Mineral composition and thermobarometry of metamorphic rocks of Western Ny Friesland, Svalbard

Introduction

The distinctive geological structure of Svalbard [1-3] and adjacent territories  [4-6] has been the subject of long debate [7-9] due to the tectonic position of the archipelago, which is located on the northwestern Barents-Kara continental margin [10] at the Svalbard Plate – North Atlantic contact. Being part of a formerly integrated fold belt, which formed completely in Middle Devonian time, Svalbard crystalline complexes, together with the Caledonides in the northern Ap-palachians, Eastern Greenland, the British Isles, and Scandinavia, are interpreted as fragments projecting now on both sides of the ocean. The tectonics and rock composition of the above com-plexes are studied to reconstruct the sequence of tectonothermal events responsible for the geologi-cal evolution of the region [11-13].

Geological background

Occurring at the base of Western Ny Friesland are metamorphic  rock complexes (Fig.1) forming a large-scale geologic structure interpreted as a near N-S trending, ~ 150 km long anticlinorium. Its axis spatially coincides with a deep long-lived fault. The core of  the anticlinorium is exposed on northern Mossel Peninsula. Its western limb consists of volcanic-sedimentary rocks making up the Paleoproterozoic (~1750 Ma) Atomfjella Series. Atomfjella rocks (gneisses, often migmatized, schists, quartzites and marbles) are thrown into isoclinals folds  with low-angle thrusts. The sequence is cut by small anatectic granite bodies and meta-ultramafic  and meta-gabbroic rock intrusions. Within the eastern limb, it is overlain with structural unconform-ity by Lower and Middle Riphean Mossel sediments. These rocks (schists and gneisses with quartz-ite and marble interbeds) form a steep monocline, which dips east and displays small open folds. The eastern boundary of the anticlinorium is indicated by the tectonic contact of the Mossel Series with the Upper Riphean Lomfjord Series, in which rocks are poorly metamorphosed, making up the  western limb of the Hinlopen synclinorium. 

Aim of studies

The region’s geology has been studied by the Polar Marine Geological Prospecting Survey. A 1:100000-scale geologic map for the northern part of the peninsula, based on the results  of geological survey, was compiled and the composition of Western Ny Friesland crystalline  complexes was described. The aim of a new stage of studies is to assess the age, the temperatures, pressures and geodynamic setting of metamorphism. The primary goals of our efforts are the detail petrography, studying composition of minerals and estimation of P-T-parameters of metamorphism. 

Methods

Representative rock samples from the Atomfjella and Mossel series, taken from the northern zone of the anticlinorium, were analyzed. In petrographic studies, attention was focused on the microstructure of the rocks and relationships between minerals. Their composition was analyzed by a JSM-6510LA scanning electron microscope equipped with a JED-2200 (JEOL) energy dispersive spectrometer. Metamorphic conditions were estimated using mineral thermobarometry. The representative compositions of the minerals used for thermobarometric calculations are shown in Table 1. 

Petrography and mineral chemistry

The samples analyzed were divided into three groups: metapelitic schists consisting of the Ms-Bt-Grt-Pl (±Ky, St) assemblage (symbols of minerals are given after [15]); calcic pelitic schists, which also contains calcite, scapolite, epidote-group minerals, titanite, and K-feldspar; amphibole-biotite plagiogneisses. 

Atomfjella Series. Rittervatnet Suite. Metapelitic schist 3912-3а

• Petrography. The rock contains Qz (35-40 vol.%), Ms (40-35 vol.%), Grt (12-14 vol.%), Bt (4-5 vol.%), Pl (4-3 vol.%), Ilm (2-1 vol.%), Chl (2-1 vol.%), Tur (1 vol.%), Ky, Kfs, carbonaceous matter (< 1 vol.%). Apatite, zircon, rutile, monazite-(Ce), allanite-(Ce) and hydroxylbastnesite-(Ce) are present as accessories. The rock matrix consists of granoblastic quartz and plagioclase grains, lamellar crystals of muscovite (up to 1 mm in size) and subordinate biotite, and scarce kyanite (Fig.2, а). Carbonaceous matter occurs in mica crystal interstices (Fig.2, b). Garnet forms porphyroblasts with S-shaped tails of quartz and other matrix mineral inclusions. Garnet porphyroblasts often display a fancy structure, which is due to an abundance of quartz inclusions (Fig.2, c). The distribution of ore mineral inclusions shows a zonal pattern: ilmenite is replaced by rutile from core to rim of the porphyroblasts [16]. Garnet crystals are surrounded by the biotite-muscovite aggregate (Fig.2, c), suggesting syntectonic porphyroblastesis. Late mineralization occurs as lenticular chlorite and sericitized plagioclase aggregates in the matrix, chlorite-albite-K-feldspar micro-veinlets with plates of carbonaceous matter cross-cutting garnet (Fig.2, d-f), and hydroxybastnesite-(Ce) rims around al-lanite crystals (Fig.2, g).
• Mineral chemistry. Plagioclase from the matrix is oligoclase and shows zoning: from core to rim, the An end-member mole fraction increases from 0.14 to 0.21 (Fig.2, h, i). It was subjected lo-cally to late albitization. The composition of plagioclase from inclusions in garnet varies from oligoclase (An₂₀) to andesine (An_32-34). Muscovite is treated as quaternary solid solution of musco-vite, paragonite, Mg-celadonite, and Fe-celadonite end members [17]. Si concentration in white mi-ca varies from 2.98-3.11 atom per formula units (apfu) (in contact with garnet) to 3.04-3.17 apfu (in the matrix), Mg content reaches 0.16 apfu and Fe content reaches 0.09 apfu. White mica is en-riched in titanium (TiO₂ 0.3-1.1 wt.%). Biotite is also enriched in titanium (TiO₂ 2.0-3.2 wt.%), dis-plays moderate Mg content [Mg# = Mg/(Mg + Fe) = 0.46-0.50] and elevated alumina content (Al₂O₃ 19-20 wt.%, Al 1.7-1.8 apfu). Chlorite is rich in Fe [Fе# = Fе/(Mg + Fе) = 0.68-89] and poor in Si (2.5-2.7 apfu). In Hey’s diagram (Hey M.H. A new review of chlorite. Mineralogical Maga-zine. 1954. Vol. 30, p. 277-292), chlorite plots the ripidolite field. Al content in chlorite is high (2.7-2.9 apfu) with close ratios of Al^IV and Al^VI. The degree of filling of octahedral positions (R^VI) is 5.8-6.0 apfu.

