Our description of petrographical characteristics and mineral compositions concentrates on the four facies-critical metabasitic rocks (eclogite, garnet amphibolite, amphibolite and greenschist) as well as on some selected rocks from the eclogite unit such as garnet micaschist, micaschist and granite gneiss. The list of the assemblages in the rock samples is given in Table 1.
Garnet Amphibole Epidote Chlorite White mica Biotite Clinopyroxene K-feldspar Albite Plagioclase Quartz Rutile Titanite Ilmenite Magnetite Calcite Deformation Eclogite and retroeclogite 27231 x x x x x x x x x x x x nd 27233 x x x x x x x x x x nd 27234 ? x x x x ? x x x x x vwf 27230 x x x x x ? x x x x x vwf 27256 ? x ? x x x x x vwf Garnet amphibolite 27236 x x x x x x x x sf 27237 x x x x x x x x x nd 27244 x x x x x x nd 27245 x x x x x x x x x x x nd Amphibolite 27273 x x x x x x x nd Greenschist 27272 x x x x x x sf Garnet micaschist 27250 x x x x x x x s Micaschist 27255 x x x x x x x x s 27232 x x x x x x x x s Granite gneiss 27241 x x x x x x x x wgn nd. not deformed; s. schistose; wgn. weakly gneissose; vwf. very weakly foliated; sf. strongly foliated.
Table 1. Assemblages and characteristics of samples in the BMC
Macroscopically, the eclogite (samples 27231, 27233) is medium to fine-grained. Grain sizes of amphibole and mica are generally a little larger than those of garnet and omphacite. Major mineral constituents and respective modal amounts are garnet (30%–40%), omphacite (5%–25%), phengite (from accessory amounts up to 15%), quartz (< 5%) and amphibole (< 5%). In retrogressively altered samples (retro-eclogite samples 27230, 27234, 27256), the amount of amphibole can reach 50% and epidote as well as carbonate are becoming major constituents. Rutile (< 3%), biotite, chlorite, titanite and magnetite are accessories. Eclogite samples are generally undeformed, but in retro-eclogite samples occasionally small volume portions are characterized by a weak foliation.
Common inclusions in garnet (grain size 0.5–0.8 mm) are epidote, rutile, amphibole, quartz and clinopyroxene; often large phengite grains form inclusions in the core of garnet that is radially fractured (Figs. 3a, 3b). Garnet may exhibit reaction rims of amphibole (±epidote and biotite). Garnet shows similar compositional variations in eclogite and retro-eclogite (Fig. 4a). For sample 27231 an element distribution map of garnet is provided in Fig. 4d and its full compositional range is as follows: And0.013–0.017Grs0.34–0.35Alm0.53–0.55Sps0.017–0.022Prp0.08–0.10 in the core to And0.007–0.014Grs0.19–0.29Alm0.50–0.54Sps0.009–0.011Prp0.18–0.25 at the rim. Mg-content strongly increases from the core towards the rim, whereas Ca- and Mn-contents decrease notably and Fe- contents decrease rather weakly.
Figure 3. Microphotographs of rock samples studied. (a) Garnet (Gt) with phengite (Ph) inclusions in omphacite (Cp)-rich matrix of eclogite sample 27231; arrows indicate amphibole-rich symplectites (plane polarized light PPL); (b) same as (a) under crossed polars (XPL); (c) coarse-grained amphibole 3 (Am) with inclusions of Gt and Cp (eclogite sample 27231; XPL); (d) Gt-Am-quartz (Qz)-biotite (Bi) matrix (garnet-amphibolite sample 27245; PPL); (e) calcite (Cc)-bearing portion of the matrix (garnet-amphibolite sample 27245; XPL); (f) Gt porphyroblast in a matrix rich in Bt, Ph, Qz, Pl (plagioclase porphyroblast; garnet micaschist sample 27250; XPL); (g) sieve structure of Gt in a matrix rich in Qz, Mc (microcline), Bt (granite gneiss sample 27241; XPL); (h) Bt-epidote intergrowth texture in matrix rich in Qz and Mc (granite gneiss sample 27241; XPL).
Figure 4. (a)–(c) Compositional variations of garnet cores and rims in almandine-pyrope-grossular triangular plots; (d) element distribution maps of garnet in eclogite sample 27231; (e) element distribution maps of garnet in garnet amphibolite sample 27245.
Omphacite occurs in grain sizes of 0.05–0.3 mm. It is often elongated and weakly aligned and occasionally exhibits amphibole-rich symplectitic rims. Omphacite occurs as matrix crystals, inclusions in garnet and in symplectite, all of which are different in composition (Fig. 5). The matrix omphacite has an average composition of 60.7 mol% diopside, 38.5 mol% jadeite and 0.8 mol% acmite components. Two out of 16 analyses lack a detectable acmite component. A chemical zoning pattern was not observed. Omphacite in symplectites has an average composition of 78.6 mol% diopside, 20.3 mol% jadeite and 3.2 mol% acmite component similar to those of inclusions in garnet and coexists with plagioclase with variable compositions (74.9 mol%–93.1 mol% albite, 6.5 mol%–24.8 mol% anorthite and 2 mol%–4 mol% orthoclase components).
Figure 5. Compositional variations of omphacite generations in eclogite samples within a jadeite-diopside-acmite triangular plot.
