Chlorine-rich potassic-ferro-pargasite with Cl-poor cummingtonite in mafic granulite from Austhovde, Lützow-Holm Complex, East Antarctica

Abstract

Blue-green potassic-ferro-chloro-pargasite containing up to 0.3 wt% BaO and 4.6 wt% Cl occurs together with Cl-poor cummingtonite in a mafic granulite composed mainly of olive-brown potassic-ferro-pargasite with subordinate orthopyroxene, Ba- and Cl-bearing biotite, calcic plagioclase, quartz, ilmenite, zircon, and Cl-rich fluorapatite. The mafic granulite is characterized by high bulk rock contents of large-ion lithophile elements such as K, Ba, Pb, and Rb. Mineral assemblages, compositions and textures suggest four stages of recrystallization to form and modify the potassic-ferro-pargasite. Stage 1 is the main granulite-facies metamorphism, whereas stages 2-4 correspond to local phenomena most probably caused by fluid infiltration along a fracture during cooling. Stage 2 includes biotite formation with concomitant compositional change of orthopyroxene and potassic-ferro-pargasite to more Fe-rich. Fluorapatite partially changed composition from Cl-rich to Cl-poor. Stage 3 was a more localized event including (1) increase of K, Cl, Al, and Fe and decrease of Si, Ti, and Mg in the potassic-ferro-pargasite, accompanied by the color change from olive-brown to blue-green, and (2) formation of Cl-poor cummingtonite and sodic plagioclase by replacement of preexisting potassic-ferro-pargasite. Stage 4 was a more localized phenomenon within blue-green potassic-ferro-pargasite. It may be expressed as a volatile-conserving reaction such as Cl-bearing potassic-ferro-pargasite → potassic-ferro-chloro-pargasite + Cl-poor cummingtonite. The mafic granulite appears not to have undergone partial melting even at stage 1 most probably because of the high Cl content of the dominant potassic-ferro-pargasite.

INTRODUCTION

Amphibole, biotite, and apatite are important rock-forming minerals of various igneous and metamorphic rocks, containing hydroxyl (OH) and halogen (F, Cl). Amphibole is the major phase of amphibolite-facies mafic metamorphic rocks, and breaks down to form orthopyroxene-bearing mineral assemblages with increasing metamorphic grade to the granulite facies. It has a flexible crystal structure characterized by a large number of sites with variable size accommodating various elements, and therefore its breakdown reaction is continuous, taking place over a wide range of temperature.

Here we report a mafic granulite composed mainly of Cl-rich amphibole from Austhovde, East Antarctica, especially focusing on Cl-rich amphibole that has been reported in a variety of rock types: skarn, calcareous metamorphic rock, calcareous pegmatite, granitic rock, submarine mylonitic gabbro, mylonite, amphibolite, rodingite, and charnockitic rock (e.g., Giesting and Filiberto, 2016; Aranovich and Safonov, 2018; Henry and Daigle, 2018 and references therein). In many cases Cl-rich amphibole is considered to be a product of metasomatism induced by infiltration of Cl-rich fluid (e.g., Giesting and Filiberto, 2016; Aranovich and Safonov, 2018). Cl-rich potassic-ferro-pargasite + Cl-poor cummingtonite assemblage has not been reported as far as we know. This paper aims to document the detailed mode of occurrence of the new mineral assemblage and discuss the possible reactions to form it.

GEOLOGICAL OUTLINE

The high-grade metamorphic rocks around Lützow-Holm Bay belong to the Late Proterozoic-Early Paleozoic Lützow-Holm Complex (LHC), which crops out between longitudes 39°E and 45°E, that is, from northeast of Syowa Station along the Prince Olav Coast and south of the station along the Sôya and Prince Harald Coasts (Fig. 1a). It is bounded by the Late Proterozoic Rayner Complex to the east and by the Late Neoproterozoic to Early Cambrian Yamato-Belgica Complex to the west.

Figure 1. (a) Index map of Antarctica showing location of Lützow-Holm Bay. (b) Map of the Prince Harald and Sôya Coasts of Lützow-Holm Bay showing the location of Austhovde in the Late Neoproterozoic to Early Cambrian Lützow-Holm Complex. (c) Geological map of Austhovde-minami Rocks showing the locality of studied mafic granulite sample (84012223) (after Shiraishi and Yoshida, 1987).

The LHC is characterized by progressive metamorphism from the upper amphibolite facies in the eastern part of the Prince Olav Coast to the upper granulite facies in the southern part of the Sôya Coast (Hiroi et al., 1983; Shiraishi et al., 1989; Hiroi et al., 1991). Rocks of the LHC experienced a clockwise P-T path, as evidenced by the presence of relict prograde kyanite included in garnet and plagioclase in both the amphibolite- and in the granulite-facies rocks of the LHC (Hiroi et al., 1983; Shiraishi et al., 1989; Hiroi et al., 1991). Suzuki and Kawakami (2019) revealed that some rocks in the central Prince Olav Coast and Sôya Coast experienced almost the same P-T conditions around the kyanite/sillimanite transition (∼ 830-850 °C/∼ 11 kbar). The LHC may be subdivided into several sub-complexes or suites based on the protolith and main metamorphic ages (Takamura et al., 2020; Dunkley et al., 2020). In addition, the Neoproterozoic-Cambrian regional metamorphic episode of the LHC can be divided into an early thermal event (either an independent single metamorphic event or the prograde stage of the main event) prior to 580 Ma, near-peak condition stage between 580 and 560 Ma, and subsequent retrograde stage after 550 Ma (e.g., Kitano et al., 2023). There is evidence that the main metamorphic event of some of the LHC exposed in the eastern Prince Olav Coast is Late Proterozoic (e.g., Kitano et al., 2023).

