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Volume 30 Issue 6
Dec.  2019
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Cathodoluminescence Microscopy of Zircon in HP- and UHP- Metamorphic Rocks: A Fundamental Technique for Assessing the Problem of Inclusions versus Pseudo-Inclusions

  • This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context. We present and discuss two well-documented ex-amples. Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex, Dominican Republic, and from UHP pyrope quartzite of the Dora Maira Massif, northern Italy, were studied using cathodoluminescence (CL) techniques, in combination with mineral inclusion and age data. In general, zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims. The magmatic cores of zircon from the jadeitite yield ages of 115-117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma. Zircon from lawsonite blueschist, representing the country rock of the jadeitite, contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite, and also contains younger metamorphic minerals such as lawsonite, albite, phengite (Si3.68), chlorite, and omphacite. Similar observations were made on the magmatic cores of zircon from the pyrope quartzite. These are about 275 Ma in age and host pyrope, phengite (Si3.55), talc, and kyanite, all of which formed during UHP metamorphism at about 35 Ma. Zircon from the biotite-phengite-gneiss country rock (metagranite) shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite, which thus reflect granitic intrusion ages. The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma. Such minerals are, for example, garnet and phengite, as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz, that point to formation at UHP metamorphic conditions around 40 kbar/750℃. Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation, but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx. Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.
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Cathodoluminescence Microscopy of Zircon in HP- and UHP- Metamorphic Rocks: A Fundamental Technique for Assessing the Problem of Inclusions versus Pseudo-Inclusions

    Corresponding author: Hans-Peter Schertl,
  • 1. Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, D-44780 Bochum, Germany
  • 2. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
  • 3. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA

Abstract: This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context. We present and discuss two well-documented ex-amples. Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex, Dominican Republic, and from UHP pyrope quartzite of the Dora Maira Massif, northern Italy, were studied using cathodoluminescence (CL) techniques, in combination with mineral inclusion and age data. In general, zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims. The magmatic cores of zircon from the jadeitite yield ages of 115-117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma. Zircon from lawsonite blueschist, representing the country rock of the jadeitite, contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite, and also contains younger metamorphic minerals such as lawsonite, albite, phengite (Si3.68), chlorite, and omphacite. Similar observations were made on the magmatic cores of zircon from the pyrope quartzite. These are about 275 Ma in age and host pyrope, phengite (Si3.55), talc, and kyanite, all of which formed during UHP metamorphism at about 35 Ma. Zircon from the biotite-phengite-gneiss country rock (metagranite) shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite, which thus reflect granitic intrusion ages. The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma. Such minerals are, for example, garnet and phengite, as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz, that point to formation at UHP metamorphic conditions around 40 kbar/750℃. Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation, but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx. Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.

  • Mineral inclusions are significant sources of information of particular importance for metamorphic petrologists. A key method for determining the pressure-temperature conditions of different stages of prograde, peak or retrograde metamorphism is to carefully analyze mineral inclusions in various growth domains of rock-forming porphyroblasts, and to apply geothermo-barometric techniques to them. For instance, typical metamorphic host minerals are garnet, kyanite, staurolite, and pyroxene. Garnet plays an especially significant role. It is widespread in metamorphic rocks and its chemical composition can change considerably during progressive growth. Assuming equilibrium between particular growth zones and the mineral inclusions found in them allows the pressure-temperature (P-T) conditions to be decoded and a PT-path to be worked out.

    In order to decipher the chronological sequence of mineral growth, rock evolution and ultimately of mountain-forming processes, additional key information is obtained from the direct dating of growth domains of metamorphic minerals. Lu-Hf and Sm-Nd are, for instance, the preferred methods for dating garnet (e.g., Baxter and Scherer, 2013; Scherer et al., 2000). However, an alternative route is to study and date suitable accessory minerals such as zircon, and to focus either on those forming inclusions in porphyroblasts such as garnet or on those occurring in the matrix, in that case correlating growth domains of garnet and matrix zircon that grew contemporaneously. Whereas only two decades ago multi-grain studies were the only way of dating zircon successfully (e.g., Kröner et al., 1994; Ames et al., 1993; Tilton et al., 1991), in-situ age dating by secondary ion mass spectrometry (e.g., SHRIMP) or laser ablation (LA) ICP-MS can now date individual growth domains of, for example, a 200 μm single zircon grain (Liu Y S et al., 2010; Gerdes and Zeh, 2006; Liu F L et al., 2006). In order to link precisely the particular age of a distinct zircon domain with the P-T conditions at which it grew, a geothermobarometric evaluation is needed from a study of the corresponding mineral inclusions, either mono- or polymineralic. Studies of mineral inclusions in zircon are of particular significance, since zircon is known to be stable and chemically resistant over a wide range of P-T conditions (Parkinson and Katayama, 1999; Liou et al., 1997; Schertl and Schreyer, 1996; Sobolev et al., 1994) and it represents one of the best container minerals for protecting inclusions from late-stage retrogression events.

    Cathodoluminescence (CL) imaging is the preferred method for distinguishing different growth domains in zircon (e.g., Rubatto and Gebauer, 2000; Vavra et al., 1999; Hanchar and Miller, 1993; Vavra, 1990). Magmatic crystallization of zircon in felsic melts typically leads to oscillatory growth zoning (Vavra, 1994, 1993, 1990), where widely-spaced oscillatory zoning patterns (often interrupted by surfaces of dissolution) are interpreted to represent a low degree of zircon saturation in the melt, whereas narrowly-spaced and uninterrupted oscillatory zoning patterns are interpreted to represent higher degrees of zircon saturation.