In general, three types of amphibole occur: Am Ⅰ is fine-grained and forms inclusions in garnet, Am Ⅱ is either part of the symplectitic rims around garnet or present in the matrix. Am Ⅲ is coarse-grained (1–1.5 mm) and overgrew garnet, quartz, phengite and omphacite during a later stage (Fig. 3c). While in the rock matrix garnet and omphacite often exhibit reaction rims due to later retrogression, inclusions of garnet and clinopyroxene in Am Ⅱ are lacking these rims, indicating a formation of Am Ⅱ prior to the retrogression of garnet and omphacite. Only if eclogite shows an intense retrogression and thus is characterized by high amounts of Am Ⅲ, a fourth amphibole generation (Am Ⅳ) is observed which forms either gridded intergrowth textures with uneven extinctions, or idioblastic crystals < 100 μm.
The four generations of amphibole also differ by compositions as can be shown in classification diagrams according to Hawthorne et al. (2012; Figs. 6a, 6b). Amphibole can be assigned either to the calcium or sodium-calcium subgroup, but most of Am Ⅰ and Ⅱ are also characterized by high occupation of the A-position (>0.50 apfu). Whereas all amphibole generations show similar ranges of minor elements and oxidation stages (Ti 0.00–0.07 apfu, exceptionally 0.36 apfu, Mn 0.00–0.04 apfu, XFe3+=Fe3+/(Fe3++Fe2+) 0.01–0.68, exceptionally 0.85), strong differences are between Am Ⅰ and Ⅱ versus Am Ⅲ and Ⅳ with respect to contents of Si, BNa, ANa and XMg=Mg/(Mg+Fe2+) as shown in the following.
Am Ⅰ inclusions yield compositions corresponding to pargasite, sadanagaite, tschermakite or to winchite with Si 5.79–6.64, BNa 0.27–0.57, ANa 0.16–0.74 apfu and XMg 0.36–0.39.
Am Ⅱ shows a similar composition, but a more limited range that comprises pargasite and sadanagaite with Si 5.64–6.53, BNa 0.13–0.43, ANa 0.00–0.66 apfu and XMg 0.46–0.61.
Larger amphibole (Am Ⅲ) is classified as magnesio- hornblende, winchite, barroisite or katophorite with Si 6.59–6.88, BNa 0.38–0.68, ANa 0.10–0.50 apfu and XMg 0.72–0.89.
Am Ⅳ has the widest compositional range. Most of the analyses are classified as tremolite and paragasite, but some grains as magnesio-hornblende, sadanagaite or tschermakite. This amphibole generation comprises the following compositional ranges: Si 6.12–7.84, BNa 0.00–0.44, ANa 0.00–0.43 apfu and XMg 0.64–0.73. A significant chemical zoning could not be observed in neither generation.
White mica (grain size < 0.6 mm) occurs in the matrix and often shows small cryptocrystalline reaction rims. It forms inclusions in garnet (see above) and Am Ⅲ. Fine-grained white mica forms intergrowth textures with Am Ⅳ. White mica composition is phengite (i.e., ≥3.2 Si apfu; Rieder et al., 1998; Fig. 7a) showing similar variations of Si-contents (3.25–3.40 Si apfu) between samples and little deviation from the ideal Tschermak substitution 2Al=Si+(Mg+Fe2+) due to Fe3+-substitution in some grains (calculated Fe3+ 0.00–0.25 apfu) and minor di/trioctahedral substitution. Decrease of Si-contents at the rims has been observed. XMg (=Mg/(Mg+Fe2+)) varies from 0.56 to 0.81. Na-contents (0.06–0.20 apfu) may rise up to 0.62 in retro-eclogite samples. Contents of Ba (≤0.026 apfu), Ti (0.002–0.091 apfu) are minor.
Figure 7. Variations of Si apfu with Al apfu in potassic white mica. The line of the ideal Tschermak's substitution is indicated. (a) Eclogite samples 27230, 27231, 27233; (b) micaschist sample 27232, granite gneiss sample 27241, garnet micaschist sample 27250.
Rutile up to 100 μm forms inclusions in garnet and clusters in the matrix. Occasionally it is enveloped by rims of titanite. Epidote, titanite and carbonates are accessories, their modal proportion, however, increases in retrogressively altered eclogite.
Garnet amphibolite is the typical metabasite of the unit that structurally overlies the eclogite unit (Fig. 2) and it can easily be distinguished from retro-eclogite by different mineral assemblages and compositions. Except for sample 27236, samples 27237, 27244 and 27245 are undeformed, fine-grained and dark greenish grey in color. Garnet, albite and biotite porphyroblasts are a few mm in size and thus can be macroscopically identified. Except for granoblastic textures in undeformed portions, rare banded intercalations of oriented greenish and whitish-grey layers (sample 27236) can be observed.
The following assemblage is documented: amphibole (45%), garnet (5%–20%), quartz (5%–30%), albite (~20%; in undeformed variety only), biotite (< 5%; in undeformed variety only), epidote (5%–20%), ilmenite (up to 10%); accessories are chlorite, apatite, titanite and calcite. This characteristic assemblage points to conditions of the epidote-albite-amphibolite facies (Fig. 2c).