  Table 1

  Mineral chemistry (wt.%) used of thermobarometry

  

  Notes. Atom per formula units (aofu) are based on 120 (garnet), 110 (biotite, muscovite), 13(С+T) amphibole, 80 (plagioclase). FeO* - total iron. 0.00 - componetn concentration below the detection limit. Location: m - matrix, r - crystal rim (garnet), contact with garnet (biotite and muscovite)

  

  Garnet is generally enriched in the grossular end member (X_Ca 0.12-0.14) and exhibits growth zoning: mole fractions of spessartine and almandine end members decrease from core to rim (X_Mn ranges between 0.05-0.06 and 0.01, X_Fe between  0.80-0.81 and 0.74-0.75). Hence, the mole fraction of the pyrope end member increases between 0.04-0.05 and 0.14-0.16. The composition of REE- and Ti-bearing accessory minerals (monazite, allanite, and ilmenite) was discussed in [16]. Hydroxybastnesite, which overgrowth allanite, inherits its compositional characteristics: at ∑REE content of 0.60-0.68 apfu, Ca 0.16-0.32 apfu, and F 0.12-0.21 apfu (O = 1.5), it shows the prevalence of Се (0.26-0.29 apfu), Nd (0.18-0.19 apfu) and La (0.13-0.15 apfu) over Pr and Sm (0.01-0.04 apfu); its ThO2 content increases to 3.3 wt.%.

Calcic pelitic schist 3912-3b

• Petrography. The rock contains Qz (15-20 vol.%), Kfs (20-15 vol.%), Ms + Ser (20-15 vol.%), Bt (15-10 vol.%), Cal (11-8 vol.%), Grt (7-10 vol.%), Scp (5-10 vol.%), Pl (4-7 vol.%), Chl (2-4 vol.%), Ttn (1 vol.%), Czo, Ep, Ilm, Rt  (< 1 vol.%). Apatite, zircon, and allanite-(Ce) are present as accessories. The rock has a finely banded structure produced by the alternation of lenticular interlayers enriched in  sericitized plagioclase and biotite,  quartz and K-feldspar, and calcite and scapolite (Fig.3, а-e). The latter forms irregular monocrystalline aggregates, up to 1-2 mm in size, which replace biotite (Fig.3, f). 

   

  

  Garnet occurs as isometric porphyroblasts, up to 1 cm in size, surrounded by matrix layers and jet-like biotite aggregates. They have lenticular «coating» consisting of K-feldspar (Fig.3, c). The porphyroblasts contain quartz, K-feldspar, plagioclase, biotite, and ilmenite inclusions, and are cut by thread-like veinlets of chlorite, clinozoisite, epidote, late K-feldspar, and calcite.  Ilmenite is often intergrown with rutile and overgrown with rims of titanite, which also evolve along cleavage cracks in chlorite. Accessory allanite is closely intergrown with REE-bearing clinozoisite, forming crystal cores with external zones consisting of clinozoisite [16].  The rock typically shows an abundance of the late chlorite-muscovite assemblage. Finely scaly mica aggregates replace plagioclase, biotite, and scapolite, and are cut by micro-veinlets of chlorite (Fig.3, f, g).

