Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B2, PAGES , FEBRUARY 10, 2000 Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle Masako Yoshikawa and Eizo Nakamura Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori, Japan Abstract. Major and trace element compositions and isotopic ratios of Sr and Nd were determined for bulk rocks and their constituent clinopyroxenes from the Horoman peridotite complex, Japan. A1203, CaO, and heavy rare earth elements (HREE) contents of peridotites generally decrease from plagioclase lherzolite through spinelherzolite to spinel harzburgite, indicating simple melt extraction from a single source. However, the extremely large variations in isotopic (87Sr/86Sr = to , œnd = -I- 110 to -10) and trac element compositions ([Ce/Yb]N = to 4.0) cannot be explained by a simple melt extraction mechanism. The samples can be divided into two groups: one suite has depleted isotopic and light REE (LREE) characteristics (DP), while the other suite shows enriched isotopic and LREE signatures (EP). Sm-Nd isotope systematics of whole-rock DP samples yield an isochron age of 833 _+ 78 Ma with an initial 143Nd/144Nd ratio of _+ 2, which is identical to the isotopi composition of mid-ocean ridge basalt (MORB) source mantle at that time. The relationship between MgO and Yb abundances of whole rocks shows that melt extraction was initiated at pressures near the garnet and spinelherzolite transition. Peridotites that formed at different depths presently occur in close proximity to each other, sometimes within tens of meters. The chemical and isotopic signatures of the EP samples can be explained by mixing between mantle residue and an isotopically and more incompatiblelement enriched fluid derived from a subducted slab. These observations suggest that the small-scale compositional layering observed in the complex may have formed in a wedge mantle by water-enhanced thinning and folding of metasomatized peridotites which had previously developed large-scale simple stratification as a result of melt extraction beneath a mid-ocean ridge. 1. Introduction On the basis of these correlations, the compositional layering in peridotite complexes has been interpreted to represent Alpine-type peridotite complexes provide direct residues left by various degrees of melt extraction from a information on the chemical composition of the upper mantle. homogeneous source mantle [e.g., Frey et al., 1985; Compared with peridotite xenoliths which only represent McDonough and Sun, 1995]. Additionally, abundances of centimeter to meter size pieces of the mantle, peridotite heavy rare earth elements (HREE) are inversely correlated complexes occur at the kilometer scale. They are thus with MgO content. This correlation is also expected in capable of furnishing more detailed geochemical and residues left after varying degrees of melt extraction from a structural information abouthe upper mantle than is provided single source [e.g., McDonough and Sun, 1995]. by xenoliths. However, it is generally accepted that the poor correlation The peridotite complexes are characterized by between light REE (LREE) and abundances of CaO, Al203, compositional layering composed mainly of peridotites, which and MgO, as well as the extreme isotopic variation of Sr, Nd, range in composition from lherzolite to dunite, with a small and Pb observed at various scales (centimeter to kilometer proportion of pyroxenite and gabbro [e.g., Menzies and order), cannot be explained by simple melt extraction alone Dupuy, 1991]. In the peridotites, abundance of MgO in bulk [e.g., Frey and Green, 1974; Polvd and Alltigre, 1980; rocks and the forsterite (Fo) content of olivine increase with Richard and Alltigre, 1980; Loubet and Alltigre, 1982; decreasing bulk rock abundances of CaO, A1203, and TiO 2. Hamelin and Alldgre, 1988]. Zindler and Hart [1986] found a positive correlation between isotopic variation and scale 1 Now at Yamanashi Institute of Environmental Sciences, Yamanashi, length of sampling in peridotite complexes and oceanic Japan. basalts and suggested that isotopic heterogeneity in the mantle existed not only on small scales but also on the largest scales Copyright 2000 by the American Geophysical Union (thousands of kilometers). To explain such chemical and Paper number 1999JB isotopic heterogeneity in peridotites, Alldgre and Turcotte /00/1999JB [1986] proposed a "marble cake mantle" model in which, 2879

2 2880 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES prior to its subduction, the oceanic mantle is progressively but variably depleted by multistage melting. Solid-state mixing of such variably depleted mantle together with oceanic crust during subsequent convection in the upper mantle results in Horoman peridotite complex. These data confirm many previous inferences based on fewer data, and further clarify the genetic relationship between melt extraction and metasomatism well as the origin of compositional layering. the marble cake composition. However, All gre and Turcotte [1986] do not preclude the possibility of a metasomatic imprint on the peridotite complexes. 2. Geological and Petrological Features of the In the Ronda peridotite, detailed isotopic studies showed Horoman Peridotite Complex that some clinopyroxenes had lower Nd isotopi compositions than the bulk earth, and this was interpreted as isotopic heterogeneity caused by metasomatism involving a LREE- The Horoman peridotite complex occurs over an ~8 x 10 km area and is more than 3 km thick [Niida, 1974, 1984] enriched component [Reisberg and Zindler, ; (Figure 1). It resides in the southern end of the Hidaka Reisberg et al., 1989]. Reisberg et al. [1989] also showed metamorphic belt [Niida, 1974, 1984], which consists of a that the isotopic compositions of clinopyroxenes were, within continental or island arc crust-mantle section and a analytical error, similar to those of orthopyroxene and plagioclase. This was taken as evidences that the Ronda peridotite was recently metasomatized in the mantle. A recent study, based on geochemical and structural data, metamorphosed oceanic crust-mantle section [e.g., Komatsu et al., 1994]. The Horoman complex is exposed along the Hidaka Main Thrust between these two sections and is in fault contact with the surrounding Hidaka metamorphic rocks [e.g., suggested that residual peridotite in the Ronda complex was Komatsu et al., 1994]. No evidence of contact metasomatized by pervasive kilometer-scale porous melt flow, metamorphism has been found in the country rocks [e.g., resulting in chemical variation and recrystallisation [Van der Niida, 1974, 1984]. Although serpentinization has Wal and Bodinier, 1996]. On the basis of trace element studies of amphibole pyroxenite vein and host peridotite in the Lherz complex, it has been inferred that metasomatism in Alpine peridotites could also be attributed to percolation of alkaline melts from the asthenosphere (pyroxenite) into developed locally at the margin and along faults and shear zones, the complex is essentially unaltered [e.g., Niida, 1974, 1984]. The Horoman peridotite complex exhibits a well-developed compositionalayering on a scale of centimeters to several residual peridotite [e.g., Bodinier et al., 1990]. However, hundred meters. The layers consists of dunite, harzburgite, such a model fails to explain the origin of the metasomatic spinel lherzolite, plagioclase lherzolite, and minor amounts of agent and to constrain tectonic setting during metasomatism. gabbro and pyroxenite as shown in Figure 1 [e.g., Komatsu To explain high LREE/HREE ratios with high Fo mol% of olivine and Cr/(Cr + Al) ratio of spinel (Cr#) in peridotire complexes, melt extraction coupled with melt influx has been suggested [e.g., Ozawa, 1988; Kelemen et al., 1992, 1995; Ozawa and Shimizu, 1995]. However, these studies do not address the origin of isotope heterogeneity in the mantle. The Horoman peridotite complex is one of the freshest orogenic peridotite complexes. On the basis of detailed petrologic and geochemical studies, it has been inferred that and Nochi, 1966; Niida, 1984; Takazawa et al., 1992; Takahashi, 1991a, b, 1992]. On the basis of symmetrical and oscillatory mineral compositional changes as a function of stratigraphic height, Niida [1984] inferred that the Horoman peridotite complex was formed by successive fractional crystallization resulting in a series of dunite, lherzolite, plagioclase lherzolite, and gabbro. Takahashi [1991a, b, 1992] subdivided the complex into three distinct rock suites: the dominant Main Harzburgite - Lherzolite this complex suffered similar chemical fractionation processes, (MHL) suite, a subordinate Spinel Rich-Dunite - Wehrlite such as melt extraction, metasomatism, and subsolidus mineral breakdown, as did other peridotite complexes [e.g., Niida, 1974, 1984; Obata and Nagahara, 1987; Frey et al., 1991; Takahashi, 1991a, b, 1992; Takazawa et al., 1992, 1995, 1996; Yoshikawa, 1993; Nakamura and Yoshikawa, 1995; Ozawa and Takahashi, 1995; Takazawa, 1996]. Thus this complex should be suitable for resolving the origin and (SDW) suite, and a minor Banded-Dunite-Harzburgite (BDH) suite. The samples used in this study were collected from the MHL suite, which consists of spinel harzburgite, spinel lherzolite, and plagioclase lherzolite. The peridotites in the MHL suite show a porphyroclastic texture [Niida, 1975b, 1984]. Plagioclase in plagioclase lherzolite usually occurs in seams associated with elongated fine-grained aggregates of timing of chemical and isotopic variations in the peridotites, spinel, small amounts of olivine and/or two pyroxene which because their compositions have not been significantly are parallel to the layering [Niida, 1984; Takahashi and Arai, disturbed by secondary alteratio near the surface. Although trace element and isotopic analyses of whole rocks provide powerful data for discussing primary processes, these data are still limited in the Horoman peridotite complex due to analytical difficulties. Furthermore, more careful selection and preparation of samples for trace element and isotope analyses are required to avoid the secondary alteration and grain boundary enrichment that may have biased the whole rock data, because of extremely low contents of so-called 1989]. Spinel lherzolite near the boundary with plagioclase lherzolite has two-pyroxene + spinel (+ plagioclase) symplectites which are found in seams similar to those in plagioclase lherzolite [Niida, 1984; Takahashi and Arai, 1989; Morishita et al., 1995; Morishita and Arai, 1997]. From plagioclase lherzolite to spinel harzburgite through spinel lherzolite in the MHL suite, the modal abundances of two pyroxenes, and the contents of Al203, CaO, and HREE decrease with increasing contents of MgO and forsterite incompatible trace elements, used previously in the discussion mole% in olivine. On the basis of these observations, the of the mantle processes. In this paper, we present a more MHL suite has been interpreted to be a residue left after comprehensive geochemical data set which includes major and trace element abundances, as well as Sr and Nd isotopic compositions of whole rocks and clinopyroxenes from the various degrees of melt extraction from a homogenousource mantle [Frey et al., 1991; Takahashi, 1991a, b, 1992; Takazawa et al., 1992].