Austhovde is located on the Prince Harald Coast (Fig. 1b) and consists of three outcrops: Austhovde-kita Rock, Austhovde-naka Rock, and Austhovde-minami Rocks (Shiraishi and Yoshida, 1987). Each outcrop is about 1.0 km in length. According to Shiraishi and Yoshida (1987), biotite gneiss and garnet-biotite gneiss are predominant, alternating with biotite-hornblende gneiss and biotite amphibolite in Austhovde-kita Rock and Austhovde-naka Rock. The foliation trends E-W and dips S in these outcrops. In contrast, Austhovde-minami Rocks are composed of alternating quartzite, siliceous garnet gneiss, marble and skarn, garnet-biotite gneiss, biotite gneiss, and pyroxene-hornblende gneiss (Fig. 1c). Minor amphibolite and mafic to ultramafic granulite occur as thin layers and lenses in gneisses. Gneissose granite is widespread, especially in the south-western part. The general layering and foliation trend NW-SE and dip SW, and the mineral lineation plunges gently W. Tsunogae et al. (2016) reported a zircon U-Pb age of 579 ± 5 Ma as the timing of high-grade metamorphism for garnet-clinopyroxene granulite similar to layer A of sample 84012223. Tsunogae et al. (2016) estimated peak temperature of 780-830 °C and pressure of 7.2-7.7 kbar for the same sample. Takahashi and Tsunogae (2017) revealed subsequent high-temperature decompression down to 5.0-3.0 kbar based on the study of carbonic fluid inclusions in garnet-clinopyroxene granulite from Austhovde-minami Rocks. Takahashi and Tsunogae (2017) also estimated peak P-T conditions of 8.0-9.0 kbar and 800-850 °C for Austhovde.

SAMPLE DESCRIPTION

Sample 84012223 is a medium-grained melanocratic rock, occurring as a part of layered mafic block in pyroxene-hornblende gneiss in the gneissose granite-dominant area (Figs. 1c and 2a). Four samples were collected from the mafic block by K. Shiraishi on the 25^th Japanese Antarctic Research Expedition in 1984. Sample 84012223 is composed of two distinct layers A and B separated by a relatively sharp boundary (Figs. 2b and 2c). Layer A consists mainly of clinopyroxene and garnet, whereas layer B consists mostly of potassic-ferro-pargasite (∼ 85 modal %) with quartz pools and veins (Figs. 2d and 2e). It is noted that layer B sample has a biotite-rich surface (Figs. 2d and 2e), and thereby layer B sample is tentatively subdivided into the ‘main part’ and thin ‘biotite-rich part’ close to the biotite-rich surface (Fig. 2e).

Figure 2. (a) Photograph showing the mode of field occurrence of layered mafic granulite block in pyroxene-hornblende gneiss. (b) Close-up view of layer B of sample 84012223 with thin skin of layer A. (c) Plane-polarized photomicrograph of boundary zone between layers A and B. (d) Close-up view of layer B with biotite-rich surface as well as quartz pool and vein. (e) Plane-polarized photomicrograph of layer B with biotite-rich surface. Layer B is tentatively subdivided into the main part and biotite-rich part close to the biotite-rich surface. The mineral abbreviations are after Warr (2021).

The main part of layer B is composed mainly of coarse-grained, olive-brown potassic-ferro-pargasite, intergrown with minor orthopyroxene, plagioclase, biotite, quartz, ilmenite, zircon, and fluorapatite (Fig. 2e). Orthopyroxene occurs mainly around quartz pools and veins. Fine-grained orthopyroxene associated with biotite and quartz is also found in small aggregates interstitially to larger potassic-ferro-pargasite grains. Brown biotite usually shows textural equilibrium relations with orthopyroxene and potassic-ferro-pargasite, although it locally penetrates coarse grains of these minerals. Fluorapatite in grains up to 0.5 mm in diameter is scattered in small clusters (Fig. 2e). Clinopyroxene, garnet and symplectitic intergrowths of orthopyroxene and plagioclase after garnet have not been found, a marked contrast to their common occurrences in layer A of sample 84012223 and other mafic granulites in the same block. K-feldspar is also absent despite the high bulk rock K content.

The biotite-rich part is characterized by the occurrences of cummingtonite and sodic plagioclase in addition to the slight increase in the amount of biotite (Figs. 3a, 3c, and 3d). Cummingtonite partially replaces potassic-ferro-pargasite and includes quartz and fine particles of an unidentified Ti-rich phase. Sodic plagioclase also contains tiny anhedral inclusions of potassic-ferro-pargasite. Moreover, olive-brown potassic-ferro-pargasite is rimmed and embayed by blue-green potassic-ferro-chloro-pargasite in the immediate vicinity of cummingtonite and sodic plagioclase [Figs. 3a, 3b and Supplementary Fig. S1 for close-up view of intergrown cummingtonite and potassic(-ferro-chloro-) pargasite; Fig. S1 is available online from https://doi.org/10.2465/jmps.230329]. These features define a distinct domain shown as an ‘altered zone’ in the biotite-rich part (Fig. 3a). Biotite commonly penetrates potassic-ferro-pargasite and orthopyroxene grains, and its arrangement and enrichment produce a planar structure of the biotite-rich surface.

Figure 3. (a) Plane-polarized photomicrograph of the area in the red rectangle in Figure 2e, showing ‘altered zone’ in the biotite-rich part. (b) Plane-polarized photomicrograph of the area in the pink rectangle in (a), showing the occurrences of blue-green potassic-ferro-(chloro-)pargasite and cummingtonite replacing olive-brown potassic-ferro-pargasite. (c) Backscattered electron (BSE) image of the area in the red rectangle in (b). (d) X-ray elemental maps for Ca (d-1), Ti (d-2), K (d-3), Cl (d-4), and Mg (d-5) of almost the same area as in part (c). Increases in the contents of these elements are indicated by change in color from blue to green to yellow to red to white in all the X-ray elemental maps. There are two different compositions for plagioclase: anorthite is dominant in Pl-1, whereas albite is dominant in Pl-2. Note the occurrences of fine particles of an unidentified Ti-rich phase and the small anhedral amphibole grains in cummingtonite and Pl-2.

