at a much higher stress (0.05 GPa). This heterogeneous shearing probably describes a single tectonic event because it began at high temperatures, producing dynamic recrystallization of olivine in the shear zone, and ended at low temperatures in the stability field of chlorite and serpentine. In a few samples, olivine shows evidence of quasi-hydrostatic recrystallization at a very high temperature. Here, we propose that this recrystallization was related to fluid/magma percolation, a process that can also account for the resorption of the orthopyroxene and for the late crystallization of diopside veins in the rock. The impregnation by fluid or magma, development of the main high-temperature, low-stress deformation, and subsequent migration recrystallization of olivine probably occurred in a mantle fragment involved in the arc formation. In addition, this mantle has preserved structures that may have formed earlier in the oceanic lithosphere upon which the arc formed. Heterogeneous ductile shear zones in the peridotites may have developed during uplift. The "cold" deformation may have taken place during diapiric rise of hot mantle that underwent subsequent serpentinization or gliding along normal faults associated with the extension of the eastern margin of the forearc. INTRODUCTION Subduction-related magmatic processes have been studied intensively, mainly by means of geochemical investigations of arc lavas. Until now, however, the deepest parts of island-arc systems have remained largely unknown because of sampling difficulties. Consequently, few data are available on peridotites from subduction-related active margins, in contrast to abundant data on peridotites from true oceanic domains, ridges, transform-fault segments of ridges, and incipient oceans. However, significant petrological, geochemical, and textural changes are predicted to occur in the supra-subduction mantle to explain the unique characteristics of arc lavas. Almost no data are available on the structure of peridotites from supra-subduction-zone settings, except those published by Shcherbakov and SavelTeva (1984) on samples from the Mariana trench. Other data concern the petrology of some peridotites dredged from the outer wall of the Puerto Rico trench (Bowin et al., 1966) and mostly from the inner slope of the Mariana (Bloomer, 1983; Bloomer and Hawkins, 1983; Ishii, 1985; T. Ishii, pers. comm., 1990) and Tonga (Bloomer and Fisher, 1987; Saraskin et al., 1983) trenches. Most of these peridotites have experienced a high degree of melting and depletion, as evidenced by the very low modal content of clinopyroxene in these rocks and by the high Mg/Mg+Fe (Mg#) and Cr/Cr+Al (Cr#) contents of spinels (Mysen and Kushiro, 1977; Jakes and Green, 1980), which have compositions close to spinels from island-arc lavas (Dick and Bullen, 1984) and from peridotites of ophiolites that are supposed to have formed in subduction-zone environments (see discussion in Pearce et al., 1984, and in Bonatti and Michael, 1989). Traces of impregnation by basaltic liquids also Fryer, P., Pearce, J. A. , Stokking, L. B. , et al., 1992. Proc. ODP, Sci. Results, 125: College Station, TX (Ocean Drilling Program). 2 IPGP-UP7, 2 Place Jussieu, F-75252 Paris Cédex 05, France. (Present address: Université de Nantes, 2 Rue de la Houssinière, F-44072 Nantes Cédex 03, France.) 3 GDRGEDO, 6 Avenue Le-Gorgeu, F-29287 Brest Cédex, France. have been reported in some ultramafic rocks from the Mariana trench that are more lherzolitic in composition (Bloomer and Hawkins, 1983). The peridotites have undergone intense serpentinization mainly from seawater circulation, but also from water resulting from the dehydration of the subducting slab (Sakai et al., 1990). Most of these ultramafic rocks are recovered with gabbros and basalts: they should be part of an oceanic lithosphere that formed in the island arc (Dietrich et al., 1978; Bloomer and Hawkins, 1983; Ishii, 1985; Johnson et al., 1989; Johnson and Fryer, 1991). During Leg 125, peridotites from a seamount located between the outer-arc high of the Mariana forearc and trench were cored at Sites 778, 779, and 780 to constrain their origin and mechanism(s) of uplift (Fryer, Pearce, Stokking, et al., 1990, Fryer et al., 1990; Shipboard Scientific Party, 1990a, 1990b). Because of their location at about 100 km to the east of the present-day active arc, these rocks might represent part of the basement of the island arc or part of a "pristine" mantle from trapped Pacific Ocean lithosphere. We present a structural study of these peridotites to define their tectonic history and to constrain their origin. The peridotite clasts in sedimentary rocks have been reworked; therefore, special attention has been devoted to the textures to constrain the magmatic and deformational evolution of the rocks during their ascent. REGIONAL BACKGROUND AND PREVIOUS DATA The Mariana Arc constitutes the southern branch of the Izu-BoninMariana arc system, marking the active boundary between the Pacific and the Philippine Sea plates (Fig. 1). This arc represents the eastern border of a complex area comprising several imbricated arcs and basins that began to form during the early-middle Eocene, along with westward subduction of the Pacific Plate. This system, more than 1000 km wide, includes, from east to west, the trench-related subduction zone; the forearc area; the active island arc; a back-arc basin (the Mariana Trough); the West Mariana Ridge, a now inactive marginal basin (the Parece Vela Basin) and a starved ridge, the Palau Kyushu Ridge.