3 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2881 N Map of Hokkaido, Japan Mt. Yoshida Dunite Harzburgite Lherzolite Plagioclase Iherzolite Mafic rocks Mylonite, gneiss & schist""iiiiiiii Metagabbro Hidaka super group Alluvium o I Fault & thrust krn -td m Bozu Stream BZ.262 BZ257 BZ146 BZ143 BZ216 BZl16 Figure 1. Lithological map of the Horoman peridotite complex showing sample localities. The lithological map is modified from Takazawa et al. [1992]. The sample locations and numbers in the Bozu Stream section correspond to those in Takazawa et al. [1992, 1996] and Takazawa [1996]. The two-pyroxene + spinel (+ plagioclase) symplectites mineral compositions and structural relationships [Takahashi, observed within seams in spinel lherzolites and plagioclase 1991a, b, 1992]. The BDH suite occurs in the MHL suite as lherzolites have been interpreted to be products of subsolidus several discontinuous bodies with sharp boundaries, and it is breakdown reactions sustained as the Horoman complex strongly deformed compared with other suites [Takahashi, experienced successive decompression from the garnet to the 1991a, b, 1992]. This suite is characterized by high Cr# plagioclase stability field [Niida, 1984; Takahashi and Arai, spineis (> 0.7) and olivine with high Fo content, and whole 1989; Ozawa and Takahashi, 1995; Takazawa et al., 1996; rocks have different major element compositions from other Morishita et al., 1995; Morishita and Arai, 1997]. In the suites. From these observations, Takahashi [1991a, b, 1992] MHL suite, alkali-metasomatism formed phlogopite veins as interpreted that the BDH was an exotic block within the MHL well as interstitial phlogopite and amphibole [Niida, 1975a; suite, derived from a high Mg magma generated in a fore-arc Arai and Takahashi, 1989]. On the basis of Rb-Sr isotope setting. systematics of the phlogopite vein and its host peridotite, Yoshikawa et al. [1993] suggested that the metasomatism 3. Sample Locations and Selection responsible for the formation of the phlogopite veins took To examine relationships between isotopic variation and place at 23 Ma and was caused by fluid derived from the surrounding metasediments during emplacement of the amplitude of layering, peridotite samples were collected from three stratigraphic sections: the, 600 m Horoman River complex. Metasomatism without the formation of hydrous minerals has also been inferred from the occurrence of LREEsection, the 140 m Bozu Stream section and the 6m Mount enriched clinopyroxenes the Bozu stream section of the Yoshida section (Figure 1). The Horoman River section is MHL suite [Takazawa et al., 1992, 1996]. The mantle composed of roughly 30 m harzburgite (HD03), 15 m dunite, metasomatism which followed melt extraction was also 15 m harzburgite (62127), 140 m spinel lherzolite (62130), discussed on the basis of extremely large isotopic variations 300 m plagioclase lherzolite (62213, ), and 70 m in clinopyroxenes of this complex [Yoshikawa, 1993; spinel lherzolite ( , m) (for detailed Nakarnura and Yoshikawa, 1995; Takazawa et al., 1995, information of samples 62127, 62130, and 62213, see Frey et 1996; Takazawa, 1996]. al., [1991]). In the Bozu Stream section, 55 m plagioclase The SDW suite, which occurs as layers in the MHL lherzolite (BZ262, BZ257, BZ146), 60 m spinel lherzolite harzburgites with sharp boundaries, is inferred to be a (BZ143), and 20 m harzburgite (BZ216, BZ 116) were cumulate related to the formation of the MHL suite based on sampled (for detailed information of sample locations, see

4 2882 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES Takazawa et al [1992, 1996] and Takazawa [1996]). Three samples were collected from the Mount Yoshida section which is composed of 2.5 m plagioclase lherzolite (YOH8), Trace element analyses of whole rocks were performed on a Yokogawa PMS 2000 ICP-MS using the flow injection method developed by Makishirnand Nakamura [1997] and 0.1 m spinel lherzolite (YOH7), 2 m harzburgite, 0.1 m Makishirna et al. [1997, 1998]. The analytical gabbro, 0.1 m harzburgite, and 1.5 m plagioclase lherzolite reproducibility (relative standard deviation, RSD) of trace (YOHB). To avoid chemical changes caused by alteration and elements except boron in peridotites, U.S. Geological Survey (USGS) and Geological Survey of Japan rock reference phlogopite-vein forming metasomatism, we selected samples material PCC-1 and JP-1, respectively, were < 20% and for analyses which were free from phlogopite veins and had minimal secondary minerals (< 5 vol%). typically <10% [Makishirn and Nakamura, 1997; Makishirna et al., 1998]. Replicate analyses of boron for USGS rock standard PCC-1 yielded RSD of < 2% [Makishirna et al., 4. Analytical Method 1997]. Trace element analysis of clinopyroxenes was carried out In order to remove surface contamination and alteration employing a Cameca ims 5f ion microprobe following the effects before analysis, the powdered whole-rock samples and techniques described by Nakamura et al. [1998] and separated clinopyroxenes were treated according to the Nakarnura and Kushiro [1998]. Clinopyroxenes from following procedure: For analysis of isotopic compositions, mantle xenoliths were used as standard materials, and these the samples were leached with 4 M HC1 at 80 øc for 1 hour. were chemically characterized using the ICP-MS. For analysis of trace elements in whole-rocks, the powder was Clinopyroxenes the thin sections were sputtered with an O- washed with 0.1 M HCI for 20 min in an ultrasonic bath. primary beam of- 10 na intensity resulting in a beam size of The sample was then rinsed twice with distilled water and - 10 txm in diameter. Positive secondary ions were collected dried at 110 øc prior to acid digestion. After these by ion counting using an energy offset of-60 V from 4500 V treatments, the whole-rock samples contained > 98 % of the acceleration with an energy bandpass of _+ 10 V. These Sm, Nd, and Sr and > 60 % of the Rb relative to unwashed operating conditions resulted in 30Si secondary ion beam samples. This indicates that while the concentration of these intensities of x 105 cps for clinopyroxenes. 17 trace elements did not change significantly owing mostly to olivine elements were determined in a run taking ~40 min including breakdown by HCI, most of the Rb of secondary origin was presputtering. Analytical reproducibility (RSD, n = 4-6) removed, as previously observed by Zindler and Jagoutz using one of the clinopyroxene standards from a spinel [1988]. The acid leaching was not, however, performed on lherzolite mantle xenolith was generally < 10%, except for Ba the samples used for major element analysis of bulk rocks, (50%), but typically < 5% except for La and Sm and Gd (10 - because the partial serpentinization of the samples had no 7%). effect on major element composition [Frey et al., 1985]. All the concentration and isotope analyses were carried out at the Pheasant Memorial Laboratory (PML), Institute for Study of the Earth's Interior, Okayama University at Misasa. Major element compositions together with NiO and Cr203 of whole rocks and clinopyroxenes were analyzed on fused disks 5. Major Element Compositions Major element compositions of the samples are given in Table 1 and selected oxides are plotted against MgO contents on Figure 2. The Horoman peridotites investigated this using a Philips PW 2400 X-ray fluorescence spectrometer study contain relatively small amounts of water (< 1.4 wt% (H.Takei and E. Nakamura, manuscript in preparation, 1999), and on thin sections using a Horiba EMAX-7000 energy dispersive X-ray spectrometer assembled into a Hitachi S- 3100H scanning electron microscope (C. Sakaguchi and E. H20+, < 0.78 wt% LOI, loss on ignition) compared with other peridotite complexes (Ronda, < 7.5 wt% H20 + [Frey et al., 1985]; Lherz, < 3.91 wt% LOI [Bodinier et al., 1988]; Lanzo, < 7.43 wt% LOI [Bodinier, 1988]). This indicates that the Nakamura, manuscript in preparation, 1999), respectively. Horoman peridotite complex has experienced only minor H20+ and H20-, and FeO for whole rocks, were determined by gravimetric and titration methods, respectively (H. Takei and E. Nakamura, manuscript in preparation, 1999). The analytical procedure for chemical separation and mass spectrometry followed Yoshikaw and Nakamura [1993] for Sr isotopic ratio and abundances of Rb and Sr and Shibata et al. [1989] for Nd isotopic ratio and abundances of Sm and Nd. Mass spectrometry was carried out on a Finnigan MAT 261 instrument equipped with 5 Faraday cups using statichydrous alteration, as is also corroborated by petrographic observations. As shown in Figure 2, SiO2 (42 to 44 wt%) and total iron (expressed as FeO*; 7.7 to 8.3 wt%) contents are relatively constant irrespective of MgO contents (38 to 47 wt%). On the other hand, abundances of A1203 (4.3 to 0.5 wt%), CaO (3.4 to 0.3 wt%), TiO2 (0.13 to 0.01 wt%), and Na20 (< 0.4 wt%) decrease with increasing MgO contents (Figure 2). Abundance of NiO (0.26 to 0.32 wt%) is positively correlated multicollection mode. Normalizing factors to correct with MgO contents, although the relationship is not plotted in isotopic fractionation for Sr and Nd are 86Sr/88Sr = Figure 2. These systematic changes essentially reflect the and 146Nd/144Nd = , respectively. Total procedural change of rock types from plagioclase lherzolite to spinel blanks for Rb, Sr, Sm, and Nd were 11, 40, 4, and 15 pg, harzburgite through spinel lherzolite, although some spinel respectively, and are considered negligible. Analytical lherzolites compositionally overlap with plagioclase reproducibility of Rb, Sr, Sm, and Nd abundances was better lherzolites. It is also indicative that major element than 2% ( 20% in n = 4) for the peridotite samples. compositions change systematically toward dunite. Our Measured ratios for standard materials were 87Sr/86Sr = compositional trends are similar to trends previously reported _+ 11 (2Om) for NIST 987 (n = 5) and 143Nd/ 44Nd = by Obata and Nagahara [1987], Frey et al. [1991], _+ 05 (2Om) for La Jolla (n - 50). Takahashi [1991a], and Takazawa et al. [1992] for the

