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

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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 jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy 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 Orco 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) garnet－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 pyroxene 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 augite in the jadeite－augite－ aegirine 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 solid－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 blueschist－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 metamorphism 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 quartz－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 MINERAL 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 minerals 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－lawsonite 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 Alpette 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 iron 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 glaucophane, actinolite, phengite, paragonite, garnet and clinozoisite (*1) Total iron as FeO. (*2) In contact with phengite. (*3) In contact with omphacite. cations for six oxygen 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 amphibole 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.
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