• Mineral chemistry. Plagioclase from inclusions in garnet is andesine-labradorite (An_42-52), while plagioclase in the matrix is more anorthitic (bytownite, An₈₂). Scapolite is sodium meionite (mizzonite) Ca₃NaAl₅Si₇O₂₄CO₃ [18], as indicated by averaged chemical formula of the mineral (n = 5): (Ca_2,94-3,06Na_0,82-0,91)_3,80-4,01(Al_5,11-4,99Si_6,90-7,01)₁₂O₂₄(CO₃)_0,84-1,05. K-feldspar contains minor Na₂O (0.4-0.8 wt.%, X_Na 0.05-0.07) and BaO (up to 1.7 wt.%). Calcite contains minor FeO (up to 2.6 wt.%), MgO (up to 1.7 wt.%), and MnO (up to 1.9 wt.%). Muscovite is enriched in Si (3.06-3.16 apfu), Mg (up to 0.11 apfu), Fe (up to 0.07 apfu) and depleted in TiO₂ (< 0.1 wt.%). White mica occasion-ally contains up to 1.9 wt.% BaO. Biotite has low Al₂O₃ (16-17 wt.%, Al 1.45-1.50 apfu), moderate MgO (Mg# 0.39-0.54), and elevated TiO₂ (2.9-3.4 wt.%). Chlorite is similar in composition to chlo-rite from sample 3912-3а. Garnet is enriched in the grossular end member (X_Ca 0.26-0.28) and shows normal Fe, Mg, and Mn zoning (core: X_Fe 0.63-0.64, X_Mn 0.06-0.07, X_Mg 0.04; rim: X_Fe 0.60-0.61, X_Mn 0.04, X_Mg 0.09-0.10). The chemical composition of accessory minerals was described in the previous publication of the authors [16]. 

   

  

Calcic pelitic schist 4072-2

• Petrography. The rock contains Qz (25-30 vol.%), Ms + Ser (25-20 vol.%), Kfs (10-15 vol.%), Bt (9-10 vol.%), Grt (8-9 vol.%), Cal (10-5 vol.%), Pl (7-5 vol.%), Chl (4-2 vol.%), Ttn (1-2 vol.%), Ilm (1-2 vol.%), Ep, Prh (< 1 vol.%). Apatite, magnetite and zircon are present as accessories. The rock shows a crenulated structure. It consists of lens-shaped aggregates of biotite, sericitized plagioclase, and prehnite, which alternate with lenses of granoblastic quartz and calcite (Fig.4, а, b). Biotite is often finely intergrown with muscovite and is partly replaced by chlorite (Fig.4, c). Also spatially associated with biotite are fine (0.1 mm) prismatic epidote crystals oriented along schistosity (Fig.4, d) and xenomorphic mesoperthitic K-feldspar (Fig.4, e-i). The matrix contains scattered ~0.6 mm thick idiomorphic titanite crystals and plate ilmenite grains of various sizes (0.1-0.5 mm); these minerals are often intergrown.Garnet forms coarse (up to 5-7 mm) isometric crystals surrounded by biotite and having  concentric zoning caused by the presence of arcuate fragments in the rims of porphyroblasts separated from their cores by quartz interlayers (Fig.4,  j). The porphyroblasts often contain concentric tails of biotite, epidote, and ilmenite inclusions.

• Mineral chemistry. Plagioclase varies in composition from andesine to bytownite (An_49-81). K-feldspar contains minor Na₂O (0.7-0.9 wt.%). Calcite contains minor FeO (up to 3.1 wt.%), MgO (up to 1.3 wt.%), and MnO (up to 0.5 wt.%). The Fe³⁺ content in epidote varies between 0.57 and 0.78 apfu. Epidote crystals often show zoning produced by appearance of Fe-depleted rims at their margins. Prehnite occurs as an iron-bearing variety: Ca_1.92-1.93(Al_0.89-0.90Fe_0.18-0.19)_1.07-1.09(Al_1.02-0.98Si_2.98-3.02)O₁₀(OH)₂ (n = 2).  Muscovite is markedly enriched in Si (3.11-3.23 apfu), Mg (up to 0.11 apfu), and Fe (up to 0.13 apfu); also, it contains TiO₂ (up to 0.8 wt.%). Biotite is poor in Al₂O₃ (15-16 wt.%, Al 1,4-1,5 apfu) and MgO (Mg# 0.31-0.33) but is rich in TiO₂ (4.1-4.7 wt.%). Garnet shows the high con-tent of Ca which increase from core (X_Ca 0.22) to rim (X_Ca 0.31). Also it has Mg and Mn growth zoning (core: X_Mg 0.04, X_Mn 0.04; rim: X_Mg 0.06, X_Mn 0.004). Ilmenite has the low MnO content (< 0.8 wt.%). Magnetite  contains minor TiO₂ (1.2 wt.%), V₂O₅ (1.2 wt.%), and Cr₂O₃ (0.4 wt.%).

Amphibole-biotite plagiogneiss 4143-1

• Petrography. The rock contains Qz (35-40 vol.%), Pl (25-20 vol.%), Bt (15-20 vol.%), Hbl  (10-8 vol.%), Grt (9-6 vol.%), Ilm (4-3 vol.%), Ep (2-3 vol.%), Ser, Chl (< 1 vol.%). Apatite,  allanite-(Ce), rutile, and magnetite are present as accessories. The rock shows the crenulated structure and a porphyroblastic texture due to crystallization of coarse (at least 3-4 mm) garnet and smaller (up to 1-2 mm) plagioclase in the fine-grained matrix. The crenulated structure is formed by the presence of jet-like dark mineral aggregates surrounded porphyroblasts and lenticular clusters of granoblastic quartz grains (Fig.5, а-c).  Biotite occurs as part of these aggregates forming 0.3-0.4 mm thick plate crystals closely intergrown with coarser (1-2 mm) poikiloblastic amphibole (Fig.5, d, e). The matrix consists of plagioclase (fine poorly sericitized granoblastic grains with polysynthetic twins), epidote (prismatic crystals, up to 0.3-0.4 mm in size) and ilmenite (irregular grains, up to 0.1-0.2 mm in size). Garnet forms poikiloblasts containing tails of quartz, ilmenite, and rutile grains; the two latter minerals are often intergrown (Fig.5, f). Garnet is corroded along grain boundaries and micro-fractures by late plagioclase, finely scaly biotite, chlorite, sericite, and ore minerals (Fig.5, g).

