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The Coexistence of Jadeite and Omphacite in an Eclogite-Facies Metaquartz Diorite from the Southern Sesia Zone, Western Alps, Italy

The Coexistence of Jadeite and Omphacite in an Eclogite-Facies Metaquartz Diorite from the Southern Sesia Zone, Western Alps, Italy

Journal of Mineralogical and Petrological Sciences, Volume 100, page 70 －84, 2005

The coexistence of and in an -facies metaquartz from the southern Sesia Zone, Western Alps,

Keisaku MATSUMOTO and Takao HIRAJIMA

Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo－ku, Kyoto 606－8502, Japan

An association of jadeite and omphacite has been newly found in an eclogite－facies metaquartz diorite col- lected from the Valley area, southern part of the Eclogitic Micaschist Complex of the Sesia Zone, Western Alps, Italy. Both the jadeite and omphacite occur as idiomorphic to subidiomorphic grains in the matrix, and some of them are in contact with each other with a sharp grain boundary. Most of the jadeite shows VI faint and irregular zoning, with a chemical variation of Xjd (Al jd /(Na + Ca)) = 0.75－0.90. Half of the omphacite grains is homogeneous, and the remainder shows various zoning patterns. Some omphacite grains exhibit pro- 3+ grade zoning with an increase in Xjd from 0.31 to 0.55 and a decrease in Xaeg (= Fe /(Na + Ca)) from 0.14 to 0.04 from the core to the rim. The rim composition of the zoned omphacite is similar to that of homogeneous

omphacite (Xjd = 0.40－0.56). The average rim composition of the jadeite－omphacite pairs in direct contact

shows an apparent miscibility gap between Xjd = 0.50 ± 0.06, Xaeg = 0.09 ± 0.03, and Xaug (= Ca/(Na + Ca)) =

0.41 ± 0.05 in omphacite, and Xjd = 0.79 ± 0.04, Xaeg = 0.08 ± 0.03, and Xaug = 0.13 ± 0.03 in jadeite. Application of Powell (1985) －clinopyroxene geothermometer gives T = 470 ± 30°C at P = 12 kbar and application of Waters and Martin (1993) garnet－omphacite－phengite geobarometer gives P = 12.4 kbar at T = 440°C and P = 12.0 kbar at T = 500°C as peak metamorphic conditions. Composition data of the P2/n and C2/c compositional fields obtained from both this study and the literature suggest that: (1) the shape of two miscibility gaps between jadeite and omphacite and between omphacite and in the jadeite－augite－ phase diagram of Carpenter (1983) is more concordant with the natural data than that of Holland (1990), (2) the miscibility gap between omphacite and augite closes at T ～ 500°C and P = 15 kbar, but the mis- cibility gap between jadeite and omphacite may still exist, and (3) the miscibility gap between jadeite and omphacite closes at T = 700－850°C and P =15－45 kbar.

Keywords: Jadeite, Omphacite, Miscibility gap, Eclogite, Sesia Zone, Western Alps.

INTRODUCTION Holland and Powell, 1996; Nakamura and Banno, 1997). The position of the miscibility gap has been discussed At low temperature, omphacite is regarded as an ordered with observations on natural metamorphic rocks, e.g., pyroxene with P2/n symmetry, and jadeite and augite are from omphacite－augite joins by Brown et al. (1978), regarded as a disordered C2/c structure. Two miscibility Carpenter (1980a), Enami and Tokonami (1984) and gaps between jadeite－omphacite and omphacite－augite Tsujimori (1997), and from jadeite－omphacite joins by exist in the jadeite－augite join at low temperatures. Carpenter (1979), Harlow (1994), Compagnoni et al. Several authors have considered the －solution prop- (1995) and D’Amico et al. (1995). However, the position erties of omphacite to construct phase diagrams, or have of the miscibility gaps has not yet been confirmed, calculated the activity of the phase components (e.g., despite many studies being conducted. Carpenter, 1980b; 1983; Banno, 1986; Davidson and We found a jadeite－omphacite association in an Burton, 1987; Holland, 1990; Carpenter et al., 1994; eclogite－facies metaquartz dioritecollected from the Orco Valley area, southern part of the Eclogitic Micaschist K. Matsumoto, [email protected]－u.ac.jp Corresponding author Complex of the Sesia Zone, Western Alps, Italy. The T. Hirajima, [email protected]－u.ac.jp specimen is poorly retrograded by a greenschist－ or The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 71 －facies overprint. In this paper, we will Compagnoni, 1977; Compagnoni et al., 1977; Rubie, describe the detailed petrography of the relevant rock, 1984; Oberha¨ nsli et al., 1985; Pognante, 1989; 1991; and provide an assessment of the theoretical phase dia- Droop et al., 1990; Castelli, 1991, Fig. 1). In the EMC, grams using the data obtained in this study and from two contrasting exhumation paths have been proposed for available data in the literature. the central and southern parts, respectively. Rubie (1984) reported that the central part of the EMC underwent GEOLOGICAL SETTING early－Alpine eclogite－facies with peak P－ T conditions of T = 500－560°C and P > 14－16 kbar. The The Eclogitic Micaschist Complex (EMC) of the Sesia metamorphic conditions evolved towards lower pressure Zone is one of the best－studied －eclogite－facies ter- (T ～ 470－490°C and P ～ 8 kbar in the blueschist－ ranes in the world (e.g., Compagnoni and Maffeo, 1973; facies). During the later stage of the exhumation, the