    Metamorphic zircon grains often show rounded or ovoid shapes due to resorption or metamorphic overgrowth phenomena (e.g., Hoskin and Black, 2002; Schaltegger et al., 1999). Since metamorphic zircon grains or zircon domains can be formed under a wide variety of PT-conditions, either by solid-state reaction or by crystallization from fluids, their internal structures can be rather heterogeneous (Corfu et al., 2003, and references therein). Some zircons contain inherited cores which can be surrounded by low-luminescence growth zones or sector-zoned as well as oscillatory zoned domains (Schaltegger et al., 1999; Vavra et al., 1999, 1996).

    Besides examination by CL microscopy, trace and rare earth element (REE) analysis as well as oxygen isotope studies on zircon (e.g., Valley et al., 1994) facilitate petrogenetic interpretation of zircon growth (Li et al., 2018; Hertwig et al., 2016; Gilotti et al., 2014; Liu F L et al., 2009), but these methods are beyond the scope of the present paper. Hoskin and Ireland (2000) as well as Rubatto (2002) have demonstrated that additional geochemical studies of trace elements and REE of specific zircon domains allow distinction between magmatic and metamorphic stages; even different metamorphic domains formed during, for instance, eclogite, granulite and greenschist facies can be characterized and distinguished. Such methods are routine and common. They help to distinguish between magmatic and metamorphic growth domains of zircon and to date the respective growth stages. Several studies have successfully demonstrated an existing interrelationship between the metamorphic age of a distinct zircon domain and the corresponding metamorphic mineral inclusions, such as UHP coesite (e.g., Liu et al., 2007, 2002; Zhang et al., 2006; Katayama et al., 2000).

    Nevertheless, all such studies on zircon must be carried out carefully in order to avoid misleading results. It must be kept in mind that minerals formed during an event postdating the zircon host can nonetheless form "inclusions" in this older zircon domain. These are not true inclusions, since they were not entrapped during the growth of the zircon and should therefore be called pseudo-inclusions. How can such pseudo-inclusions be identified? How can they be distinguished from real inclusions? What are the consequences of an incorrect interpretation? To address these and related questions, the present article introduces two studies that combine CL microscopy, petrological investigation of mineral inclusions, and age dating of zircon. The first study deals with an occurrence of jadeitite from the Rio San Juan Complex (RSJC) in the northern part of the Dominican Republic, initially described by Schertl et al. (2012). The second study focuses on UHP-metamorphic pyrope quartzite (also called whiteschist) from the Dora-Maira Massif in northern Italy, the "type locality" of metamorphic coesite discovered by Chopin (1984).

    All the mineral abbreviations in this paper are taken from Whitney and Evans (2010).

  • Various types of jadeitite and jadeite-rich rock occur as loose boulders and as intercalations in blueschist in serpentinite mélanges of the RSJC, Dominican Republic (Schertl et al., 2012). The mélanges represent remnants of a Cretaceous to Paleocene subduction channel (Krebs et al., 2008), active from 120 Ma or earlier until 55 Ma. With respect to the formation of jadeitite and jadeite-rich rock in general, two different scenarios are currently being discussed. Yui et al. (2010) distinguished jadeitite formed by a metasomatic replacement process, in which a suitable protolith such as plagiogranite or trondhjemite is transformed into a jadeite-rich rock (e.g., Hertwig, 2014; Compagnoni et al., 2012), from jadeitite formed by vein precipitation, i.e., direct precipitation from an aqueous fluid (see also Fu et al., 2010). Tsujimori and Harlow (2012) introduced the terms "R-type" (metasomatic replacement) for the former and "P-type" (fluid precipitate) for the latter. Both types of formation were documented for occurrences in the RSJC (Hertwig et al., 2016; Hertwig and Maresch, 2015; Hertwig, 2014; Schertl et al., 2012). Jadeite quartzite and jadeite-lawsonite quartzite can display evidence for both "R-type" and "P-type" origins (Fig. 1a), whereas end-member jadeitite is "P-type" (Fig. 1b).

    Figure 1.  Representative rock samples of (a) concordant jadeitite and (b) discordant jadeite quartzite, Rio San Juan Complex (RSJC), Dominican Republic, as well as (c) fine-grained pyrope quartzite and (d) coarse-grained pyrope rock from Case Ramello, Parigi, Dora Maira Massif, Italy. Microphotographs of jadeitite from Magante, RSJC under (e) crossed polarizers (XPL); (f) same field of view as (e) but as cathodoluminescence (CL) image. (g), (h) CL microphotographs of jadeite with oscillatory zoning.

    For the current study we focus on two jadeitites. Sample 30071 is from a boulder in the Rio San Juan River (Schertl et al., 2012; their Fig. 1, locality 2b), and sample 30046 forms concordant, decimeter-thick layers in a lawsonite blueschist (sample 30045; see Fig. 1a). The latter rock association in turn occurs as meter-sized blocks in serpentinite mélange (Escuder-Viruete et al., 2011; Draper and Nagle, 1991). The mélange contains additional blocks of various lithologies such as eclogite, orthogneiss, marble, metapelite, and cymrite-bearing rocks, all of which were metamorphosed at high pressures (P) and low temperatures (T). Krebs et al.(2011, 2008) presented a model derived from 14 PT-paths of different types of blocks that documents the thermal evolution, i.e., cooling, of the subduction channel with time. Early eclogite blocks indicate peak conditions at about 800℃/26 kbar, omphacite-bearing blueschist blocks indicate 520℃/7 kbar and jadeitite- and/or lawsonite-bearing blueschist blocks suggest < 400 ℃/16 kbar. For details on the different serpentinite mélanges that can be differentiated within the complex (Jagua Clara and Arroyo Sabana mélanges) and their respective rock inventory, the reader is referred to Draper and Nagle (1991) and Krebs et al.(2011, 2008).