In undeformed portions of the rock, garnet is generally fine-grained, isometrical and roundish and 0.2–0.5 mm in size. Within the layered intercalations (sample 27236), the grain size of garnet is 1.5–4 mm. Whereas in the greenish, amphibole-dominant layers garnet forms isometrical shapes, the quartz- and chlorite- dominant layers exhibit sieve-like structures of garnet. Many garnet grains show parallel sets of fractures documenting brittle deformation. Inclusions of epidote, amphibole, titanite, ilmenite and rare rutile are observed in garnet. A typical photomicrograph showing intergrowth textures of amphibole, garnet, and biotite in a quartz-rich matrix is presented in Fig. 3d. Typical compositional variations of garnet in garnet amphibolite sample 27245 are And0.012–0.034Grs0.313–0.357Alm0.582–0.622Sps0.009–0.060Prp0.022–0.041 in the core to And0.008–0.014Grs0.281–0.307Alm0.620–0.633Sps0.005–0.014Prp0.041–0.62 at the rim (see also Figs. 4b, 4e). From core to rim, Mg strongly and Fe weakly increase, whereas contents of Ca and Mn notably decrease. Mn shows a typical bell-shaped growth zonation.
Amphibole is idioblastic in shape, inclusion-free, intensively pleochroitic (with a typical hornblende colour absorption bluish-green to light green to light yellow) and occasionally zoned. Its grain size is about 50–100 μm partly forming clusters of 1.2–2.5 mm size. In banded portions, the amount of amphibole varies widely. Whereas in the greenish layers (which are dominated by amphibole) the proportion reaches up to 70%, in whitish-grey layers (rich in chlorite and quartz) it is up to 30%. In the banded rocks (sample 27236) amphibole forms a lineation. Amphibole is invariably classified as magnesio-hornblende and tschermakite as shown in Fig. 6c. A chemical zoning could not be determined, although a core of lighter coloured pleochroism is observable in several grains.
The average grain size of quartz is 20 μm. In granoblastic portions the amount of quartz reaches 5%, in layered portions 30%. In the layered portions a weak undulatory extinction is observed. Almost pure albite mostly forms xenoblastic, isometrical porphyroblasts 0.3–1.5 mm in size. These late porphyroblasts are rich in inclusions and envelope all earlier formed phases. Biotite occurs in undeformed portions of garnet amphibolite and forms flakes of 0.3–1 mm in size; occasionally it is partly replaced by chlorite. Biotite has an intermediate composition between phlogopite and annite (XMg 0.49–0.55) with minor Ti 0.09–0.011, Mn < 0.017, Ba 0.006–0.01 and Na 0.03–0.04 apfu.
In undeformed rocks, ilmenite which is typically associated with titanite, shows sieve-like structures. In the banded rock portions which are lacking titanite, ilmenite forms tabular morphologies. Grain size is 0.4–0.6 mm. Epidote (< 0.5 mm) may occur as a major constituent (< 20%) or as an accessory phase in undeformed garnet amphibolite; in the banded sample portions modal abundances are about 5%. Rare calcite in the amphibole-rich matrix is shown in Fig. 3e.
Two types of metabasite could be identified in the upper unit of the BMC, both characterized by the absence of garnet.
An undeformed coarse grained amphibolite (sample 27273) shows macroscopically visibile amphibole grains up to 0.5 mm and amgydules (0.2–1 mm in size) in hand specimen. Main mineral constituents are amphibole (~50%) and epidote (45%), whereas biotite and retrograde chlorite are minor constituents and titanite, quartz and calcite are present as accessories.
Amphibole is strongly pleochroitic (bluish-green to green to light yellow). Its composition varies from magnesiohornblende to tschermakite (Fig. 6c) with following compositional variations: Si 6.78–7.74 apfu, XMg 0.55–0.69, ANa < 0.30, BNa < 0.41, Ti < 0.16, Mn 0.02–0.07 apfu. No compositional zonation was detected.
Epidote occurs as single grains, as local metasomatic monomineralic clusters and as fillings of amgydules. It is weakly pleochroitic (pale yellow-colourless) and its average grain size is about 0.5 mm. Fe3+ content is 0.48–0.52 apfu and slightly decreases from core to rim. Biotite occurs occasionally as flakes of 0.3–0.5 mm in size with a slightly Fe-rich composition (XMg 0.44–0.47). Like amphibole, biotite is often replaced by chlorite. The amphibolite sample lacks plagioclase, but its tschermakitic amphibole allows an assignment to the classical amphibolite facies.
By contrast, the strongly foliated metabasite (27272) of the upper BMC unit has a typical greenschist facies assemblage with chlorite, amphibole and epidote each in a modal abundance of ca. 20%. Abundant quartz and albite occur as further major constituents of around 10% each. Slightly greenish and oriented chlorite with anomalous interference colours (XMg 0.49; Si 5.32–5.46 apfu) forms platy grains of a length of around 0.05–0.1 mm and defines a pronounced foliation as oriented amphibole. Weakly coloured amphibole grains are generally larger than chlorite grains with an average grain size of 0.5 mm. Individual crystals can reach sizes of up to 1 mm. Its composition varies from actinolite to magnesio-hornblende (Fig. 6c; Si 7.176–7.852 apfu, XMg 0.58–0.72; ANa < 0.15, BNa 0.10–0.25, Ti 0.002–0.018, Mn 0.034–0.053 apfu). Epidote grains (Fe3+ 0.45–0.52 apfu) have a relatively uniform size (around 0.1 mm) and mostly hypidiomorphic or nodular habits. Quartz and albite grains in polygonal aggregates and nodular aggregates of titanite have sizes of 0.1–0.5 mm and modal abundances of ~10% each. Apatite, zircon and opaque minerals occur as accessory phases.