Figure 1. Tectonic sketch map of the Sesia Zone (modified after Castelli et al., 1994). 1, Eclogitic Micaschist Complex (EMC); 2 , Gneiss Minuti Complex (GMC); 3, Second Diorito－Kinzigitic zone (DK), Vasario (VA); 4, Post－orogenic Oligocene intrusives of Brosso－ Traversella (BT) and Valle del Cervo (C); 5, Rocca Canavese Unit (RCT), Canavese line (CL). The star denotes the sample locality. 72 K. Matsumoto and T. Hirajima

EMC was overprinted by a greenschist－facies metamor- PETROGRAPHY AND CHEMISTRY phism (T ～ 400°C and P ～ 4－5 kbar). On the other hand, Pognante (1989) reported that the southern part of The chemical analysis of the was carried out the EMC underwent early－Alpine eclogite－facies meta- using a Hitachi S550 scanning electron microscope morphism with peak P － T conditions of T = 500－550°C equipped with a Kevex energy dispersive analytical sys- and P = 13－20 kbar, and then underwent a blueschist－ tem at Kyoto University, Japan. The accelerating voltage facies metamorphism during an early stage of the exhu- and beam current were maintained at 20.0 kV and 0.5 nA, mation, which was characterized by a significant tempera- respectively. The detail of the analytical method follows ture drop at high pressures (T < 450－500°C, at pressures Mori and Kanehira (1984) and Hirajima and Banno in the jadeite－ stability field). During a later (1991). The backscattered electron images were taken by stage of the exhumation, pumpellyite and albite grew a Hitachi S3500H scanning electron microscope at Kyoto under P－T conditions around of T < 350°C and P < 5 University. The bulk rock composition was determined kbar. using X－ray fluorescence spectroscopy employing a The studied specimen was collected along a road cut Rigaku Simultix－3550 spectrometer at Kyoto University. between and Cuorgne in the Orco Valley area of The analytical procedure follows Goto and Tatsumi the EMC (Fig. 1). The sampling locality is mainly occu- (1991; 1992). Abbreviation of minerals follows those pied by glaucophanite and eclogite. Pognante (1991) con- used in Kretz (1983), except for phengite (phn). sidered that glaucophanite and eclogite were derived The Fe2+/Fe3+ estimation for clinopyroxene (jadeite from basic rocks with different CaO/Na2O. and omphacite) leads to a substantial error in the mg# (= Mg/(Mg + Fe2+)). One of the common calculation meth- ods for Fe3+ estimation in clinopyroxene is based on an ideal structural formulae and charge balance, i.e., four

Table 1. The chemical composition of clinopyroxene

( *1) Total as FeO. * ( 2) Rim compositions of jadeite－omphacite pairs with the sharp grain boundary. The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 73

Table 2. The chemical compositions of , actinolite, phengite, paragonite, garnet and clinozoisite

(*1) Total iron as FeO. (*2) In contact with phengite. (*3) In contact with omphacite. cations for six atoms. However, this method propagates all analytical errors to the Fe3+ content. Table 3. Bulk rock Therefore, in this study, the Fe3+ content in the clinopy- chemistry of the 3+ VI VI study rock roxene was estimated using Fe = Na － Al jd (where Al jd total VI total IV IV = Al for Si > 2.00, or Al jd = Al － 2Al (where Al = 2 － Si) for Si < 2.00). The enstatite and ferrosilite compo- nents are very small under high－pressure conditions, and so they are ignored. In our study, we therefore treat clino- pyroxene as having three components: jadeite, aegirine and augite. The proportions of the jadeite, aegirine and VI augite components were calculated as: Xjd= Al jd /(Na + 3+ Ca), Xaeg = Fe /(Na + Ca) and Xaug = Ca/(Na + Ca). The Fe3+ content in the sodic was estimated as Fe3+ VI VI total VI = Na － Al gln (where Al gln = Al for Si > 8.00, or Algln = Altotal － 2AlIV (where AlIV = 8 － Si) for Si < 8.00). The chemical compositions of the representative minerals are shown in Tables 1 and 2, and the bulk rock (*1) Total iron as Fe O . 2 3 chemistry of the study rock is shown in Table 3. All iron

of the bulk rock chemistry is assumed to be Fe2O3. 74 K. Matsumoto and T. Hirajima

Figure 2. Backscattered electron images of: a) jadeite (Jd) and omphacite (Omp) in contact with sharp grain boundary, and b) Tiny omphacitic region in jadeite. The compositions of the numbered white circles are shown in Figure 3a. Grt, garnet; Phn, phengite; Czo/Zo, clinozoisite/zoisite.

