the Parece Vela Material Exposed in

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1 118 Proc. Japan Acad., 72, Ser. B (1996) [Vol. 72(B), Peridotites from Upper Mantle the Parece Vela Material Exposed in Rift in the Philippine Sea an Extinct Back-arc Basin By Yasuhiko OHARA, *) Shigeru KASUGA,*) and Teruaki ISH11**) (Communicated by Yoshibumi TOMODA, M. J. A., June 11, 1996) Abstract : Serpentinized peridotites and gabbros were dredged from the axial zone of the Parece Vela Basin during a cruise of S/V Takuyo in The central zone of the Parece Vela Basin is characterized by the NS trending chain of depressions forming right-step en-echelon alignment. Our petrological studies suggest that the peridotites are residues of extensive partial melting of primitive mantle peridotites. Presence of hydrous phases in the peridotites indicates that the peridotites were subjected to metasomatic event in the upper mantle beneath the Parece Vela Rift. Key words : Parece Vela Rift; back-arc basin; peridotite; upper mantle; metasomatism. Introduction. The recovery of serpentinized peridotites and gabbros from back-arc basins has been very limited. Serpentinized spinel-lherzolites and gabbros were dredged for the first time in back-arc basins during the cruise of R/V Dmitry Mendeleev (USSR) in the Parece Vela Rift in 1976, although no detailed petrological studies on these peridotite and gabbro samples have been published yet. 1)2) In 1984, the cruise of R/V Akademik Vinogradov (USSR) recovered dunite and troctolite assemblage in the Parece Vela Rift at 17 54'N.3~ Recently, Martinez et al. (1994) reported peridotites and gabbros from the northern part of the Mariana Trough, one of the active back-arc basins. 4) The Parece Vela Basin is an extinct back-arc basin located to the south of the Shikoku Basin in the Philippine Sea. It is bordered by the Kyushu-Palau Ridge on its west and by the West Mariana Ridge on its east (Fig. 1). The Hydrographic Department of Japan (JHD) has conducted geophysical surveys in the southern waters of Japan by S/V Takuyo since From 1992 to 1995, JHD has obtained SeaBeam swath bathymetric data, seismic reflection profiles, magnetic and gravity anomalies, and several dredge hauls in the middle to northern part of the Parece Vela Basin (Kasuga and Ohara, in prep.). The samples described in this paper are serpentinized peridotites and gabbros dredged at 16 23'N *) Hydrographic Department Japan. **) Ocean Research Institute Tokyo 164, Japan. of Japan, Tsukiji, Tokyo 104,, University of Tokyo, Nakano, along the Parece Vela Rift during a cruise of S/V Takuyo in We report preliminary analyses of the peridotites and gabbro samples in this paper. It should be noted that this paper describes the petrology of peridotites from back-arc basins for the first time. Tectonic setting of the Parece Vela Basin and dredge location. The Philippine Sea is divided into two portions by roughly NS trending Kyushu-Palau Ridge. To the east of the Kyushu-Palau Ridge, there exist to be a eastward progression of sequentially younger basins, that is, extinct Parece Vela Basin that merges to the north with the Shikoku Basin, and the active Mariana Trough (Fig. 1). According to the previous studies, the Parece Vela Basin has a typical oceanic crust.2),5> Most part of the basin are roughly 5 km deep. Axial zone of the Parece Vela Basin is characterized by the NS trending chain of the depressions forming right-step en-echelon alignment (Fig. 2). These depressions are typically diamond-shaped and bordered by steep escarpment with 1000 to 1500 m in relative height. Presence of a series of discrete deeps with depths locally exceeding 7 km was first pointed out by Bogdanov et al. (1977).1) Mrozowski and Hays (1979) named it Parece Vela Rift.5~ Seismic profiles show that these depressions have very thin sediment cover forming V-shaped cross sections. The samples described in this paper come from a steep escarpment cutting a diamond-shaped depression (16 23'N, 'E; 2659 m depth) (Fig. 2A). The dredge contains about 4 kg of serpentinized peridotites and gabbros (Table I).