ANALYTICAL METHODS

Concentrations of the major constituents and 4 trace elements (Cr, Ni, V, and Zr) in layer B of sample 84012223 were determined using a Rigaku RIX-2000 X-ray fluorescence spectrometer at the Department of Geosciences, Shimane University, Matsue, Japan. Analysis was made on a glass bead prepared in an automatic bead sampler, using an alkali flux consisting of 80% lithium tetraborate and 20% lithium metaborate, with a sample to flux ratio of 1:2. Analytical procedures are described by Kimura and Yamada (1996).

Concentrations of other trace elements were determined using inductively coupled plasma-mass spectrometry (ICP-MS) at Actlabs, Ltd., Canada, and ICP-MS combined with laser ablation system (LA-ICP-MS) at the Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, Fukuoka, Japan. Analysis at Actlabs was performed by a lithium metaborate/tetraborate fusion technique.

Minerals in layer B of sample 84012223 were analyzed using a field emission electron probe microanalyzer (JEOL JXA-8500F FE-EPMA) at Kyushu University. Analyses were performed with a 15 kV accelerating voltage, 12 nA beam current, and a 2 µm beam diameter. Synthetic oxides and natural minerals were used as standards for major elements. The raw data were corrected using a JEOL oxide-ZAF program. Backscattered electron (BSE) images and X-ray compositional maps were also obtained on the same instrument using a 15 kV accelerating voltage, beam currents up to 300 nA, and a 1-2 µm beam diameter. Counting times were up to 50 milliseconds.

BULK ROCK AND MINERAL CHEMISTRY

Bulk rock composition

In Table 1 bulk rock composition of layer B including the biotite-rich part is presented together with that of the clinopyroxene-garnet granulite sample Ts11011405A, which is composed of the same minerals as layer A. Although quartz is present, both rocks are olivine-normative. They both have low X_Mg [= mol MgO/(MgO + total Fe as FeO)] < 0.4. Compared to sample Ts11011405A, layer B contains an order of magnitude more K, Rb, Ba, and Th; its Pb content is also relatively high.

Table 1. Bulk compositions of mafic granulites from Austhovde

Sample No.	84012223B (Potassic-ferro- pargasite granulite)	Ts11011405A* (Garnet-clinopyroxene granulite)
Major elements, wt%
SiO2	40.82	42.50
TiO2	2.23	4.25
Al2O3	14.52	12.88
FeO†	19.62	20.27
MnO	0.34	0.36
MgO	6.46	6.47
CaO	9.99	9.73
Na2O	1.35	1.22
K2O	1.99	0.26
P2O5	0.17	0.53
H2O	0.25	0.10
Total	97.74	98.57
XMg#	0.370	0.338
Trace elements, ppm
Ba	800	72
Ce	54	77
Cr	86	40
Ga	5
Nb	25	35
Ni	63	<20
Pb	24
Rb	32	3
Sr	226	96
Th	17	2
V	637	684
Y	59	108
Zr	106	111
C.I.P.W NORM
Q	0.00	0.00
C	0.00	0.00
or	9.80	1.54
ab	0.00	10.32
an	27.68	28.90
lc	1.54	0.00
ne	6.19	0.00
kp	0.00	0.00
di	17.60	13.45
hy	0.00	19.50
ol	30.05	15.46
il	4.23	8.07
ap	0.39	1.23
Total	97.48	98.47

^* Tsunogae et al. (2016)

^† Total Fe as FeO

^# Mole MgO/(MgO + total Fe as FeO)

Mineral compositions

Representative EPMA analyses of amphiboles and other minerals in layer B are presented in Tables 2 and 3, respectively [see also Supplementary Table S1 for amphibole formula assignments by the spreadsheet of Locock (2014); Table S1 is available online from https://doi.org/10.2465/jmps.230329].

Table 2. Representative EPMA analyses of amphiboles in layer B of mafic granulite sample 84012223

Name	Potassic-ferro- pargasite	Potassic-ferro- pargasite	Potassic-ferro- pargasite	Potassic-ferro-chloro- pargasite	Cummingtonite
Part (Fig. 2e)	Main	Bt-rich	Bt-rich	Bt-rich	Bt-rich
	Olive-brown Amp	Olive-brown Amp-1 in Figures 3d and 5a	Blue-green Amp-2 in Figures 3d and 5a	Blue-green Amp-3 in Figures 3d and 5a
SiO2	38.96	39.07	38.10	35.38	53.12
TiO2	2.31	2.40	1.09	0.45	0.03
Al2O3	12.69	12.76	13.06	14.66	0.91
Cr2O3	0.05	0.06	0.00	0.03	0.03
FeO*	20.13	20.92	21.62	25.03	26.19
MnO	0.15	0.15	0.16	0.16	0.56
MgO	7.09	6.72	5.95	2.94	15.97
CaO	11.44	11.60	11.48	11.11	0.30
Na2O	1.29	1.33	1.17	1.09	0.01
K2O	2.38	2.38	2.49	2.75	0.01
BaO	0.11	0.21	0.29	0.30	0.00
P2O5	0.00	0.12	0.01	0.06	0.00
F	0.22	0.19	0.18	0.00	0.06
Cl	1.78	1.79	2.28	4.65	0.13
-O	−0.49	−0.48	−0.59	−1.05	−0.05
Total	98.11	99.22	97.29	97.56	97.27
O	23	23	23	23	23
Si	6.117	6.089	6.121	5.906	7.910
P	0.000	0.016	0.001	0.008	0.000
Al	1.883	1.895	1.878	2.086	0.090
Total T	8.000	8.000	8.000	8.000	8.000
Al	0.465	0.448	0.595	0.798	0.070
Ti	0.273	0.281	0.132	0.056	0.003
Cr	0.006	0.007	0.000	0.004	0.004
Mg	1.660	1.561	1.425	0.732	3.545
Fe*	2.596	2.703	2.848	3.410	1.378
Total M1-M3	5.000	5.000	5.000	5.000	5.000
Fe	0.047	0.023	0.056	0.084	1.883
Mn	0.020	0.020	0.022	0.023	0.071
Ca	1.924	1.937	1.898	1.893	0.046
Na	0.009	0.020	0.000	0.000	0.000
Total M4	2.000	2.000	2.000	2.000	2.000
Ca	0.000	0.000	0.078	0.094	0.002
Na	0.384	0.382	0.364	0.353	0.003
K	0.477	0.473	0.510	0.586	0.002
Ba	0.007	0.013	0.018	0.020	0.000
Total A	0.868	0.868	0.970	1.053	0.007
Sum	15.868	15.868	15.970	16.053	15.007
F	0.109	0.094	0.091	0.000	0.028
Cl	0.474	0.473	0.621	1.315	0.033
OH	1.417	1.433	1.288	0.685	1.939
Total W	2.000	2.000	2.000	2.000	2.000
XMg#	0.386	0.364	0.329	0.173	0.521