5 YOSHIKAWA AND NAKAMU : EVOLUTION OF HOROMAN PERIDOTITES 2883 Table 1. Major Element Compositions of Peridotites in the Horoman Peridotite Complex Plagioclase Lherzolite YOHB BZ262 YOHS* BZ BZ257 SiO TiO A Cr Fe FeO MnO MgO NiO CaO Na K P H H20' Total Spinel Lerzolite Spinel Harzburgite * BZ143 YOH7* m BZ216 HD03* BZl16 SiO TiO A Cr Fe FeO MnO MgO NiO CaO Na K P H:O H O' Total Total iron expressed as FeO. Dash indicates element below the detection limit. *Samples not measured for H20 + and H20' by gravimetric method and ferrous iron by wet chemistry. Horoman peridotite complex (Figure 2). Major element composition, although our MgO - FeO trend is slightly trends observed in other peridotite complexes are also shown in Figure 2 (Ronda[Frey et al. 1985], Lherz [Bodinier et al. different format calculated bulk compositions of abyssal peridotites, as reconstructed from modal and mineral 1988], Lanzo [Bodinier, 1988]). Such major element compositions by Niu et al. [1997]. systematics have been generally explained as residual compositions left after variable degrees of melt extraction 6. Trace Element Compositions from a compositionally homogeneous peridotite [e.g., Frey et al., 1985]. Trace element compositions of whole rocks are given in Among the analyzed samples, the least refractory Table 2 and plotted on MORB-normalized diagrams together plagioclase lherzolite ( ) has MgO = wt%, AI20 3 = 4.31 wt%, and CaO = 3.39 wt%, which is nearly identical to the estimated mantle source for MORB, MgO = with ion microprobe analyses of clinopyroxene porphyroclasts in Figure 3. A positive Pb anomaly in most of the peridotites and a positive Sr anomaly in some peridotites, which have not 38.5 wt%, A120 3 = 3.94 wt%, and CaO = 3.19 wt% [Kinzler been reported previously, are observed in this study. and Grove, 1992]. This similarity indicates that as in other peridotite complexes [e.g., Frey et al., 1985; Bodinier et al., Plagioclase lherzolites, except for sample YOH8, show relatively depleted incompatible element patterns, although 1988], the source of the Horoman peridotites is chemically Rb and Ba are disturbed (Figure 3a). Spinel lherzolites show similar to the MORB source mantle in terms of major element two different types of patterns: convex downward patterns

6 2884 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES o $i02 4 n AI MgO (wt%) 3 Ronda Horoman.rm Lherz EZ3 Lanzo MgO (vv't%) Figure 2. Major element variation against MgO contents in the Horoman peridotite complex. The range of previous studies of the Horoman peridotite [Obata and Nagahara, 1987; Takazawa et al., 1992] are also shown. The oxide variations are compared with those from other peridotite complexes of Ronda [Frey et al., 1985], Lherz [Bodinier et al., 1988], and Lanzo [Bodinier, 1988]. Symbols are square, plagioclase lherzolite; circle, spinelherzolite; triangle, spinel harzburgite. Total iron is given as FeO*. with abrupt slope changes at Nd for less refractory (fertile) spinel lherzolites (BZ143 and ), and patterns with inflections on the side of elements more compatible than Eu for refractory spinel lherzolites (YOH7 and rn) (Figure 3b). The exceptional plagioclase lherzolite YOH8 has a pattern rather similar to that of refractory spinel lherzolites. Spinel harzburgites have trace element patterns characterized by a change of slope in the vicinity of Y-Ho-Er (Figure 3c). Relative to the inflections in spinelherzolite, these inflections occur on the side of the diagram of the more compatible elements. Thus MORB-normalized trace element patterns of the bulk peridotites in the entire Horoman complex tend to display more relative enrichment in incompatiblelements as peridotites become more refractory. These convex downward patterns of whole rocks have not been observed previously in the Horoman complex, although Frey et al. [1991] displayed the LREE depleted and flat patterns of seven peridotites. These results indicate that the trace element compositions of the Horoman complex cannot be explained by simple melt extraction; otherwise, the trace element patterns of the residue would become increasingly more depleted in the more incompatible elements with increasing degree of melt extraction. Instead, these results indicate that residues were affected by enrichment of incompatible elements, most likely associated with a metasomatic event. A similar conclusion was also made on the basis of more limited REE data of whole rocks in the Horoman River [Frey et al., 1991] and trace element compositions of clinopyroxenes in the Bozu stream [Takazawa et al., 1992, 1996] from the Horoman complex. Convex downward REE patterns with an abrupt slope change at the middle REE have also been observed in peridotite xenoliths [Downes, 1990], peridotites in ophiolite complexes [Prinzhofer and All gre, 1985] and other peridotite complexes [Bodinier et al., 1988] and were explained by a combination of melt extraction and mantle metasomatism or disequilibrium melting. Abundances of highly incompatiblelements (e.g., La and Ce) in the Horoman peridotites show no correlation with MgO contents (Figure 4). On the other hand, less incompatible elements such as Yb and Lu are inversely correlated with MgO contents (Figure 4). Although Frey et al. [1991] showed an inverse correlation between highly incompatiblelements and MgO contents of whole rocks in the Horoman River, some refractory peridotites (YOH7, BZ216, rn) have higher abundance of Ce than those of refractory plagioclase lherzolites. A similar result was reported on the whole rocks in the Bozu Stream from the Horoman complex [Takazawa, 1996]. These results suggest that the enrichment of highly incompatiblelements occurred heterogeneously over a wide area in the Horoman complex. Such a heterogeneous enrichment may be caused by various degrees of enrichment by a component with variable element abundances as suggested by Yoshikawa [1993] and Takazawa [1996]. Frey et al. [1991] and Takazawa [1996] also reported a well-defined negative trend between HREE and MgO suggesting that the variations in HREE contents were not affected by the enrichment and were caused by variable degrees of melt extraction. The highest Yb concentration in the Horoman complex (0.38 ppm) is similar to that estimated for the MORB source mantle (Yb = 0.42 ppm [Woodhead, 1989]). This suggests that originally, the Horoman peridotites were residues left after melt extraction from MORB source mantle. Such an interpretation is also consistent with the major element data described in the previousection and has also been used to explain the REE data from the Ronda peridotite complex [Frey et al., 1985]. MORB-normalized trace element patterns of clinopyroxene porphyroclasts are generally parallel to those of whole rocks, indicating that the clinopyroxenes are the major reservoir of lithophile trace elements in the peridotites, although pronounced negative Zr spikes are observed in most of the analyzed clinopyroxenes (Figure 3). Clinoyroxenes plagioclase lherzolites also have negative Sr anomalies (Figure 3a). The magnitude of the Sr and Zr anomalies tends to diminish from rim to core. Similar observations have also been reported from two samples in the Bozu stream [Takazawa et al., 1996]. An increase of Sm/Nd ratio from core to rim and the negative Eu spikes at the rim area observed (Figure 3, Table 3) and were also found by Takazawa et al. [1996]. Furthermore, a clinopyroxene porphyroclast with a size of ~600 tm in diameter in a

7 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2885 Table 2. Whole-Rock Trace Element Composition in the Horoman Peridotites Obtained by ICP-MS Plagioclase Lherzolite YOHB BZ262 YOH8 BZ BZ257 Rb Ba 0.26 B Nb La Ce 0.24 Pb Pr Sr 9.2 Nd 0.42 Sm 0.21 Eu Gd 0.36 Tb Dy 0.56 Y 3.6 Ho 0.13 Er 0.35 Tm Yb 0.36 Lu [Ce] [Yb]N 0.12 [Ce/Yb] n.d n.d Spinel Lherzolite Spinel Harzburgite BZ143 YOH lm BZ216 HD BZl16 Rb Ba B Nb La Ce Pb Pr Sr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu [Ce] [Yb] [Ce/Yb] Subscript N shows MORB [Sun and McDonough, 1989] normalized ratio. Dash indicates element below the detection limit. N.d. not determined element concentrations. Composition in ppm. plagioclase lherzolite BZ 257 (Figure 5) shows the same increase in the abundances of REE (especially HREE), Y and chemical zoning of major and trace elements from core to rim Zr from core to rim. The previously unreported Eu* values, which has been reported in previou studies [Niida, 1984; which measure the intensity of Eu anomalies, increase from Ozawa and Takahashi, 1995; Takazawa et al., 1996]. The rim to core and are compatible with the zoning in Sr main zonation is a decrease in abundance of Si and an abundances (Figure 5).