    The major constituent of the fine-grained and light-green jadeitite layer 30046 is jadeite (~90 vol.%; Jd~92–99); quartz, glaucophane, calcite, and phengite (Si~3.5–3.65) sum up to about 10 vol.%, and apatite, titanite, and zircon are accessories. The surrounding lawsonite blueschist essentially consists of glaucophane (~70 vol.%), lawsonite (~20 vol.%), and jadeite (~10 vol.%; Jd~92–99), while phengite (Si~3.5–3.65), apatite, titanite, and zircon are accessory phases. The jadeitite boulder 30071 is phengite-rich, dark green, and consists of jadeite (~85 vol.%), omphacite (~5 vol.%), phengite (~5 vol.%), albite (~3 vol.%), biotite (~2 vol.%), and epidote/clinozoisite (~1 vol.%), with accessory apatite, titanite, and zircon.

  • Pyrope quartzite forms boudins and lenses up to about 30 by 100 m within biotite-phengite-gneiss country rock (metagranite). The major constituents of pyrope quartzite are pyrope, quartz, kyanite, and phengite, while talc and coesite are minor constituents. Accessories are chlorite, vermiculite, ellenbergerite, Mgdumortierite, sodium amphibole, scapolite, jadeite, dravite, zircon, rutile, and monazite. For further details see Chopin (1984) and Schertl et al. (1991). Note that in the pyrope quartzite a fine-grained variety with small pyrope crystals up to about 2 cm in size (Fig. 1c) can be distinguished from a coarse-grained type with pyrope megacrysts up to about 25 cm (Fig. 1d). This is of major importance, because pyrope in these rocks is formed by two different pyrope-forming reactions (Schertl et al., 1991). Ferrando et al. (2009) also considered a third type of garnet formed during retrogression. Whereas the megacrysts contain inclusions of chlorite, talc, kyanite, ellenbergerite, Mg-dumortierite, amphibole with a glaucophane-like composition, dravite, vermiculite, zircon, monazite, and rutile (and never coesite or quartz), the small pyrope crystals contain quartz, coesite, phengite, talc, kyanite, zircon, rutile, monazite (and never ellenbergerite or Mg-dumortierite). Garnet reaches 99 mol.% pyrope end-member in composition and is thus very pale rose-colored. Phengite is characterized by high Si-contents up to about 3.55 Si per formula unit (pfu) and very high XMg-ratios of about 0.99. The country rocks are deformed biotite-phengite gneiss that only in rare cases shows undeformed portions where the magmatic structure of the former granitic protolith has been preserved (Compagnoni et al., 1994; Chopin et al., 1991). Major constituents are plagioclase, K-feldspar, quartz, biotite, and phengite, while epidote, allanite, zoisite, jadeite, kyanite, garnet, zircon, rutile, tourmaline, clinopyroxene are accessory.

    Peak metamorphic conditions are reported to be in the range of 37–44 kbar and 750℃ (Ferrando et al., 2009; Hermann, 2003; Schertl et al., 1991). The occurrence of true coesite inclusions in pyrope, kyanite, jadeite, and zircon proves the UHP nature of these rocks; microdiamonds have not (yet) been observed in any of the UHP lithologies of the coesite-bearing unit (called the Brossasco-Isasca Unit by Compagnoni et al., 1994). A widely accepted age for the UHP metamorphism is ~35 Ma, as documented by different studies using various minerals and isotopic systems (Rubatto and Hermann, 2001; Duchêne et al., 1997; Gebauer et al., 1997; Tilton et al., 1991, 1989). The granitic country rock from which the pyrope quartzite formed due to metasomatic interaction with Mg-rich fluids (Chen et al., 2017; Schertl and Schreyer, 2008; Compagnoni et al., 1994) yields an intrusion age of about 275 Ma (Gebauer et al., 1997). For a detailed historical outline of dating of the Dora-Maira HP- and UHP-metamorphic events see Schertl and Hammerschmidt (2016).

  • Mineral analyses were performed using a Cameca SX 50 electron microprobe at Ruhr-University Bochum (RUB), Bochum, Germany; standards used were pyrope (Si, Al, Mg), rutile (Ti), andradite glass (Ca, Fe), K-glass (K), jadeite (Na), spessartine (Mn). Operating conditions were 15 kV and 15 nA, with a beam diameter of ca. 3 μm. Black and white CL images were obtained using the CL detector of the electron microprobe. Colored cathodoluminescence (CL) images were obtained using a 'hot cathode' scanning-electron microscope of the type HCL-LM at RUB; operating conditions were 14 keV beam energy and ca. 9 μA mm-2 current beam density (for further details see Schertl et al., 2005, 2004).

  • One key method to study and display internal structures and growth zones of jadeite crystals is cathodoluminescence (CL) microscopy (Sorensen et al., 2006; Harlow and Sorensen, 2005; Schertl et al., 2004). Vein precipitation or "P-type" jadeite crystals from the RSJC are typically characterized by oscillatory growth zones clearly seen in CL images (Figs. 1e, 1f; see also Hertwig et al., 2016; Schertl et al., 2012). Figure 1f documents three different jadeite generations with early olive-colored cores, bluish to reddish luminescent mantle zones, and intense yellowish rim domains. Note that the intense yellowish luminescent domains of the outermost zone of the jadeite crystal shown in Fig. 1f (Jd98; jadeite rim) are nearly identical in composition to the bluish to reddish domains (Jd99). Specific activator elements such as REE can cause different luminescence colors, although their concentration might be below the detection limit of the electron microprobe (e.g., Schertl et al., 2018). The internal structures in jadeite crystals are quite complex; particular crystal faces (hkl) can be terminated during growth and later replaced by other crystal faces (Fig. 1g). In general, jadeite crystals of different jadeite-bearing rocks are dominated by green luminescence colors (Fig. 1h); in rare cases bluish and also reddish luminescence colors are observed.

