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Multiple Growth Mechanisms of Jadeite in Cuban Metabasite

Multiple Growth Mechanisms of Jadeite in Cuban Metabasite

Eur. J. . 2012, 24, 217–235 Jadeitite: Published online December 2011 new occurrences, new data, new interpretations

Multiple growth mechanisms of in Cuban metabasite

1, 1,2 3 1 WALTER V. MARESCH *,CHRISTIANE GREVEL ,KLAUS PETER STANEK ,HANS-PETER SCHERTL 4 and MICHAEL A. CARPENTER

1 Institut fu¨r Geologie, Mineralogie, Geophysik, Ruhr-Universita¨t Bochum, Universita¨tsstr 150, 44801 Bochum, Germany *Corresponding author, e-mail: [email protected] 2 TU¨ V Rheinland, Am Grauen Stein, 51105 Cologne, Germany 3 Institute of Geology, TU Bergakademie Freiberg, Bernhard-von-Cotta-Street 2, 09596 Freiberg, Germany 4 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Abstract: Samples of rocks reported in the literature to be jadeite jade from the -zone complex of the Escambray Massif in central Cuba have been studied by optical and transmission electron microscopy, electron microprobe and hot-cathode cathodolu- minescence (CL) microscopy. Although these rocks are indeed rich in jadeite, the bulk rock composition generally conforms to MORB, with Na2O enriched by . 3 wt% and CaO depleted by .2 wt%. Al2O3 contents are unchanged. These changes are attributed to early pre-subduction spilitization of the ocean-floor protolith. Relics of magmatic preserving an ophitic texture are common. Disequilibrium textures are the rule. Extensively recrystallized rocks show fine, felty intergrowths of predominantly Al-rich and jadeite, the latter with rims and patches of . TEM observations indicate extensive replacement of by . Glaucophane developed rims of magnesiokatophorite and edenite. Chlorite and are also present. Late development of actinolite, chlorite, epidote and albite is observed. is present. Less recrystallized samples with numerous large (.1.5 mm) grains of augite show several types of sodic and sodic-calcic clinopyroxene development: (1) Topotactic replacement of magmatic pyroxene by jadeite and omphacite along a broad front encroaching upon the augite grain from the rock matrix. Jadeite dominates where presumably plagioclase was formerly present. Omphacite dominates where augite is internally replaced along and fractures. Late chlorite, taramite and ferropargasite replace these pseudomorphs. (2) Former plagioclase laths of the ophitic fabric are replaced by jadeite together with lesser omphacite in epitactic relationship with the enclosing augite. Former plagioclase-augite grain boundaries remain preserved. Late pumpellyite is associated with the omphacite. (3) Jadeite þ omphacite þ pumpellyite þ chlorite with irregular grain boundaries dominate in the rock matrix between the augite relics, with idiomorphic of epidote scattered throughout and in chlorite–epidote clusters. Pumpellyite is interpreted to be a late retrograde product. Quartz is present. (4) Jadeite þ omphacite þ chlorite assemblages, in which monomineralic sheaf-like jadeite aggregates are common, fill very thin (500–1500 mm) fractures criss-crossing the sample, including ophitic augite remnants. Cathodoluminescence microscopy shows that jadeite in the veins is distinctly different from CL in the other types of jadeite, showing features like oscillatory growth zoning indicative of crystallization from a fluid. Generally omphacite develops irregularly along jadeite rims, but recrys- tallization may lead to pairs with straight grain boundaries suggestive of phase equilibration. Comparison with published solvus relationships suggests temperatures of 425–500 C. This unusual occurrence of different types of jadeite in a metabasic rock suggests two contrasting sources. The first – in the rock matrix, as topotactic alteration of igneous pyroxene and as plagioclase replacement epitactically growing on augite – can be explained as due to local domain equilibration in a rapidly subducted ‘‘spilitized’’ gabbroic rock. The second, in very thin fillings, conforms to an origin as a crystallization product from a pervasive fluid. Conceivably, ‘‘pooling’’ of the fluids flowing through the fractures in larger cavities could lead to larger masses of jadeitite. These have not yet been conclusively documented in the Escambray Massif. Key-words: jadeite, jadeitite, omphacite, pyroxene topotaxy, pyroxene epitaxy, Escambray Massif, Cuba.

1. Introduction Schertl et al., 2007, 2008; Garcı´a-Casco et al., 2009; Ca´rdenas-Pa´rraga et al., 2010; Baese et al., 2010). In the Jadeite jade is a rare rock type world-wide. In a recent New World, jadeite jade was probably already known and summary, Harlow et al. (2007) cite 14 described occur- used for tools and ornaments in Mesoamerica (southern rences. Recent findings in Iran, Cuba and Hispaniola can Mexico through Guatemala to Honduras and Nicaragua) be added to this still very short list (Oberha¨nsli et al., 2007; almost 3500 years ago (Harlow et al., 2011, and references

0935-1221/12/0024-2179 $ 8.55 DOI: 10.1127/0935-1221/2012/0024-2179 # 2011 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 218 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter therein). The most important source of New World jade is ophitic texture in epitactic continuity with the augite, as a in the middle Motagua Valley, Guatemala, where it occurs pervasive recrystallization product in the matrix, and as a in serpentinite me´langes on both sides of the Motagua presumed precipitate from aqueous fluid in late rock frac- Fault. Harlow et al. (2011) have provided a recent com- tures, all in a single sample. Considering that access to the prehensive update of these occurrences and show that sample area is now very difficult, we will present the known sources north of the fault extend over 200 km, and available microanalytical, cathodoluminescence and elec- over 11 km to the south. tron microscope data in this paper. As recounted by Harlow et al. (2011), knowledge of Guatemalan jade sources was lost by the original inhabi- tants of Mesoamerica. Early re-discoveries in the early 1950s then ushered in a new era of research on jadeitites 2. Geological setting and jadeite-bearing rocks that gained momentum after hurricane-induced new outcrops and new archeological The Escambray Massif is located in central Cuba at the finds (e.g., Seitz et al., 2001; Harlow et al., 2003, 2004, northern margin of the Caribbean Sea (Fig. 1). In geody- 2007; Harlow & Sorensen, 2005). This interest is two-fold. namic terms, the geology of Cuba represents a key element Harlow & Sorensen (2005), Harlow et al. (2007), and in our attempts to understand the complex plate tectonic Sorensen et al. (2006, 2010) present cogent arguments evolution of the Caribbean area (e.g., Garcı´a-Casco et al., for the formation of jadeite jade by precipitation from a 2008; Pindell & Kennan, 2009; Stanek et al., 2009; Pindell high-pressure aqueous fluid in a subduction zone channel. et al., 2012, and numerous references therein). During the Consequently jadeitites can be viewed as records of fluid- Late Cretaceous and Paleogene the oceanic Great driven or fluid-assisted mass transfer processes in such Antillean island arc and its associated subduction-accre- settings. On the other hand, pre-Columbian jade (or possi- tion complex interacted and collided with the Yucata´n bly jade) artifacts have been mentioned from a number of margin and an intra-oceanic sedimentary prism extending Antillean islands (e.g., Harlow et al., 2006; Garcı´a-Casco to the southeast of it (‘‘Caribeana’’ of Garcı´a-Casco et al., et al., 2009, and references therein), raising the question of 2008), before docking onto the Bahamas platform in the whether Guatemala represents the only source for this Eocene. This suture zone is well-preserved in Cuba. In the archeological jade. As already surmised by Harlow et al. north and northwest of Cuba, the rocks can be related to the (2006) on the basis of geological and petrological simila- continental margins of the Bahamas Platform and the rities to Guatemala, recent findings in Cuban (Garcı´a- Yucata´n Peninsula, whereas in the south rocks of the Casco et al., 2009; Ca´rdenas-Pa´rraga et al., 2010) and Cretaceous oceanic island arc and associated ophiolites Hispaniolan (Schertl et al., 2007, 2008; Baese et al., are found. 2010) serpentinite me´langes now extend possible source To the rear of this continent–arc collisional suture, and areas further to the east along the Greater Antilles. associated with the Cretaceous island arc, tectonic win- In their description of the new Cuban occurrences in dows expose metamorphic rocks throughout the southern eastern Cuba, Garcı´a-Casco et al. (2009) also point to a part of the Island of Cuba (e.g., Garcı´a-Casco et al., 2008; short petrographic report of rare jadeitite pebbles, cobbles Stanek et al., 2009). With an exposed area of about 1800 and boulders in river deposits of the Escambray Massif in km2, the Escambray Massif (Fig. 1) is the largest such central Cuba by Milla´n & Somin (1981), suggesting that metamorphic complex of the Greater Antilles. The massif this occurrence may have been largely overlooked so far. forms two morphological domes, the western Trinidad and Based on optical microscopy, Milla´n & Somin (1981) the eastern Sancti Spiritus dome (Somin & Milla´n, 1974). described essentially monomineralic rocks composed of Considerable discussion has accompanied attempts to pro- jadeitite with minor clinozoisite, and albite. vide a tectono-metamorphic nomenclatural framework for Some samples contain relics of magmatic clinopyroxene the Escambray Massif, with ideas evolving from early in part replaced by fine-grained aggregates of jadeite, lithological concepts to the recognition that the Massif leading Milla´n & Somin (1981) to suggest that the jadei- represents a complex nappe pile (Somin & Milla´n, 1981; tites formed by transformation of small bodies of basic Milla´n et al., 1985a, b; Milla´n-Trujillo, 1997; Stanek et al., intrusives. 2000, 2006, 2009; Schneider et al., 2004). On the basis of Additional petrographic and analytical data of a suite of interdisciplinary structural, petrological and geochronologi- samples from one of the localities described by Milla´n& cal work in the Sancti Spiritus dome, Stanek et al. (2006) Somin (1981) have been available in an unpublished thesis recognized four major tectono-metamorphic nappes (Fig. 1), (Grevel, 2000) or in abstract form for some time (Grevel which can be discriminated on the basis of their contrasting et al., 1998; Maresch et al., 2007). None of these samples pressure-temperature-time-paths. From bottom to top these collected together with G. Milla´n at this site can be con- are the intermediate- to high-pressure Pitajones, Gavilanes sidered to be true jadeitites; they are jadeite-bearing meta- and Yayabo nappes, which are in turn overlain by the low- gabbroic rocks. Nevertheless, in the context of current pressure Mabujina nappe. The lower three represent an discussions on the origin of jadeitite, these samples have accretionary complex composed of both oceanic and con- become important. Sodic and sodic-calcic pyroxene can be tinental margin sedimentary rocks. The uppermost observed as a topotactic replacement of igneous ophitic Mabujina nappe represents the basal section of the augite, as a replacement of the former plagioclase in the Cretaceous volcanic arc (see summaries by Stanek et al., Jadeite formation in metabasic rock 219