^* Total Fe as FeO

^# X_Mg = Mg/(Mg + Fe^*)

Table 3. Representative EPMA analyses of orthopyroxene, biotite, plagioclase, apatite, ilmenite, and unidentified mineral in layer B of mafic granulite sample 84012223

Mineral	Opx		Bt			Pl				Ap					Ilm	Uidp†
Part (Fig. 2e)	Main	Bt-rich	Main	Bt-rich	Bt-rich	Main	Main	Bt-rich	Bt-rich	Main	Main	Bt-rich	Bt-rich	Bt-rich	Bt-rich	Bt-rich
			1 in Table 4	2 in Table 4	3 in Table 4	Interior	Rim	Pl-1 in Figure 3d	Pl-2 in Figure 3d	1 in Table 4	2 in Table 4	3 in Table 4	4 in Table 4	5 in Table 4
SiO2	50.61	50.08	36.10	34.35	35.20	54.54	48.70	48.91	58.83	0.07	0.27	0.19	0.24	0.18	0.08	7.19
TiO2	0.08	0.14	5.67	5.20	3.57	0.00	0.01	0.02	0.02	0.00	0.04	0.00	0.02	0.00	50.83	73.95
Al2O3	1.11	1.03	13.85	13.92	13.80	29.40	32.86	32.42	25.63	0.00	0.01	0.00	0.02	0.05	0.03	1.55
Cr2O3	0.00	0.02	0.00	0.01	0.00	0.00	0.00	0.00	0.02	0.00	0.01	0.00	0.00	0.00	0.00	0.04
FeO*	29.68	31.05	20.38	20.02	21.00	0.15	0.44	0.09	0.14	0.27	0.29	0.26	0.19	0.24	47.92	10.80
MnO	0.70	0.76	0.06	0.04	0.10	0.00	0.04	0.00	0.01	0.13	0.02	0.07	0.03	0.04	0.05	0.19
MgO	16.41	15.57	9.68	9.45	9.86	0.01	0.01	0.03	0.01	0.00	0.00	0.00	0.00	0.03	0.02	0.98
CaO	0.53	0.56	0.04	0.04	0.01	12.14	16.39	15.69	7.88	55.13	55.07	55.26	55.87	55.86	0.06	0.51
Na2O	0.00	0.01	0.04	0.09	0.11	4.75	2.35	2.72	7.15	0.03	0.03	0.04	0.03	0.10	0.00	0.06
K2O	0.00	0.00	9.30	8.72	9.15	0.17	0.03	0.06	0.14	0.00	0.00	0.00	0.00	0.00	0.00	0.11
BaO			0.80	2.01	0.89	0.02	0.00	0.00	0.03	0.00	0.00	0.02	0.00	0.08	0.00	0.28
P2O5			0.02	0.00	0.01	0.03	0.01	0.02	0.03	41.94	41.28	41.33	42.20	43.68	0.00	0.06
F			0.60	0.53	0.55	0.00	0.00	0.03	0.00	2.26	2.39	2.56	2.96	3.20	0.00	0.00
Cl			1.04	1.18	1.87	0.00	0.00	0.00	0.00	2.69	2.36	2.10	1.27	0.82	0.00	0.01
-O			−0.49	−0.49	−0.65	0.00	0.00	−0.01	0.00	−1.56	−1.54	−1.55	−1.53	−1.53	0.00	0.00
Total	99.12	99.22	97.09	95.07	95.47	101.21	100.84	99.98	99.89	100.96	100.23	100.28	101.30	102.58	98.99	95.73
O	6	6	22	22	22	8	8	8	8	12.5	12.5	12.5	12.5	12.5	6	6
Si	1.975	1.969	5.550	5.457	5.578	2.438	2.216	2.238	2.634	0.006	0.023	0.016	0.020	0.015	0.004	0.308
Ti	0.002	0.004	0.656	0.621	0.425	0.000	0.000	0.001	0.001	0.000	0.003	0.000	0.001	0.000	1.948	2.384
Al	0.051	0.048	2.509	2.606	2.577	1.549	1.762	1.749	1.352	0.000	0.001	0.000	0.002	0.005	0.002	0.078
Cr	0.000	0.001	0.000	0.001	0.000	0.000	0.000	0.000	0.001	0.000	0.001	0.000	0.000	0.000	0.000	0.001
Fe*	0.969	1.021	2.620	2.659	2.783	0.006	0.017	0.003	0.005	0.019	0.020	0.018	0.013	0.016		0.387
Fe3+ **															0.097
Fe2+ **															1.943
Mn	0.023	0.025	0.008	0.005	0.013	0.000	0.002	0.000	0.000	0.009	0.001	0.005	0.002	0.003	0.002	0.007
Mg	0.955	0.912	2.219	2.238	2.329	0.001	0.001	0.002	0.001	0.000	0.000	0.000	0.000	0.004	0.002	0.063
Ca	0.022	0.024	0.007	0.007	0.002	0.581	0.799	0.769	0.378	4.977	5.007	4.969	4.990	4.888	0.003	0.023
Na	0.000	0.001	0.012	0.028	0.034	0.412	0.207	0.241	0.621	0.005	0.005	0.007	0.005	0.002	0.000	0.005
K	0.000	0.000	1.824	1.767	1.850	0.010	0.002	0.004	0.008	0.000	0.000	0.000	0.000	0.000	0.000	0.006
Ba			0.048	0.125	0.055	0.000	0.000	0.000	0.001	0.000	0.000	0.001	0.001	0.000	0.000	0.005
P			0.003	0.000	0.001	0.001	0.000	0.001	0.001	2.992	2.966	2.937	2.979	3.021	0.000	0.002
F			0.292	0.266	0.276	0.000	0.000	0.004	0.000	0.602	0.641	0.679	0.780	0.827	0.000	0.000
Cl			0.271	0.318	0.502	0.000	0.000	0.000	0.000	0.384	0.339	0.299	0.179	0.114	0.000	0.001
Cation total	3.997	4.005	15.456	15.514	15.647	4.998	5.006	5.008	5.003	8.001	8.027	7.953	8.013	7.953	4.001	3.269
XMg#	0.496	0.472	0.459	0.457	0.456
XF or Xan			0.073	0.067	0.069	0.579	0.793	0.758	0.375	0.602	0.641	0.679	0.780	0.827
XCl or XAb			0.068	0.080	0.126	0.411	0.205	0.238	0.617	0.384	0.339	0.299	0.179	0.114
XCl or XAb			0.859	0.854	0.806	0.010	0.002	0.004	0.008	0.014	0.020	0.022	0.041	0.059