The Achmerovo granite gneiss (sample 27241) from the eclogite unit basically consists of quartz, K-feldspar, biotite and plagioclase likely belonging to its magmatic precursor assemblage. In addition, white mica, garnet and epidote occur which are constituents formed during the metamorphic overprint. Weakly oriented micas define a faint foliation which is best observed macroscopically. In detail, the list of modal amounts is as follows: quartz (30%), K-feldspar (30%), plagioclase (15%), biotite (15%); the metamorphic minerals white mica, garnet and epidote sum up to about 10%. Accessories are titanite, zircon and opaques.
Quartz yields a bimodal distribution with coarse grains (~500 μm) with intense undulatory extinction, and smaller grains (~100 μm) which are weakly undulatory. K-feldspar (average grain size of 400 μm) shows a cross-hatched twinning which is characteristic of microcline. The average grain size of plagioclase is around 500 μm. Plagioclase is pure albite (< 1 mol% anorthite component) showing that magmatic plagioclase was completely overprinted during metamorphism.
Biotite grains show a light olive-green to dark brown pleochroism and reach sizes of 2 mm. Biotite has an Fe-rich composition (XMg 0.147–0.156), but enhanced Ti-contents (0.10–0.13 apfu).
Flakes of white mica reach 400 μm in size. White mica is phengite with Si-contents ranging between 3.20 and 3.39 apfu (Fig. 7b). However, one grain was found to display an exceptionally high Si-content of 3.62 apfu. Further compositional ranges are as follows: calculated Fe3+-contents 0.00–0.23 apfu, Fe2+ 0.24–0.39 apfu, Mg 0.05–0.16 apfu, XMg (=Mg/(Mg+Fe2+) 0.15–0.33, Ti 0.00–0.02 apfu, Ba 0.00–0.03 apfu, Na 0.02–0.03 apfu.
Garnet up to 2 mm in size forms sieve structures and preferentially grew along grain boundaries of quartz and feldspar (Fig. 3g). Garnet zonation is little pronounced showing a range of end member compositions of And0.028–0.042Grs0.400–0.412Alm0.426–0.455 Sps0.099–0.118Prp0.003–0.004 in the core and And0.024–0.034Grs0.402–0.428 Alm0.422–0.445Sps0.100–0.117Prp0.004–0.005 at the rim. Only pyrope and grossular contents slightly increase from core to rim.
Epidote occasionally contains cores of allanite, typically occurs in assemblage with biotite and yields grain sizes of 50 to 400 μm. Figure 3f shows intergrowth with biotite. Epidote has Fe3+-contents of 0.70–0.74 apfu.
Macroscopically, garnet micaschist (sample 27250) within the eclogite unit is characterized by garnet pophyroblasts within a groundmass of pronouncedly oriented white mica. Major constituents are white mica (30%–40%), quartz (20%–30%), plagioclase (15%–25%), biotite (~15%), garnet (~10%) and chlorite (~5%); tourmaline and apatite are accessories.
Oriented flakes of white mica are 0.4–0.6 mm long and define the predominant cleavage. Smaller undeformed recrystallized grains occur at microfold hinges of a crenulation cleavage. White mica shows a remarkably wide range of compositions (Fig. 7b) from muscovite to phengite (Si 3.05–3.33 apfu) with XMg 0.56–0.75, Ti 0.014–0.025 apfu and Na 0.085–0.184 apfu.
Biotite flakes (0.1–0.5 mm) are either mimetically aligned parallel to the direction of the major deformation characterized by white mica or are oriented randomly. Biotite has an intermediate composition between annite and phlogopite (XMg 0.57–0.58) with Ti 0.06–0.08 apfu and Na 0.03–0.04 apfu.
Chlorite is light green in color; grain size and textural behaviour resemble biotite. Areas of large quartz grains (0.2–0.5 mm) exhibit an undulatory extinction and curved grain boundaries, whereas areas of fine grained quartz (average grain size of ~100 μm) exhibit straight grain boundaries and represent recrystallization structures.
Porphyroblastic plagioclase displays grains mimetically oriented parallel to the major deformation that is dominated by oriented mica and chlorite (Fig. 3f). It shows patchy extinction patterns and contains trails of biotite, chlorite, white mica, apatite, tourmaline and fluid inclusions. Albite twins are observed only rarely, saussuritisation is common. Plagioclase has 18 mol%–24 mol% anorthite component.
Garnet is 1.3–2 mm in size, typically yields poikilitic sieve structures and is surrounded by flakes of oriented biotite, white mica and chlorite (Fig. 3f). Inclusions are dominantly quartz and, to a lesser extent, also plagioclase and white mica. Occasionally inclusion trails define an earlier foliation. As particular inclusions in garnet chloritoid with an XMg of 0.25 was observed. Replacement textures of chlorite (in parts also of biotite) after garnet are common. Elemental zoning in garnet is less distinct than in the eclogite or the garnet amphibolite. Its end member compositions vary from And0.010–0.020Grs0.200–0.232Alm0.646–0.653Sps0.020–0.045 Prp0.097–0.119 in the core to And0.010–0.020Grs0.173–0.190Alm0.681–0.73 Sps0.011–0.020Prp0.109–0.113 at the rim showing faint increase of Mg and Fe and decrease of Mn and Ca from core to rim. Highest spessartine contents of 0.05 apfu can be observed in the outermost rim of garnet.