Figure 3. a) Chemical composition of the clinopyroxenes in the Jd－Aug－Aeg phase diagram. open diamonds, jadeite and omphacite that are not in contact with each other; grey diamonds, jadeite and omphacite contacted at sharp grain boundary; open circles, tiny omphacitic region in jadeite (the positions of the numbered point are given in Figure 2b). b) A close up of jadeite－omphacite pairs in direct contact denoted by the solid tie－lines. The average rim composition of the jadeite－omphacite pair in direct contact is regarded as being an apparent miscibility

gap (denoted by the dashed line). (Solid diamond: XJd = 0.79, XAeg = 0.08 and XAug = 0.13 in jadeite, and XJd = 0.50, XAeg = 0.09 and XAug = 0.41 in omphacite).

The study rock is a potassium－poor tonalitic grano- white－, pale coffee－, and green and red－colored, with a diorite (Table 3) following the classification of Cox et al. few millimeters in diameter. These domains are com- (1979) and Wilson (1989) and it shows a granoblastic posed of high－pressure mineral aggregates. texture characterized by various colored domains, i.e., The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 75

Figure 4. Chemical variation of garnet in the Prp－(Alm + Sps)－Grs phase diagram. The open circles denote garnet in contact with omphacite. The open squares denote garnet that is not in contact with omphacite. The solid diamond denotes the average rim composition of garnet.

White domain (igneous quartz) (domain size = 4-25 Green and red-colored domain ( phases and pla- mm2 ) gioclase) (domain size >25 mm2 )

This domain (～ 23 modal %) consists of either fine－ This domain (～ 42 modal %) mainly consists of ompha- grained polycrystalline quartz aggregate (up to 100 m cite, jadeite and garnet with minor amounts of sodic in diameter) or large grains of quartz (1－2 mm in diame- amphibole, zoisite/clinozoisite, phengite, quartz and ter) showing undulatory extinction. This domain is . derived from the original igneous quartz domain/grain. The omphacite mainly occurs as idiomorphic pris- matic grains in contact with jadeite and/or quartz. Half of Pale coffee-colored domain (igneous feldspar and the omphacite is homogeneous, whereas the biotite)(domain size = 10-25 mm2 ) remainder shows various zoning patterns. Some ompha-

cite grains show prograde zoning, with an increase in Xjd

This domain (～ 35 modal %) mainly consists of phen- from 0.31 to 0.55, and a decrease in Xaeg from 0.14 to gite (40－100 m in the longitudinal direction), clinozoi- 0.04 from the core to the rim. Most of the zoned ompha- 3+ 3+ site (YFe3+ (=Fe /(Fe + Al)) = 0.1－0.2, 25－100 μ m in cite forms aggregates with garnet, along with minor the longitudinal direction), zoisite (YFe3+ ～ 0.05, 25－100 amounts of zoisite/clinozoisite and quartz. These ompha- m in the longitudinal direction) and garnet (up to 70 cites show an irregular zoning. The rim composition of m in diameter), with minor amounts of paragonite (up the zoned omphacite, however, is similar to that of homo- to 15 m in the longitudinal direction), quartz, titanite geneous omphacite (Xjd = 0.40－0.56). On the whole, the and . This domain can be originated from K－feld- omphacite composition is in the range: Xjd = 0.31－0.55, spar, plagioclase and/or biotite. The phengite shows a Xaeg = 0.04－0.14, Xaug = 0.37－0.56 (Fig. 3a). zoning, with an increase in Si content from the core (Si = The jadeite mainly occurs as idiomorphic to subidio- 3.3 for O = 11) to the rim (Si = 3.4). In each domain, morphic grains up to 500 m long. Most of the jadeite phengite and zoisite define a rough foliation, whose direc- shows faint and irregular zoning with a chemical varia- tion varies from one domain to another. tion from Xjd = 0.75－0.90 (Fig. 3a). The jadeite contains

inclusions of zoisite (YFe3+ ～ 0.05, up to 20 m in the

longitudinal direction), clinozoisite (YFe3+ ～ 0.1, up to 20 76 K. Matsumoto and T. Hirajima

m in the longitudinal direction), quartz (up to 20 m in (1) The jadeite－quartz association provides a minimum the longitudinal direction), garnet (up to 50 m in diame- pressure of P = 12.0 kbar at T = 450°C, and P =13.5 kbar ter) and phengite (up to 50 m long dimension). Some at T = 500°C (Holland, 1983). 2+ jadeite grains are in contact with omphacite in the matrix (2) The temperature is determined using the Fe －Mg with a sharp grain boundary (Fig. 2a). The rim composi- exchange equilibrium in the coexisting garnet－clinopy- tion of each jadeite－omphacite pair in direct contact is roxene and garnet－phengite pairs. In the absence of an connected by tie－line in Figure 3b. The average composi- independent determination, all Fe in the phengite are tion of these jadeite and omphacite is Xjd = 0.79 ± 0.04, assumed to be divalent. When garnet and omphacite are