    First studies on zircon from the jadeitite sample 30071 (Schertl et al., 2012) also documented oscillatory growth zones (Fig. 2a) which mimic those of jadeite crystals from the same rock. Occasional sector zoning was observed. CL images of zircon, which usually show CL intensities on a greyscale, are routinely performed prior to in-situ U-Pb dating (e.g., Liu and Liou, 2011; Rubatto and Gebauer, 2000; Gebauer et al., 1997).

    Figure 2.  CL-images of (a) oscillatory zoned zircons separated from concordant jadeitite (Fig. 1a) and (b) oscillatory zoned zircon from adjacent blueschist with lawsonite inclusion. Note (see arrows) the thin annealed fluid pathways in zircon and the small "pocket" of zircon characterized by a greyish CL color compared to the bluish one of the major portion of zircon.

  • Besides the oscillatory zoning of zircon which mimics that observed in jadeite crystals, the presence of certain mineral inclusions in zircon led Schertl et al. (2012) to conclude that zircon in this sample had formed by precipitation from an aqueous fluid. The authors discovered inclusions of jadeite and omphacite (Table 1, Nos. 1–2) which have similar chemical compositions to jadeite and omphacite occurring in the matrix of this jadeitite sample (Fig. 3).

    No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
    Mineral Jadeite Omphacite Albite Lawsonite Albite Phengite Chlorite Omphacite Pyrope Phengite Talc Kyanite Phengite Garnet
    Sample/locality 30071 30071 30046 30045 30045 30045 30045 30045 Parigi-Zr Parigi-Zr Parigi-Zr Parigi-Zr Parigi-Zr Tapina-Zr
    Rock Jadeitite Jadeitite Jadeitite Blueschist Blueschist Blueschist Blueschist Blueschist Pyrope-quartzite Pyrope-quartzite Pyrope-quartzite Pyrope-quartzite Bi-Ph-gneiss Bi-Ph-gneiss
    SiO2 59.24 56.17 69.16 38.32 68.05 55.03 28.95 55.12 43.78 52.85 62.44 36.73 50.62 38.01
    Al2O3 21.04 11.92 19.43 0.10 18.51 22.70 18.34 9.00 24.71 24.53 0.38 62.70 23.68 20.23
    TiO2 0.04 0.09 0.05 32.28 0.26 0.12 0 0.12 0 0.32 0.02 0 0.74 0.08
    Fe2O3 5.12 0.17 0.02 1.87
    FeO 2.09 5.19 0.01 0.20 1.12 1.82 19.80 4.20 3.08 0.06 0.30 6.90 17.8
    MnO 0.01 0.17 0.04 0.03 0.04 0.06 0.34 0.13 0.10 0.05 0 0 0.01 0.85
    MgO 1.92 6.36 0.02 0.01 0.69 4.84 19.43 6.56 26.51 5.84 31.00 0.02 1.54 0.37
    CaO 3.20 11.97 0.09 17.78 0.35 0.02 0.04 11.69 1.46 0.01 0.02 0.04 0 20.11
    Na2O 13.02 7.58 9.87 0.02 11.17 0.03 0.04 7.55 0.06 0.10 0.15 0.01 0.01 0.10
    K2O 0 0.01 0.07 0 0.04 8.37 0.03 0.00 0.10 11.22 0.04 0.01 10.66 0
    Total 100.56 99.46 98.74 88.65 100.23 92.99 86.97 99.49 99.97 94.98 94.35 99.53 94.16 99.42
    Si 2.02 2.00 3.03 2.00 2.98 3.68 5.95 2.00 2.99 3.51 3.99 0.99 3.50 3.01
    Al[4] 0 0 0 0 0 0.32 0 0.00 0.01 0.49 0.01 0.01 0.50
    Al[6] 0.84 0.50 1.00 1.99 0.96 1.47 4.44 0.38 1.98 1.43 0.02 1.99 1.43 1.88
    Ti 0 0 0 0 0.01 0.01 0 0 0 0.02 0 0 0.04 0.01
    Fe3+ 0.15 0.01 0.11
    Fe2+ 0.06 0.16 0 0.01 0.04 0.10 3.40 0.12 0.18 0 0.02 0 0.40 1.12
    Mn 0 0 0 0 0 0 0.06 0.00 0.01 0 0 0 0 0.06
    Mg 0 0.34 0 0 0.04 0.48 5.95 0.35 2.73 0.58 2.95 0 0.16 0.04
    Ca 0.12 0.46 0 0.99 0.02 0 0.01 0.45 0.11 0 0 0 0 1.70
    Na 0.86 0.52 0.84 0 0.95 0 0.01 0.53 0.01 0.01 0.02 0 0 0.02
    K 0 0 0 0 0 0.71 0.01 0 0 0.95 0 0 0.94 0
    O* 6 6 8 8 8 11 28 12 12 11 11 5 11 12

    Table 1.  Representative microprobe analyses of pseudo-inclusions in zircon from rocks of the RSJC (1-8) and the Dora Maira Massif (9-14). Mineral names and respective rock types are indicated

    Figure 3.  Quad-jadeite-aegirine clinopyroxene diagram after Morimoto et al. (1988) showing jadeite and omphacite compositions from the matrix of jadeitite 30071 and from pseudo-inclusions in zircon from that sample.