8 2886 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES (a) BZ ry" 0.01 o E YOHB... *'"135 = 290 Sr Zr Eu Tb Y Er Yb RbBaLacePbpr Nd Sm Gd Dy Ho Tm Lu _ BZ262 a b r Zr Eu Er b Ba Ce Pr Nd Sm Gd Dy Ho Tm Lu Figure 3. MORB-normalized trac elements patterns of whole rocks and coexisting clinopyroxenes from the (a) plagioclase lherzolites, (b) spinel lherzolites, and (c) spinel harzburgites. Solid squares, solid circles, and open circles represent whole rocks and cores and rims of clinopyroxenes, respectively. The trace element data are normalized to the MORB values of Sun and McDonough [1989]. Numbers indicated against the legends for BZ257 and BZ143 correspond to the relative distance in micrometers from the rim to core of the clinopyroxene porphyroclasts given in Table 2. Takahashi and Arai [1989] noted the polygonal outlines of of Sm and Nd along grain boundaries in the reacting rock symplectite in orthopyroxene porphyroclasts, which mimic a system. This yields a decrease of Sm/Nd ratio in garnet cross section, and the similarity of major element composition between bulk symplectite in the most fertile plagioclase lherzolite and mixtures of pyrope-rich garnet + olivine, and attributed these features to garnet breakdown during decompression for the Horoman peridotite complex clinopyroxene porphyroclast from rim to core as a result of diffusion (see also the discussion in relation to isotope systematics). Furthermore, decompression from the spinel to the plagioclase stability field is suggested by many features including the two-pyroxene + spinel symplectite inclusions from the garnet to the spinel stability field. The subsolidus orthopyroxene porphyroclasts in plagioclase lherzolite breakdown of garnet has also been inferred from the increase of Ca, A1, and HREE abundances and the decrease of Si abundance in pyroxene porphyroclasts from core to rim, the negative Zr anomalies and the depletion of HREE in the core of clinopyroxene porphyroclasts [Ozawa and Takahashi, 1995; Takazawa et al., 1996]. The increase of Sm/Nd ratio in clinopyroxene porphyroclasts from core to rim also supports this suggestion. The breakdown of garnet ( 5 times higher Sm/Nd than that of clinopyroxene [Becker, [Takahashi and Arai, 1989], occurrence of plagioclase [Ozawa and Takahashi, 1995], abrupt decreases of A1 contents in the outer most rim of pyroxene porphyroclasts, increase in the abundances of Si, Ti, REE, Y, and Zr, and a diminishing in the intensity of the negative Sr anomalies in clinopyroxene porphyroclasts from rim to core, and an increase in the An contents of plagioclase from core to rim [Ozawa and Takahashi, 1995; Takazawa et al., 1996]. The negative Eu anomalies in clinopyroxene porphyroclasts in 1993]) increases both the Sm/Nd ratio and the concentration plagioclase lherzolites also supporthis suggestion. Thus it

9 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2887 (b) I I I I I I I YOH7! I I I I I I I I! i I I I I I I I! I 1 o o.ool lo o. 1 Dis!ance (tzm) - ; BZ , - = 610, r., rn O.Ol o.ool RbLa Pb Sr Zr Eu Tb Y Er Yb Ba Ce Pr Nd Sm Gd Dy HoTm Lu RbLa Pb Sr Zr Eu Tb Y Er Yb Ba Ce Pr Nd Sm Gd Dy Ho Tm Lu (c), - ", BZ HD03 BZl16 o. Ol o. OOl o. oool RbLa Pb Sr Zr Eu Tb Y Er Yb Ba Ce Pr Nd Sm Gd Dy Ho Tm Lu RbLa Pb Sr Zr Eu Tb Y Er Yb Ba Ce Pr Nd Sm Gd Dy Ho Tm Lu Figure 3. (continued) is concluded that the compositional zoning of major and trace elements in clinopyroxenes are a result of successive breakdown reactions during decompression from the garnet stability field to the plagioclase stability field as has been discussed in previous studies [Ozawa and Takahashi, 1995; Takazawa et al., 1996]. The similarity of MORB-normalized trace element patterns between clinopyroxenes and whole rocks (except for the changes in some elements induced by the subsolidus breakdown reactions discussed above) indicates that the enrichment of incompatibl elements in whole rocks in the Horoman complex did not originate from secondary minerals formed by alteration. The enrichments had occurred before the uplift of the complex, because compositional zoning of porphyroclasticlinopyroxenes inferred to have been caused by subsolidus breakdown reactions during decompression was observed in samples with both depleted and enriched incompatiblelement patterns. Recently, on the basis of the similar relative enrichment of LREE in the core and rim of clinopyroxene, Takazawa et al., [1996] have also suggested

10 2888 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES previous studies [Yoshikawa, 1993; Yoshikawa and 0.10 Nakamura 1994; Nakamura and Yoshikawa, 1995; Takazawa et al. 1995; Takazawa, 1996]. Especially for Nd isotopic compositions, the variation is larger than that reported from D 0.08 any other mantle-derived materials [e.g., Sharma et al., 1995]. The ranges of isotopic compositions are end to -10 and 87Sr/86Sr = to in the Horoman River section, [Ce]N end = +60 to -6 and 87Sr/86Sr = to in the Bozu 0.06 Stream section and end = +17 to-1, and 87Sr/86Sr = to in the Mount Yoshida section. Sr isotopic compositions among the three sections overlap with each other. The Nd isotopic range of the Mount Yoshida section 0.04 O may have less variation than observed in the other two O 0 sections, although only three samples were analyzed. There is no relationship between isotopic heterogeneity and amplitude of layering as suggested by Zindler and Hart 0.02 D [1986]. 0 It is now widely recognized that whole rock St, and to a lesser extent Nd, isotopic compositions of peridotites are 0 perturbed by crustal contamination [Menzies and Murthy, 1978; Polvd and All gre, 1980]. Thus the previous researchers analyzed clinopyroxenes in order to obtain 0.12 isotopic information of pristine peridotite [e.g., Reisberg and Zindler, ; Reisburg et al., 1989; Yoshikawa, 1993; Yoshikawa and Nakamura 1994; Takazawa et al., 1995; a [Yb]N Takazawa, 1996]. After acid leaching, most whole rocks from the Horoman peridotites yield isotopic compositions 0.08 which are nearly identical with those of clinopyroxenes R (Figure 6). Clinopyroxenes from the spinel lherzolite have a much higher end value than the host whole rock. This may be because the very low abundance of Nd in the clinopyroxene (< 0.01 ppm) makes it more susceptible to secondary alteration. However, the isotopic compositions of 0.04 most of the whole rocks in the Horoman complex essentially o display isotopic signatures of the mantle with minimal secondary crustal contamination. Theoretically, the more refractory peridotite should have more depleted Sr and Nd isotopic compositions compared with the fertile source, because depleted isotopic characteristics are caused by higher Sm/Nd and lower Rb/Sr ratios, which in turn are caused by a larger extent of melt MgO (wt%) extraction. In addition, the present Sr isotopic composition of the residue should not have changed significantly from the Figure 4. MgO versus [Yb]N and [Ce]N for the whole initial Sr isotopic composition because of the extremely low rocks in the Horoman peridotite complex. Symbols are as in Rb/Sr (, 0) left after melt extraction, a consequence of the Figure 2. Yb and Ce are normalized to the values of MORB extremely low partitioning of Rb into the residual phases [e.g., and the normalization factors are from Sun and McDonough Polvd and All gre, 1980]. Thus it is expected that spinel [1989]. lherzolites and harzburgites should have higher end values than those of plagioclase lherzolites, whereas differences in Sr that the enrichment preceded the formation of the HREE isotopic compositions among them should be small. zoning and was caused by a process subsequento melt However, the observed isotopic compositions in the Horoman extraction, although Takazawa et al. [1992] and Takazawa peridotites do not conform to the predicted pattern. [1996] suspected that enrichment and melt extraction were Plagioclase lherzolites, except YOH8, have depleted isotopic contemporaneous. From these considerations, we conclude signatures (end > +11, 87Sr/86Sr <0.7026) relative to MORB that the enrichment of incompatibl elements was caused by (Figure 6). Two fertile spinel lherzolites (BZ143 and mantle metasomatism ) have more depleted Nd isotopic compositions than the plagioclase lherzolites, which have the highest end values (end > +50) ever measured in whole-rock peridotite samples 7. Isotopic Compositions (Figure 6). On the other hand, two refractory spinel The overall variations of Nd and Sr isotopic compositions lherzolites (YOH7 and m) and spinel harzburgites of whole rocks and their constituent clinopyroxenes are have enriched isotopic compositions (end = +6 to -10, extremely large, end = +110 to -10 and 87Sr/86Sr = to 87Sr/86Sr = to ) compared with those of (Table 4, Figure 6) even compared with those of plagioclase lherzolites. Instead, the isotopic signatures are

11 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2889 Table 3. Trace Element Compositions of Clinopyroxenes from Plagioclase Lherzolites Obtained by Ion Microprobe Plagioclase Lherzolite YOHB BZ262 YOH8 Core Rim Core Rim Core Rim Core Rim Core Rim Rb Ba La Ce Pr Sr Nd Zr Sm Eu Gd Dy Y Er Yb Lu Sm/Nd Plagioclase Lherzolite BZ146 Core Rim Rim f 55* 135' BZ257 Core Rim 210' 290* 385* 465* 585* Rb a La Ce Pr Sr Nd Zr Sm Eu Gd Dy Y Er Yb Lu Sm/Nd Dash indicates element below the detection limit. Composition in ppm. * Distance from rim (micrometers). largely consistent with the observed trace element patterns. MORB strongly suggests that some peridotites in the In the previousection, we argued that the enrichment of Horoman complex have preserved their original isotopic and incompatible elements was caused by mantle metasomatism. trace element signature imprinted by melt extraction. Trace Thus it is possible that the isotopic enrichment in the element and isotopi compositions of bulk rocks are better Horoman peridotites was caused by the same mantle indicators of melt extraction and metasomatism than are metasomatic event which caused relative enrichment in clinopyroxenes, because trace element evidence of melt incompatible elements. extraction and metasomatism in clinopyroxene has been The depleted Nd isotopic signature of plagioclase slightly disturbed by the late subsolidus breakdown reaction, lherzolites and fertile spinel lherzolites can largely be as discussed in the following sections. considered to reflect time integrated evolution of a melt extraction residue, although the Nd isotopic composition of 8. Discrimination Between Melt Extraction and sample was slightly affected by secondary Mantle Metasomatism alteration as mentioned above. Melt extraction is also compatible with their LREE depleted patterns. Therefore the It has been difficult to determine the age and tectonic depletion of Nd isotopic and REE compositions relative to setting of melt extraction in the mantle based on the