Garnet-free micaschist (sample 27232) from the eclogite unit shows similar characteristics as the garnet-micaschist. The amount of white mica, however, is higher and its grain size generally is smaller and uniformly around 100 μm. Likewise grain size of quartz is smaller (50–100 μm). White mica is phengite (Si 3.20–3.33 apfu; Fig. 7b) with Ti 0.02–0.04 apfu, Na 0.03–0.04 apfu and XMg 0.74–0.79. Biotite is similarly Mg-rich (XMg 0.70–0.71) with Ti 0.06–0.07 apfu and Na 0.01–0.02 apfu. Plagioclase has 27 mol%–32 mol% anorthite component.
For reconstruction of the P-T history we focussed on four critical rock samples: eclogite sample 27231, garnet amphibolite sample 27245, garnet micaschist sample 27250 and granite gneiss sample 27241. The latter two occur within the eclogite unit in close vicinity of eclogite occurrences. Mineral abbreviations used are as follows: Ab, albite; Am, amphibole; Ar, aragonite; Bt, biotite; Cc, calcite; Cd, chloritoid; Ch, chlorite; Cp, clinopyroxene; Do, dolomite; Ep, epidote; Gt, garnet; Im, ilmenite; Kf, potassic feldspar; Ky, kyanite; Lw, lawsonite; Mt, magnetite; Om, omphacite; Pa, paragonite; Ph, phengite; Pl, plagioclase; Qz, quartz; Rb, riebeckite; Ru, rutile; SI, sillimanite; St, staurolite; Tt, titanite; V, H2O as hydrous fluid; Wm, potassic white-mica.
First four PT pseudosections and isopleths of mineral composition for the selected rock samples in the PT-range of 5–20 kbar, 400–700 ℃ were calculated using the PERPLE_X software package (Connolly, 2005, 1990; latest version downloaded from www.perplex.ethz.ch). The thermodynamic data set of Holland and Powell (1998, updated 2002) for minerals and aqueous fluid was used. Calculations were performed using the following solid-solution models: for white-mica, epidote, amphibole, garnet, plagioclase, omphacite, chloritoid, staurolite, carbonate, chlorite and biotite by Holland and Powell(2003, 1996) and Powell and Holland (1999). Only for eclogite the amphibole model by Diener et al. (2007) was applied. These models were selected from the distributed version of the PERPLE_X solid-solution model file. Albite, K-feldspar, quartz, titanite, H2O and paragonite were considered as pure phases. The fluid equation of state used is a compensated Redlich-Kwong (CORK) by Holland and Powell (1998). For the calculation of the pseudosections the major element compositions analysed by XRF were simplified to a 11-component system (SiO2-TiO2-Al2O3-FeO-MgO-MnO-CaO-Na2O-K2O-H2O- O2 and additional CO2 for calcite-bearing sample 27245) and normalized to 100% (Table 2). Water contents were augmented to excess water conditions that are considered to have prevailed during peak PT-conditions. The amount of O2 used was the one extracted from the original analyses. The amount of CaO was corrected for the amount of CaO present in apatite.
(a) 27231 27245 27241 27250 (b) 27231 27245 27241 27250 SiO2 47.5 44.6 69.5 59.2 SiO2 46.5 44.1 69.6 59.4 TiO2 1.37 2.71 0.39 0.72 TiO2 1.34 2.68 0.39 0.72 Al2O3 13.8 13.5 11.9 18.6 Al2O3 13.5 13.4 11.9 18.7 Fe2O3 1.42 2.40 1.16 1.26 FeO 11.7 16.1 2.82 5.06 FeO 10.7 14.1 1.77 3.91 MnO 0.20 0.28 0.06 0.05 MnO 0.20 0.28 0.06 0.05 MgO 6.68 5.33 0.27 3.15 MgO 6.81 5.38 0.27 3.14 CaO 10.7 8.29 0.74 1.11 CaO 11.1 8.81 0.81 1.24 Na2O 2.13 1.93 3.04 2.05 Na2O 2.17 1.95 3.04 2.04 K2O 0.54 0.37 5.14 3.81 K2O 0.55 0.37 5.13 3.80 CO2 0.81 P2O5 0.12 0.32 0.05 0.10 H2O 6.50 6.50 5.89 5.88 CO2 0.82 O2 0.15 0.20 0.11 0.12 LOI 0.97 0.62 2.75 Sum 100.0 100.0 100.0 100.0 Sum 95.6 96.2 94.7 96.9 LOI. Loss of ignition.