Xaeg = 0.08 ± 0.03 and Xaug = 0.13 ± 0.03 and Xjd = in contact with each other, their mg# at the rim are almost

0.50 ± 0.06, Xaeg = 0.09 ± 0.03 and Xaug = 0.41 ± constant at mg# = 0.73 ± 0.04 in omphacite (Fig. 3) and 0.05, respectively (Fig. 3b). We tentatively regard that mg# = 0.04 ± 0.03 in garnet (Fig. 4). The application of these average compositions show an apparent miscibility Powell (1985) garnet－clinopyroxene geothermometer to gap at peak metamorphic conditions of the study rock. these rim－rim pairs gives T = 470 ± 30°C at P = 12 kbar Some jadeite grains contain tiny omphacitic regions (the average value of 15 pairs). The application of Green (Fig. 2b). The brightness in the backscattered electron and Hellman (1982) phengite－garnet geothermometer to image of this region is higher than for jadeite and lower the rim－rim pairs of phengite (mg# = 0.67 ± 0.03 and Si than for omphacite. The EPMA data of this region show = 3.35 ± 0.03) and garnet (mg# = 0.04 ± 0.03) gives T that Xjd varies from 0.52 to 0.76, and some of them exist = 480 ± 30°C at P = 12 kbar (the average value of 11 in our proposed miscibility gap, shown in Figure 3b. We pairs). consider that the EPMA data of the tiny omphacitic (3) The pressure is determined using the + 2 gros- region can be affected by the host jadeite, mainly owing sular + 3 celadonite = 6 + 3 muscovite equilib- to the resolution limit of the scanning electron rium in the coexisting garnet－omphacite－phengite pairs. microscope. Therefore these data are not discussed fur- The mean value of each rim composition of garnet, ther here. omphacite and phengite is used, because each rim compo- The garnet commonly occurs as idiomorphic grains sition is almost constant, as mentioned－above. Waters up to 100 m in diameter. Rarely, coronitic garnet rims and Martin (1993) calibration of this geobarometer gives rutile－titanite or omphacite aggregates. Some idiomor- P = 12.4 kbar at T = 440°C and P = 12.0 kbar at T = phic in the omphacite aggregates show faint zon- 500°C. 2+ 2+ ing, with a decrease in Xalm (= Fe /(Fe + Mn + Mg + These constraints indicate that the study rock was 2+ Ca)) and an increase in Xgrs (= Ca/(Fe + Mn + Mg + once equilibrated in the temperature range 440－500°C Ca)) from the core to the rim, and the other idiomorphic and pressure range 12.0－12.4 kbar. These estimated tem- garnets, which is in contact with jadeite, phengite and peratures close to those of a previous study by Pognante quartz, show an opposite zoning with an increase in Xalm (1989), who reported T = 520 ± 45°C at P = 14 kbar, and a decrease in Xgrs from the core to the rim. However, where the temperature was determined using Powell both types of zoned garnet have an identical rim composi- (1985) garnet－clinopyroxene geothermometer. 2+ tion, i.e., Xalm = 0.61, Xsps (= Mn/(Fe + Mn + Mg + Ca)) 2+ = 0.01, Xgrs = 0.34 and Xprp (= Mg/(Fe + Mn + Mg + DISCUSSION

Ca)) = 0.04 (Fig. 4). Sodic rarely occur in the matrix, and most of them are homogeneous glauco- Assessment of the phase diagrams of the jadeite- 3+ 3+ VI phane (mg# = 0.43－0.47, Fe /(Fe + Al gln) < 0.12). augite join from the jadeite-omphacite association However, a few sodic amphiboles include an actinolitic core. Rarely, the sodic amphibole is replaced by a retro- Since omphacite is confirmed to be the ordered phase gressive actinolite and albite symplectite. Albite (Xab > with P2/n at low temperature, several models

0.8 and Xan < 0.2) of secondary origin, which develops at have been proposed for the jadeite－augite solid－solution the jadeite rim, is also scarce. In the all domains, titanite properties. Based on pioneering TEM work, Campness overgrows the rutile or occurs as isolated grains. (1973) proposed that the order－disorder transformation of omphacite is first－order. However, Carpenter (1980b) car- P-T ESTIMATION ried out TEM observations on omphacite and interpreted that the order－disorder transformation of omphacite The temperature and pressure conditions of the study should be second－order in character. Based on this idea, rock from the southern part of the EMC were estimated Carpenter (1983) drew a phase diagram of the C2/c and from the following data. P2/n compositional fields along the jadeite－augite join The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 77

Figure 5. The binary jadeite－omphacite and omphacite－augite sol- vus proposed by Carpenter (1983) and by Holland and Powell (1996).