12 2890 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTH'ES Table 3. Trace Element Compositions of Clinopyroxenes From Spinel Lherzolites and Spinel Harzburgites Obtained by Ion Microprobe. Spinel Lherzolite BZ143 Rim Core Core Rim 60* 145' 230* 315' 435* 525* 610' 695* Rb Ba La Ce Pr Sr Nd Zr Sm Eu Gd Dy Y Er Yb Lu Sm/Nd Spinel Lherzolite Spinel Harzburgite YOH lm BZ216 HD Core Rim Core Rim Core Core Core Rim Core Rb Ba La Ce Pr Sr Nd Zr Sm Eu Gd Dy Y Er Yb Lu Sm/Nd Dash indicates element below the detection limit. Composition in ppm. * Distance from rim (micrometers). geochemistry of peridotites, because it has not always been possible to classify the individual events such as melt extraction, metasomatism and subsolidus reactions which occurred in the mantle without combining detailed petrological and geochemical data. Figure 7 shows the relationship between Nd isotopicompositions and [Ce/Yb]N ratios of whole rocks for the Horoman peridotites. There are two distinct trends in the relationship between end and [Ce/Yb]N ratios. One (referred to as DP) is characterized by higher end values (end > +10) with more depleted LREE patterns ([Ce/Yb]N < 0.5) compared to the MORB source (end = +10 [White and Hofmann, 1982] and [Ce/Yb]N = 0.66 [Sun and McDonough, 1989]). The other one (referred to as EP) consists mostly of spinelherzolites and spinel harzburgites with enriched isotopicharacteristics (encl < +6) and relatively variable and enriched LREE patterns. The enriched isotopic and REE signatures of the EP samples are the result of metasomatism as discussed in the previousections. If the metasomatism was caused by meltsolid interaction that triggered larger degrees of melt extraction as has been suggested by Bodinier et al. [1991] and Kelernen et al. [1992, 1995], it is likely that similarly refractory peridotites would show similar isotopic and REE characteristics. However, some plagioclase and spinel lherzolites with similarefractory compositions (i.e., MgO = 42 to 43 wt%) in the Horoman complex exhibit both enriched

13 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES IJ I 'ljl ' I I I I I I I I...'1... Si # Sr (ppm) 1.90 I I I! I I I I Yb (ppm) 1 o I I I I I I I I I!! I Distance (prn) ß ;;: "... % u*' ::'-: : : ;': %,..:' ' :':%:'%:%!i. -: -:. : -.-,.-:...?..::::;:...:??;: ':" -;;'::!:'{}. :: L. : :..-::?*. --.:, - '? L.¾%,, ' ; -- ß. ß.. -.:: [? :::.?: :,;... 2'2'%... :., :4..; :." :... :' : : : : : :-..-:., ::, : * '".'... %* / ; ;; ::;;; '*:.::'". :.:.;,. %. ß *:' -... '?...,a:.. -:*% ,......:½.,-.: ;., [Aq,ø I "";'½'.;...,Z;... :.. oj,,,,,,,,,,, Zr(ppm) Figure 5. Compositional zonings of Si, Yb, Sr and Zr and Eu anomaly in clinopyroxene porphyroclast in the plagioclase lherzolite BZ257. An electron back scattered image taken after ion microprobe analysis is also shown at a similar scale to the zoning patterns. The dots in the clinopyroxene porphyroclast are the analyzed spots sputtered by the ion microprobe. The clinopyroxene porphyroclast which also contain orthopyroxene lamellae is surrounded by fine-grained neoblastic olivine, orthopyroxene, plagioclase and spinel. Si n expresses the number of Si per 6 oxygens in clinopyroxene. The Eu anomaly is given by Eu* = [(2 x Eu)/(Sm + Gd)] N (the subscript N indicating that all elements are normalized to the MORB values). (YOH7, YOH8) and depleted (BZ143, BZ146, BZ257, ) isotopic and REE signatures. This suggests that component, which caused mantle metasomatism in the Horoman complex, was mainly derived from crustal materials. the metasomatism occurred after melt extraction. On the basis of high-pressurexperiments [Kogiso et al., Furthermore, we do not believe the harzburgites are reaction 1997] and isotopic and trace element systematics of island arc products between olivine saturated melt and wall peridotitc as suggested by Kelemen et al. [1992, 1995], because the harzburgites near the dunitc (HD03, 62127, BZl16) do not basalts [Shibata and Nakamura, 1997a], it has been inferred that Sr and Pb are more mobile in aqueous fluid than other elements of similar incompatibility during magmatic have the most enriched isotopic and trace element signatures processesuch as Ce, Pr, and Nd [Hoffman et al., 1986; Sun in the Horoman complex. Takahashi [1992] also concluded and McDonough, 1986]. If so, fluid derived from the crustal that the Horoman dunitc and surrounding harzburgite do not have a replacive origin, on the basis of systematic decrease in materials must have positive spikes of Sr and Pb, resulting in positive Sr and Pb anomalies in the metasomatized peridotitc. the modal abundance of orthopyroxene and Cr# of spinel This is consistent with the positive Sr and Pb spikes observed from plagioclase lherzolite to harzburgite, as well as abrupt in the whole rock samples of the Horoman peridotitcomplex changes in mineral composition observed at the contact with (see Figure 3). dunitc. If the EP trend was formed by two-component Applying the B/Nb ratio in the same way as the Pb/Ce ratio mixing between an enriched component and the residual for discriminating the fluid-related process, B/Nb-Pb/Ce peridotites, then the enriched component must have had more enriched isotopic and REE characteristics than the most enriched peridotitc ([Ce/Yb]N = 2.7, ENd = -10). Only systematics are examined for the Horoman peridotitc samples (Figure 8). B is considered to be strongly partitioned into aqueous fluid compared with fluid-immobile Nb [e.g., continental crustal material shows such isotopic Ishikawa and Nakamura, 1994], although these elements are characteristics [e.g., McCulloch and Chappell, 1982]. Fluid or melt derived from the continental crust (an average of almost identical in partitioning behavior during magmatic processes resulting in nearly constant B/Nb ratio [Ryan and [Ce/Yb]N = 4.0 [Taylor and McClenan, 1985]) should have Lagmuir, 1987]. Fluid-related processes must lead to a [Ce/Yb]N > 4.0, because in comparison with HREE, LREE are preferentially partitioned into fluid or melt [Tatsumi et al., significant variation and higher B/Nb ratio in the fluid than in the source material [Ishikawa and Nakamura, 1994]. Thus 1986; Bau, 1991]. Thus we infer that the enriched the B/Nb - Pb/Ce systematics can be a good indicator for

14 2892 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES Table 4. Elementral and Isotopic Analyses of Whole Rocks and Clinopyroxenes Weight Rb Sr 87Rb/86Sr 87Sr/86Sr Sm Nd 147Sm/144Nd 143Nd/144Nd ENd mg ppm ppm ppm ppm Plagioclase Lherzolites WR Cpx _ _ _ WR _ _ YOHB WR Cpx _ _ BZ262 WR _ Cpx* _ _ _ YOH8 WR _ Cpx _ _ _ BZ146 WR _ Cpx* _ _ _ WR _ _ BZ WR Cpx BZ143 WR Cpx* YOH m WR _ Cpx* _ WR Cpx WR Cpx Spinel Lherzolites _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Spinel Harzburgites _ _ BZl16 WR _ _ Cpx* _ _ HD03 WR _ _ WR _ _ BZ216 WR _ _ Cpx* ] _ _ Isotopic fractionation normalized to 86Sr/88Sr = , 146Nd/144Nd = Analytical precisions for isotope dat are 2o mean. Here en, displays deviation in parts per 104 from the chondritic reference reservoir. * Data are from Takazawa et alo [1995] and Takazawa [1996]. identifying fluid-related processes. The Horoman peridotites are island arc magma and MORB-type mantle and these exhibit a wide range of B/Nb (1-270) and Pb/Ce ( ), components interact in the mantle, it is difficult to explain the which are clearly discriminated from MORB and oceanic large variations of B/Nb and Pb/Ce ratios obtained in the island basalt (OIB) with higher B/Nb and Pb/Ce ratios. The Horoman complex that exceed those of IAV. We therefore B/Nb ratios of the peridotites are also clearly higher than conclude that the Horoman peridotites were metasomatized by those of crustal materials (Figure 8). Variations in B/Nb fluid originating mainly by dehydration of crustal materials, ratios observed in island arc volcanics (IAV) are significantly rather than by the crustal melt or IAV magma. This smaller than that of the Horoman complex and are considered speculation is supported by geological setting of the Horoman to be caused by systematic addition of fluids derived from a peridotite complex in the Hidaka metamorphic belt [e.g., subducting oceanic slab to the mantle wedge [e.g., Ishikawa Komatsu et al., 1994]. Peridotite complexes, including the and Nakamura, 1994; Shibata and Nakamura, 1997a, b; Horoman complex, generally outcrop in metamorphosed Moriguti and Nakamura, 1998]. Such variations are clearly sedimentary rock along major fault systems within chains included in the variation of the Horoman peridotites (Figure where interaction between oceanic and continental crust has 8). Metasomatism involving either melts derived from occurred [e.g., Loubet and All gre, 1982; Kunugiza, 1995; S. crustal materials or island arc magmas cannot produce such Maruyama, personal communication, 1991]. On the basis of elevated and wide variations in B/Nb an Pb/Ce ratios in the petrological and geochronological investigations, Kunugiza Horoman peridotites. Even if the end-member components [1995] suspected that the Alpe Arami peridotite complex was