^† Uidp, unidentified phase

^* Total Fe as FeO

^** Recalculated assuming stoichiometry

^# X_Mg = Mg/(Mg + Fe^*)

Amphiboles. The olive-brown amphibole constituting ∼ 85% of layer B is potassic-ferro-pargasite containing ∼ 2.4 wt% K₂O, ∼ 1.8 wt% Cl, and ∼ 0.2 wt% F. Its X_Mg (∼ 0.39) in the main part is slightly higher than that (0.36) in the biotite-rich part. The blue-green amphibole occurring locally in the biotite-rich part is potassic-ferro-(chloro-)pargasite containing up to 2.7 wt% K₂O and 4.6 wt% Cl. The Cl content is as high as those known as the typical high-Cl amphiboles (e.g., Giesting and Filiberto, 2016; Aranovich and Safonov, 2018) (Fig. 4). The blue-green potassic-ferro-(chloro-)pargasite contains more Al (up to 14.7 wt% Al₂O₃) and Fe (up to 25.0 wt% Fe as FeO) and less Si, Ti, Mg, and F than the olive-brown potassic-ferro-pargasite (Fig. 3d). Fluorine content varies inversely with Cl content so that F/Cl ratio decreases from ∼ 0.2 in the main part to <0.01 in the biotite-rich part. Another noteworthy feature of the potassic-ferro-(chloro-)pargasite, especially in the biotite-rich part, is its content of Ba (up to 0.3 wt% BaO). The potassic-ferro-(chloro-)pargasite occurring in the biotite-rich part can be divided into three groups: low-Cl, intermediate-Cl, and high-Cl (Fig. 5a). The boundaries between the groups within the same grain are relatively sharp (Fig. 3d). The olive-brown potassic-ferro-pargasite, represented by Amp-1, belongs to the low-Cl group. Blue-green potassic-ferro-(chloro-)pargasite belongs both to the intermediate-Cl group represented by Amp-2 and to the high-Cl group represented by Amp-3. Compositions of the potassic-ferro-(chloro-)pargasite obeys the Mg-Cl avoidance rule (Rosenberg and Foit, 1977; Munoz, 1984) (Fig. 5a).

Figure 4. Diagram showing correlations between Cl (wt%) and K (atoms per formula unit; apfu) contents of K-Cl-rich calcium amphiboles from granulite- and amphibolite-facies areas.

Figure 5. Diagrams showing compositional variations in biotite, potassic-ferro-(chloro-)pargasite and cummingtonite in layer B of sample 84012223. (a) X_Cl = Cl/(F + Cl + OH) versus X_Mg = Mg/(Mg + total Fe). (b) TiO₂ (wt%) versus X_Mg. (c) X_F = F/(F + Cl + OH) versus X_Mg. Potassic-ferro-(chloro-)pargasite in the biotite-rich part is subdivided into three groups, low-Cl, intermediate-Cl, and high-Cl, which correspond to three stages of modification of potassic-ferro-pargasite formed at the peak granulite-facies metamorphism. Compositional variations in potassic-ferro-(chloro-)pargasite appear to conform to the Mg-Cl avoidance rule.

Cummingtonite is poor in Ca and Al despite the coexistence with potassic-ferro-(chloro-)pargasite. It has the highest and almost constant X_Mg (0.52) among the Mg-Fe minerals and the lowest Cl content among the minerals containing the volatile components Cl, F, and OH (Fig. 5a).

Orthopyroxene. Orthopyroxene is enstatite containing only up to 1.1 wt% Al₂O₃ and 0.55 wt% CaO (Table 3). Its X_Mg (∼ 0.50) in the main part is slightly higher than that (0.47) in the biotite-rich part.