• Mineral chemistry. The composition of plagioclase is andesine to labradorite (An_38-53). Some crystals show patch zoning indicated by irregular areas composed of oligoclase (An_24-26). Amphibole is magnesio-tschermakite [20] with Mg/(Mg + Fe²⁺) of 0.50-0.58 and TiO₂ content of 0.7-1.2 wt.%. Biotite typically contains moderate MgO (Mg# 0.49-0.51), low Al₂O₃ (15-16 wt.%, Al 1.35-1.45 apfu), and high TiO₂ (3-3.5 wt.%). The Fe³⁺ content in epidote is 0.50-0.56 apfu (O = 12.5). ∑REE concentration in allanite (Ce) is 0.7-1.1 apfu and Ce concentration is 0.34-0.46 apfu. Garnet is enriched in the grossular end member (X_Ca 0.18-0.21) and depleted in the spessartine end member (X_Mn 0.02-0.03). It shows XMg increase between core and rim from 0.10 to 0.16.

Mossel Series. Floen Suite. Metapelitic schist 3885-1

• Petrography. The rock contains Qz (25-30 vol.%), Ms (40-35 vol.%), Bt (15-20 vol.%), Grt (9-7 vol.%), Pl (8-5 vol.%), St (1-2 vol.%), Ilm (2-1 vol.%), Rt, Chl, carbonaceous matter  (< 1 vol.%). Zircon, apatite, and monazite-(Ce) are present as accessories. The rock shows the crenulated structure formed by wave-curved aggregates of muscovite and biotite alternating with lenticular layers of quartz and plagioclase (Fig.6, а). The intergranular interstices of mica aggregates host plate ilmenite and rutile up to 100-150 µm lengthwise. The mineral typically present in the ma-trix is staurolite. It forms fine (< 0.1 mm) idiomorphic crystals of prismatic habit or clusters of grains oriented conformably with or at an angle to schistosity (Fig.6, b).

  

  

  Micaceous aggregates surround coarse (up to 1 cm) garnet porphyroblasts included in a mantle of granoblastic quartz. An isometric shape of the porphyroblasts is often complicated because their branches are  produced through the selective replacement of muscovite interlayers by garnet. The garnet is cut by chlorite micro-veinlets.

• Mineral chemistry. Plagioclase is oligoclase (An_14-15) or less anorthitic (An_2-4) in late albitization zones. Muscovite is enriched in Na₂O (1.4-1.6 wt.%, Na 0.17-0.20 apfu), TiO₂ (0.27-0.30 wt.%), and SiO₂ (Si 3.11-3.14 apfu). Biotite contains moderate MgO (Mg# 0.52-0.53), elevated Al₂O₃ (19.4-20.4 wt.%), and low TiO₂ (1.5 wt.%). Staurolite is Fe-rich (Fe# 0.84) and contains minor ZnO (1.8 wt.%, Zn 0.38 apfu). Monazite (Ce 0.41-0.44 apfu) shows enrichment of ThO₂ (1.7-5.3 wt.%) and UO₂ (0.5-0.8 wt.%).Garnet exhibits well-defined zoning due to core-rim depletion in X_Mn (in the range of 0.05-0.00) and enrichment in X_Mg (in the range of 0.04-0.15). Also the core of garnet porphyroblasts is more calcium (X_Ca 0.19-0.20) compared to the rim (X_Ca 0.05-0.06).

Mossel Series. Mosseldalen Suite. Metapelitic schist 4032-1

• Petrography. The rock contains Qz (35-40 vol.%), Ms (30-25 vol.%), Bt (22-20 vol.%), Grt (10-11 vol.%), Ilm (2-3 vol.%), Chl (1 vol.%), Rt (< 1 vol.%). Zircon, apatite, monazite-(Ce) and sulphides are present as accessories. The rock displays the crenulated structure and consists of lenticular interlayers of quartz and plagioclase alternating with jet-like aggregates of muscovite and biotite which surround coarse (up to 1 cm in size) garnet porphyroblasts. Interstices of the micaceous aggregate host numerous elongated ilmenite and rutile grains up to 50-100 µm in size (Fig.6, c). Chlorite locally replaces biotite. Garnet porphyroblasts have idiomorphic or more complex S-shaped contours with tails of ilmenite and rutile inclusions, which inherit a schistosity pattern. Some of the porphyroblasts are overgrown with the quartz mantle; in this case, porphyroblasts have reticular rims instead of rectilinear boundaries. Additionally, in some places, garnet is cut by thread-like chlorite veinlets oriented at an angle to schistosity (Fig.6, d).