a

5 km

b

C

Fig. 1. Geological setting. (a) Regional situation in the Caribbean area. [G] Guatemalan jadeitite occurrences; [E] Escambray massif; [C] Sierra del Convento jadeitite occurrences; [R] Rio San Juan Complex jadeitite occurrences. (b) Geological sketch map of the eastern Escambray massif (Sancti Spiritus dome) with sample locality. (c) Simplified geological profile (line N-S in part (B)) with sample locality.

2006, 2009; Garcı´a-Casco et al., 2008; and references summaries by Garcı´a-Casco et al., 2006, 2008; Stanek therein). The nappe pile is characterized by multiple thrust et al., 2006). The Yayabo unit is restricted to the north- planes as well as out-of-sequence thrusts. eastern part of the Sancti Spiritus dome (Fig. 1), but inter- The tectonically lowermost Pitajones nappe consists of calated slivers of this unit have also been reported from the monotonous carbonate- and quartz-mica schists with abun- upper parts of the Gavilanes unit (Somin & Milla´n, 1981). dant boudins of marble, metagabbro, and greenschist in its The Yayabo unit comprises fine- to coarse-grained, epi- upper part. Maximum preserved equilibration pressures dote-bearing - with barroisitic amphi- are about 8 kbar, while correlated temperatures vary boles. Maximum pressures of 13–15 kbar are indicated between 410 and 520 C (Grevel, 2000; Stanek et al., (Grevel, 2000). Low-pressure/high-temperature, amphibo- 2006). The overlying Gavilanes nappe, considered as the lite- to greenschist-facies rocks characterize the Mabujina source of the samples investigated in this study (see unit, the tectonically highest nappe of the Escambray below), is a tectonic mega-me´lange containing quartzite Massif. Peak PT-conditions of 7 kbar at 620–700 C (occasionally with the rare high-pressure mineral deerite), (Grevel, 2000) support the regional temperature estimates , , , and serpentinized ultra- of Somin & Milla´n (1981). basic rocks in a general matrix of impure metacarbonates, quartz-mica schists, and marbles. Preserved maximum pressure (P)-temperature (T) conditions in the various 3. Sampling vary from 15 to 25 kbar and 500 to 660 C (Grevel, 2000, Schneider et al., 2004; Garcı´a-Casco et al., Guided by G. Milla´n we took samples in 1994 from a 2006). Lenses of ‘‘high-grade’’ serpentinite with antigorite small, dried-out, 3–4 m wide tributary of the Unimazo are common (Auzende et al., 2002), indicating that these River, at ca. 450 m elevation in a densely forested area serpentinized ultramafic rocks shared the same meta- (Fig. 1; UTM coordinates of the locality are 637 418, morphic history as the other lithologies. They are often 242 2851). The slightly slumped banks of the tributary associated with metabasic lithologies, and serpentinite expose friable greenschists of the Pitajones nappe, which me´langes with blocks of eclogite in serpentinite matrix can be interpreted to represent the in situ nappe unit. These have been described (Schneider et al., 2004; Garcı´a- rocks contain the typical greenschist assemblage of chlor- Casco et al., 2006; Stanek et al., 2006). Therefore this ite, actinolite, epidote, albite and , quartz, nappe contains both rocks of a subducted passive margin white mica, . Characteristic porphyroblasts of and fragments of subducted oceanic lithosphere (see albite can impart a typical spotted appearance. No 220 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter clinopyroxene or blue amphibole is observed. In contrast, KAPPA optoelectronics GmbH) with a high sensitivity at the subangular to rounded cobbles filling the dry stream low light conditions, requiring only 5–10 s per exposure. bed present tough, massive, fine-grained, grey- to bluish- At a resolution of 1300 1030 points a maximum of 10 green rocks. Thin (1.5 mm) light-coloured veinlets irre- frames/s is available in progressive scan mode. gularly cross-cutting the rocks, light-coloured diffuse The TEM samples were cut from standard petrographic blotches and scattered small dark crystals 1.5mm in thin sections, using 2.3 mm copper discs for support, and diameter are the most conspicuous macroscopic feature. ion-beam thinned. They were then examined in a JEOL According to Milla´n (Personal communication, 1994), JEM 100CX microscope operating at 100 kV. these were the rocks described by Milla´n & Somin (1981) as jadeitites. They are the subject of this report. Based on mapping and structural analysis (Stanek et al., 2006), the source of these blocks is assumed to be the 5. Petrography overlying Gavilanes nappe, which in this area represents a klippe-like remnant (Fig. 1). No in situ exposures of these Mineral abbreviations used in this paper follow the rocks were found. Although lawsonite is known from sev- updated compilation of Fettes & Desmons (2007): Ab ¼ eral localities in the Gavilanes unit (Grevel, 2000), and albite, Act ¼ actinolite, Amp ¼ amphibole, An ¼ described by Milla´n & Somin (1981) from their jadeite- anorthite, Aug ¼ augite, Bt ¼ biotite, Chl ¼ chlorite, Czo bearing samples, no lawsonite was found in the present ¼ clinozoisite, Di ¼ , Ep ¼ epidote, Fe2-Prg ¼ suite of samples. ferropargasite, Gl ¼ glaucophane, Jd ¼ jadeite, Mg-Ktp ¼ magnesiokatophorite, Ne ¼ nepheline, Omp ¼ omphacite, Pmp ¼ pumpellyite, Qtz ¼ quartz, Tmt ¼ taramite, Ttn ¼ titanite 4. Analytical methods Thin-section observation confirms the suggestion of Milla´n & Somin (1981) that these rocks represent former Bulk rock chemistry was obtained by X-ray fluorescence basic magmatic intrusives. The macroscopically recogniz- with a Philips PW 1400 spectrometer. Uncertainties are able dark crystals are seen to be relict magmatic augite estimated to be 1 % for major and 10 % for minor with recognizable ophitic texture (Fig. 2a). Sample M592 elements of importance. FeO was determined potentiome- is an example with abundant augite relics (Fig. 2b), trically (Ungethu¨m, 1965), and Fe O by difference. H O whereas sample M591 appears to represent a more highly 2 3 2 overprinted variety with only a few remnants of augite and and CO2 were analyzed coulometrically (Johannes & Schreyer, 1981). Microprobe analyses were obtained with a Cameca SX 50 at 15 kV acceleration voltage and 10 nA beam current, 20 s counting time (10 s on background). High-resolution element distribution maps of small sample areas were obtained in beam-scan mode at 15 kV, 40 nA, and 140 ms counting time per pixel. Data correction followed the PAP procedure (Pouchou & Pichoir, 1984). The standards employed were natural for Si, Al, Mg; natural spessartine for Mn; natural jadeite for Na; natural rutile for Ti; synthetic andradite for Fe, Ca; synthetic potassium silicate and barium silicate glass for K and Ba, respec- tively; synthetic oxides of Cr, Ni, Cu, and Zn for the respective elements. Calculation procedures for mineral formulae were as follows: pyroxene – 6 and 4 cations; sodic and sodic-calcic amphibole – 23 oxygens and 13 cations without Ca,Na,K; actinolite – 23 oxygens and Fetot ¼ Fe2þ; epidote-group – 12.5 oxygens tot 3þ tot 2þ and Fe ¼ Fe ; chlorite – 28 oxygens and Fe ¼ Fe ; Fig. 2. (a) Augite relic with conspicuous ophitic texture. Former tot 2 biotite – 22 oxygens and Fe ¼ Fe þ; pumpellyite – 8 plagioclase laths now consist of jadeite þ omphacite þ pumpellyite cations without hydrogen. (M592; XPL; width of image ¼ 750 mm). (b) Microphoto of M592 The cathodoluminescence (CL) examinations were done showing augite relics in a fine-grained matrix consisting mainly of using a ‘‘hot cathode’’ CL microscope (type HC1-LM) jadeite þ omphacite þ pumpellyite þ chlorite þ epidote (XPL; width of image ¼ 1250 mm). (c) Microphoto of M591 showing developed at the Ruhr-University Bochum (Neuser, large glaucophane with frayed edges and chlorite þ calcic 1995). This device allows a comparative investigation of amphibole alteration. The felty matrix consists mainly of glauco- thin sections using transmitted light and an electron beam. phane þ jadeite as well as omphacite þ chlorite þ epidote (PPL; We employed a beam energy of 14 keV and a beam current width of