2 No. 6] Peridotites from the Parece Vela Rift in the Philippine Sea 119 Fig. 1. shaded Major geological features in the Philippine Sea. The area is shown in Fig. 2. Table I. Recovered of rocks in the dredge S/V Takuijo in 1995 haul of a cruise Petrography and mineral chemistry. We report the modal and mineral compositions of two peridotite samples, 0' and ; and a gabbro sample, in this paper. Mineral compositions were analyzed at Ocean Research Institute, University of Tokyo with JEOL EPMA model JCXA-733. The correction procedures are after Bence and Albee (1968).6) Total iron is assumed to be equal to Fe2+ except for spinel. Fe3+ content of spinel was calculated from charge balance considerations. Only core compositions are used in this paper to characterize the primary mineralogy. The primary modal compositions are shown in Table II and representative chemical compositions of minerals in Table III. In addition to the samples mentioned above, the sample was also analyzed for the composition of the hydrogarnet after plagioclase. Peridotites. The dredged rocks are angular pieces ranging in diameter from 1 to 10 cm. Most are slightly weathered on all sides with relatively thin Mn-oxide coating. All of the dredged peridotites are highly serpentinized; especially, olivine and plagioclase are totally serpentinized. Because the alteration products form characteristic pseudomorphs after the primary phases, it is often possible to determine the primary mineral assemblage even when no relict primary mineral remains. The peridotites include irregular dark-brown patches of hydrogarnet with Fig. 2. A) SeaBeam bathymetric map of the middle part of the Parece Vela Basin. Contours are in 500-m intervals. The solid circle shows the location of the dredge haul of a cruise of S/V Takuz0o in 1995 (16 23'N, 'E; 2659 m depth). B) Computer-aided topographic relief map illuminated from N45 E.

3 120 Y. OHARA, S. KASUGA, and T. ISHII [Vol. 72(B), Table II. Modal compositions of the peridotites and gabbro from the Parece Vela Rift Table III. Representative chemical composition of constituent minerals in the peridotites and gabbro from the Parece Vela Rift high Ca, Al, and Fe contents, that appear to be altered plagioclase (Table III). We refer to the peridotites as harzburgite on the basis of primary modal abundances; they have fairly uniform modal compositions of 71 to 75% of olivine, 21 to 26% of orthopyroxene, 0.9 to 1.3% of clinopyroxene, 1.3 to 2.4% of plagioclase, 0.4 to 0.7% of spinel, and trace amounts of amphibole and phlogopite (Table II). The primary relict minerals are orthopyroxene, clinopyroxene, spinel, amphibole, and phlogopite. Spinel is generally dark reddish-brown in color, and exhibits holly-leaf habit (Fig. 3A). In most cases, orthopyroxenes have thin lamellae of exsolved clinopyroxene (Fig. 3B). Clinopyroxene occurs as clots or patches at the margins or the inside of orthopyroxene (Fig. 3B). Amphibole (hornblende) occurs as small patches at the rim of orthopyroxene (Fig. 3C). Traces of pale brown phlogopite are present in some samples. Phlogopite blades are usually less than 0.2 mm across and occur as inclusions in orthopyroxene (Fig. 3D). Spinel. A plot of Mgl(Mg+Fe) ratio vs. Cr/ (Cr+AI) ratio in spinel is shown in Fig. 4. Spinel shows Cr/(Cr+AI) ratio ranging from 0.45 to 0.53, and relatively high Fe`' content. Ti02 content is relatively constant and considerably high, ranging from 0.32 to 0.46 wt%; but it is not correlated with Cr/(Cr+AI) ratio. Pyroxenes. The pyroxene compositions are plotted in the pyroxene quadrilateral in Fig. 5. The composition of orthopyroxene is considerably variable, and falls both in the enstatite and the outside of the enstatite field. The Mg/(Mg+Fe) ratio of orthopyroxene ranges from 0.91 to The A1203 content of orthopyroxene ranges from 2.96 to 4.65 wt% and Ti02 content from 0.08 to 0.18 wt%. The composition of clinopyroxene also variable, but falls within the diopside field (Fig. 5). The Mg/(Mg+Fe) ratio of clinopyroxene ranges from 0.91 to The Ti02, Na20, and Cr20; contents of clinopyroxene are variable; Ti02 content ranges from 0.21 to 0.53 wt%, Na20 content from 0.43 to 0.79 wt%, and Cr203 content from 0.96 to 1.84 wt%.