Biotite. The compositional variation of biotite is small compared to that of associated minerals (Table 3). It is relatively rich in Ti (∼ 5.0 wt% TiO₂), but not in F (only up to 0.6 wt% F). Some grains in the biotite-rich part contain up to 2.0 wt% BaO and up to 1.9 wt% Cl, although most grains contain ∼ 0.9 wt% BaO and ∼ 1.1 wt% Cl.

Plagioclase. Plagioclase commonly shows compositional heterogeneity (Table 3). Anorthite (An) content is generally higher in the rim (up to An₈₃) than in the core (An₅₈) in both the main and biotite-rich parts. Some grains in the biotite-rich part are distinctly less calcic (An₃₈) (Figs. 3c and 3d).

Fluorapatite. Formulae calculated for fluorapatite give negligible OH as Cl + F exceed 0.9 (Table 3). Compositions plot in two groups: one high-Cl and a second with less Cl (Fig. 6c). The high-Cl flourapatite is compositionally nearly homogeneous, showing minor variation from grain to grain and an inverse relationship between Cl and F (Figs. 6b and 6c). The low-Cl flourapatite is present in the vicinity of the ‘altered zone’ in the biotite-rich part and surrounded by orthopyroxene, plagioclase, and ilmenite (Fig. 3a). It is characterized by heterogeneous distribution of Cl (Fig. 6a). The boundary between higher and lower Cl parts within the low-Cl flourapatite grains is not so sharp as reported by Putnis and Austrheim (2010), Higashino et al. (2013), and Harlov (2015) (Fig. 6a).

Figure 6. (a) X-ray elemental map for Cl of low-Cl flourapatite grain-3 in biotite-rich zone in Figure 3a. Increase in Cl content is indicated by change in color from blue to green to yellow to red. (b) Ternary Cl-OH-F plot for fluorapatite, assuming Cl + F + OH = 1. Note that fluorapatite in the main part is extremely rich in halogens. (c) F (wt%) versus Cl (wt%) diagram, showing compositional variation of fluorapatite, which is subdivided into high-Cl and low-Cl groups, corresponding to the occurrences in the main and biotite-rich parts.

Ilmenite. Ilmenite contains <5 mol% hematite component. It commonly shows partial to complete decomposition to fine-grained intergrowths of rutile and an unidentified hydrous silicate mineral.

DISCUSSION

Halogen fugacities

The analyses give X_F generally increasing Amp < Bt < Fap and the inverse for X_Cl, which tends to increase Fap < Bt < Amp (Figs. 5a and 5c). Biotite and fluorapatite can be used for estimating the fugacities of F and Cl in associated fluids. The fugacities are commonly expressed as the ratios f_HF/f_H2O, f_HCl/f_H2O, and f_HF/f_HCl and can be calculated by the equations that were derived from experimental and thermodynamic data for biotite (e.g., Zhu and Sverjensky, 1991, 1992; Munoz, 1992) and apatite (e.g., Yardley, 1985; Brenan, 1993; Piccoli and Candela, 1994, 2002; Spear and Pyle, 2002; Webster et al., 2009).

Calculated log(f_HCl/f_H2O) and log(f_HF/f_H2O) are summarized in Table 4. Calculation was performed assuming 850, 830, and 800 °C and 7.5 kbar based on the estimation by Tsunogae et al. (2016) and Takahashi and Tsunogae (2017) for the peak metamorphic conditions. A pressure of 3.5 kbar and temperatures of 700 and 600 °C are assumed for the P-T conditions after high-temperature decompression based on Takahashi and Tsunogae (2017). The stability of cummingtonite (e.g., Evans and Ghiorso, 1995) is also used to assume the temperature during its formation with potassic-ferro-(chloro-)pargasite. Temperatures using the amphibole-plagioclase thermometers of Holland and Blundy (1994) are not adopted here because of the large discrepancies (>200 °C) between the edenite-tremolite and edenite-richterite thermometers. The scatter in Cl and F contents of biotite is relatively small (Figs. 5a and 5c), and consequently, the calculated log(f_HCl/f_H2O) and log(f_HF/f_H2O) values vary over a relatively narrow range: from −2.3 at 800 °C to −2.7 at 600 °C and from −4.1 at 850 °C to −4.8 at 600 °C. In contrast, the scatter in Cl content of fluorapatite is larger (Fig. 6), and log(f_HC1/f_H2O) calculated from fluorapatite ranges from −0.9 at 850 °C and 7.5 kbar to −2.5 at 600 °C and 3.5 kbar. The scatter in F for fluorapatite is not so large as Cl (Fig. 6) and calculated log(f_HF/f_H2O) ranges from −3.6 at 850 °C and 7.5 kbar to −5.0 at 600 °C and 3.5 kbar. These results show the following three points; (1) log(f_HCl/f_H2O) and log(f_HF/f_H2O) values calculated from biotite are always lower than those calculated from fluorapatite, especially in the main part, (2) the calculated log(f_HCl/f_H2O) from the high-Cl fluorapatite at 800 °C are almost the same throughout layer B, and (3) the calculated log(f_HCl/f_H2O) from the low-Cl fluorapatite in the biotite-rich part at 600 °C and 3.5 kbar is close to that from biotite. These points may be due to the difference in sensitivity of the minerals to the changing P-T conditions and composition of coexisting fluid. Apatite in general is thought to be less susceptible than most minerals to resetting of halogen content by hydrothermal fluids (e.g., Piccoli and Candela, 1994). Such resistance to resetting could explain the heterogeneous and variable compositions of low-Cl fluorapatite grains in the biotite-rich part, which introduce ambiguity in the evaluation of calculated log(f_HCl/f_H2O) value. As Nijland et al. (1993) pointed out, log(f_HC1/f_H2O) values calculated from biotite are also problematic, because the incorporation of F and Cl into biotite is a function of Fe/Mg ratio (e.g., Munoz, 1984; Kullerud, 1995). In addition to these mineralogical considerations, Nijland et al. (1993) argued that when fluid/rock ratios are low, halogen-bearing fluids were controlled over a very restricted scale during peak as well as during retrograde metamorphism.