image ¼ 280 mm). (d) Microphoto of M592 with jadeitite density of 9 mA/mm2 on the sample surface. The pictures veinlets consisting of jadeite þ omphacite þ chlorite (XPL, width of were taken with a digital camera system (DX30 C from image ¼ 3.3 cm). Jadeite formation in metabasic rock 221 with the characteristics of a blueschist (Fig. 2c). As described below, both samples are characterized by abun- dant examples of disequilibrium textures. Grain boundaries are complex and irregular and replacement textures con- trolled by the crystal chemistry of precursor minerals are common. Both samples exhibit irregular cross-cutting vein- lets up to 1.5 mm in thickness rich in jadeite (Fig. 2d). The following descriptions will focus on these two samples. Because of the very fine-grained nature of these rocks, the optical microscope by itself is not a sufficient tool for a petrographic description, and therefore the results from qua- litative microprobe data and cathodoluminescence micro- scopy (e.g., Schertl et al., 2004) will be integrated into this section. Augite relics up to 1.5 mm in size constitute a major component of sample M592 (Fig. 2a, b). The rock matrix between the augite relics is composed of finely and irregu- larly intergrown jadeite, omphacite, pale pumpellyite and chlorite with complex grain boundaries. Idiomorphic crys- tals of epidote are found scattered throughout the matrix and also in distinct epidote-chlorite clusters. The latter tend to occur where augite relics are less abundant, suggesting that these clusters may have evolved from the breakdown of magmatic pyroxene. Minor albite appears to be a late secondary product. Titanite is an accessory phases. Rare grains of quartz are found in the matrix between the augite relics with no obvious spatial relationships to any of the other matrix minerals. The most distinctive feature of this rock is the replacement of magmatic augite and of the plagioclase laths of the ophitic intergrowth by jadeite and omphacite (Fig. 3c–e). Augite, jadeite and omphacite are in optical continuity (Fig. 2a). Such topotactic replacement of magmatic augite by omphacite was already documented by electron microscopy in a blueschist from Turkey (Carpenter & Okay, 1978). Petrographic descriptions exist from other localities (e.g., Essene & Fyfe, 1967; Black, 1974). In the present case it is noteworthy that the sodic pyroxene topotactically replacing augite is in optical continuity not only with the magmatic pyroxene, but also with sodic pyroxene replacing plagioclase, the latter case then representing epitactic growth on the augite/sodic-pyr- oxene pseudomorph. Furthermore, in sample M 592 omphacite only predominates where augite grains are altered along cleavage planes and fractures from within (Fig. 3b). Jadeite predominates in the former plagioclase laths, with omphacite and associated pumpellyite generally only patchily developed at jadeite grain boundaries. The original contacts between augite and plagioclase are pre- served, thus mimicking and preserving the original ophitic texture (Fig. 3c, d). Irregular aggregates of jadei- Fig. 3. Back-scattered electron (BSE) images: (a) Matrix of sample teþomphacite and minor pumpellyite encroach upon the M591, indicating complex irregular intergrowth of constituent miner- augite crystal along an irregular reaction front from the als; (b) Augite relic with predominant omphacite alteration (M592); rock matrix (Fig. 3e), which was presumably plagioclase- (c, d) Jadeite-dominated replacement of plagioclase mimicking igneous ophitic texture and also encroaching on augite from rich in the original metabasic rock. Quartz has not been rock matrix at the right-hand side of the image in d (M592); found associated with jadeite growth from the augite relics (e) Remnants of zoned augite (note variations in BSE brightness in or the enclosed plagioclase laths. Late blue-green sodic- augite due to primary variations in Mg/Fe2þ ratios – see Fig. 7) with calcic amphibole and chlorite replace the complex augite Jd-rich replacement of plagioclase inclusion in augite (right-center), pseudomorphs, with amphibole growth again controlled by Jd þ Omp replacement surrounding both augite remnants and ompha- the pyroxene structure. The crystallographic c-axes of both cite-dominated replacement from within grain to the left (M592). 222 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter phases are parallel and amphibole growth occurs preferen- predominate, suggesting compositional differences, but tially in this direction. Augite is preferentially affected, but the distribution is very irregular (Fig. 4a, b). The mottled amphibole may also replace topotactically intergrown appearance is likely due to the finely intergrown ompha- sodic pyroxene. A younger jadeite þ omphacite þ chlorite cite, which should not show optical CL due to the presence assemblage fills the macroscopically evident fractures of acting as a quencher element. Jadeite in the matrix cross-cutting the rock (Fig. 2d). These can even be seen of sample M591 is much more homogeneous in colour, to dissect the ‘‘ophitic’’ augite relics. No obvious interac- with greenish hues predominating (Fig. 4c). In contrast, tion between these vein fillings and the rest of the rock is jadeite growing in the veinlets cross-cutting the rock are evident. coarser, forming interlaced sheaves (Fig. 4d). The CL col- The more bluish macroscopic colour of sample M591 is ours are homogeneous, with a violet to blue growth genera- due to abundant glaucophane, both as xenomorphic discrete tion clearly discernible from a green one (Fig. 4e, f). crystals and together with intimately intergrown jadeite and Although the two are in places intimately intergrown, the omphacite to constitute the bulk of the felty rock matrix (Fig. green generation appears younger, being concentrated in 2c). TEM analysis (see below) indicates that glaucophane has the centre of the veins and growing at the terminations of to a large extent replaced earlier magmatic augite and sodic violet-blue sheaves (Fig. 4f). pyroxene. Chlorite and hypidiomorphic to xenomorphic epi- dote-group minerals are also present. The back-scattered electron microprobe image of Fig. 3a illustrates the pervasive 6. Characterization by transmission electron lack of an equilibrium fabric. Grain boundaries are complex microscopy (TEM) and irregular intergrowth textures are wide-spread. Omphacite and jadeite are finely intergrown, but omphacite The very fine scale of the disequilibrium and replacement also tends to form thin, irregular rims at jadeite crystal mar- features observed optically with the polarizing microscope gins, suggesting that omphacite replaced earlier jadeite. suggests that further more detailed characterization by Glaucophane crystal edges are frayed and show reaction to transmission electron microscopy (TEM) is called for. younger chlorite and calcic amphibole. Albite also appears to The pyroxene in M591 has been partially replaced by be a late secondary product filling interstices in the matrix. amphibole giving complex microstructures, the abundance Titanite, biotite, apatite, chalcopyrite and pyrite are accessory of which suggests that this sample at one time contained phases. Only a few grains of quartz have been identified in the much more pyroxene than at present. Some of the pyroxene rock, with no obvious preferred spatial relationship to any of is certainly C-face-centered, i.e., magmatic augite and/or the other phases. Only minor relics of magmatic augite , 10 metamorphic jadeite. Some of the pyroxene may be P-type mm in size are observed. ordered omphacite, but it is not possible to be conclusive Cathodoluminescence microscopy (Fig. 4a–f) reveals about this on the basis of electron diffraction data alone, that jadeite replacing ophitic augite and jadeite growing because of overlapping reflections from P omphacite and in the matrix of sample M592 show very similar CL char- C amphibole. Abundant mixed layer chlorite can be acteristics. Greenish and reddish to violet hues identified.