• Mineral chemistry. The plagioclase composition corresponds to oligoclase (An_14-17). It has been subjected locally to late albitization. Muscovite is enriched in Na₂O (1.6-2,1 wt.%, Na 0.18-0.25 apfu) and TiO₂ (0.21-0.41 wt.%) while it shows Si values between 3,01 and 3,12 apfu. Contents of celadonite and Fe-celadonite end members in white mica do not exceed 0.11 and 0.07 apfu, respectively. Biotite contains moderate MgO (Mg# 0.43-0.51), elevated Al₂O₃ (18.9-19.3 wt.%) and low TiO₂ (1.2-1.6 wt.%). Chlorite (ripidolite) exhibits enrichment in Fe (Fe# 0.47-0.70) and Al (2.5-2.7 apfu) and depletion in Si (2.6-2.8 apfu).Garnet is enriched in the grossular end member (from core to rim, X_Ca ranges between 0.20-0.23 and 0.06-0.09). Simultaneously, garnet shows normal Mg and Mn zoning (core: X_Mg 0.03-0.04, X_Mn 0.03; rim: X_Mg 0.10, X_Mn < 0.01).Ilmenite contains no more than 0.4 wt.% MnO. Monazite is characterized by the prevalence of Ce (0.40 apfu) over other rare earth elements (Nd 0.29, La 0.20, Pr 0.04, Sm 0.03 apfu) and contains 5.1 wt.% ThO₂.

Results of mineral thermobarometry

• A garnet-biotite geothermometer, garnet-biotite-plagioclase-quartz, garnet-kyanite/sillimanite-quartz-plagioclase and garnet-rutile-ilmenite-plagioclase-quartz thermobarometers. For estimation of pressure and temperature at which minerals in metapelites and similar rocks were equilibrated, a geothermometer, based on an exchange reaction
  Py + Ann = Alm + Phl GB,
  which describes the partitioning Fe and Mg between garnet and biotite when temperature rises, and thermobarometers, based on net transfer reactions
  $$\begin{array}{c}\text{6An + 3Ann = Alm + Grs + 3Sid + Qz }\left(\text{GBPQ}\right)\text{;}\\ \text{3An = Grs + 2 Als + Qtz }\left(\text{GASP}\right)\text{;}\\ \text{3An + 6Ilm + 3Qz = 2Alm + Grs + 6Rt }\left(\text{GRIPS}\right)\end{array}$$
  , which show the partitioning Ca between garnet and plagioclase when pressure rises, are widely used. Modern calibrations of these thermobarometers [21-23] are well-supported thermodynamical-ly, based on experimental data and natural evidence, and take into account nonideality of garnet, biotite, and plagioclase solid solutions [24-26]. The application of the GASP geobarometer is limited by parageneses containing Al₂SiO₅ polymorphs and those of the GRIPS geobarometer by parageneses with ilmenite and rutile.The garnet-biotite geothermometer showed that the peak metamorphic temperature of Atomfjella series metapelitic and calcic pelitic schists was 670-690 ^oС. Matrix biotite and rim of garnet with the highest MgO content were equilibrated at this temperature (Table 2). The pressure, calculated for this temperature with different thermobarometers, is variable. Higher pressure esti-mates (9.5-13.5 kbar) were obtained for the metapelitic schist and lower ones (7.2-8.2 kbar, GBPQ; 10.5-12 kbar, GRIPS) for calcic pelitic schists.

  Table 2

  Representative temperature and pressure estimates obtained with the garnet-biotite geothermometer and garnet-biotite-plagioclase-quartz (GBPQ), GASP, and GRIPS geobarometers

  Sample	Analysis				Mole fractions								T, °C		P, kbar
  	Grt	Bt		Pl	XcaGrt	XMnGrt	XMgGrt	XTiBt	XMgBt	XCaBt	XCaPl	XFeI lm	GBKM04	GBG10	GBPQ	GASP	GRIPS
  Atomfjella Series Metapelitic schist
  3912-3а	001	068		070	0.128	0.000	0.160	0.039	0.165	0.38	0.21	0.99	677	690	11.6	13.5	10.9
  	013	005		008	0.091	0.000	0.154	0.051	0.128	0.38	0.21	0.99	667	673	9.5	11.5	10.2
  Calcic pelitic schist
  3912-3b	034	069		070	0.270	0.045	0.096	0.057	0.083	0.47	0.82	0.88	609	601	8.2	–	10.5
  4072-2	025	038		061	0.276	0.004	0.063	0.099	0.041	0.26	0.81	0.99	693	690	7.2	–	12.0
  	028	006		079	0.312	0.004	0.057	0.083	0.058	0.28	0.49	0.99	676	676	8.2	–	11.2
  Amphibole-biotite plagiogneiss
  4143-1	001	011		016	0.194	0.013	0.137	0.057	0.075	0.40	0.38	0.96	715	705	8.5	–	11.2
  	084	046		047	0.170	0.013	0.158	0.058	0.056	0.45	0.32	0.96	708	692	9.4	–	10.9
  	084	046		048	0.170	0.013	0.158	0.058	0.056	0.45	0.40	0.96	708	692	8.6	–	10.7
  Mossel Series Metapelitic schist
  3885-1	001	063		050	0.048	0.000	0.146	0.028	0.165	0.42	0.15	0.98	580	590	7.6	8.8	11.3
  	033	067		024	0.073	0.000	0.139	0.028	0.149	0.44	0.14	0.98	580	585	9.6	10.6	11.2
  4032-1	001	034		037	0.140	0.001	0.087	0.024	0.167	0.40	0.14	0.99	555	560	10.6	–	9.4
  	010	053		042	0.090	0.002	0.099	0.031	0.138	0.42	0.14	0.99	540	545	8.9	–	9.9
  	025	032		014	0.075	0.002	0.115	0.024	0.139	0.38	0.16	0.99	600	605	9.5	–	9.9