15 -- YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES O loo 8o 6O DP Whole Rock 6o C040 4O co 20 Present-Day MORB Mantle Source 2O Ronda Metasomatic Agent derived from the continenatl crust MORB i i i i i i Sr/86Sr hydrated by an aqueous fluid derived from dehydrated crustal material at a subduction zone. Metasomatism in the Horoman peridotite complex may have analogously resulted from fluids derived from the dehydration of subducted sediments which were similar in origin to the Hidaka metamorphic rocks when the Horoman complex formed part of the wedge mantle. Although there is no clear distinction in the B/Nb and Pb/Ce ratios between DP and EP samples, the spinel lherzolites and harzburgite show higher ratios than those of fertile plagioclase lherzolites. This may be due to a larger flux of infiltrated fluid in refractory peridotites than fertile peridotites. A melt or fluid forms a continuous network of grain-edge tubules when the dihedral angle (0)at fluid-solid triple junctions is less than 60 ø [Bulau et al., 1979; Watson et al., 1989]. The dihedral angle for fluid-olivine triple junctions over various P-T conditions were determined by Watson et al. [1989], suggesting that water-rich fluid can achieve interconnectivity at temperatures just below the peridotite solidus. However, we cannot evaluate the effect of modal composition on the connectivity of fluid, because the 0 values for fluid-pyroxene junctions have not been [Ce/Yb]N Figure 7. The ENd versus [Ce/Yb]N for whole rocks from Figure 6. The end versus 87Sr/86Sr diagram for whole rocks the Horoman peridotite complex. The normalization factors and their constituent clinopyroxenes. The tie lines between to MORB are from Sun and McDonough [1989]. Symbols whole rocks (open symbol) and clinopyroxene (solid symbol) are as in Figure 2. A star indicates [Ce/Yb]N and end of the indicate the same sample. Symbols are as in Figure 2. present-day MORB source mantle estimated by Sun and Some samples in these data were obtained by Yoshikawa McDonough [1989] and White and Holmann [1982], [1993], Yoshikawa and Nakamura [1994], Nakamura and respectively. The isotopically and LREE depleted (DP) and Yoshikawa [1995], Takazawa et al. [1995], and Takazawa enriched (EP) samples have distinctively discrete trends, [1996]. The ranges of MORB [Cohen et al, 1980; Cohen which are emphasized by pale and dark shaded areas, and O'Nions, 1982; White and Holmann, 1982], oceanic respectively. The wide arrow indicates the direction to a island basalts (OIB) [Zindler and Hart, 1986], and the Ronda metasomatic agent responsible for the EP trend through peridotite complex [Reisberg and Zindler, ; mixing. Reisberg et al, 1989] are also shown. Dashed lines indicate the present-day Nd and Sr isotopic compositions of the bulk earth [Faure, 1986] : Bulk Oceanic - Continental Sedi & Crust,& o IAV '."1... i... i"00... i' '00'';I B/Nb Figure 8. Pb/Ce versus B/Nb diagram for whole rocks from the Horoman peridotite complex. Symbols are as in Figure 2. Open and shaded symbols representhe DP and EP samples, respectively. The ranges of MORB [Moriguti and Nakarnura, 1998; T. Moriguti, unpublishe data, 1999], OIB [T. Nakano, unpublished data, 1999; Hanyu and Nakarnura, 1999], Oceanic Sediment [Ishikawa and Nakarnura,!993; T. Nakano, unpublished data] and island arc volcanic rocks (IAV) [Shibata and Nakarnura, 1997a, b; T. Moriguti, unpublishedata, 1999] are also shown. A cross indicates the bulk continental crust value estimated by Taylor and McLennan [1985].

16 2894 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES i (a) (b) Z -o.55 z I I Rb/86Sr 147Sm/144Nd Figure 9. The (a) 87Rb/86Sr versus 87Sr/86Sr and (b) 147Sm/144Nd versus 143Nd/144Nd diagrams for whole rocks (open symbols) and coexisting clinopyroxeses (solid symbols) from the Horoman peridotite complex. Symbols are as in Figure 2. Data of whole rock and clinopyroxene from the same sample are tied by a line. Some samples in these data were obtained by Yoshikawa [1993], Yoshikawand Nakarnura [1994], Nakarnurand Yoshikawa [1995], Takazawa et al. [1995], and Takazawa [1996]. Pale and dark shaded areas represent the variations in the DP and EP samples, respectively. In Figure 9b only EP samples are plotted and the DP samples are separately plotted in Figure 10 for more detaile discussion. A regression line for the whole rock EP samples is also indicated in Figure 9b. obtained. In the mantle, an aqueous fluid phase can be a solvent of trace and major elements [Eggler, 1988; Iizuka and 9. Age of Melt Extraction Nakamura, 1995] and could become similar to melt in In Figure 9, 87Sr/86Sr and 143Nd/144Nd ratios are plotted chemistry and structure [e.g., Shen and Keppler, 1997]. against 87Rb/86Sr and 147Sm/144Nd ratios for whole rocks and Thus we may assume that mineral-melt and mineral-aqueous their coexisting clinopyroxenes, respectively. fluid grain boundary energetics may be similar at mantle The 87Sr/86Sr ratios of the DP samples are lower than those conditions. If this assumption is correct, then dihedral of the EP samples. No correlation is observed either in the angles at fluid-olivine triple junctions may be significantly EP or DP samples between 87Rb/86Sr and 87Sr/86Sratios in smaller than those at triple junctions of other minerals at Figure 9a. These observations suggest that the Sr isotopic similar mantle conditions [Torarnaru and Fujii, 1986]. Thus compositions and highly incompatible elements (including Rb the connectivity of fluid along grain edges may be favored in and Sr abundances) were strongly affected by metasomatism olivine-rich peridotites relative to pyroxene-rich peridotites. and subsequent subsolidus phase change in the mantle. Such Consequently, it is likely that the olivine-rich refractory metasomatism may have preferentially affected the isotopic lherzolites and harzburgite suffered a larger flux of signatures of spinelherzolites and harzburgites which were metasomatic fluid than fertile lherzolites, and thus attained a higher concentration of highly incompatiblelements than expected from melt extraction processes in the mantle. Isotopic and chemical features of the DP samples are originally extremely depleted in incompatible trace elements due to their having experienced larger degrees of melt extraction compared to the plagioclase lherzolites (the DP samples). In addition to such a buffering effect to the consistent with those of residues from the MORB source incompatible trace elements, a larger flux of infiltrated mantle, that is, they show an increase in œnd values with metasomatic fluid into the refractory peridotites than the decreasing [Ce/Yb]N ratios relative to the original source fertile peridotites could have resulted in their EP depending on the degree of melt extraction and time characteristics as discussed above. The difference in integrated effects (Figure 7). These observations indicate 87Sr/86Sr ratios between the clinopyroxenes and the whole that the REE and Nd isotopicompositions of the DP samples rocks is relatively small (Figure 9a). have preserved geochemical information related to the In the Sm-Nd isochron plots of Figures 9b and 11, samples extraction of melt from the MORB source mantle, although generally show a positive trend. Similar but less apparent more incompatible elements (e.g., Rb, Ba, St, B, and Pb) were positive correlations have also been observed in affected by later metasomatism. clinopyroxenes from Ronda [Reisberg and Zindler, 1986/7]

17 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2895 Metasomatized( Residues,EP) Metasomati½ Component 475m/44Nd Figure 10. Schematic 147Sm/144Nd-143Nd/144Nd diagram illustrating the formation of EP samples by the mixing between residual peridotites and a metasomatic agent derived from the continental crust. Open triangles represent residual peridotites with higher but different degrees of melt extraction than the DP samples. Solid triangles represent the isotopic compositions resulting from the differential addition of the metasomatic fluid to the residual peridotites forming the EP samples. The EP samples consequently define a very steep regression line which is quite distinct from the trend obtained from the DP samples. Z lower 147Sm/144Nd and 143Nd/144Nd ratios. The residual peridotites such as spinel lherzolites and spinel harzburgites that formed by high degrees of melt extraction should have had extremely low concentrations of Nd and Sm with very high Sm/Nd ratios. These geochemical characteristics could have evolved into remarkably high 143Nd/144Nd ratios with time. However, the subsequent metasomatism by a larger fluid flux with a much higher concentration of trace elements and low 143Nd/144Nd ratio, substantially shifted the isotopic compositions of the refractory peridotites close to the composition of the metasomatic component, as shown in Figure 10. Variable mixing of the metasomati component with different residual compositions of the refractory peridotites resulted in the steep EP trend which is quite different from the DP trend (Figure 10). When the DP samples, which mostly consist of plagioclase lherzolite are investigated more detail (excluding the whole rock sample of ), the clinopyroxene separates tend to show an older age (1148 +_ 350 Ma ) than the whole rock (833 e 78 Ma), although the difference is not well resolved at the!lclinopyroxene% Ma l (143Nd/144Nd)i l = l MSWD = 66 J Whole Rock 833 :78 Ma (43Nd/44Nd)i and whole rocks from Zabargad [Brueckner et al., 1988], and these were interpreted as both isochrons and mixing trends. Although 147Sm/144Nd ratios of some clinopyroxenes are = significantly different from those of whole rocks, there is MSWD =27 almost no difference in 143Nd/144Nd ratios between clinopyroxenes and whole rocks, except for the sample (Figure 11). This may suggest that these peridotites were isotopically reequilibrated quite recently, an Sm/144Nd 1.0 observation which is also consistent with the Rb-Sr isotope Figure 11. The 147Sm/144Nd-143Nd/144Nd isochron systematics. Such an isotopic reequilibration could be related to subsolidus phase changes caused by uplift of the Horoman peridotite complex as discussed below. The EP and DP samples show different trends in 143Nd/144Nd - 147Sm/144Nd space (Figure 9b), although the variation of Nd isotopic compositions and Sm/Nd ratios in the EP samples is very small compared to that shown by the DP samples. As discussed in the previous sections, the EP samples consist of refractory spinel lherzolites and spinel harzburgites which were subsequently strongly and preferentially metasomatized. As shown in Figure 10, it may therefore be inferred that the EP trend is a consequence of mixing between the residual peridotites with extremely high diagram for whole rocks (open symbols) and coexisting clinopyroxeses (solid symbols) in the DP samples. Symbols are as in Figure 2. Whole rock-clinopyroxene pairs from the same sample are tied by a line. Some samples in these data were obtained by Yoshikawa [1993], Yoshikawa and Nakarnura [1994], Nakarnura and Yoshikawa [1995], Takazawa et al. [1995], and Takazawa [1996]. The ages are determined employing the open file report [Ludwig, 1994] based on the method of York [1969] assuming that the regression lines from the whole rocks and the clinopyroxenes represent isochrons. Here X = 6.54 x 10-12yr is used as the decay constant of 147Sm [Lugrnair and Marti, 1978]. A star indicates the present-day MORB values [White and Hofrnann, 143Nd/144Nd ratios evolved from the higher 147Sm/144Nd 1982; Sun and McDonough, 1989]. Errors quoted for age ratios, and an isotopically enriched metasomatic fluid with and initial ratio are at the 95% confidence level.