Fig. 4. Cathodoluminescence (CL) images. (a) Relict augite crystal (cf., Fig. 2a) with luminescing jadeite-rich laths after plagioclase in augite with no CL (M592, width of image ¼ 1.3 mm). (b) CL overview of M592 with augite relics (black) and luminescing jadeite-rich matrix (width of image ¼ 2.8 mm). (c) Green luminescence of jadeite in M591 rock matrix (width of image ¼ 2.8 mm). (d, e) Same view of a jadeite- rich veinlet in M592 under crossed polarizers (d) and in CL mode (e) (width of image ¼ 2.8 mm). (f) CL image of jadeite-rich veinlet in M592 showing zoned crystals (width of image ¼ 1.3 mm). Jadeite formation in metabasic rock 223

In sample M592, TEM study confirms that C-face-cen- tered augite, C-face-centered jadeite and P omphacite have the same crystallographic topotactic relationship, i.e., these give identically oriented diffraction pat- 1 µm terns. The textures observed between these three pyrox- enes are very similar to those described by Carpenter & Okay (1978) in a low-temperature ( 350 C) blueschist sample from Turkey, where augite was replaced by aegir- ine-rich omphacite. In the present case, the replacement mechanism of augite by omphacite and/or jadeite also appears to have been the formation of veins at an advan- cing front through each crystal. Dark-field images obtained with hþk odd reflections from the omphacite regions show fine-scale antiphase domain textures with rather variable antiphase boundary distributions (Fig. 5). In diffraction patterns from these areas, the hþk odd reflections are weak and slightly diffuse; there appears to be some inten- sity at positions violating the n-glide of the expected P2/n . This observation would be in line with the Fig. 6. Bright-field image of C-centered pyroxene (i.e., augite or Turkish example, where Carpenter & Okay (1978) showed jadeite) with fine-scale spinodal exsolution texture in two orienta- that omphacite originally formed as a metastable disor- tions. Sample M592. dered phase with probable C2/c symmetry. Ordering, and the C ! P lattice transformation, then proceeded irrever- miscibility gap during cooling. Exsolution in jadeite-rich sibly via the metastable intermediate P2/c and P2 struc- crystals during slow cooling of the Dabie ultra-high pres- tures before attaining stable ordered P2/n symmetry. sure terrain has been reported by Wu et al. (2002), but the In sample M592 an exsolution texture was also observed textural relationship between the coexisting phases was not in some areas of C-type pyroxene that resembles the early described. In sample M592 the amplitude of the composi- stages of spinodal decomposition (Fig. 6). Without addi- tion modulations is low, as indicated by the low contrast in tional analytical data it is not possible to decide whether Fig. 6, so that the exsolution process has been caught at an this unusual feature is found in the magmatic augite relics earlier stage than is represented by the spinodal textures in or in jadeite-rich pyroxene. Both possibilities would repre- omphacite described by Carpenter (1980). sent rare examples. Exsolution is conceivable within an original magmatic augite that has just cooled through the limb of the quadrilateral pyroxene solvus (e.g., Lindsley, 1980). Alternatively, it could be occurring within a jadeite- 7. Mineral chemistry rich crystal which entered the jadeite-omphacite Representative mineral analyses are given in Table 1.

7.1. Pyroxene

The compositions of the magmatic and metamorphic pyrox- enes in samples M591 and M592 are summarized in Fig. 7 and 8 according to the classification scheme of Morimoto et al. (1988). Pyroxenes classified as ‘‘Quad’’ are shown in 1 µm both figures. The magmatic relics in M592 are augite with a primary magmatic zonation. Homogeneous cores and more Mg-rich homogeneous rims are separated by an abrupt transition. Augite chemistry is typical for ocean- floor or volcanic-arc (Nisbet & Pearce, 1977). There is a slight tendency towards diopside in Fig. 8. Although some mixed analyses due to the pervasive fine- scale alteration to jadeite and omphacite cannot be ruled out, this shift reflects compositional equilibration toward omphacitic compositions (e.g., Fig. 3), and in the Quad- Jadeite- plot of Fig. 8 there is a clear spread of analyses away from typical magmatic compositions Fig. 5. Dark-field image (hþk odd reflection) showing antiphase toward jadeite and aegirine. This effect is even more domains in a region of omphacite surrounded by C-centered pyrox- obvious in the few very small , 10 mm relics that could ene (black). Sample M592. be measured in sample M591. In the ‘‘Quad’’ projection of 224

Table 1. Representative microprobe analyses.

Mineral Auga Jda Ompa Amphibole b c b b b Analysis 11302/18 11153/82 11306/9 11335/55 11303/4 11307/16 1971229c/10:Gln 11305/8:Act 11303/6: Mg-Ktp 11335/47 Tmt 11335/46 Fe2-Prg Sample M591 M592 M591 M592 M591 M592 M591 M591 M591 M592 M592 Carpenter M.A. Schertl, H.-P. Stanek, K.P. Grevel, C. Maresch, W.V. SiO2 52.08 53.11 57.54 58.35 55.52 55.23 56.04 53.11 45.80 39.15 40.19 TiO2 0.04 0.31 0.52 0.14 0.13 0.22 0.44 0.25 0.29 0.30 0.10 Al2O3 1.99 1.51 18.70 24.78 9.69 10.81 9.43 3.66 10.06 17.08 13.71 Cr2O3 0.00 0.14 0.05 0.07 0.00 0.02 0.09 0.00 0.06 0.06 0.07 Fe2O3 4.21 0.93 2.98 0.00 4.19 4.63 3.02 0.00 3.31 4.12 3.64 FeO 6.45 4.92 1.07 0.77 2.88 3.94 10.21 11.99 13.01 16.35 18.36 MgO 10.66 18.55 2.39 0.16 7.35 5.39 9.26 14.69 10.84 5.61 6.27 MnO 0.42 0.15 0.00 0.04 0.21 0.20 0.30 0.34 0.32 0.19 0.30 CaO 22.18 19.47 4.02 0.54 12.43 10.87 1.08 11.41 9.36 8.14 9.97 Na2O 1.69 0.15 12.68 14.59 7.42 8.32 7.02 1.89 4.23 5.52 4.22 K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.22 0.33 0.28 0.37 Total 99.72 99.24 99.94 99.44 99.82 99.62 96.93 97.56 97.61 96.80 97.20 Si 1.955 1.949 1.991 1.983 1.993 1.992 Si 7.882 7.687 6.779 5.998 6.207 AlIV 0.045 0.051 0.009 0.017 0.007 0.008 AlIV 0.118 0.313 1.221 2.002 1.793 2.000 2.000 2.000 2.000 2.000 2.000 8.000 8.000 8.000 8.000 8.000 AlVI 0.043 0.015 0.754 0.976 0.403 0.452 AlVI 1.446 0.311 0.534 1.082 0.702 Ti 0.001 0.009 0.014 0.004 0.003 0.006 Ti 0.046 0.027 0.032 0.034 0.012 Cr 0.000 0.004 0.001 0.002 0.000 0.001 Cr 0.010 0.000 0.007 0.007 0.009 Fe3þ 0.122 0.026 0.078 0.000 0.113 0.126 Fe3þ 0.320 0.000 0.372 0.475 0.423 Mg 0.596 1.015 0.123 0.008 0.393 0.290 Mg 1.941 3.169 2.392 1.281 1.444 Fe2þ 0.208 0.151 0.031 0.022 0.087 0.119 Fe2þ 1.201 1.451 1.624 2.095 2.371 Mn 0.013 0.005 0.000 0.001 0.006 0.006 Mn 0.036 0.042 0.040 0.025 0.039 Ca 0.892 0.766 0.149 0.020 0.478 0.420 5.000 5.000 5.000 5.000 5.000 Na 0.123 0.011 0.851 0.968 0.516 0.582 Ca 0.163 1.770 1.484 1.337 1.650 2.000 2.000 2.000 2.000 2.000 2.000 NaB 1.837 0.230 0.516 0.663 0.350 2.000 2.000 2.000 2.000 2.000 NaA 0.078 0.300 0.698 0.977 0.913 K 0.008 0.040 0.062 0.054 0.072 0.086 0.340 0.760 1.031 0.985 Table 1.