Table 2. (a) Whole rock analyses and (b) simplified compositions for the calculation of PT pseudosections
This forward modeling method has the advantage that (1) phase relationships can be predicted over a wide PT-field, (2) peak PT conditions can be directly extracted by comparison with the observed mineral assemblage in the studied rock, (3) parts of PT-paths can be derived by compositional isopleth intersections e.g., for garnet core and rims or with compositional isopleths of other minerals within the PT field of the observed assemblage, and (4) evidence for the form of the retrograde PT path can be extracted. The disadvantage of the method is that retrograde phases do not grow from the effective whole rock composition and hence tend not to be correctly represented in the PT pseudosection. For garnet rim compositions of the eclogite sample 27231 convergence of isopleths were rather wide. Hence we calculated equilibria by intersection of multivariant equilibria using the software winTWQ (Berman, 1991) based on the database of Berman (1988) in the system NCKFMASH. We used the databases DEC06.SLN and DEC06.DAT and calculated activities for the end members anorthite, almandine, pyrope, grossular, tremolite and clinozoisite for garnet rims in contact with amphibole, plagioclase and epidote. Albite, quartz and water were treated as pure phases. The list of the calculated equilibria and the graphical representation is given in ESM2.
The PT pseudosection calculated for eclogite sample 27231 (Figs. 8a–8c) shows a relatively wide PT field for the observed peak metamorphic assemblage garnet-omphacite-chlorite-white mica-epidote-quartz-rutile within the PT range of 11–18 kbar, 490–560 ℃ (Fig. 8a). This field is close to the limits of the PT fields of lawsonite (towards higher pressure), epidote (towards higher temperature), rutile (towards lower temperature; Fig. 8c), and amphibole (towards lower pressure). The latter confirms the observed retrograde nature of amphibole in this eclogite. However, albite, plagioclase and biotite would occur at much lower pressures than the peak PT metamorphic conditions derived. Intersection of isopleths of four garnet end members (Fig. 8b) for the range of garnet core compositions show optimal intersection within the higher PT part of the observed assembage just below the isopleth of the maximum Si content (3.40 apfu) in phengite. This overlap and additionally the isopleths of the range of Na-contents (0.34–0.37 apfu) in the omphacite narrow the PT conditions during growth of the garnet core to 16.5–18.5 kbar/520–555 ℃, which represents the peak pressure conditions (stage Ⅰ).
Figure 8. (a) PT pseudosection of the Beloretzk eclogite sample 27231 in the system SiO2-TiO2-Al2O3-FeO-MgO-MnO-CaO-Na2O-K2O-H2O-O2. Shading of PT fields from white towards dark grey represents increasing variance from bivariant, trivariant, quadrivariant, quintvariant to sixtvariant assemblages. The field of the observed peak metamorphic assemblage is marked in red. (b) Isopleths of garnet end member fractions. The stippled field marks the optimal convergence for the garnet core compositions. (c) PT fields of single mineral phases. PT-path defined by stage Ⅰ (vertically ruled field) with the overlap of garnet core compositions, the compositional range of omphacite and the field of the observed assemblage, stage Ⅱ by garnet rim compositions calculated (A) by conventional phengite-garnet-clinopyroxene geobarometry and (B) by multivariant reaction including symplectite compositions and by fields and finally by fields of retrograde assemblages.
According to the inclusions in garnet (quartz, mainly tschermakitic to pargasitic amphibole, low-Na clinopyroxene, epidote, phengite, rutile) with compositions similar to those of the later symplectite stage garnet overgrew an assemblage that had formed at elevated temperature and lower pressure. However, it is unlikely that the inclusion assemblage represents a prograde assemblage, because garnet cores nucleated at peak pressures and garnet cores are relatively free of inclusions. The isopleths for the garnet rim compositions do not yield reasonable intersections.They merely point to higher temperature during growth of the garnet rim. Instead we calculated a PT point at approximately 13 kbar, 600 ℃ (Fig. 8c, point A) by conventional geothermo- barometry (clinopyroxene-garnet thermometer of Krogh Ravna (2000); garnet-clinopyroxene-phengite barometry of Waters and Martin (1993)) using adjacent rim compositions of garnet and phengite. Multivariant reactions were calculated using the winTWQ software for adjacent amphibole and garnet rim compositions and symplectitic clinopyroxene and plagioclase. Nineteen reactions of three independent equilibria intersect at 600±15 ℃ and 12±0.5 kbar (point B in Fig. 8c; see ESM2). Both points are close within the general error range of ~±1 kbar, ~±25 ℃ at 1σ level assumed for any geothermobarometry method (Spear, 1993). Hence the range of 11.5–13.0 kbar, 585–615 ℃ for stage Ⅱ at peak temperature conditions gives an approximate range for the end of garnet growth and beginning of symplectite formation, when the PT path had reached the PT-field of amphibole. However, isopleths of Na-contents of symplectitic clinopyroxene (0.14–0.19) do not intersect this PT-range, but plot at lower pressure. This is likely due to the fact that the whole rock composition is not the effective one anymore for retrograde reactions. Nevertheless these isopleths can further limit the geometry of the retrograde PT-path. This late partial PT path is characterized by slight decompression and cooling into the PT-fields, where plagioclase, biotite and titanite are present. They occur as minor late phases and titanite frequently forms tiny rims around rutile (Fig. 8c).