    Oscillatory zircon domains of sample 30071 were reported to yield ages of 114.9±2.9 Ma (Schertl et al., 2012) while the rim domains are younger and document a poorly defined intercept age of 93.3±6.9 Ma. Hertwig et al. (2016) conducted a comprehensive SIMS U-Pb dating, REE and trace-element as well as oxygen-isotope study on zircon of different jadeite-rich rocks and blueschist country rocks from the RSJC. Based on distinct U-Pb ages, zircon geochemistry and oxygen-isotope ratios of zircons from the concordant jadeitite 30046 (see Fig. 1a), the authors distinguished three age groups: old (117.1±0.9 Ma), intermediate (three dates only: 90.6, 97.3, 106.0 Ma) and young (77.6±1.3 Ma). Note that their old age group coincides in age with the old age group reported by Schertl et al. (2012) and that their intermediate age group coincides in age with the poorly defined age of 93.3±6.9 Ma. In zircon of jadeitite sample 30046 an inclusion of albite was found (Table 1, No. 3).

  • Zircon U-Pb dates from the lawsonite-blueschist (sample 30045) country rock of the jadeitite layer 30046 discussed in Section 3.2 define a single age group, yielding a weighted mean 206Pb/238U age of 113.6±1.2 Ma (n=13; Hertwig et al., 2016). This age coincides with the old age group derived from zircon of the adjacent jadeitite 30046 (117.1 Ma). Hertwig et al. (2016) found a lawsonite inclusion (Fig. 2b; Table 1, No. 4) within an oscillatory zoned zircon domain that gave 119.3±5.3 Ma in age. Note that chondrite-normalized REE patterns of these zircons are similar to those of the old age group of zircons from the adjacent jadeitite layer 30046 (e.g., positive Ce and negative Eu anomalies; for further details see Hertwig et al., 2016). Further minerals detected within zircon from the same lawsonite-blueschist sample are albite, phengite, chlorite and omphacite (Table 1, Nos. 5–8).

  • Fine-grained pyrope is characterized by bluish-reddish CL colors. In general, pyrope shows an increase of brightness from core to rim and an oscillatory zoning pattern (Schertl et al., 2018, 2004). Since the amount of iron in the outermost domains of pyrope is higher than in the core and mantle regions, and iron serves as a quencher element, luminescence of this outermost domain is subdued (Fig. 4a; see also Schertl et al., 2004). Recent analyses using CL-microscopy and spectroscopy techniques showed that the REEs Dy3+, Sm3+, and Tb3+ are important activator elements of the Dora-Maira pyrope, causing the intense CL colors observed (Schertl et al., 2018 and Fig. 5 therein). Zircon from the fine-grained pyrope quartzite that was studied with a CL-microscope displays bluish-green to yellow luminescent cores and thin white rims (Fig. 4b). As observed in pyrope, parts of the zircon cores also show oscillatory zoning patterns (Fig. 4b, right-hand crystal). Since pyrope also displays oscillatory growth zoning, it is possible to construct a genetic link between oscillatory zircon and pyrope domains. Gebauer et al. (1997), who performed U-Pb SHRIMP dating on zircon from different lithologies of the coesite-bearing Brossasco Isasca Unit of the Dora-Maira Massif, also observed oscillatory zoning patterns in core domains of zircon from fine-grained pyrope quartzite. They were interpreted as representing magmatic domains that are distinct from the small metamorphic domains that generally occur along the rims of zircon grains.

    Figure 4.  The CL-images of (a) oscillatory zoned pyrope, fine-grained pyrope quartzite; (b) zircon from fine-grained pyrope quartzite—note the different zircon domains and the pseudo-inclusion of a quartz pseudomorph after coesite (arrow). Back scattered electron (BSE) images of (c) euhedral zircon forming inclusions in pyrope megacrysts and (d) anhedral zircon separated from fine-grained pyrope quartzite. Oscillatory zoned zircon from biotite-phengite-gneiss country rock of the pyrope quartzite lenses as (e) CL image and (f) BSE image, the latter with a pseudo-inclusion assemblage of garnet (Grt), K-feldspar (Kfs) and quartz (Qz). Grain boundaries are indicated; note the crack close to the pseudo-inclusion assemblage.

    Figure 5.  Phengite analyses from matrix and from pseudo-inclusions in zircon of pyrope quartzite and biotite-phengite country rock from the Dora-Maira rocks, plotted in a Mg/(Mg+Fe) versus Si per formula unit (pfu) diagram.

  • A first compilation of mineral inclusions in heavy mineral separates (monazite, zircon, rutile) from fine-grained pyrope quartzite was presented by Schertl and Schreyer (1996). These authors documented inclusions of kyanite, talc, pyrope, phengite and coesite/quartz in zircon. Figure 4b (arrow) shows a pseudomorph after coesite. The garnet inclusion in zircon is pure pyrope (Table 1, No. 9); Table 1 (Nos. 11, 12) also gives the mineral chemistry of talc and kyanite inclusions in zircon. Note that phengite enclosed in oscillatory-zoned zircon contains high amounts of Si per formula unit (pfu) (about Si3.55) and exceptionally high XMg ratios of about 0.99 (Table 1, No. 10). In Fig. 5 (top), phengite inclusions in zircon from the fine-grained pyrope quartzite are compared with compositions of phengite from the rock matrix. The exceptionally high XMg ratios of phengite are typical for the fine-grained pyrope quartzite where whole-rock studies also document a lack of iron (Schertl and Schreyer, 2008). Since phengite in the biotite-phengite gneiss that represents the precursor of the pyrope quartzite has much lower XMg values (Fig. 5), this phengite inclusion in zircon cannot represent an entrapped relic from the precursor rock. Gebauer et al. (1997) also observed inclusions of phengite and coesite in zircon from fine-grained pyrope quartzite.