Mineral Czod Epidoted Chloritee Biotitef Pmpg Analysis 11302/16 11302/17 11335/76 11306/3 Al-poor 11391/2 Al-rich 11154/1 Al-poor 11151/9 Al-rich 11306/10 11499/32 Sample M591 M591 M592 M591 M591 M592 M592 M591 M592 SiO2 39.00 38.06 38.13 28.37 25.40 28.00 25.29 37.61 37.35 TiO2 0.08 0.06 0.25 0.03 0.03 0.01 0.08 0.30 0.03 Al2O3 32.85 27.98 26.47 17.22 20.73 17.08 20.47 14.09 26.79 Cr2O3 0.00 0.00 0.00 0.08 0.00 0.00 0.02 0.00 Fe2O3 1.09 7.59 9.30 FeO 22.45 23.35 23.68 25.41 18.96 4.47 rock metabasic in formation Jadeite MgO 0.00 0.00 0.02 18.79 16.13 17.18 15.26 13.43 1.46 MnO 0.25 0.17 0.03 0.38 0.34 0.26 0.34 0.12 0.09 CaO 24.59 24.42 24.34 0.04 0.17 0.14 0.13 0.29 22.07 Na2O 0.03 0.04 0.04 0.00 0.25 0.86 K2O 0.02 0.00 0.05 0.00 9.47 0.02 Total 97.86 98.28 98.54 87.40 86.19 86.45 86.99 94.52 93.14 Si 2.971 2.963 2.979 Si 5.905 5.405 5.936 5.392 Si 2.890 Si 2.980 AlIV 0.029 0.037 0.022 AlIV 2.095 2.595 2.064 2.608 AlIV 1.110 Ti 0.002 3.000 3.000 3.000 8.000 8.000 8.000 8.000 4.000 Al 2.519 AlVI 2.921 2.531 2.416 AlVI 2.130 2.605 2.204 2.535 AlVI 0.166 Mg 0.174 Cr 0.000 0.000 0.000 Ti 0.005 0.005 0.001 0.013 Ti 0.017 Fe2þ 0.298 Fe3þ 0.063 0.445 0.546 Cr 0.013 0.000 0.000 0.003 Cr 0.000 Mn 0.006 2.984 2.976 2.962 Mg 5.831 5.118 5.431 4.850 Mg 1.538 5.979 Ti 0.004 0.004 0.014 Fe2þ 3.908 4.155 4.199 4.530 Fe2þ 1.218 Ca 1.886 Mg 0.000 0.000 0.003 Mn 0.067 0.061 0.047 0.062 Mn 0.008 Na 0.134 Mn 0.016 0.011 0.002 Ca 0.009 0.038 0.033 0.029 2.948 K 0.002 Ca 2.008 2.037 2.037 Na 0.012 0.016 0.016 0.000 Ca 0.024 2.022 2.028 2.052 2.056 K 0.005 0.000 0.014 0.000 Na 0.037 11.980 11.998 11.944 12.022 K 0.928 0.989

Act, actinolite; Aug, augite; Czo, clinozoisite; Fe2-Prg, ferropargasite; Gln, glaucophane; Jd, jadeite; Mg-Ktp, magnesiokatophorite; Omp, omphacite; Pmp, pumpellyite;Tmt, taramite. Normalization procedures for formula calculation: a6 oxygens and 4 cations; b23 oxygens and 13 cations without Ca,Na,K; c 23 oxygens and Fetot ¼ Fe2þ; d 12.5 oxygens and Fetot ¼ Fe3þ; e 28 oxygens and Fetot ¼ Fe2þ; f22 oxygens and Fetot ¼ Fe2þ; g8 cations without hydrogen. ( Continued 225 ) 226 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter

Ca2Si2O6 topotactic alteration products of augite, as matrix compo- nents or as vein fillings. Figures 9 and 10 indicate that with time irregular jadeite/omphacite intergrowths may recrys- tallize to take on a quasi-equilibrium fabric with regular grain boundaries. Analyses of coexisting sodic pyroxene pairs from settings such as in Fig. 10 probably represent the M 591 M 592 closest approach to local equilibrium available. Two such ‘‘quasi-equilibrium’’ pairs are indicated in Fig. 8. For M591 these are two omphacite compositions correspond- Diopside ing to Jd72Di15Hd5Aeg8 and Jd43Di35Hd11Aeg11. For cores M592 a jadeite s.s. with composition Jd Di Hd Aeg rims 81 9 2 8 coexists with an omphacite corresponding to Augite Jd45Di34Hd14Aeg7 (Fig. 8). Figure 9 also demonstrates that although the contacts between augite relics and the jadeite-rich lamellae are generally sharp, small embay- ments of omphacite are seen to grow locally on both Clinoenstatite Clinoferrosilite sides of the contact. Fe Si O If equilibrium can be assumed, coexisting sodic pyrox- Mg Si O 2 2 6 2 2 6 enes offer the possibility of using the solvus to obtain Fig. 7. Classification of relict magmatic pyroxenes according to information on the temperatures of equilibration. In their Morimoto et al. (1988). Note the distinct core-rim zonation in mag- detailed study of coexisting sodic pyroxenes in jadeite jades matic pyroxenes of sample M592. of the Sierra del Convento in Cuba, Garcı´a-Casco et al. (2009) showed that the sodic pyroxene populations there Quad follow linear trends from almost pure jadeite to fictive end- 100 member compositions between Di90Hd5Aeg5 and 10 90 Di80Hd10Aeg10. These trends are shown in projection in

20 the Quad-Jd-Aeg diagram of Fig. 8. Although there is con- 80 siderable scatter of sodic pyroxene compositions in Fig. 8, B C 30 70 A especially in jadeite, a similar tendency appears indicated in 40 Fig. 8, so that in Fig. 11 the two ‘‘quasi-equilibrium’’ sodic 60 pyroxene pairs of Fig. 8 are compared to the pseudobinary 50 50 M 591 T-X sections calculated by Garcı´a-Casco et al. (2009). M 592 60 Notwithstanding the considerable approximations involved, 40 the M592 pair suggests equilibration temperatures of 70 30 Aegirine- 475–500 C. The M591 omphacite suggests 425–450 C, Omphacite Augite 80 whereas the coexisting jadeite-rich pyroxene offers no plau- 20 sible fit to any of the pseudosections. This composition does, 90 10 Jadeite Aegirine however, also deviate considerably from the assumed linear 100 Jd to Di-Hd-Aeg trend (Fig. 8).