(the solid line in Fig. 5) and in the jadeite－augite－ aegirine ternary diagram (Fig. 6a). The solid－solution properties of omphacite have been frequently discussed on the basis of the Bragg－Williams approximation, and qualitative and quantitative phase diagrams have been theoretically proposed (e.g., Banno, 1986; Davidson and Burton, 1987; Nakamura and Banno, 1997). Recently, the Landau theory was applied to omphacite (e.g., Holland, 1990; Carpenter et al., 1994), and the order－disorder transformation of omphacite has come to be considered as being tricritical (Carpenter et al., 1990). Holland Figure 6. a) The Jd－Aug－Aeg phase diagram at 350°C, as pro- (1990) proposed a phase diagram for the jadeite－augite－ posed by Carpenter (1983). b) The Jd－Aug－Aeg phase diagram T aegirine system at = 500°C using the Landau theory at 500°C, as proposed by Holland (1990). The numbered nine (Fig. 6b), but the geometry of this diagram is quite differ- lines refer to the jadeite activity. ent from that proposed by Carpenter (1980b; 1983) (cf. Fig. 6a, b). Carpenter (1983) proposed the ternary phase diagram using the available natural data at that time for Samples J9467 and E6330 of DAmico et al. (1995) are an approximate temperature T ～ 350°C. Holland and plotted in the P2/n field of Carpenter (1983) diagram, but Powell (1996) reconsidered the solid－solution properties in the miscibility gap between jadeite and omphacite of of the jadeite－augite join using the symmetric formalism Holland (1990) diagram. Some data from prehistoric rem- method, and recalculated the phase diagram along the nants in the Western Alps (Compagnoni et al., 1995) are jadeite－augite join (the dotted line in Fig. 5). Thus, located in the miscibility gap of both diagrams. although various phase diagrams have been proposed, the Compagnoni et al. (1995) described that jadeite and position of the miscibility gaps in the jadeite－augite join omphacite grew at different stages in Sample NAR439, are still controversial. In the following section, we will and the clinopyroxene in Sample ALBA16 shows compo- assess the validity of the proposed phase diagrams based sition zoning from jadeite to omphacite. As there is the on the compositional range of the natural jadeite－ompha- possibility that these clinopyroxenes were crystallized at cite association. different stages, we will not discuss these data further First, we compare the ternary diagrams proposed by here. Carpenter (1983) and Holland (1990) using the natural Ferric contents are somewhat contained in these data (Fig. 7, also see Appendix 1). Most reported data are natural clinopyroxenes. The systematic discrepancy in the located in the P2/n (omphacite) and the C2/c (mainly Fe3+ content estimation is caused by several calculation jadeite) fields in both diagrams. Omphacite data obtained methods from the EPMA data, and is one of main from Sample No. 97503 of Carpenter (1979) and ambiguous factor to determine the P2/n and C2/c bound- 78 K. Matsumoto and T. Hirajima

Figure 7. Comparison between the natural association of jadeite－omphacite and the Jd－Aug－Aeg phase diagram, as proposed by Carpenter (1983) and Holland (1990). ary in the ternary system. We compared the data set Assessment and interpretation of the phase diagrams obtained using two Fe3+ estimation methods: (1) a stoi- from the natural clinopyroxene data chiometric method (four cations for six oxygen atoms), and (2) using a simplified equation where Fe3+ = Na － In the following section, we compile clinopyroxene data VI Al jd. As a result, the proportion of jadeite and aegirine equilibrated at both T ～ 500°C and P = 15 kbar and T = varies 0.02－0.05, but that of augite is almost similar using 700－850°C and P = 15－45 kbar, and provide an estimate both correction methods. Consequently, we suggest that of the position of the miscibility gap between jadeite and the ternary diagram of Carpenter (1983) is more concor- omphacite, and between omphacite and augite using dant with the natural data than that of Holland (1990). these data.

Clinopyroxene data equilibrated at T ～ 500°C and P = 15 kbar

We show a compilation of the literature data of the Sesia The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 79

Figure 8. The clinopyroxene data from the Eclogitic Micaschist Complex of the Sesia Zone.

Figure 9. A KD versus Xgrs plot showing the 400, 500 and 600°C isotherms at a pressure of 15 kbar obtained from Ellis and Green (1979, EG),

Powell (1985, P) and Krogh (1988, K) garnet－clinopyroxene geothermometers. The KD －XGrs data from this study, and from Ungaretti et al. (1983), Oberha¨nsli et al. (1985), Pognante et al. (1987) and Castelli (1991) are also plotted.

Zone in Figure 8 (Oberha¨ nsli et al., 1985; Castelli, 1991; SE18) deformed from granitoid from the Monte Mucrone Boffa Ballaran et al., 1998). Oberha¨ nsli et al. (1985) area, central part of the EMC of the Sesia Zone (Fig. 1). reported jadeite in fine－grained pelitic gneiss (Sample Castelli (1991) reported clinopyroxene compositions in SE14A) within a metagranitoid body and omphacite from impure marble from several EMC localities shown in both metagranitoid (Sample SE14) and mylonite (Sample Figure 1. At the Pont Canavese, Quassolo and Colma di 80 K. Matsumoto and T. Hirajima

Figure 10. The clinopyroxene data from the Eclogitic Micaschist Complex of the Sesia zone. The symbols and the composition ranges of the clinopyroxene are the same as those shown in Figure 8. The proposed miscibility gap at T = 470°C is denoted by the dotted line.