    Zircon grains that form inclusions in pyrope megacrysts are distinct from those occurring in the fine-grained pyrope quartzite in that they show euhedral morphologies (Fig. 4c) and contain inclusions of ellenbergerite, but never any SiO2 polymorphs (Schertl and Schreyer, 1996). In contrast, zircon grains from the fine-grained pyrope quartzite are rounded (Fig. 4d), and frequently contain inclusions of quartz and coesite, but never ellenbergerite.

    Using U-Pb SHRIMP dating, Gebauer et al. (1997) documented that oscillatory zircon cores from the pyrope quartzite belong to an old age group of about 275 Ma, while the rim zones belong to a young age group of about 35 Ma. Similar ages were obtained by Chen et al. (2017), who determined concordant U-Pb dates varying from 247±3 to 269±8 Ma for the oscillatory-zoned magmatic zircon cores, while the metamorphic rim domains yielded a U-Pb lower intercept date of 34.4±1.1 Ma. Zircon fission-track data of 29.9±1.4 Ma (Gebauer et al., 1997) mark the time when the coesite-bearing unit had reached temperatures of about 290℃ during exhumation to shallow crustal levels. Several independent geochronological studies on different minerals with different isotopic systems have shown that the peak UHP metamorphic event was at around 35 Ma (Rubatto and Hermann, 2001; Duchêne et al., 1997; Gebauer et al., 1997; Tilton et al., 1991) and also document that the rim zones of zircon formed during UHP metamorphism. Thus, as in the zircon studied in jadeitite from the Dominican Republic, some of the Dora-Maira zircons are seen to contain inherited magmatic cores (Fig. 4b). They are 275 Ma in age but may enclose younger (35 Ma) UHP-metamorphic minerals.

  • Zircon from the country rock of the pyrope quartzite, the biotite-phengite gneiss, typically displays oscillatory growth patterns (Fig. 4e). In a zircon core domain, Schertl and Schreyer (1996) found an inclusion containing a metamorphic mineral assemblage of garnet+K-feldspar+quartz (Fig. 4f). Burchard (1999) described one zircon with an inclusion containing a garnet+phengite+clinopyroxene+quartz intergrowth assemblage. Table 1 shows analyses (13, 14) of phengite and garnet inclusions in zircon. Note that phengite inclusions in zircon also contain high amounts of Si pfu (~3.55); the respective Mg/(Mg+Fe) ratio of around 0.3, however, is much lower than that of phengite from the pyrope quartzite (~0.99) (Fig. 5). SHRIMP analyses of Gebauer et al. (1997) indicate a magmatic crystallization age of the biotite-phengite gneiss protolith of 275 Ma. Recent SIMS and LA-ICPMS studies on oscillatory zircon from the metagranite by Chen et al. (2017) yielded a weighted mean age of 270±2 and 267±3 Ma, respectively. A further sample gave similar results, with weighted mean ages of 268±6 (SIMS) and 262±5 Ma (LA-ICPMS).

  • To allow interpretation of the petrological context of its formation, zircon should be investigated using CL techniques prior to age dating, in order to distinguish between different zircon domains. Such domains might include cores that were inherited from a magmatic protolith and younger overgrowths which correspond to zircon growth during one or more metamorphic overprints. Furthermore, in order to link the age of the different domains to potential magmatic or metamorphic events, detailed studies of mineral inclusions and assemblages in the respective zircon domains are required. Additional investigations, for example zircon geochemistry and oxygen isotope ratios of different zircon domains, can contribute to a more detailed classification of the domains and their link to specific magmatic, metamorphic, or metasomatic events.

    However, even if zircon age dating is performed in combination with CL imaging and mineral inclusion studies, misleading interpretations of the results are still possible, and the two examples presented in this paper illustrate this problem well.

    In the RSJC example the 115–117 Ma old zircon from the jadeitite layer 30046 contains inclusions of 78 Ma old metamorphic minerals. In the case of the Dora-Maira UHP rocks, 275 Ma old zircon contains inclusions of metamorphic minerals that formed 35 Ma ago. Given the excellent age control generated in these studies, this observation clearly appears counter-intuitive. However, it is obvious that the host-inclusion relationship can easily per se lead to the incorrect conclusion that the 115–117 Ma age derived from RSJC zircon domains is interpreted as the formation age of jadeitite, and the 275 Ma age of Dora-Maira zircon domains as the age of the UHP metamorphism. How can such erroneous interpretations be avoided and how can the correct interrelationship between zircon host and inclusion best be clarified?