100 90 80 70 60 50 40 30 20 10 Jadeite Aegirine 7.2. Other minerals Fig. 8. Classification of all analyzed pyroxenes according to Morimoto et al. (1988). Connected large open circles: ‘‘quasi-equi- librium’’ sodic-pyroxene pair in M591. Connected large filled As a general feature, amphibole dominates over pyroxene squares: ‘‘quasi-equilibrium’’ jadeite-omphacite pair in M592. in M591, but is only a minor constituent in M592. Following Lines A, B, C represent, as discussed in the text, the projections of the classification scheme of Leake et al. (1997), the amphi- linear compositional trends from end-member jadeite to boles of sample M591 are Al-rich glaucophane, i.e., Al- Di90Hd5Aeg5, Di80Hd10Aeg10 and Di60Hd20Aeg20, respectively. richer than the ‘‘crossite’’ of classical usage. Occasional rims of magnesiokatophorite or edenite (i.e., straddling the Fig. 7 these plot as diopside compositions, but in Fig. 8 boundary of the calcic to sodic-calcic amphibole groups) are they are seen to be quite Na- and Al-rich. observed. Secondary actinolite replaces glaucophane at Possible problems of mixed analyses are also evident frayed crystal edges. Late amphibole replacing augite in from Fig. 8, and are a logical consequence of the intimate sample M592 is taramite of the sodic-calcic group, but Ca- intergrowths observed between jadeite and omphacite contents lie between 1.3 and –1.5 apfu, and gradation to a (Fig. 3, 9, 10). Although there are compositional clusters ferroparagasite composition is observed. conforming to jadeite and omphacite, intermediate ana- Defining clinozoisite as Ca2Al3Si3O12(OH) and epidote 3þ lyses are found. Pyroxenes reach Jd99 in sample M592, as Ca2Al2Fe Si3O12(OH) (e.g., Armbruster et al., 2006), but only Jd79 in M591. No systematic compositional trends then the zoned epidote-group crystals of sample M591 are could be identified for jadeite and omphacite occurring as clinozoisite with epidote contents ranging from 6 mol% in Jadeite formation in metabasic rock 227

Fig. 9. Element distribution maps obtained in beam-scan mode (sample M592, image dimensions 82 82 mm; 256 256 pixels; colour scale bar indicates total counts per pixel). Detail of augite grain in contact with matrix and with a plagioclase lamella pseudomorph (lower left). Note irregular jadeite/omphacite intergrowth (cf. Fig. 10). the cores to 45 % in the rims. The idiomorphic epidote- The Tschermak’s substitution accounts for most of the group minerals in sample M592 are very homogeneous, compositional variability observed (Fig. 12). No systema- with epidote contents ranging from 54 to 68 mol%. tic trends with respect to mode of occurrence (associated Considering that amphibole and pumpellyite (see minerals, matrix vs. late alteration of glaucophane, etc.) below) compositions are generally quite homogeneous in could be found in either sample. Chlorite is Mg-dominant, each sample, it is noteworthy that the compositions of the as expected for metabasic rocks (Zane et al., 1998), with ferromagnesian mineral chlorite are not. Different genera- chlorites from sample M591 clustering at higher Mg/ tions of growth or domain equilibria could be a reason. As (MgþFe) values than those from M592. This difference outlined in the following section, there is clear evidence is actually the opposite of what might be expected from the that the protolith was affected by spilitization prior to bulk Fe and Mg contents of the two samples. subduction, and a first generation of chlorite could already Pumpellyite is found only in sample M592, both in the have formed at this time. TEM observations also suggest rock matrix and in the pseudomorphs after ophitic augite, that mixed-layer chlorite is abundant, so that EMPA results where it is closely associated with omphacite. The composi- of microscopically identified chlorite should be interpreted tion is homogeneous and Al-rich (Table 1). Following Hatert with some caution. Nevertheless, in almost all cases AlIV et al. (2007,2008), a typical mineral formula can be written as VI 3þ 2þ (Al þ 2Ti þ Cr), so that octahedral vacancies or Fe (Ca1.89Na0.13)2.02(Al0.52Fe 0.30Mg0.17Mn0.01)1.00Al2.0(SiO4) contents appear to be negligible (e.g., Zane et al., 1998). (Si2O7)(OH)2.450.55H2O. 228 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter

Fig. 10. Element distribution maps obtained in beam-scan mode (sample M592, image dimensions 82 82 mm; 256 256 pixels; colour scale bar indicates total counts per pixel). Detail of jadeite-omphacite intergrowth with beginning development of regular grain boundaries suggestive of approach toward an equilibrium fabric (cf. Fig. 9).

Al always slightly exceeds 50 % of the M(1) site, thus elsewhere in the rock appears to be even more intimately defining the composition unequivocally as pumpellyite-(Al) tied to omphacite, suggesting that pumpellyite may be an (see discussion in Hatert et al. 2008). Within analytical alteration product of this phase throughout the rock. Such a error, no Fe3þ is required to maintain pumpellyite stoichio- possibility places critical constraints on the metamorphic metry. The composition and homogeneity of pumpellyite evolution of the rock as discussed further below, are significant features for a metabasic rock. Although the Plagioclase in both samples is almost pure albite formation of Al-rich pumpellyite seems to be favoured (Ab99An01). Rare small flakes of biotite (,10 mm in toward higher pressures into the blueschist facies and to size) in sample M591 are slightly Mg-dominant phlogo- higher temperatures transitional to the greenschist facies pite-annite solutions displaced approximately 20–25 (Cortesogono et al., 1984, and references therein), these mol% toward siderophyllite-eastonite (Rieder et al., 1998). trends are often obscured by the local chemical environ- ment. Relict plagioclase as a precursor phase offers such a favourable local environment for the growth of Al-rich 8. Bulk rock composition pumpellyite (Cortesogono et al., 1984, and references therein). In sample M592 Al-rich pumpellyite is found in Available chemical data on samples M591 and M592 are such plagioclase pseudomorphs, but its occurrence there and listed in Table 2. Schneider et al. (2004) have presented Jadeite formation in metabasic rock 229

800 0.65

Omp 0.60 Diopside Jd M591 Pn2/ 2/ 700 Cc2/ Cc 0.55

T (°C) 0.50 Mg/(Mg+Fe) M592 600 0.45

0.40 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 Al(IV)-2(apfu) 500 M592 Fig. 12. Compositional variation of chlorites in terms of Mg/ M591 (MgþFe) vs. the Tschermak’s substitution (referred to ideal 2þ ? (Mg,Fe )6Al2(Si6Al2)O20(OH)16). 400