18 2896 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 95% confidence level (Figure 11). We assume that Nd isotope compositions were equilibrateduring melting and interpreted the 850 Ma whole-rock age to reflecthe time of melt extraction. However, we have demonstrated that clinopyroxene grains have trace element zoning, and their Nd isotope compositions may be zoned as well. Therefore we believe that the Nd isotope systematics of the clinopyroxene separates (Figure 11) are likely affected by mixing between isotopically distinct cores and rims, and that the regressed age of = 1150 Ma is not meaningful. If the 833 _+ 78 Ma corresponds to the time of melting in the mantle, then the initial Nd isotopic ratio of _+ 2 (95% confidence level) is that of the original peridotite source. This initial ratio is identical within error to the MORB-type mantle value of 143Nd/144Nd = _+ 1 at the same age calculated using a present day 143Nd/144Nd ratio of for MORB [White and Holmann, 1982]. This further supports our arguments, based on major elements and REE compositions, that the Horoman peridotites originated from the MORB source mantle. On the basis of the Cr# of spinels in the peridotites, Takahashi [1991a, b, 1992] proposed that the MHL suite in the Horoman complex was a residue left after extraction of an arc magma. However, spineis of the abyssal peridotites have Here ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; gar, garnet; sp, spinel. D "a'm " of Cr# (= 0.1 to 0.6 [Arai, 1994]) similar to those of the spinel is extrapolated along D "d'm " and D Nd ø"d' " peridotites in the MHL suite (Cr# to 0.7), and the D ' "a' " are extrapolated along Dr7y 'a *" and D 'a'm*". peridotites in the MHL suite may have undergone melt Data are from Nakamura et al. [ 1989] and Johnson et al. extraction in either an arc or ocean floor setting. [1990]. Furthermore, our result contradicts models for recent melt extraction occurring almost simultaneously with uplift and accompanying subsolidus breakdown [Takahashi, 1992, 1997; Morishita et al., 1995]. Thus we conclude that the Melt extraction in the spinel stability field was suggested various rock types in the MHL suite of the Horoman complex on the basis of the observation of a large range in HREE in were formed by various degrees of melt extraction from the MORB source mantle at -850 Ma. Source rocks of the the Horoman peridotites [Takazawa et al., 1995, Takazawa, Hidaka metamorphic rocks have ages which range from late 1996]. Furthermore, negative Eu anomalies are not observed Cretaceous to early Tertiary [e.g., Komatsu et al., 1994] and in the Horoman bulk peridotites, as would be expected in thus are much younger than the Horoman peridotite complex. rocks in which melt extraction occurred in the plagioclase This also shows that the Horoman complex did not originate peridotite facies as suggested by Prinzhofer and Alldgre from lithospheric mantle under the Hidaka crustal section as [1985] for New Caledonian ophiolite. Thus we assume that has been suggested by the P-T history of uplift for this the source mantle consisted of garnet and spinel lherzolite. complex [Ozawa and Takahashi, 1995]. The modal compositions of source and melt used in the model calculations are also shown in Table 5. Calculations are 10. Melt Extraction Process carried up to 25% melt extraction which, according to mass In the Horoman peridotite complex, we observed a balance calculation of major element compositions [Frey et compositional gap in [Yb]N between fertile plagioclase al., 1991; Takahashi, 1992], corresponds to a spinel lherzolites ([Yb]N >0.1) and other peridotites ([Yb]N <0.06) harzburgite residue. The distribution coefficient of MgO (Figure 4). Because Yb has not been significantly affected (D ' /melt ) was obtained from P-T-D relationships [Yamashita by later metasomatic events [Frey et al., 1991], we infer that et al., 1996] based on the experimental data of the KLB-1 this gap is caused by a melt extraction process. Thus we peridotite xenolith [Hirose and Kushiro, 1993]. The model the effect of variable degrees of melt extraction from a estimated values for D ' g /me" of garnet peridotite (30 kbar, homogeneous MORB-type mantle on the Yb and MgO contents of the residual peridotites. For the model 1550øC) and spinel peridotite (10 kbar, 1230øC) are 2.51 and calculation, parameters such as source composition, element 2.83, respectively. partition coefficients, and mineral modes of source and melt Our calculations eliminate the simple batch model as a are required. The REE composition of is used as the viable mechanism for melt extraction in the Horoman source composition, because this plagioclase lherzolite shows peridotites, because the largest permissible Sm/Nd ratios with the highest HREE content without significant Sr and Pb the batch melt extraction model is 0.7, whereas two DP anomalies and has a fertile major element composition. samples (BZ143 and ) have higher Sm/Nd 0.77 and 1.5, respectively. Such high ratios cannot be produced from a source with a Sm/Nd ratio of 0.41 by simple batch melt Distribution coefficients of REE used in this calculation are listed in Table 5. Table 5. Distribution Coefficients and Modal Compositions of Source and Melt Used in Models ol opx cpx gar sp Distribution Coefficients La Ce Nd Sm Eu Gd Dy Yb Lu Garnet Peridotite Modal Compositions Source Melt Spinel Peridotite Source Melt

19 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES 2897 Calculated curves of melt extraction are shown in Figure The Yb content of residue in the garnet stability field does not change significantly with different degrees of melt 0.10 extraction, because HREE are strongly partitioned into garnet 15%, ] lncre_'mental in the residual peridotite [e.g., Shimizu and Kushiro, 1975]. In the spinel stability field, the estimated [Yb]N of the residue decreases more rapidly with increasing degree of melt o.o8,..o elt Extraction extraction than in the garnet stability field (Figure 12). Therefore fertile plagioclase lherzolite ( , 62213, Fractional YOHB, and BZ262) with relatively constant [Yb]N-0.1 can, Melt Extraction be explained by melt extraction in the garnet stability field. Conversely, it is clear that, owing to their low [Yb]N values, 0.04 spinelherzolite and harzburgite cannot be produced by melt 15 % 25 o/ lo% extraction only in the garnet stability field, and can be 0.02 % 20% 25% residues of melt extraction in the spinel stability filed (Figure 12). Thus we infer that the Horoman peridotite complex %' J 15% 20%,1 underwent polybaric (garnet and spinel stability fields) near,,,, fractional melt extraction, and that the observed gap in [Yb]N between fertile plagioclase lherzolites ([Yb]N >0.1) and other peridotites ([Yb]N <0.06) can be easily explained as a MgO (wt%) consequence of the release of HREE with melt extraction from garnet to spinel stability field. A similar gap in [Yb]N Figure 12. Variation of MgO and Yb in the model residual has also been observed in the spinel tectonite in the Ronda peridotites after fractional and incremental melt extraction in complex [Van der Wal and Bodinier, 1995] and may also be the garnet and spinel stability fields. The Yb content of the result of polybaric melt extraction. For example, Frey et MORB for the normalization is from Sun and McDonough al. [1985] concluded that some peridotites underwent melt [1989]. Symbols are same as in Figure 2. The relationship extraction in the garnet stability field, while other peridotities among the pressure (P), temperature (T), and distribution underwent melt extraction in the spinel stability field. The coefficient of major element (a sølid/melt ) were formulated by Horoman peridotite complex shows lower [Yb]N(0.002 to fitting a simple equation ln D søiid/melt-- a 0 'l' al/t + a2p/t 0.13) compared to other peridotite complexes ([Yb]N = 0.01 [Yamashita et al., 1996], where a o, a 1, and a 2 are constants. to 0.15 [Loubet and Allbgre, 1982; Ottonello et al., 1984; The melting experiment data on the KLB-1 peridotite [Hirose Frey et al., 1985; Bodinier et al., 1988; Bodinier, 1988]). and Kushiro, 1993] were applied to constrain the P-T- This may indicate that a larger proportion of melt extraction D 'ø""/m 't relationships, and thereby the constants a o, a 1, and a 2 occurred in the spinel stability field, whereas other peridotite for -,,o are obtained to be 0.777, 1440 and -39.5, complexes may have experienced more melt extraction in the respectively. r. ol,d/=c,, ß in the garnet and spinel stability field are calculated at 30 kbar, 1500 øc and 25 kbar, 1450 øc, garnet stability field. This is generally consistent with recent geochemical studies which suggest that MORB is formed by respectively. The REE concentration of the plagioclase near fractional melting starting in the garnet stability field lherzolite sample were used as a source composition for the modeling. Batch, fractional and incremental melt [Salters and Hart, 1989; Johnson et al., 1990; Beattie, 1993; Iwamori, 1994; Bourdon et al., 1996]. extractions were performed on the REE using the above conditions together with the D REE " /=e ' given in Table 5. The melt extraction equations adopted for the calculation of REE 11. Formation of Compositional Layering in the contents in the residues are from Gust [1968] and Show Orogenic Lherzolites [1970]. Increments in the incremental melting are assumed In the Horoman peridotite complex, cyclic repetition of to be 1%. Model results are presented for the cases of rock types, such as spinel harzburgite - spinel lherzolite - fractional and incremental melt extraction under conditions of plagioclase lherzolite - spinel lherzolite - spinel harzburgite, is spinel and garnet stability fields. Ticks with percent on the melt extraction curves indicate the degrees of melt extraction well developed on a scale ranging from hundreds of meters to centimeters [e.g., Komatsu and Nochi, 1966; Niida, 1974, from the source. 1984; Obata and Nagahara, 1987]. To explain such compositional layering, three models have been proposed: (1) successive fractional crystallization [Niida, 1984], (2) permeable flow of partial melt from an ascending extraction. Theoretically, fractional or incremental batch melt extraction [Gust, 1968; Shaw, 1970] are more effective at raising the Sm/Nd ratio of the residue. Therefore we apply a model of fractional or incremental batch melt extraction from a homogeneous mantle source to the Horoman peridotite. The increment used in the incremental melting is assumed to be 1%, because, according to the homogeneous mantle [Obta and Nagahara, 1987], and (3) melt segregation by suction with local melting along a fracture in the ascending mantle [Takahashi, 1991b, 1992]. Recently, Toramaru [1997] suggested the necessity of stretching and bending during deformation to explain the scale-invariant symmetric and asymmetric layering structure that is characteristic of the Horoman complex, although he theoretical modeling of McKenzie [1984], the maximum melt could not constrain when the deformation occurred. As fraction in contact with residue varies from 2 to 3%. discussed in the previousection, peridotite layers that formed