Mombarone localities, the omphacites are poorly zoned net－clinopyroxene geothermometers are shown in the KD Grt Cpx and have low aegirine content (up to 0.06). At (= (Fe/Mg) /(Fe/Mg) ) － Xgrs diagram in Figure 9. For Fontainemore, the clinopyroxenes are also unzoned in example, the application of Powell (1985) geothermome- each grain, but their composition varies from Xjd = 0.10－ ter systematically gives about 20－30°C lower temperature 0.48. Castelli (1991) interpreted that the compositional than using Ellis and Green (1979) (Fig. 9) through the variation of clinopyroxene was due to differences in the wide range in KD and Xgrs. The temperature obtained chemical composition between different domains within using Powell (1985) geothermometer, however, is system- the same outcrop. Conversely, at Pont S. Martin, the cli- atically 20－40°C higher than that obtained using Krogh nopyroxenes fall into two groups: (i) sodic augite with Xjd (1988) geothermometer for less calcic garnet (Xgrs <

= 0.02－0.14 and Xaeg = 0.06－0.15 mainly occurring at the 0.35), but it gives a significantly higher temperature for rim of the pre－Alpine augite, and (ii) omphacite with Xjd calcic garnet (Xgrs > 0.35). As Carswell et al. (1997) > 0.20. At Torre Pramotton, the omphacites coexisting pointed out that temperature estimation is better using with garnet are zoned, with the core being richer in jade- Powell (1985) geothermometer for less calcic garnet (Xgrs ite with more aegirinic rim. Except for the rim composi- < 0.35), and Krogh (1988) geothermometer for calcic gar- tion at Torre Pramotton, Castelli (1991) considered that net (Xgrs > 0.35), and we concur with their suggestion. the composition of clinopyroxene, ranging from sodic The KD－Xgrs relationships used to the temperature estima- augite to omphacite in the impure marble, is due to the tion of the Sesia samples in the literature is also plotted in local bulk composition formed under isofacial conditions. Figure 9 (Ungaretti et al., 1983; Oberha¨ nsli et al., 1985; Boffa Ballaran et al. (1998) determined the space groups Pognante et al., 1987; Castelli, 1991). Castelli (1991) of the clinopyroxenes derived from representative litho- determined the peak metamorphic conditions as T = 575 types of the EMC of the Sesia Zone (see their Table 1): ± 45°C at P = 15 kbar, using a sample from Torre the jadeite with a composition of Xjd = 0.87－0.97, Xaug = Pramotton sample (near Pont S. Martin), central part of

0.01－0.08 and Xaeg = 0.01－0.05 has space group C2/c, the EMC, using the Ellis and Green (1979), Ganguly and the omphacite with Xjd = 0.50－0.56, Xaug = 0.42－0.48 (1979) and Saxena (1979) geothermometers. However, and Xaeg = 0.01－0.02 has space group P2/n. application of the new criteria to the data set of Castelli The equilibrated temperature conditions of clinopy- (1991) gives the peak metamorphic conditions as T = 430 roxene are important in the following discussion. The ± 40°C at P = 15 kbar. The same recalculation at P = 15 peak metamorphic temperature in the EMC of the Sesia kbar to the data set of Ungaretti et al. (1983) gives the Zone has been determined using various geothermome- peak metamorphic temperature as T = 510 ± 25°C in the ters, e.g., Ra゜heim and Green (1974), Ellis and Green Monte Mucrone area, and T ～ 540°C in the Alpe Artorto (1979), Ganguly (1979), Saxena (1979), Dahl (1980) and area. When applied to the data of Oberha¨ nsli et al. (1985), Powell (1985). These data include the systematic discrep- the peak metamorphic temperature is calculated to be T = ancy, mainly caused by the different expression of the 540 ± 70°C in the Monte Mucrone area, and when

Xgrs in the geothermometers. Therefore, it is necessary to applied to the data of Pognante et al. (1987), the peak unify the temperature estimation. The isotherms of T = metamorphic temperature is calculated to be T = 500 ± 400, 500 and 600°C at P = 15 kbar obtained using Ellis 30°C in the Pont Canavese area. The peak metamorphic and Green (1979), Powell (1985) and Krogh (1988) gar- temperature in the EMC is considered to be T = 500－ The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite 81

Figure 11. The clinopyroxene data in the ternary system equilibrated at T ～ 700°C. The thin line shows the miscibility gap at T = 350°C in the ternary system proposed by Carpenter (1983). The dotted line shows a possible phase boundary occurring between the P2/n and C2/c phases.

550°C (Droop et al., 1990). The re － estimated tempera- suggest that the miscibility gap between jadeite and ture conditions applied using the new criteria are in good omphacite shrinks with increasing metamorphic tem- agreement to the Droop’s idea, except for the Torre perature.