The best approach for the field of the observed assemblage in the PT pseudosection calculated for garnet amphibolite sample 27245 (Fig. 9a) is the field of the calculated assemblage garnet- amphibole-(riebeckite)-biotite-chlorite-epidote-carbonate-titanite in the PT-range of 11.5–14.5 kbar, 480–520 ℃. Here albite does not appear, confirming the observed retrograde nature of albite as late porphyroblasts. In the PT pseudosection albite appears only below 9 kbar and 500 ℃. The calculated "riebeckite" is an artefact that appears in some calculated assemblages at temperatures below 500 ℃ due to bad adaptation of Fe3+ in the chosen solution model. Nevertheless the field of the observed assemblage is well restricted as it is close to the upper temperature and lower pressure limit of the PT field of omphacite, the lower temperature limits of the PT-fields of rutile, biotite and epidote as well as the upper temperature limits of the PT fields of lawsonite and chlorite. Ilmenite which is well represented in the natural assemblage only appears in the PT pseudosection in PT fields at slightly lower pressure.
Figure 9. (a) PT pseudosection of garnet amphibolite sample 27245 in the system SiO2-TiO2-Al2O3-FeO-MgO-MnO-CaO-Na2O-K2O-H2O-CO2-O2. Shading of PT fields as in Fig. 8. The field of the observed peak metamorphic assemblage is marked in red. (b) Isopleths of garnet end member fractions. The stippled field marks the optimal convergence for the garnet core compositions, the dotted field for the rim compositions. (c) PT fields of single mineral phases. PT-path defined by stage Ⅰ (vertically ruled field) with the overlap of garnet core compositions with the field of the observed assemblage, stage Ⅱ by the field of garnet rim compositions and finally by fields of retrograde assemblages.
The isopleths of the garnet end-member molar fractions for the garnet core only show a weak convergence in this PT pseudosection. As a result, minima and maxima of the core analyses overlap in a wide field (Fig. 9b). However, the overlap of this field with the PT field of the observed peak metamorphic assemblage restricts the peak pressure conditions at 11.7–14.5 kbar, 480–510 ℃ (stage Ⅰ) for garnet amphibolite sample 27245. On the other hand, isopleths converge much better for the garnet rim compositions at 9.5–11.0 kbar, 535–560 ℃. This field entirely overlaps with two assemblages realized in sample 27245: the first one is garnet-amphibole- biotite-chlorite-epidote-calcite-ilmenite, the second contains additional magnetite. Hence the PT conditions of this field restrict the peak temperature conditions (stage Ⅱ). The further retrograde PT path can be well confined by the PT field of albite < 9 kbar, < 500 ℃, where the conspicuous late albite porphyroblasts of the rock grew, which enclosed all priorly formed mineral phases. The phenomenon is a well known feature occurring on the retrograde decompression path of metapelitic and metabasic high pressure rocks. Jamieson and O'Beirne-Ryan (1991) could show semiquantitatively calculating activity diagrams of fluid species that the stability field of albite would be enlarged during pressure and temperature decrease and hence further albite growth was triggered.
The PT pseudosection calculated for the granite gneiss sample 27241 (Fig. 10a) shows that the dominant assemblage garnet- biotite-phengite-epidote-albite-K-feldspar-quartz-titanite-magnetite is a metamorphic assemblage occurring in a wide PT-field at intermediate temperature and intermediate to high pressures around 5–16 kbar, 450–650 ℃. The field is well bounded by the lower temperature limit of garnet and magnetite, the lower pressure limit of omphacite, which is also the upper pressure limit of albite and biotite, the lower pressure limit of epidote, which is also the upper pressure limit of plagioclase. Surprisingly, isopleths of the garnet end-member molar fractions for the garnet core converge in a very restricted PT field at high pressure and temperature at 15.6–17.0 kbar, 660–690 ℃ (Figs. 10b, 10c). This field merges into the PT-field of omphacite, which is not observed. Hence the overlap with the field of the observed assemblage restricts conditions for stage Ⅰ to even 15.6–16.2 kbar, 660–675 ℃. Isopleths for garnet rim composition converge in a narrow close PT field, at 15.0–15.6 kbar, 650–670 ℃. Garnet obviously grew along a very short early cooling-decompression path that is subsequently followed towards low pressure and temperature within the wide field of the observed assemblage.
Figure 10. (a) PT pseudosection of granite gneiss sample 27241 in the system SiO2-TiO2-Al2O3-FeO-MgO-MnO-CaO-Na2O-K2O-H2O-O2. Shading of PT fields as in Fig. 8. The field of the observed peak metamorphic assemblage is marked in red. (b) Isopleths of garnet end member fractions. The stippled field marks the optimal convergence for the garnet core compositions, the dotted field for the rim compositions. (c) PT fields of single mineral phases. PT-path defined by stage Ⅰ (vertically ruled field) with the overlap of garnet core compositions and the field of the observed assemblage, stage Ⅱ by the field of garnet rim compositions and finally by fields of retrograde assemblages.