elements, but Zr, Cr, and V conform to the isocon; Ba is enriched with respect to N-MORB, but contents are still 300 low. All these features could be the result of hydrothermal 0.00 0.20 0.40 0.60 0.80 1.00 ocean-floor alteration processes (e.g., Staudigel, 2003), Diopside fluid-driven metasomatic exchange during subduction (A) XJd Jadeite (B) and high-pressure (e.g., Sorensen et al., (C) 1997; Bebout, 2007), or a combination of both. Analyzing these trends with the data at hand is not straight- Fig. 11. Comparison of the two ‘‘quasi-equilibrium’’ pyroxene pairs forward. Following Sorensen et al. (1997), we note that of Fig. 8 with phase relations calculated for the binary join Di-Jd although the Ba contents and Ba/TiO2 ratios (Table 2, Fig. (Green et al., 2007) and various T-X locations of the omphacite- 13) are somewhat higher than in pristine N-MORB, this jadeite solvus limbs as calculated by Garcı´a-Casco et al. (2009) for enrichment is still in the general range of subduction-zone pseudobinary trends Di0.9Hd0.05Aeg0.05 (curves A), Di0.8Hd0.1Aeg0.1 (curves B) and Di0.6Hd0.2Aeg0.2 (curves C). A, B and C correspond MORB protolith and can be explained by an at most 1 % to the projected trends shown in Fig. 8. A and B taken from Fig. 7 of contamination of a sediment-equilibrated subduction-zone Garcı´a-Casco et al. (2009), C interpolated from the results of Garcı´a- fluid (Sorensen et al., 1997). Casco et al. (2009) and the phase relations Jd-Di-Aeg of Green et al. Although K O should be positively correlated with Ba 2 (2007) at 500 C. during low-temperature hydrothermal alteration of oceanic crust (Staudigel, 2003) or syn-subduction metasomatic pro- cesses (Sorensen et al., 1997; Bebout, 2007), this is not much more extensive major, trace element, REE as well as observed in samples M591 and M592. However, potassium Nd and Sr isotope data for several metabasites () is leached from basalts by hydrothermal high-temperature from the Gavilanes unit, from which samples M591 and alteration reactions (Staudigel, 2003), and we suggest that M592 are considered to have been derived. Schneider et al. the large increase in Na2O and decrease in CaO is a dis- (2004) conclude that metabasites in the Gavilanes unit are tinctive chemical characteristic produced by classical pro- of two distinct types. Those found in a serpentinite-me´l- cesses of spilitization of a MORB protolith (e.g., Graham, ange context derive from a MORB-type protolith. Other 1976; Fettes & Desmons, 2007), where magmatic plagio- samples from intercalations within metasedimentary rocks clase is albitized and the ferromagnesian minerals are lar- appear to possess a distinctly different calc-alkaline affi- gely replaced by chlorite. The magmatic fabric of the rock is nity. The Al2O3 contents of samples M591 and M592 preserved. It is also important to note that the increase in compare well with the 14.84 wt% of the MORB-type Na2O is not accompanied by a similarly significant increase sample studied by Schneider et al. (2004) and are distinctly in Al2O3, so that no net gain of a ‘‘jadeite component’’ with lower than the Al2O3 contents (20.12 and 20.16 wt%) of respect to N-MORB can be seen in M591 and M592. The the calc-alkalic types, suggesting a MORB-type protolith effect of the thin jadeitite veinlets on the bulk composition for the present samples as well. of the rock appears to be negligible. In Fig. 13 the bulk rock compositions of M591 and We conclude from the data on bulk chemistry that the M592 are compared to average N-MORB, using an isocon protolith of M591 and M592 was spilitized chlorite-rich plot adapted from Grant (1986). For both samples the ocean-floor , with abundant albitized plagioclase major oxides Al2O3, SiO2, MgO and FeO* all lie on or available as a precursor mineral for jadeite formation during close to a 1:1 isocon, i.e. suggesting relatively constant subduction metamorphism via local domain reequilibration. mass during any modification of a presumed N-MORB Although the assumption of a spilite protolith subjected to protolith. In both samples Na2O is highly enriched with essentially isochemical recrystallization provides a bulk- respect to N-MORB and CaO is clearly lower, as are TiO2, compositional template for exploring isochemical pres- P2O5, and K2O. There is considerable scatter in some trace sure-temperature (pseudosection) phase diagrams, this 230 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter

Table 2. Major (wt%) and selected minor (ppm) element concentra- tions for samples M591 and M592. 100 M 591 Cu M 592 B M591 M592 Zn Zn loss Zr SiO2 49.65 49.24 5 Al O 0.5 x N-MOR 23 TiO2 1.04 1.23 Ni/2 Ni/2 Al2O3 15.01 14.84 Fe2O3 3.16 2.92 5CaO 2Y FeO 6.12 6.46 Cr/5 constant mass MnO 0.21 0.15 SiO Cr/5 50 Sr/2 2 MgO 6.62 7.60 Co CaO 9.07 9.07 5MgO Sr/2

N-MORB Na O 6.11 6.38 200K2 O 2 -MORB K2O 0.05 0.01 2 x N 20TiO2 3FeO* P2O5 0.09 0.00 þ H2O 2.04 2.39 V/10 10Na2 O CO2 0.26 0.14 100P25 O gain Sum 99.43 100.43 Ba Ba 47 64 Co 69 69 0 Cr 126 277 0 50 100 Cu 70 80 Ga 15 12 jadeite-bearing metabasalts Nb 5a 3a Ni 52 106 Fig. 13. Isocon diagram (Grant, 1986) comparing element concen- a a trations of samples M591 and M592 (Table 2, anhydrous basis) to N- Pb 9 4 Rb 9a 12 MORB. Major-element oxides are given in wt% oxide and trace Sn 6a 9a elements in ppm. Scaling factors are indicated. N-MORB data are taken from Basaltic Volcanism Study Project (1981) and Sun & Sr 194 64 McDonough (1989). V 282 258 Y 34 37 Zn 33 76 Zr 69 72 former plagioclase. Although omphacite growth may have

a been in part coeval with jadeite, it is generally concentrated Note: Value within uncertainty limits. in the interstices between jadeite grains and forms rims on these. The internal omphacite-dominated replacement of augite grains is also inferred to post-date the early jadeite- approach has not been successful. Abundant partially altered dominated replacement front encroaching on augite from relict phases at all stages of metamorphic development, the rock matrix. In the latter, finely and irregularly inter- especially of magmatic augite in M592, make it difficult to grown jadeite, omphacite, pumpellyite, chlorite and epi- quantify effective bulk compositions during pressure-tem- dote are found together, but these phases can not represent perature evolution. If the bulk rock compositions in Table 2 an equilibrium assemblage. As in the augite relics, much of are used without modification, Na2O-enrichment appears to the omphacite appears to be replacing jadeite. The cross- lead to a wide P-T stability field for two pyroxenes (ompha- cutting idiomorphic epidote-group minerals and the chlor- cite and an aegirine-rich sodic pyroxene) for a hypotheti- ite-epidote clusters grew at a later stage. Thus an early cally completely equilibrated rock. jadeite þ chlorite omphacite growth stage appears indi- cated, although chlorite should already have formed in the original spilite. The occurrence of pumpellyite is enig- matic. Although it is found intimately intergrown with 9. Discussion other minerals of the rock matrix, the thermal history of M592 outlined in the next section indicates that the P-T 9.1. Summary of inferred phase relationships stability field of this phase must have been overstepped by at least 100 C, so that we are led to consider pumpellyite Considering the many stranded reaction relationships with to be a late phase formed mainly from omphacite on the their relict reactants and the evidence for domain equili- retrograde path. Late albite could also belong to this retro- bration as a driving force for recrystallization, defining grade growth stage. The Al-rich taramite and possible equilibrium assemblages and reaction relation- ferropargasite and associated chlorite replacing augite ships between them is difficult and in part speculative. At relics probably represent recrystallization at the highest best, a sequence of phase growth can be outlined for each temperatures reached, suggestive of the epidote-amphibo- sample. In M592 the augite relics appear to represent lite facies. No formation of any amphibole until this very domains where local equilibrium predominated. Jadeite is late stage in metamorphic development is observed in a dominant phase that replaces both augite and especially metabasite M592. Jadeite formation in metabasic rock 231