Pramotton sample of Castelli (1991). The Xgrs (= 0.4－0.6) and Xsps (= 0.2－0.3) values of the Torre Pramotton sam- Clinopyroxene data equilibrated at T = 700-850°C and ple are significantly higher than those of the other data, P = 15-45 kbar and so the temperature estimation for this data has to be viewed with caution. Therefore, we conclude that the mis- Carpenter and Smith (1981) analyzed the compositions cibility gap between omphacite and augite closes at T = and space groups of clinopyroxenes from eclogite, clino- 500－550°C at P = 15 kbar in the EMC (Fig. 8). and orthopyroxene－eclogite from the Nybo¨ Our jadeite－omphacite association suggests that the area in the Western Gneiss Region, Norway (see their miscibility gap exists between Xjd = 0.79 ± 0.04, Xaug = Figs. 1 and 2). The peak pressure－temperature conditions

0.13 ± 0.03 and Xaeg = 0.08 ± 0.03 in jadeite, and Xjd = of the Nybo¨ eclogite have been estimated to be T = 700－

0.50 ± 0.06, Xaug = 0.41 ± 0.05 and Xaeg = 0.09 ± 0.03 850°C at P = 15－45 kbar (Lappin and Smith, 1978). We in omphacite at T = 470 ± 30°C and P = 12.0－12.4 kbar. plot the selective clinopyroxene compositions of We tentatively propose the position of the miscibility gap Carpenter and Smith (1981) in Figure 11. They con- in ternary system under the relevant pressure－tempera- cluded that the clinopyroxene composition obtained from ture conditions, as shown in Figure 10. However, the the Nybo¨ area fills the natural composition gap in jadeite－ omphacite data of Sample SE14 of Oberha¨ nsli et al. rich omphacite with up to 3－12 % aegirine component, (1985) from the Monte Mucrone area is plotted in our pro- confirming a complete miscibility under the pressure－ posed miscibility gap. The rock with the jadeite－ompha- temperature conditions of the Nybo area (> 700°C). cite association in this study was collected from the Orco Boffa Ballaran et al. (1998) reported the composi- Valley area, i.e., located at the southern edge of the EMC, tions and space groups of clinopyroxenes from the Dora－ and the estimated temperature of this rock is 50－100°C Maira massif in the Western Alps, from Mu¨nchberg in lower than that of the Monte Mucrone samples. The Germany and from the Nybo¨ area (Fig. 11 and see their omphacite composition of Oberhansli et al. (1985) may Table 1). The eclogite in the Dora－Maira massif was col- 82 K. Matsumoto and T. Hirajima lected from the Brossasco－Isasca ultra－high pressure Journal of the Japanese Association of Mineralogists, Complex, which was formed under peak metamorphic Petrologists and Economic Geologists, 81, 281－288 (in conditions of T ～ 720°C at P = 36 kbar obtained using Japanese with English abstract). Boffa Ballaran, T., Carpenter, M.A., Domeneghetti, M.C. and Powell (1985) garnet－clinopyroxene geothermometer Tazzoli, V. (1998) Structural mechanisms of solid solution (e.g., see Nowlan et al., 2000). OBrien (1989) estimated and cation ordering in augite－jadeite . I: A macro- the peak temperature of Munchberg eclogite as being T = scopic perspective. American Mineralogist, 83, 419－433. 710 ± 40°C at P = 15 kbar using Powell (1985) garnet－ Brown, P., Essene, E.J. and Peacor, D.R. (1978) The mineralogy and petrology of manganese－rich rocks from St. Marcel, clinopyroxene geothermometer. The Mu¨ nchberg data of , Italy. Contributions to Mineralogy and Petrology, Boffa Ballaran et al. (1998) suggest that there is no misci- 67, 227－232. bility gap between omphacite and augite at T = 710 ± Ca´ mara, F., Nieto, F. and Oberti, R. (1998) Effects of Fe2+ and 40°C and P = 15 kbar, and that the P2/n and C2/c phase Fe3+ contents on cation ordering in omphacite. European － boundary exists at Xjd ～ 0.35 and Xaug ～ 0.65. Journal of Mineralogy, 10, 889 906. Carpenter, M.A. (1979) Omphacites from Greece, Turkey and Ca´ mara et al. (1998) reported the compositions and Guatemala: composition limits of cation ordering. American space groups of omphacite from －eclogite from Mineralogist, 64, 102－108. the Nevado－Fil´ a bride complex, Spain (Fig. 11), where the Carpenter, M.A. (1980a) Composition and cation order variations peak eclogitic conditions were estimated to be T = 630°C in a sector－zoned blueschist pyroxene. American at P = 16 kbar (Puga et al., 1989). Ca´ mara et al. (1998) Mineralogist, 65, 313－320. determined the P2/n and C2/c phase boundary in the Carpenter, M.A. (1980b) Mechanisms of exsolution in sodic pyroxenes. Contributions to Mineralogy and Petrology, 71, omphacite region; omphacite with Xaeg < 0.20 has space 289－300. group P2/n, and omphacite with Xaeg > 0.20 has space Carpenter, M.A. (1983) Microstructures