    The jadeite crystals which frequently show an oscillatory zoning pattern under the CL microscope (e.g., Schertl et al., 2012; Harlow and Sorensen, 2005) were the starting point for studies on the RSJC jadeitite. Our preliminary observations (Schertl et al., 2012) led to the idea that small amounts of REE below the detection limit of the electron microprobe, but known to cause oscillatory zoning patterns in various rock-forming minerals such as jadeite (Figs. 1f1h), may also have caused the oscillatory growth zoning in zircon (Fig. 2a). If this were true, and a mineral such as jadeite occurs as an inclusion in an oscillatory-zoned zircon domain, then this domain should consequently have formed more or less contemporaneously with the jadeite. As a logical consequence, the age of about 115–117 Ma for these zircon domains (Schertl et al., 2012), was interpreted as representing the age of the metasomatic event that produced jadeitite in the RSJC of the Dominican Republic. However, later extensive and detailed studies which focused on zircon and its domain structure from different jadeite-bearing rocks and their country-rock blueschist (Hertwig et al., 2016) clarified the situation considerably. Hertwig et al. (2016) were able to demonstrate that the oscillatory-zoned zircon domains 117.1±0.9 Ma in age shown in Fig. 2a clearly crystallized in a magmatic event and not due to precipitation from an aqueous fluid. They also showed that younger metamorphic and/or metasomatic overgrowth domains occur that cluster primarily at 77.6±1.3 Ma. How then can the apparent observation of younger HP jadeite inclusions in 117 Ma old zircon domains be explained? An elegant and logical answer is given by the CL images presented in Fig. 6a. The zircon grain contains microcracks along which fluids were able to penetrate into the inner domains of the zircon and interact with former inclusions or even crystallize new minerals dispersed along the cracks themselves. The cracks in the zircon may anneal, but since they were healed at different times by fluids of different chemical compositions, the CL color and/or intensity will generally be distinct. Hence, fluid pathways are often easily identified in CL images (see Fig. 6b, arrows). However, depending on how the zircon is cut, the cracks can be very small and difficult to identify. It is thus highly recommended that CL images of zircon be studied very carefully, in order to identify the presence of such important microstructures.

    Figure 6.  The CL images of (a) oscillatory-zoned zircon from jadeitite from the RSJC showing crack-related, annealed fluid-pathways (arrows) leading to jadeite/omphacite pseudo-inclusions. (b) Coesite pseudo-inclusion in a magmatic zircon domain of a fine-grained pyrope quartzite (protolith age of ~275 Ma; see Gebauer et al., 1997) with former fluid pathways annealed by a later zircon generation. Note the small pockets with 41 and 38 Ma, which reflect the UHP event.

    Considering the two end-member formation mechanisms of jadeitite, that is, fluid precipitate (P-type) versus metasomatic replacement (R-type), the oscillatory zoning pattern observed in many zircon grains in jadeite-rich rocks from the RSJC does not necessarily stem from zircon precipitating from a fluid. Zircon exhibiting oscillatory zoning can, for instance, be inherited from a trondhjemitic or plagiogranitic protolith that was transformed into a jadeitite by metasomatic replacement processes (Compagnoni et al., 2012); granitic zircon crystals are well known to display oscillatory growth features (e.g., Vavra, 1994, 1993, 1990). Jadeite and omphacite crystals within the oscillatory zircon domains thus represent pseudo-inclusions that were not entrapped during the magmatic growth of zircon, but were introduced during a later metamorphic or metasomatic stage. Note that Hertwig et al. (2016) discovered a crystal of lawsonite in one of the oscillatory-zoned zircon crystals separated from the lawsonite blueschist that forms interlayers with the jadeitite sample 30046. This lawsonite (Fig. 2b) likewise represents a pseudo-inclusion rather than an inclusion, since the age of the related zircon domain clearly predates HP metamorphism. Note that here as well the CL-image documents very small cracks (Fig. 2b, upper and lower arrows) as well as dark luminescent pockets (Fig. 2b, right arrow) and rims directly adjacent to the lawsonite inclusion, which are not large enough for obtaining reliable U-Pb ages, but which likely formed during HP metamorphism.

    In case of the Dora-Maira UHP-metamorphic pyrope quartzite and related rocks, which represents our second example, the initial study of Schertl and Schreyer (1996) focused on mineral inclusions in zircon, monazite, and rutile. In zircon separated from the fine-grained pyrope quartzite, inclusions of pyrope, kyanite, talc, pyrope, phengite (with high Si-contents and Mg# of about 0.99), and coesite/quartz were analyzed. These minerals also represent the major rock-forming minerals present in the rock matrix. Thus, the inclusions were interpreted to predate or to be contemporaneous with the growth of zircon. Ensuing studies on zircon using U-Pb SHRIMP dating (Gebauer et al., 1997) required modification of the interpretation of Schertl and Schreyer (1996). How can the discrepancy caused by the presence of a 35 Ma old UHP-metamorphic mineral in a 275 Ma old zircon host be explained? The CL-image that provides the solution for this specific problem is shown in Fig. 6b (modified from Gebauer et al., 1997), where a zircon crystal from the fine-grained pyrope quartzite encloses a crystal of coesite (dark). The major part of the zircon is oscillatory zoned and the respective ages (267 and 274 Ma) are in the range of the granitic intrusion age. Small dark grey pockets with dates of 38.4±3.7 and 41.2±9.1 Ma, respectively, are documented adjacent to the coesite by SHRIMP dating (Gebauer et al., 1997). The errors are quite high because of the very limited size of the luminescent pockets. These dates coincide with the age of the UHP-metamorphic stage. A further important observation is the existence of very small fluid pathways (Fig. 6b, arrows). Gebauer et al. (1997) already came to the conclusion that the coesite must actually represent a pseudo-inclusion in the zircon single crystal, and that SiO2 must have been introduced along cracks of the old zircon to form new coesite inside its core. Note that inclusions such as phengite, talc or kyanite also occur, which are also connected to the same type of "cracks" acting as former fluid pathways. In essence, through the study of CL-images of different zircon grains in pyrope quartzite, and in combination with in-situ dating techniques, it has become obvious that in the case of the studied Dora-Maira rocks zircon also contains many pseudo-inclusions. Within the same rock type (fine-grained pyrope quartzite or whiteschist) Gauthiez-Putallaz et al. (2016) also studied zircon displaying oscillatory core and one to two unzoned rim domains. These authors found phengite, talc, florencite, monazite and apatite hosted essentially in the core domains, so that these minerals were also interpreted to be of secondary origin.