  Notes. Calibrations: garnet-biotite geothermometer [23] (GB_KM04), [24] (GB_G10); garnet-biotite-plagioclase-quartz geobarometer [25] (GBPQ), garnet-A₂SiO₅-plagioclase-quartz [22] (GASP) geobarometer, garnet-rutile-ilmenite-plagioclase-quartz [27] (GRIPS) geobarometer.

  The peak metamorphic temperature for Mossel series metapelitic schists was 580-600 ^oC. Most of the pressure values obtained for this and lower temperatures reflecting mineral re-equilibration conditions at a retrograde stage (540-560 ^oC), were 9-11 kbar. Ti-in-biotite and Ti-in-muscovite geothermometers and the phengite thermobarometer. Two empirical geothermometers are based on temperature dependences of minor Ti in micas from metapelite [27, 28]. The peak metamorphic temperature calculated for the studied rocks with these geothermometers are similar to those obtained with the garnet-biotite geothermometer. They were 660-730 ^oC for the Atomfjella Series and 550-580 ^oC for the Mossel Series (Table 3). At these temperatures, Ti-rich micas in the matrix were crystallized. The overall spread of temperature estimates is much wider (Fig.7, а, b). With respect to biotite, decrease in calculated temperature (reaching tens of degrees) is typical for the mica crystals contacting with garnet and could be due to the partial loss of Ti pro-voked presumably by the exchange of femic components between two minerals at the retrograde stage.  With respect to muscovite, such a decrease is most likely due to the presence of a low-temperature generation (< 500 ^oC) of white mica, which crystallized under greenschist facies conditions. It should be noted that the lowest temperature value (360 ^oC) calculated with the Ti-in-muscovite geothermometer (which was calibrated over the range 450-800 ^oC), is consistent with TiO2 content of 0.13 wt.% close to the sensitivity threshold of the electron probe method (~0.1 wt.%). Because near one quarter of the white mica analyses have shown lower TiO2 contents, no temperature esti-mates for them are available. This means that low-temperature white micas are more abundant than those shown on the histogram in Fig.7, b.

  Table 3

  Representative temperature and pressure estimates obtained with Ti-in-biotite geothermometer, Ti-in-muscovite geothermometer and phengite geobarometer

  Sample	Analysis			Position	Mg#Bt	Crystallochemical coefficients (O = 11)						T, °C		P, kbar
  	Bt	Ms				TiBt	TiMs	SiMs	AlMs	MgMs	FeMs	BH05	MWC15	MMS87	MCT08	MK15
  	Atomfjella Series Metapelitic schist
  3912-3а	077	076		m	0.48	0.172	0.037	3.17	2.58	0.157	0.090	677	650	7.1	11.1	9.7
  	078	075		m	0.50	0.156	0.055	3.10	2.68	0.112	0.066	670	730	6.3	10.9	10.4
  	068	066		c	0.48	0.112	0.025	3.04	2.83	0.078	0.069	605	565	3.0	2.4	6.2
  Calcic pelitic schist
  3912-3b	024	077		m	0.39	0.200	0.007	3.10	2.79	0.074	0.015	690	370	2.3	–	1.2
  	069	075		m	0.54	0.165	0.000	3.10	2.84	0.056	0.024	685	–	–	–	–
  	022	–		c	0.41	0.053	–	–	–	–	–	360	–	–	–	–
  4072-2	006	071		m	0.32	0.240	0.040	3.23	2.50	0.089	0.128	708	660	8.6	13.9	9.8
  	038	062		m	0.31	0.280	0.000	3.14	2.78	0.000	0.043	728	–	–	–	–
  Amphibole-biotite plagiogneiss
  4143-1	026	–		m	0.51	0.152	–	–	–	–	–	665	–	–	–	–
  	086	–		m	0.50	0.200	–	–	–	–	–	705	–	–	–	–
  	Mossel Series Metapelitic schist
  3885-1	067	068		m	0.53	0.082	0.015	3.11	2.74	0.102	0.070	555	485	3.9	2.7	4.9
  4032-1	053	046		m	0.51	0.090	0.025	3.05	2.81	0.073	0.062	570	555	3.3	2.7	5.8
  	032	034		c	0.46	0.070	0.018	3.01	2.88	0.058	0.065	490	510	2.0	–	4.8

  Notes. Calibration: Ti-in-biotite geothermometer [27] (B_H05); Ti-in-muscovite geothermometer [28] (M_WC15); phengite geobarometer [29, 30] (M_MS87), [31, equation 7] (M_CT08), [29, equation 4] (M_K15). Location of mica crystals: m – matrix, c – contact with garnet. Mg#^Bt = Mg/(Fe+Mg) in biotite.