in sodic pyroxenes; impli- group C2/c. cations and applications. Periodico di Mineralogia, 52, 271－ From the clinopyroxene data equilibrated at T = 700－ 301. Carpenter, M.A. and Smith, D.C. (1981) Solid solution and cation 850°C and P = 15－45 kbar, no miscibility gap exists in ordering limits in high－temperature sodic pyroxenes from the jadeite－augite join (see the bold line in Fig. 11). the Nybo¨ eclogite pod, Norway. Mineralogical Magazine, Consequently, the phase diagrams shown in Figures 10 44, 37－44. and 11 are proposed on the basis of the petrographical Carpenter, M.A., Domeneghetti, M.C. and Tazzoli, V. (1990) and mineralogical data. To discuss the validity of those Application of Landau theory to cation ordering in phase diagrams, more detailed work is required in future omphacite. I. Equilibrium behaviour. European Journal of Mineralogy, 2, 7－18. investigations. Carpenter, M.A., Powell, R. and Salje, E.K.H. (1994) Thermodynamics of nonconvergent cation ordering in miner- ACKNOWLEDGMENTS als; I, An alternative approach. American Mineralogist, 79, 1053－1067. Dr. Hidehiko Shibakusa carried out the field survey and Carswell, D.A., OBrien, P.J., Wilson, R.N. and Zhai, M. (1997) Thermobarometry of phengite－bearing in the collected samples in the southern Sesia Zone during the Dabie Mountains of central China. Journal of Metamorphic summer of 1994. These samples and field reports were Geology, 15, 239－252. kindly donated to us by his family after his death, and we Castelli, D. (1991) Eclogitic metamorphism in carbonate rocks: express our sincere thanks to Dr. Shibakusa' s family. The the example of impure marbles from the Sesia－Lanzo zone, petrological study of these samples was initiated as the Italian Western Alps. Journal of Metamorphic Geology, 9, 61－77. graduation thesis study at Osaka Prefecture University Castelli, D., Compagnoni, R. and Nieto, J.M. (1994) High pres- supervised by Prof. Hirokazu Maekawa, and the first sure metamorphism in the continental crust: eclogites and author wishes to thank Prof. Maekawa for his eclogitized metagranitoids and paraschists of the Monte encouragement. We also wish to acknowledge the contri- Mucrone area, Sesia Zone. In High pressure metamorphism butions of Daisuke Nakamura, Shohei Banno, Roberto in the Western Alps (Compagnoni, R., Messiga, B. and Castelli, D. Ed.). Guide－book to the B1 field excursion of Compagnoni, and Daniele Castelli for their constructive the 16th General International Mineralogical Association discussion and the critical reading of the manuscript, and Meeting, Pisa, 4－9 September 1994, 107－116. Tomoyuki Kobayashi for carrying out the XRF analysis. Champness, P.E. (1973) Speculation on an Order－Disorder We thank Profs. Masaki Enami and Tadao Nishiyama for Transformation in Omphacite. American Mineralogist, 58, their constructive reviews. 540－542. Compagnoni, R. 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Saxena, S.K. (1979) Garnet－clinopyroxene geothermometer. Appendix 1. Description of natural jadeite－omphacite association Contributions to Mineralogy and Petrology. 70, 229－235. Tsujimori, T. (1997) Omphacite－diopside vein in an omphacitite Natural jadeite and omphacite association has been reported in block from the Osayama serpentinite melange, Sangun－ blueschist (Sample Nos. 97503 and 97614) from Syros, Greece, Renge metamorphic belt, southwestern Japan. Mineralogical which was formed at peak metamorphic conditions of P ～ 13 Magazine, 61, 845－852. kbar and T = 450－500°C (Carpenter, 1979), in jadeitite and metaba- Ungaretti, L., Lombardo, B., Domeneghetti, C. and Rossi, G. site from Guatemala, which were metamorphosed at P = 5－11 kbar (1983) －chemical evolution of amphiboles from eclo- and T = 350－400°C (Carpenter, 1979; Harlow, 1994), and in pre- gitised rocks of the Sesia－Lanzo Zone, Italian Western Alps. historic stone axes composed of eclogite and Na－pyroxenite Bulletin de Mineralogie, 106, 645－672. derived from the Western Alps (Compagnoni et al., 1995; DAmico Waters, D.J. and Martin, H.N. (1993) The garnet－clinopyroxene－ et al., 1995). Sample ALBA16 in Compagnoni et al. (1995) was phengite barometer. Terra Abstract, 5, 410－411. estimated to have been formed about T = 420－500°C at P = 12 Wilson, M. (1989) Igneous petrogenesis. pp. 466, Unwin Hyman, kbar using Ellis and Green (1979) geothermometer, but the meta- London, United Kingdom. morphic conditions of the other samples are not known.

Manuscript received September 10, 2004 Manuscript accepted December 3, 2004 Manuscript handled by Tadao Nishiyama

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