    It is important to note that the formation of annealed fluid pathways is only possible if enough fluid is available. Small pockets of metamorphic zircon can form only if that fluid contains enough zirconium and silicon or is able to redistribute Zr and Si via solution/recrystallization processes. If this was not the case, or the HP- and UHP-metamorphic overprints proceeded under fluid-deficient conditions, then metamorphic zircon domains may not form at all. Of course, the primary prerequisite is that the metamorphic overprint must occur at temperatures high enough to allow zircon growth. Indeed, Gebauer et al. (1997) did not find any evidence of 35 Ma old metamorphic zircon domains in biotite-phengite-gneiss country rock. They showed that the oscillatory growth patterns of zircon are of magmatic origin, and that the ages around 275 Ma represent the intrusion stage. Thus the polymineralic inclusions of garnet+K-feldspar+quartz (Fig. 4f) and garnet+phengite+ clinopyroxene+quartz (Burchard, 1999) must represent "young" metamorphic formation inside "old" magmatic zircon. For the grt+ph+cpx+qz assemblage Burchard (1999) derived metamorphic conditions of about 40 kbar and 750℃, which also fits the 35 Ma peak metamorphic conditions of the fine-grained pyrope quartzite (e.g., Rubatto and Hermann, 2001; Duchêne et al., 1997; Gebauer et al., 1997; Tilton et al., 1991, 1989). Note the fissure in zircon close to the polymineralic inclusion of Fig. 4f which likely served as the influx channel for fluids leading to the formation of grt+kfs+qz.

    Similar observations of "young" metamorphic inclusions in "old" zircon domains were described by Zhang et al. (2009). These authors reported coesite and eclogite-facies minerals such as phengite and jadeite in magmatic zircon cores of UHP gneiss and schist interlayers within eclogite from the Chinese Continental Scientific Drilling Main Hole; these minerals in fact also represent pseudo-inclusions.

    The results of the current study are compiled in a PT diagram in Fig. 7. Two PT-paths are shown for the Rio San Juan rocks, which were both derived from blueschists representing the country rocks of the jadeitite (Krebs et al., 2011); the PT-path for the Dora-Maira Massif UHP rocks is from Schertl et al. (1991). The significant "zircon events" are indicated along the PT-paths of both localities, i.e., the protolith and prograde stages, the peak metamorphic and pseudo-inclusion formation stages, as well as a zircon fission-track age for Dora-Maira zircon (Gebauer et al., 1997).

    Figure 7.  Compilation of the results shown in a PT diagram. The two PT-paths from blueschists of the RSJC are from Krebs et al. (2011); the ages are from Hertwig et al. (2016). For the PT-path from the Dora-Maira pyrope quartzite (Schertl et al., 1991), the given ages are from Gebauer et al. (1997).

  • Only on the basis of very careful correlated studies linking the ages of different zircon domains with the mineral inclusions entrapped in them during growth can pressure-temperature-time paths of metamorphic rocks be reconstructed. The precise evaluation of such small-scale observations, in turn, is one of the best and most robust ways to decipher large-scale mountain-forming processes. A number of studies have shown that CL imaging of zircon is a fundamental method for displaying the internal structure of zircon, and for recognizing the distinction between inherited magmatic domains, later metamorphic and/or metasomatic domains, and late alteration-related or hydrothermal domains. Misinterpretation in linking mineral inclusions and corresponding growth zones of zircon can lead to incorrect interpretation of large-scale geotectonic events. With respect to the current study on jadeitite from the Rio San Juan Complex, Dominican Republic, metamorphic omphacite and jadeite crystals formed at ~78 Ma were observed in 115–117 Ma magmatic oscillatory-zoned domains of zircon (e.g., Hertwig et al., 2016). Zircon grains separated from the fine-grained pyrope quartzite of the Dora-Maira Massif contain oscillatory-zoned core domains, which yield crystallization ages of the magmatic protolith of ~275 Ma (e.g., Chen et al., 2017; Gebauer et al., 1997). These domains host coesite, pyrope, phengite, talc, and kyanite which were shown to have formed during UHP metamorphic conditions at 35 Ma. If not treated with caution, such interrelationships could suggest that jadeitite formation in the Rio San Juan Complex occurred at 115–117 Ma, and that an UHP metamorphic event in the Dora-Maira Massif occurred at 275 Ma. However, these conclusions are clearly wrong. We have documented that inclusions in zircon that were entrapped during growth must be distinguished from pseudo-inclusions which were introduced into an already existing zircon during a later metamorphic or metasomatic event, and all the minerals hosted by zircon and mentioned above are then pseudo-inclusions. Careful interpretation of internal structures of zircon, visualized by CL studies, allows the characterization of typical features of pseudo-inclusions, a term which we introduce here. These are small-scale cracks and fissures in zircon that represent pathways for fluid influx. Such fluid pathways may have also affected former inclusions that became metasomatically transformed to other minerals or mineral assemblages. A further possibility is the formation of new minerals along these micro-cracks. We demonstrated that such microcracks often anneal with new zircon of a different, younger generation which can also be visualized by CL studies, because they show different CL properties and thus allow easy identification.

  • The current study was financially supported by the German Research Foundation (No. SCHE-517/10-1). We are grateful to two anonymous reviewers who helped to improve our paper with their comments. The final publication is available at Springer via

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