  Phengite thermobarometer takes into account the variations of Si, Fe, and Mg contents in white micas described by Tschermak's substitution Al^IVAl^VI = Si + (Mg, Fe²⁺). Experiments [30] have shown that muscovite is enriched the high Si (celadonite) component in a low temperature and high pressure range. The thermobarometer equation based on these experiments was calibrated for the paragenesis of Ms-Phl-Kfs-Qz. When phengite mica is a part of other assemblages, this equation allows to estimate the lower limit of pressure [30]. More recent calibrations of the thermobarometer consider the results of the physical and chemical modeling of mineral parageneses in metapelites [31] and the results of the empirical generalization concerning temperature and pressure dependenc-es of natural and synthetic phengite compositions [29].

  

  For the peak temperatures calculated with the Ti-in-muscovite geothermometer, most of the pressure estimates obtained with the phengite thermobarometer for Atomfjella Series schists (Table 3) vary in the range of 6-8.5 kbar (M_MS87) or 9.5-11 kbar (M_CT08, M_K15). The latter result is in good agreement with the values obtained with the garnet-plagioclase thermobarometers (see Table 2). In the case of Mossel Series schists, similar values (3-6 kbar) were found to be underestimated. This could be due to the effect of more recent (relative to final stages of the porphyroblast formation) processes, which often contribute to the white mica recrystallization at low temperatures and pressures [30].
• Geothermometers and geobarometers considering variations in the calcium amphibole composition. An amphibole-plagioclase geothermometer, proposed for amphibolites and amphibole-biotite gneiss, is one of the more effective tool. It is based on a net transfer reaction which describes the appearance of increasingly aluminous amphibole in rocks containing quartz and plagioclase when temperature rises. Tschermak's substitution A + ^T1Si = ^ANa + ^T1Al, where  is vacancy, is consistent with this reaction in the amphibole structure. The geothermometer equation was calibrated on the base of experimental data and models deal with nonideality of amphibole and plagioclase solid solutions [32]. The calculated temperatures, at which hornblende and plagioclase were equilibrated in the studied am-phibole-biotite gneiss sample (695-745 ^oС, Table 4), are stable and may be accepted as peak values.

  Table 4

  Results of thermobarometry of amphibole-biotite gneiss (sample 4143-1) obtained using calcium amphibole composition

  Analysis			Crystallochemical coefficients in the amphibole (O = 23)										T, °C			P, kbar
  Hbl	Grt	Pl	AlT1		AlM2	Ti	Fe3+	Fe2+	Mg	NaM4	NaA	K	HP*BH94	GH**GP84	HPL21	HP*M15	GHPQ**
  041	071	016	1.75		0.67	0.093	0.83	1.25	2.13	0.179	0.135	0.110	695	620	665	8.5	8.3
  042	084	047	1.92		0.66	0.105	0.64	1.63	1.94	0.019	0.289	0.138	745	660	690	9.3	8.2
  043	071	047	1.75		0.64	0.097	0.65	1.54	2.07	0.015	0.261	0.163	712	640	675	8.9	9.2
  101	071	047	1.91		0.67	0.131	0.70	1.46	1.99	0.104	0.219	0.161	725	645	725	8.9	9.6
  102	071	016	1.79		0.72	0.078	0.59	1.79	1.80	0.0544	0.215	0.162	720	695	640	9.5	8.7

  Notes. Calibrations: amphibole-plagioclase geothermometer [32] (HP_BH94); garnet-amphibole geothermometer [33] (GH_GP84); Ti-in-amphibole geothermometer [34] (HP_L21); amphibole-plagioclase thermobarometer [35] (HP_M15), garnet-amphibole-plagioclase-quartz thermobarometer [36] (GHPQ). Crystallochemical coefficients were calculated using the structural formula of amphibole A(M₄)₂(M1₃)₃(M₂)₂(T₂)₄(T1)₄O₂₂(O,OH,F)₂ [20]. Symbols ^* and ^** show thermobarometers for which pressure and temperature estimates are mutually conformable.