4 No. 61 Peridotites from the Parece Vela Rift in the Philippine Fig. 3. A) Holly-leaf spinel in sample Olivine is totally serpentinized. Orthopyroxene and clinopyroxene in sample Note the clinopyroxene Sea 121 plane-polarized light. B) exsolution lamellae in orthopyroxene. Clinopyroxene also occurs as a inclusion of orthopyroxene. light. Crossed polarizers. C) Hornblende sample Hornblende is interstitial to orthopyroxene. plane-polarized light. D) Phlogopite sample Phlogopites occur as inclusions in orthopyroxene. Orthopyroxene is totally light. Crossed polarizers. Abbreviation of mineral names are, ol: olivine; opx: orthopyroxene; spinel; hb: hornblende; phg: phlogopite. Fig. 4 (left). Plot of Mg/(Mg+Fe'') vs. Cr/(Cr+Al) for spinel of the Parece Vela Rift peridotites. in in bastitized. cpx: clinopyroxene; sp: The field for abyssal peridotites are given by Dick and Bullen Fig. 5 (center). Composition of pyroxenes (1984),`" and for fore-arc peridotites of the Parece Vela Rift peridotites by Ohara and Ishii (in prep.). ( , ) and gabbro ( ). Fig. 6 (right). Leake's diagram": expressed in structural formula vs. Si for amphibole of the Parece on the basis of 23 oxygen atoms. Vela Ca+Na+K calculated Rift peridotite. Elements are

5 122 Y. OHARA, S. KASUGA, and T. ISHII [Vol. 72(B), Amphibole. The primary amphibole is mostly hornblende according to the classification of Leake (1968).7 The compositions are plotted in the region of pargasite pargasitic hornblende in the Leake's diagram (Fig. 6). Some hornblende is altered into tremolitic composition, which is possibly from the late stage alteration. The Mg/(Mg+Fe) ratio of pargasite pargasitic hornblende ranges from 0.90 to The K20 content of pargasite pargasitic hornblende is generally higher than 0.74 wt% and up to 0.94 wt%. The Ti02 content is extremely high; it ranges from 3.34 to 4.55 wt%. Phlogopite. Phlogopite contains 1.53 to 1.90 wt% of Ti02. The Fe0 content in phlogopite ranges from 3.17 to 3.52 wt%. These chemical characteristics are comparable to those for primary phlogopite in garnet lherzolites.8 The phlogopites are relatively rich in A1203 content ranging to wt%, and have approximately 5.5 Si atoms for 22 oxygen atoms on an anhydrous basis. A1203 contents are distinctively higher than those in primary phlogopites in garnet lherzolite xenoliths in kimberlites, but are almost within the range of phlogopites in spine! to plagioclase peridotites. Gabbros. All of the dredged gabbros are relatively fresh and contain clinopyroxene and plagioclase as primary relict minerals. Olivine is totally altered into the talc and tremolite assemblage. The composition of clinopyroxene falls within the boundary field between diopside and augite (Fig. 5). The Mg/(Mg+Fe) ratio of clinopyroxene ranges from 0.79 to Some plagioclase grains have kink-banding and exhibit wavy extinction. In general, plagioclase exhibits tabular subhedral to euhedral morphologies. Plagioclase composition ranges from An 57 to 49. Brown interstitial hornblende occurs as patches at the rim of clinopyroxene and plagioclase. The modal composition of the sample is shown in Table II. Discussion. Degree of depletion of the upper mantle beneath the Parece Vela Rift. The variations of spine! Cr/Al ratio could be caused by the peridotites having undergone different degrees of partial melting, which varies in a different tectonic setting.9~'10~ Bonatti and Michael (1989) compiled the petrological and mineralogical characteristics of mantle-derived spinelperidotites from various geo-tectonic settings. 10) They concluded that the degree of depletion of the peridotites increases from pre-oceanic rifts through passive margins and mature oceans to subduction-related active margins. In the spinel Mg/(Mg+Fe) ratio vs. Cr/(Cr+AI) ratio plot (Fig. 4), the Parece Vela Rift peridotites fall in the field characterized by moderate to relatively high Cr/(Cr+AI) ratio defined by the abyssal peridotites from the Mid-Atlantic Ridge fracture zones. The Parece Vela Rift peridotites are as refractory as the most refractory peridotites from the Mid-Atlantic Ridge fracture zones. Thermal regime of the upper mantle beneath the Parece Vela Rift. The Ca-rich nature of the orthopyroxene and Ca-poor nature of the clinopyroxene indicate high nominal temperature for two-pyroxene geothermometry. The relatively high A1203 content of both orthopyroxene and clinopyroxene, together with the Cr203-rich nature of clinopyroxene also indicate that the upper mantle beneath the Parece Vela Rift could be as hot as that beneath the Mid-Atlantic Ridges. Fig. 7 shows that the relationships between Cr/(Cr+AI) ratio in spine! and A1203 content in orthopyroxene. The high Al partition coefficient between orthopyroxene and spinel in the Parece Vela Rift peridotites relative to fore-arc peridotites could signify that the former equilibrated at higher temperature than the latter (Fig. 7). We estimated equilibration temperatures for the Parece Vela Rift peridotites using two-pyroxene geothermometry calibrated by Wells (1977). 11) The estimated temperature is approximately 990 C for the averaged compositions of both pyroxenes (Table IV). Metasomatized mantle domain beneath the Parece Vela Rift. The Parece Vela Rift peridotites are as refractory as the most refractory peridotites from the Mid-Atlantic Ridge fracture zones, as discussed above. However, their mantle-equilibrated phases have relatively high concentrations of incompatible Fig. 7. Plot of Cr/(Cr+AI) of spinel vs. A12O3 content in orthopyroxene of the Parece Vela Rift peridotites. The field for abyssal peridotites are given by Bonnatti and Michael (1989),10 and for fore-arc peridotites by Ohara and Ishii (in prep.).