Calculation of a PT pseudosection for garnet micaschist sample 27250 (Fig. 11a) which occurs within the eclogite unit in close vicinity to an eclogite as well as a garnet amphibolite occurrence (Fig. 2b) yields relatively well defined PT fields for the observed peak metamorphic assemblages garnet-chlorite- biotite-plagioclase-magnetite and with additional paragonite at 6.5–9.0 kbar, 565–610 ℃. Paragonite was not observed, but could be present in small amounts. The PT field is well restriced to the close upper pressure limits of the PT fields of plagioclase and magnetite, lower pressure limit of the clinopyroxene field, lower temperature limit of the garnet and staurolite fields and upper temperature limit of the chlorite field (Fig. 11c). Also the isopleth of the maximum Ca-content of 0.24 apfu in the plagioclase occurs only slightly outside the field of the observed assemblage. However, isopleth of the maximum Si-content of 3.33 apfu in white mica occurs at much higher pressure near the PT-field of chloritoid above 17.5 kbar that was observed as inclusion in garnet. The isopleth of XMg of 0.25 in this chloritoid also plots close. The high-Si phengite and the chloritoid inclusions define higher pressure relicts (stage Ⅰ). Isopleths of the garnet end-member molar fractions for core and rim converge in a rather wide field partly overlapping with the field of the observed assemblage (Fig. 11b). However, the isopleth of the maximum grossular content of 0.23 plots also near the relict phengite and chloritoid composition. Hence a field of 17–18.5 kbar, 475–525 ℃ would define an approximate location of a relict high pressure stage Ⅰ. On the other hand, the overlap of the field of most garnet compositional isopleths with the observed assemblage can restrict the field for the formation of garnet during a later strong overprint at 7.5–9.0 kbar, 555–610 ℃ (stage Ⅱ; Fig. 2c). The retrograde path occurred during decompression and cooling well within the PT field of plagioclase and confirms the late porphyroblastic growth of plagioclase mimetically across schistosity and crenulation hinges. The PT field of albite, which is not observed as matrix phase, occurs in the calculated PT pseudosection at ≤~9 kbar, ≤470 ℃.
Figure 11. (a) PT pseudosection of garnet micaschist sample 27250 in the system SiO2-TiO2-Al2O3-FeO-MgO-MnO-CaO-Na2O-K2O-H2O-O2. Shading of PT fields as in Fig. 8. The field of the observed peak metamorphic assemblage is marked in red. (b) Isopleths of garnet end member fractions. The stippled field marks the optimal convergence for the garnet core and rim compositions. (c) PT fields of single mineral phases. Stage I is defined by few relict mineral compositions, stage Ⅱ (vertically ruled field) with the overlap of garnet core and rim compositions and the field of the observed assemblage and by fields of retrograde assemblages.
For eclogite sample 27231 we separated three grain size fractions of white mica, one apatite fraction and three omphacite fractions. Omphacite was separated into two density fractions (ρ>3.26, corresponding to more Fe-rich omphacite, and ρ < 3.26, comparatively Fe-poor omphacite) and one fraction of particularly small grain size (< 160 μm). All mineral fractions were meticulously purified; special care was taken to obtain amphibole-free omphacite separates. Deformed white mica oriented parallel to a joint plane in the sample was separated, purified and used as an additional fraction. Analytical data are given in Table 3. Seven fractions from the eclogite matrix (except for the deformed white mica) define an isochron with a date of 532.2±9.1 Ma (MSWD=7.1, Fig. 12). The elevated MSWD of 7.1 is due to very minor Sr-isotopic disequilibria between apatite and the omphacite fractions. The reason for the disequilibria remains unclear; however, this does not affect the slope of the regression line, and exclusion of either apatite or omphacite fractions does not change the resulting age information to any significant extent. It is notable that no correlation exists between grain size and apparent age for the white mica fractions in the eclogite matrix. This indicates that no relict grains from earlier metamorphic stages are present, that the formation of the dated assemblage was a short-term process (cf., Angiboust et al., 2014), the duration of which is covered by the uncertainty of the calculated Rb-Sr date. Hence we interpret the 7-point-isochron date as the age of crystallisation of the here analyzed peak metamorphic eclogite assemblage. Formation of narrow, lower-Si white mica rims during retrograde metamorphism and also the formation of the compositional range of omphacite must have occurred within the time range given by the uncertainties of the age.
Sample No. Material Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr 87Sr/86Sr 2σm PS3243 White mica 250–160 μm 541 94.2 16.9 0.844 324 0.002 0 PS3244 White mica >250 μm 487 84.7 16.8 0.839 937 0.001 9 PS3245 White mica 160–90 μm 477 83.1 16.8 0.841 782 0.002 4 PS3246 Apatite 0.205 261 0.002 27 0.716 413 0.004 4 PS3247 Omphacite ρ>3.26 4.32 41.3 0.303 0.718 817 0.002 1 PS3248 Omphacite < 160 μm 9.96 45.2 0.639 0.721 296 0.003 2 PS3250 Omphacite ρ < 3.26 17.7 45.3 1.13 0.724 904 0.002 8 PS3249 White mica in vein 483 79.4 17.8 0.842 460 0.001 4
Table 3. Rb/Sr isotope data of mineral fractions in eclogite sample 27231
Figure 12. Seven-point Rb/Sr mineral isochron of the eclogite assemblage in sample 27231 defined by grain size fractions of white mica of phengite composition, grain size and density fractions of omphacite and an apatite fraction. An additional fraction of deformed white mica points to a younger age.
The additional fraction of deformed white mica plots below the isochron and points to a younger age of formation of this texturally peculiar white mica. An apatite-white mica isochron would formally yield an age of 504±7 Ma. No clear assignment of this weakly defined age to a specific event can be given. This age only indicates that a weak late- to post-Timanian overprint affected that particular sample.