  An amphibole-plagioclase thermobarometer [35] based approximately on the same grounds as the amphibole-plagioclase geothermometer. Being empirical, it takes into account the pressure-dependent variations of Al/Si ratio in amphibole and coexisting plagioclase. Pressure calculated with this thermobarometer and matched to temperatures calculated with the amphibole-plagioclase geothermometer was 8.5-9.5 kbar.One of the major minerals present in amphibolite and amphibole-biotite gneiss is garnet. Therefore, thermobarometry of such rocks can also be performed with a garnet-amphibole geothermometer based on the exchange of Fe and Mg between garnet and hornblende, and with a garnet-amphibole-plagioclase-quartz thermobarometer based on a net transfer reaction, during which (while pressure rises) plagioclase is replaced by grossular garnet and released Al incorporates in amphibole. The equations for the thermobarometers were calibrated using metabasic rocks and take into account nonideality of  mineral solid solutions [33, 36].The equilibrium temperatures calculated with the garnet-amphibole geothermometer for the studied rock (620-660 ^OС in most cases) are much lower than peak temperatures. This discrepancy is probably due to the fact that  Fe and Mg are more mobile than Al in the amphibole structure at low temperatures. This assumption is indirectly supported by the temperatures derived from a recently proposed empirical Ti-in-amphibole geothermometer [34]. Although the highest temperatures calculated with this geothermometer are close to peak values (690-725 ^OС), another part of temperature estimates in the range of 640-675 ^OС could be accepted as an evidence for the partial loss of relatively mobile Ti by amphibole when temperature falls.Pressure calculated with the garnet-amphibole-plagioclase-quartz thermobarometer are the same as those calculated with the amphibole-plagioclase thermobarometer (8.5-9.5 kbar). It is noteworthy that peak temperature and pressure estimates obtained with thermobarometers taking into account variations in the composition of calcium amphibole agree well with those obtained with garnet-biotite and Ti-in-biotite geothermometers, as well as GBPQ and GRIPS thermobarometers (see Table 2 and 3).

  Table 5

  Representative compositions of chlorites and the results of chlorite geothermometry, wt.%

  Series	Atomfjella					Mossel
  Sample	3912-3а		3912-3b			3885-1		4032-1
  Analysis	021		037	048	053	026	027	045	058
  SiO2	23.12		26.44	26.33	23.56	24.58	25.70	24.41	25.37
  TiO2	0.00		0.00	0.00	0.00	0.00	0.24	0.00	0.00
  Al2O3	19.70		18.96	17.32	20.96	19.92	21.95	21.98	20.72
  FeO*	40.66		28.69	30.89	34.48	30.32	23.61	23.58	26.94
  MnO	0.16		0.10	0.45	0.75	0.22	0.00	0.00	0.00
  MgO	2.73		12.33	10.90	7.37	10.27	15.24	14.80	12.90
  Sum	86.37		86.43	85.89	87.12	85.31	86.74	84.77	85.93
  Crystallochemical coefficients (O = 14)
  Si	2.714		2.866	2.931	2.647	2.748	2.705	2.639	2.751
  AlIV	1.286		1.134	1.069	1.353	1.252	1.295	1.361	1.249
  T	4.000		4.000	4.000	4.000	4.000	4.000	4.000	4.000
  AlVI	1.439		1.297	1.204	1.422	1.372	1.427	1.441	1.398
  Fe	3.991		2.611	2.877	3.239	2.835	2.078	2.133	2.443
  Mn	0.015		0.010	0.042	0.071	0.021	0.000	0.000	0.000
  Mg	0.478		2.001	1.809	1.234	1.712	2.391	2.386	2.085
  Sum	5.924		5.919	5.932	5.966	5.940	5.896	5.960	5.926
  T, oC	335		268	249	492	353	319	466	325

  Note. T – temperature calculated with a chlorite geothermometer[37].

• Chlorite geothermometry. Empirically, it is based on the regular increase of Al^IV content in chlorite at rising temperature [37]. To approach this problem more rigorously, calibrations based on reactions involving both di- and tri-octahedral components of chlorite solid solutions should be used. One calibration considering the temperature dependence of the reaction 2Cln + 3Sud = 4Ame + 7SiO₂ + 4H₂O and assuming that ΣFe = Fe²⁺ [38] was used in the present study. Temperatures calculated with this geothermometer for studied rocks vary from 250 to 500 ^OС (Table 5). The distribution of temperature values is discrete: most of them range between 260-370 ^OС while minority of them range between 460-500 ^OС (Fig.7, c). The result obtained, together with the Ti-in-muscovite geothermometry data, indicate the presence of low-temperature chlorite and finely scaly muscovite (sericite) assemblage in the schists. This assemblage is a part of the late paragenesis Ms-Chl-Ep-Ab-Prh-Ttn that is characteristic of the low greenschist facies and the transition from greenschist- to prehnite-pumpellyite facies condition

Conclusion

As a result of the research, mineral composition and metamorphic conditions of rocks in Western Ny Friesland have been elucidated. Peak temperature and pressure during metamorphism of the Atomfjella Series were consistent with the high-pressure part of the upper amphibolite facies (690-720 ^OС, 9-12 kbar), while the same values of the Mossel Series were consistent with the high-pressure part of the lower amphibolite facies (580-600 °С, 9-11 kbar). In addition to the high-temperature parageneses Ms-Bt-Grt-Pl (±Ky, St), Bt-Grt-Pl-Kfs-Cal (±Scp), and Bt-Hbl-Ep-Grt-Pl, the rocks of both series host the low-temperature mineral assemblage Ms-Chl-Ep-Ab-Prh-Ttn which was formed upon transition from greenschist to prehnite-pumpellyite facies conditions (260-370 ^OС).  In contrast to the data reported by the earlier workers [14], our results were obtained with up-to-date mineral thermobarometers and take into account the maximum possible number of co-existing minerals. One of the main conclusions drawn from our study is that peak pressure obtained was higher than previously thought, which was achieved during the metamorphism of both series. 

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