20 2898 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTITES by various degrees of melt extraction at different conditions Sakhalin, and the southern margin of Siberia to the Kuril (the garnet and spinel stability fields) occur closely in space, trench by trapping of the Okhotsk oceanic plate in the early within a few ten meters of each other in the Horoman Tertiary as suggested by Kirnura [1994]. Subsequently, the complex. Therefore we believe that the cyclic layering Horoman complex was entrained in the Hidaka observed in the complex was formed under subsolidus metamorphosed accretionary complex, which consists of a conditions by thinning and folding of large-scale simple compositional bands, which were originally caused by polybaric melt extraction. Our belief is consistent with the textural development of the Horoman complex, that is, formation of a layered massif, followed by plastic deformation of coarse olivine and recrystallization of large amount of terrigenous sediments derived from an intracontinent collision zone [Kirnura, 1994]. Final uplift from the upper mantle occurred as a result of the uplift of the Hidaka metamorphic belt arising from the collision between the Eurasian and North American plates at -20 Ma as suggested by Yoshikawa et al. [1993]. polygonal olivine and finally the mylonitization of fine olivine [Niida, 1975b]. 12. Conclusions We have argued in the previous section that the Horoman peridotite complex was metasomatized by aqueous fluid dehydrated from a subducted slab. Further support for this On the basis of the above discussion, the following conclusions are made regarding the formation of the Horoman view is provided by relic H20-rich fluid inclusions observed peridotite: in olivine from the Horoman complex. These are interpreted 1. Major element and HREE compositions in the Horoman to have been derived from the oceanic slab in the mantle peridotite complex systematically change from plagioclase wedge, in contrast to CO2-rich fluid inclusions which are common in olivine from peridotite xenoliths trapped in alkaline basalt and kimberlite [e.g., Hirai and Arai, 1987]. lherzolite to spinel harzburgite and these trends suggesthat the peridotites in the Horoman complex are residues left after variable degrees of melt extraction, as has been reported in Addition of aqueous fluid to the peridotites could have caused previous studies [Obata and Nagahara, 1986; Frey et al., mantle metasomatism and induced deformation of peridotite 1991; Takazawa et al., 1992, 1995, Takazawa, 1996]. Both resulting in thinning and folding, becaus even trace amounts of water significantly reduces the creep strength of olivine at upper mantle conditions [Karato et al., 1986]. Hydration prior to subsolidus deformation has also been suggested in other peridotite complexes, on the basis of the relationship between P-T condition of deformation obtained from recrystallized pyroxene pairs and crystallization of Tipargasite which occur as interstitial grains [l ssers et al., 1991; FabriCs et al., 1991]. The initial subsolidus P-T conditions of the Horoman peridotite complex were above the wet solidus but below the dry solidus conditions [Ozawa and Takahashi, 1995]. Thus if sufficient addition of H20 accompanied metasomatism, the Horoman complex could have melted. As we had argued previously, metasomatism in the Horoman complex occurred after the melt extraction events which formed the various peridotite facies. Thus we conclude that only a small amount of H20, insufficien to cause melting, was added to the Horoman peridotite during the metasomatic episode. This idea is consistent with Takahashi's [1997] suggestion of incipient melting under H20-bearing conditions including isolation-type and veintype plagioclases associated with quartz, orthopyroxene, and Ti-pargasite. We therefore conclude that peridotite with a simple stratification caused by the extraction of MORB was subsequently incorporated into a mantle wedge, although the timing of this emplacement is unclear. Subsequent thinning and folding facilitated by the addition of aqueous fluid dehydrated from the subducted slab established the present structure and chemistry of the Horoman peridotite. This further suggests that contrary to the view of Polvd and All gre [1980] and All gre and Turcotte [1988], the repeated cyclic layering of the Horoman peridotite complex, and possibly other Alpine peridotite complexes, are not artifacts of normal mantle convection. Rather, the Horoman complex might have been incorporated into a mantle wedge setting as a result of change of subduction point and then metasomatized by fluid supplied from the subducting slab. Such a process could be related to a subduction jump zone from Hokkaido, major and trace element compositions of the most fertile peridotite are nearly identical to those of the MORB source mantle. 2. Trace element patterns both of whole rocks and their constituent clinopyroxenes tend to become progressively more enriched with relative incompatiblelements from plagioclase lherzolites to spinel harzburgites. A similar observation was made on the basis of more restricte data by Frey et al. [1991] and Takazawa et al. [1992, 1996]. Most peridotite show positive anomalies of Pb and Sr in their trace element patterns. The peridotites have more heterogeneous isotopic compositions (œnd = +110 to -10 and 87Sr/86Sr = to ) than previously reported [Yoshikawa et al., 1993; Yoshikawa, 1993; Nakarnura and Yoshikawa, 1995; Takazawa et al., 1995; Takazawa, 1996] with the most refractory peridotites showing more enriched isotopic characteristics. 3. On a [Ce/Yb]N - œn diagram, the peridotites can be divided into two groups: The DP with depleted isotopic and LREE signatures and the EP with enriched isotopic and LREE characteristics. The characteristics of the EP can be explained by addition of incompatible elements by a metasomatic fluid, possibly derived from the dehydration of the subducting slab occurred into an isotopically evolved residual peridotite. 4. The old age (= 850 Ma) obtained from Sm-Nd isotope systematics for the DP whole-rock samples in the Horoman peridotite complex suggests that, in these samples, the isotopic signature of the chemical fractionation caused by melt extraction has been preserved for a long time. Initial Nd isotopicomposition derived from the data is consistent with that of the MORB source mantle at the same time. 5. Model calculations of REE and major element abundancesuggesthat melt extraction in the Horoman peridotite complex occurred by near fractional melting under polybariconditions ranging from the garneto the spinel stability fields. 6. The compositional layering presently observed in the

21 YOSHIKAWA AND NAKAMURA: EVOLUTION OF HOROMAN PERIDOTlIES 2899 peridotite complex can be explained by a combination of melt extraction, which occurred beneath a mid-ocean ridge, and Mechanism of mantle metasomatism: geochemical evidence from the Lherz orogenic peridotite, J. Petrol., 31, , metasomatic and deformation processes governed by Bourdon, B., A. Zindler, T. Elliott, and C.H. Langmuir, Constraints subduction of an oceanic slab in a wedge mantle under water- on mantle melting at mid-ocean ridges from global 238U-230Th bearing conditions. disequilibrium data, Nature, 384, , Brueckner, H.K., A. Zindler, M. Seyler, and E. Bonatti, Zabargard 7. In combination, these observationsuggesthat the and the isotopic evolution of the sub-red Sea mantle and crust, Horoman peridotite complex is not a piece of normal Tectonophysics, 150, , convecting mantle. Bulau, J.R., H.S. Waff, and J.A. Tyburczy, Mechanical and thermodynamiconstrains on fluid distribution in partial melts, J. Acknowledgments. We thank T. Shibata, A. Makishima, K. Geophy. Res., 84, , Kobayashi, H. Takei, C. Sakaguchi, and T. Nakano for their Cohen, R.S., and R.K. O'Nions, Identifications of recycled technical helps on Sm-Nd isotope analysis, ICP-MS analysis, ion continental material in the mantle from St, Nd and Pb isotope microprobe analysis, XRF analysis, SEM-EDX analysis and B investigations, Earth Planet. Sci. Lett., 61, 73-84, content analysis, respectively. We also thank T. Moriguti, T. Nakano, and T. Hanyu for offering unpublishedata. We are Cohen, R.S., N.M. Evensen, P.J. Hamilton, and R.K. O'Nions, U-Pb, grateful to T. Shibata, M. Maboko, K. Ozawa, I. Kushiro, N. Shimizu, Sm-Nd and Rb-Sr systematics of mid-ocean ridge basalt glasses, Nature, 283, , S. Yamashita, and people of the PML for valuable discussion and encouragement. We are deeply indebted to N. Takahashi, E. Downes, H., Shear zones in the upper mantle-relation between Takazawa, and M. Obata for donating samples and providing geochemical enrichment and deformation in mantle peridotites, petrological and geological information. We also thank K. Niida Geology, 18, , for his pioneering study of the Horoman peridotite complex and his help on collecting samples. E.N. is especially grateful to M. Obata for allowing us the opportunity to study the Horoman peridotite complex. We are also grateful to E. Ito and K. Ozawa for their support. We thank M. Maboko, M. Walter, and I. Jabeen for improving the text. We thank H. Asada for providing many excellent rock thin sections. We would like to express our appreciation to the Associate Editor, M.. Kohn for his tremendous effort to improve the quality of this paper, and P.B. Kelemen and F.A. Frey are thanked for critical reviews. This research was supported Eggler, D.H., Solubility of major and trace elements in mantle metasomatic fluid: Experimental constraints, in Mantle Metasomatism, edited by M.A. Menzies and C.J. 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