Table 4. log(f_HCl/f_H2O) and log(f_HF/f_H2O) for coexisting fluid calculated from biotite and fluorapatite

log(fHCl/fH2O)	Biotite
	Main part (1)			Bt-rich part (2)
	850 °C	−2.58		800 °C	−2.55
	830 °C	−2.60		700 °C	−2.63
	800 °C	−2.62		600 °C	−2.74
				Bt-rich part (3)
	See Table 3 for biotites 1-3.			800 °C	−2.33
				700 °C	−2.41
				600 °C	−2.52
	Fluorapatite
	Main part (1)	7.5 kb	3.5 kb	Bt-rich part (3)	7.5 kb	3.5 kb
	850 °C	−0.86	−0.97	800 °C	−1.04	−1.15
	830 °C	−0.90	−1.01	700 °C	−1.26	−1.39
	800 °C	−0.96	−1.07	600 °C	−1.53	−1.67
	Main part (2)	7.5 kb	3.5 kb	Bt-rich part (4)	7.5 kb	3.5 kb
	850 °C	−1.09	−1.24	800 °C	−0.99	−1.10
	830 °C	−1.13	−1.28	700 °C	−1.21	−1.34
	800 °C	−1.19	−1.32	600 °C	−1.48	−1.62
	Fluorapatites in biotite-rich part			Bt-rich part (5)	7.5 kb	3.5 kb
	3 = Fap-1 in Figures 3a and 6c			800 °C	−1.85	−1.96
	4 = Fap-3-5 in Figures 3a and 6a, 6c			700 °C	−2.07	−2.19
	5 = Fap-3-2 in Figures 3a and 6a, 6c			600 °C	−2.34	−2.48
log(fHF/fH2O)	Biotite
	Main part (1)			Bt-rich part (2)
	850 °C	−4.06		800 °C	−4.22
	830 °C	−4.11		700 °C	−4.49
	800 °C	−4.18		600 °C	−4.83
				Bt-rich part (3)
	See Table 3 for biotites 1-3.			800 °C	−4.17
				700 °C	−4.45
				600 °C	−4.72
	Fluorapatite
	Main part (1)	7.5 kb	3.5 kb	Bt-rich part (3)	7.5 kb	3.5 kb
	850 °C	−3.61	−3.62	800 °C	−3.61	−3.66
	830 °C	−3.69	−3.70	700 °C	−4.12	−4.12
	800 °C	−3.83	−3.84	600 °C	−4.74	−4.74
	Main part (2)	7.5 kb	3.5 kb	Bt-rich part (4)	7.5 kb	3.5 kb
	850 °C	−3.45	−3.46	800 °C	−3.25	−3.27
	830 °C	−3.53	−3.55	700 °C	−3.76	−3.77
	800 °C	−3.67	−3.68	600 °C	−4.38	−4.40
	Fluorapatites in biotite-rich part			Bt-rich part (5)	7.5 kb	3.5 kb
	3 = Fap-1 in Figures 3a and 6c			800 °C	−3.90	−3.91
	4 = Fap-3-5 in Figures 3a and 6a, 6c			700 °C	−4.40	−4.42
	5 = Fap-3-2 in Figures 3a and 6a, 6c			600 °C	−5.03	−5.04

Temperatures and pressures are assumed after Tsunogae et al. (2016) and Takahashi and Tsunogae (2017). See text for details.

Metamorphic evolution of the amphibole assemblages

The compositional subdivision (Fig. 5a) and textural features of the amphiboles suggest four stages of formation and modification. Stage 1 is the major granulite-facies metamorphism resulting in the crystallization of the assemblage olive-brown potassic-ferro-pargasite + orthopyroxene + biotite + calcic plagioclase + quartz + ilmenite + Cl-rich fluorapatite. Stage 2 is a local event to form the biotite-rich part by infiltration of K-rich fluid along a fracture after the peak but still under the granulite-facies conditions. New biotite may have formed through the reaction;

  

\begin{align} &{\text{low-Cl potassic-ferro-pargasite} + \text{Opx} + \text{Qz}} \\&\qquad + {\text{Na and K in fluid} \pm \text{H$_{2}$O}} \\ &\quad {\rightarrow \text{Bt} + \text{more Fe-rich low-Cl potassic-ferro-pargasite}} \\&\qquad + {\text{more Fe-rich Opx}} \end{align} (1).

Because potassic-ferro-(chloro-)pargasite and biotite in the biotite-rich part contain more Ba than potassic-ferro-pargasite and biotite in the main part, Ba may have also derived from infiltrated fluid at this stage. Stage 3 is a more localized event taking place in the biotite-rich part to form the new mineral assemblage of the ‘altered zone’ (Fig. 3a) under the amphibolite facies conditions through the reaction:

  

\begin{align} &\text{low-Cl potassic-ferro-pargasite} + \text{Qz} \\&\qquad+ \text{Na and K in fluid} \pm \text{H$_{2}$O} \\ &\quad \rightarrow \text{Cl-poor Cum + sodic Pl} \\&\qquad+ \text{intermediate-Cl potassic-ferro-pargasite} \\&\qquad\pm \text{Bt} \end{align} (2).

At stage 4 the intermediate-Cl potassic-ferro-pargasite became high-Cl potassic-ferro-chloro-pargasite in the immediate vicinity of Cl-poor cummingtonite possibly by the following volatile-conserving continuous reaction:

  

\begin{align} &\text{intermediate-Cl potassic-ferro-pargasite} \\ &\quad \rightarrow \text{high-Cl potassic-ferro-chloro-pargasite} \\&\qquad + \text{Cl-poor Cum} \end{align} (3).