6 No. 6] Peridotites from the Parece Vela Rift in the Philippine Sea 123 Table IV. Temperatures of equilibration peridotites estimated by geothermometry calibrated for the the two-pyroxene by Wells (1977)" elements such as Ti and Na that partition into melt. Another remarkable feature of the Parece Vela Rift peridotites is that they have small amounts of pseudomorphed plagioclase and hydrous phases such as pargasite '- pargasitic hornblende and phlogopite. Peridotite with plagioclase is common among the abyssal peridotites from the Mid-Atlantic Ridge fracture zones.9)' 12) Presence of plagioclase and hydrous phases could be accounted for entrapment of a melt fraction or infiltration of H20-rich fluid. Dick and Bullen (1984) pointed out that spinel in abyssal plagioclase peridotites is richer in Ti and Fe3+ than that in the normal abyssal spinel peridotites.9) They also noticed that spinel and pyroxenes in plagioclase peridotites are richer in Cr and Ti, respectively. It seems likely that relatively high Ti content in spinels, orthopyroxenes, and clinopyroxenes of the Parece Vela Rift peridotites resulted from the reaction with a melt or H20-rich fluid in t,hp unnar mant,1p_ ThP Ti(l2- and K20-rich chemistry of pargasite pargasitic hornblende of the Parece Vela Rift peridotites is similar to that of the pargasite from the Zabargad metasomatized peridotites which represent undepleted parental mantle materials of the Red Sea lithosphere. 13) We suggest the presence of metasomatized mantle domain beneath the Parece Vela Rift. Emplacement of mantle-derived rocks in the Parece Vela Rift. Exposures of serpentinized peridotites are common in the slow-spreading ridges along the walls both of the fracture zones and the axial valleys away from fracture zones. 12),14) In many cases, fracture zone peridotites are exposed along active transform walls near their intersection with the ridge axis, in regions where peridotites also outcrop along the axial walls. Thus, peridotite exposure along fracture zones can be emplaced at ridge-transform intersections which are the magma-poor parts of the ridge. 15) The axial zone of the Parece Vela Basin is characterized by the NS trending chain of the depressions forming right-step en-echelon alignment. These depressions are aligned in diamond-shape and bordered by steep escarpments with 1000 to 1500 m in relative heights (Fig. 2). It is noted that these escarpments have S-shaped curved trend and their configuration is symmetric relative to the axial depressions (Kasuga and Ohara, in prep.). They concluded that these depressions and escarpments are a topographic expression of extinct spreading axes and S-shaped transform faults, respectively. The topography of the rift and the mineral chemistry of the dredged peridotites in the Parece Vela Rift indicate that the emplacement mechanism of mantle-derived peridotites could be the same as in the slow-spreading ridges. Acknowledgements. We acknowledge the help of the captain and crews of the S/V Takuyo. Thanks are due to Prof. Kazuhito Ozawa and Dr. Kazuo Kobayashi for critical reading of the manuscript. This work was supported in part by funds from the Cooperative Program (No. 66, 1996) provided by Ocean Research Institute, University of Tokyo. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) References Bogdanov N., Bouchez J. L., Lapierre E., Desmons J., Houshmanzadeh A. et al. (1977) Ofioliti 2, Dietrich V., Emmermann R., Oberhansli R.,and Puchelt H. (1978) Earth Planet. Sci. Lett. 39, Shcheka S. A., Vysotskiy S. V., S'edin V. T., and Tararin I. A. (1995) In Geology and geophysics of the Philippine Sea (eds. Tokuyama H., Shcheka S. A., and Isezaki N.). Terra Scientific Publishing Company, Tokyo, pp Martinez F., Stern R. J., Yamazaki T., and Bloomer S. H. (1994) EOS Trans., Am. Geophys. Union 75, 673. Mrozowski C. L., and Hayes D. (1979) Earth Planet. Sci. Lett. 46, Bence A. E., and Albee A. L. (1968) J. Geol. 76, Leake B. E. (1968) Geol. Soc. America Spec. Paper 98, Delaney J. S., Smith J. V., Carswell D. A., and Dawson J. B. (1980) Geochim. Cosmochim. Acta 44, Dick H. J. B., and Bullen T. (1984) Contrib. Mineral. Petrol. 86, Bonatti E., and Michael P. J. (1989) Earth Planet. Lett. 91, Wells P. R. A. (1977) Contrib. Mineral. Petrol. 62, Sci. Bonatti E., Peyve A., Kepezhinskas P., Kurentsova N., Seyler M., Skolotnev S., and Udintsev G. (1992) J. Geophys. Res. 97, Bonatti E., Ottonello G., and Hamlyn P. R. (1986) J. Geophys. Res. 91, Cannat M. (1993) J. Geophys. Res. 98, Cannat M., and Seyler M. (1995) Earth Planet. Sci. Lett. 133,