In sample M591, jadeite þ chlorite omphacite also appear to have dominated primary metamorphic recrystal- lization from the presumed spilitized magmatic protolith. Omphacite may in part be coeval with but definitely replaces jadeite. Recrystallization is much more pervasive than in sample M592, and TEM observations indicate that early sodic pyroxene and augite were later extensively altered to glaucophane. Clinozoisite overgrows the felty glaucophane þ jadeite þ chlorite matrix. Minor albite, actinolite and a second generation of chlorite are clearly younger, replacing glaucophane and sodic pyroxene. The jadeite þ omphacite þ chlorite assemblages in the late veinlets are the youngest features observed. In their early petrographic account, Milla´n & Somin (1981) describe a rock composed essentially of single- phase jadeite with isolated grains of clinozoisite, lawsonite and albite. Such assemblages were not encountered in this study. Fig. 14. Summary of available P-T paths for samples from the Gavilanes unit from Grevel (2000), Grevel et al. (2006) and 9.2. Metamorphic evolution of the jadeite-bearing Schneider et al. (2004). Pyroxene reaction curves from Waterwiese metabasites et al. (1995) and Gasparik & Lindsley (1980). Pumpellyite-out curve for Ca4Al5MgSi6O21(OH)7 composition from Schiffman & Liou Although the Gavilanes unit is a heterogeneous mega- (1980). Lawsonite-out curve from Harley & Carswell (1990), based on Chatterjee et al. (1984). me´lange, a number of P-T paths are available for various blocks in the me´lange that help to constrain possible P-T trajectories for samples M591 and M592. These data are summarized in Fig. 14. Maximum reported pressures for for metabasite compositions the stability field of pumpel- the various rocks vary between 15 and 25 kbar. Maximum lyite is even more restricted. The calculations of Willner reported temperatures vary between 500 and 660 C. et al. (2009) and Baziotis et al. (2009), for instance, show Prograde trajectories are poorly defined, but the P-T paths for various different metabasite compositions that during are of ‘‘hair-pin’’, clockwise type, so that prograde prograde development pumpellyite should normally react geotherms between 5 and 10/km are indicated for many out between 300 to 350 C and 5 to 10 kbar. Considering of them. These lie in the stability field of jadeite in the that Al-rich amphiboles grew in M592, such conditions presence of quartz (Fig. 14). The various members of the seem unrealistically low, and we consider pumpellyite to me´lange were collected during exhumation, and the exhu- be a phase that crystallized during the retrograde path (see mation paths can be seen to converge to a common trend above). (Fig. 14) that shows cooling during depressurization along As noted in the previous section, attempts to calculate a geotherm of 12 /km. The ‘‘coolest’’ path is indicated isochemical P-T diagrams, i.e. pseudosections, were not by rare deerite-bearing quartzites of the metasedimentary successful, mainly because the effective bulk composition me´lange matrix (Grevel et al., 2006). The ‘‘warmest’’ path could not be properly constrained. Nevertheless, classical has been deduced for an eclogite from a serpentinite me´l- metamorphic-facies field distributions (e.g., Evans, 1990) ange by Garcı´a-Casco et al. (2006). Thus exhumation show that the P-T trajectories of Fig. 14 pass through a during active subduction is indicated (Schneider et al., critical region between 8–12 kbar and 400–500 C where 2004; Grevel et al., 2006; Garcı´a-Casco et al., 2006, 2008). epidote-blueschist, epidote-amphibolite and greenschist- The above regional constraints allow some reasonable facies fields are in close proximity. Al-rich amphiboles conclusions to be reached for the P-T development of M591 found in both samples suggest that conditions of the epi- and M592. Jadeite-omphacite equilibration (Fig. 11) pro- dote-amphibolite facies were reached. Depending on bulk vides information on minimum temperatures reached. A and mineral compositions, the actual phase field bound- temperature of 475–500 C is suggested for sample M592, aries can be quite variable in this P-T region (e.g., Evans, although it is not clear whether such equilibration occurred 1990; Baziotis et al., 2009). The trend of the P-T trajec- during the prograde or the retrograde path of the rock. For tories in Fig. 14 allows the reasonable conclusion that the sample M591 a somewhat lower temperature of 425–450 prograde development of M591 and M592 occurred above C is obtained, but only the omphacite provides a reason- or at last very close to the Ab ¼ Jd þ Qtz reaction curve, ably fit to the sodic pyroxene solvus. Figure 14 indicates although this conclusion cannot be unequivocally corrobo- that the intergrown pumpellyite in the matrix of sample rated from petrographic observation. Minor quartz is pre- M592 could be stable up to 400 C on the basis of the end- sent in the matrix of both samples, but its relationship to member experiments of Schiffman & Liou (1980), but jadeite is unclear. No quartz is seen as a reaction product in available pseudosection calculations actually indicate that the plagioclase pseudomorphs, and SiO2 must have been 232 W.V. Maresch, C. Grevel, K.P. Stanek, H.-P. Schertl, M.A. Carpenter transported to and consumed in the rock matrix by other (4) The samples were exhumed along steep P-T trajec- mineral reactions, perhaps those forming chlorite and epi- tories of approximately 12 /km during active subduction. dote. Early jadeite þ chlorite omphacite growth is This may be the reason for the excellent preservation of assumed for both samples during this prograde path, with jadeite and omphacite. the formation of jadeite presumably facilitated by the pre- (5) During exhumation, brittle fractures cross-cutting the sence of albitized plagioclase. Sample M591 equilibrated rocks were filled by jadeite-rich assemblages. These display to a large extent in the epidote-blueschist facies before the typical features of open-system precipitation from an passing through the epidote-amphibolite and then the aqueous fluid known from well-studied jadeite jade occur- greenschist facies. Sample M592 shows no evidence of rences (e.g., Harlow & Sorensen, 2005; Sorensen et al., prograde amphibole development, be it sodic or calcic, 2006, 2010; Harlow et al., 2007). Cathodoluminescence until epidote-amphibolite conditions are reached. imaging indicates the growth of more than one generation However, the formation of sodic-calcic taramite there of jadeite in these veins. These jadeite jade veinlets suggest calls for conditions close to the blueschist facies. that a pervasive fluid rich in jadeite-component must have Although lawsonite was not observed in our samples been available, but the timing of their formation and the (cf., Milla´n & Somin, 1981), Fig. 14 indicates that the source of the fluids is unclear. The jadeitite-filled brittle stability field for this phase is traversed by most prograde fractures cut through the rock matrix as well as the augite P-T trajectories and encountered by many retrograde P-T pseudomorphs with no obvious interaction between the vein paths between 500 and 400 C. Given appropriate bulk material and the enclosing rock. There are no obvious com- compositions, lawsonite-bearing assemblages are possible. positional gradients in the rock toward the veins. Without additional information from REE spectra and trace element distributions it is not possible to decide whether the fluids were derived from a completely external source, were expelled from the rock itself at near-peak conditions, or 10. Conclusions were ‘‘re-injected’’ at lower temperatures from an aqueous fluid equilibrated at higher temperatures with the metaba- On the basis of the available data, we suggest the following sites themselves. evolutionary scenario for samples like M591 and M592: We conclude that aqueous fluids capable of precipitat- ing jadeite jade were present during exhumation of the (1) The protolith corresponds to MORB, but spilitization Gavilanes unit of the Escambray Massif, but we could is indicated by a clear increase in Na2O, coupled with a not find any jadeitite veinlets exceeding 1–2 mm in thick- depletion of CaO in the rock (e.g., Graham, 1976). ness. The interpretation of Milla´n & Somin (1981) that Spilitization is a complex process, but albitization of metabasic rocks were in the process of being transformed igneous plagioclase is a characteristic feature. Augite relics to jadeite jade reflects domain equilibration and enhanced of 1.5 mm are abundant in M592, so that original grain early jadeite growth in a spilitized, Na2O-enriched meta- sizes corresponding to a fine-grained appear likely. basic rock. Even if the enrichment in Na2O could lead to Ophitic texture is common. stabilization of omphacite and aegirine-rich jadeitite in (2) The rocks were not deformed during subduction, and such a metabasic rock after reaching full equilibrium, a jadeite grew readily at the expense of albitized plagioclase. possibility that could not be unequivocally tested in this The enrichment of Na2O and depletion in CaO during spili- study, there was clearly no net enrichment of jadeite com- tization may also have provided an effective local bulk ponent in the bulk composition, and no trend from a meta- composition between abundant augite relics that was con- basic toward a jadeite jade composition. ducive to jadeite nucleation and growth. Domain equilibra- tion appears to have dominated, and ophitic textures were preserved. Jadeite and minor omphacite crystallized in for- Acknowledgements: We thank H.-J. Bernhardt and mer plagioclase laths enclosed in augite in epitactic relation- R. Neuser, Bochum, for invaluable help with the electron ship to the host augite. Jadeite and more abundant omphacite microprobe and with hot-cathode cathodoluminescence growing from plagioclase in the rock matrix encroached microscopy, respectively. Arne Willner gave decisive input upon augite grains from the outside. Omphacite dominated on the ‘‘pumpellyite enigma’’. We are also grateful to Tadao when augite was replaced along cleavage planes or fractures Nishiyama, Antonio Garcı´a-Casco and George Harlow for from within, as described by Carpenter & Okay (1978). All their thoughtful reviews that helped us clarify some impor- three pyroxenes are generally in crystallographical (optical) tant points and improve the presentation as a whole. continuity, either as a result of the direct topotactic replace- ment of augite, or because of the epitactic overgrowth on augite of pyroxene replacing plagioclase. References (3) At increasing temperatures, more omphacite replaced earlier jadeite along grain boundaries. Where sufficient Armbruster, T., Bonazzi, P., Akasaka, M., Bermanec, V., Chopin, C., water became available, reequilibration to amphibole-rich Giere´, R., Heuss-Assbichler, S., Liebscher, A., Menchetti, S., assemblages as in M591 was possible; in other cases the Pan, Y., Pasero, M. (2006): Recommended nomenclature of pyroxene-rich assemblages persisted. epidote-group minerals. Eur. J. Mineral., 18, 551–567. Jadeite formation in metabasic rock 233

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