The high-Cl potassic-ferro-chloro-pargasite has a higher and more variable content of K and Al and lower content of Si, Ti, and Mg than the intermediate-Cl variety (Fig. 3d). In contrast, Cl-poor cummingtonite produced by reactions (2) and (3) has almost the same composition despite the highly variable composition of the associated potassic-ferro-(chloro-)pargasite.

In proposing reactions 1 to 3, we considered two scenarios for the occurrence of Cl-poor cummingtonite in the biotite-rich part: (1) it was formed together with the K-Cl-rich blue-green potassic-ferro-(chloro-)pargasite, replacing olive-brown potassic-ferro-pargasite, and (2) it was formed by hydration of orthopyroxene during local infiltration of low-Cl fluid. The first scenario is supported by the presence of inclusions of fine particles of an unidentified Ti-rich phase and anhedral potassic-ferro-(chloro-)pargasite grains in cummingtonite (Figs. 3c and 3d). In addition, Cl-poor cummingtonite is closely accompanied by sodic plagioclase that also contains small anhedral grains of amphibole, possibly potassic-ferro-(chloro-)pargasite (Figs. 3c and 3d). The second scenario appears to be consistent with the formation of high-Cl fluid from low-Cl fluid (Kullerud, 1996). However, the second scenario is not supported by the mineral assemblage and textures. Orthopyroxene has not been observed to be replaced by cummingtonite in the studied sample.

Similar assemblages and amphibole compositions have been reported in other areas. For example, Oen and Lustenhouwer (1992) and Henry and Daigle (2018) reported cummingtonite and grunerite associated with a Cl-rich calcium amphibole, but they regarded the Fe-Mg amphiboles to have formed separately from Cl-rich calcium amphibole. K-Cl-rich potassic hornblende was reported to replace Cl-free hornblende as the result of interaction with KCl-rich fluid after the peak of metamorphism (e.g., Belyanin et al., 2014; Fig. 11.9 of Aranovich and Safonov (2018)).

In general, potassic-ferro-(chloro-)pargasite and other Cl-rich amphiboles obey the Mg-Cl avoidance rule (Fig. 5a), a feature reported from numerous localities (Suwa et al., 1987; Morrison, 1991; Enami et al., 1992; Makino et al., 1993; Oberti et al., 1993; Kullerud, 1996; Kullerud and Erambert, 1999; Makino, 2000; Giesting and Filiberto, 2016; Aranovich and Safonov, 2018; Henry and Daigle, 2018; Jenkins, 2019). Reaction 3 may have proceeded from the ‘feedback mechanism’ of Henry and Daigle (2018), although K-Cl-rich calcium amphibole-cummingtonite immiscibility was not considered in the mechanism. Henry and Daigle (2018) argued that a feedback mechanism can be generated such that the more Cl available from a fluid, the more Fe²⁺-rich the amphibole can become, and this produces a crystal structure that can accommodate more Cl, which makes this amphibole more favorable for Fe²⁺ incorporation, and the cycle continues until Cl saturation in the crystal structures is reached or aqueous Cl⁻ is fully equilibrated with the amphibole. The formation of Cl-poor and relatively magnesian cummingtonite with a higher X_Mg ratio than the potassic-ferro-(chloro-)pargasite may have played an essential role for the K-Cl-rich pargasite becoming even more K- and Cl-rich in the present case (Fig. 5a). The restricted spatial extent of reactions (2) and (3) in the ‘altered zone’ (Fig. 3a) and the incomplete compositional change of low-Cl fluorapatite (Figs. 6a and 6c) are indicative of low fluid/rock ratios even in the biotite-rich part next to the biotite-rich surface.

Precursor to layer B

The mineral assemblages of layer A and layer B of sample 84012223 differ significantly from one another. Hiroi et al. (2023) concluded that clinopyroxene + garnet assemblage of layer A formed by incongruent partial melting reactions consuming hornblende and plagioclase under relatively high-pressure conditions. Thus, the potassic-ferro-pargasite + calcic plagioclase assemblage of layer B could correspond to the reactant of the partial melting reactions inferred in layer A. However, there is neither evidence of the former presence of clinopyroxene, garnet and/or symplectitic intergrowth of orthopyroxene + plagioclase nor any mineral textures suggestive of partial melting in layer B. Both potassic-ferro-pargasite and fluorapatite in the main part of layer B are notably rich in Cl and calculated log(f_HCl/f_H2O) is high (about −1.0 at 830 °C and 7.5 kbar). Thus, the main reason for the absence of partial melting in layer B can be the abundance of halogens (primarily Cl), which is known to expand the stability of Ca amphibole to higher temperatures and lower the H₂O fugacity in fluid (e.g., Aranovich and Safonov, 2018). Smith and Yardley (1999) considered the situation in which Cl-rich detrital apatite is present in a rock and suggested that the exchange of Cl with a limited volume of metamorphic fluid could change the concentration of Cl in the fluid by several orders of magnitude. It is well known that apatite in mafic igneous rocks, especially layered intrusions, is Cl-rich (e.g., Webster and Piccoli, 2015). Therefore, igneous Cl-rich apatite could be one of the sources of Cl in layer B prior to granulite-facies event. The occurrence of blocks and thin layers of layered mafic and ultramafic granulites in other felsic and intermediate granulites in the LHC are interpreted to be tectonically fragmented layered gabbros (Hiroi et al., 1986; Suda et al., 2006, 2008; Tsunogae et al., 2016; Takahashi and Tsunogae, 2017; Takamura et al., 2020), and it is likely that layers A and B in sample 84012223 also originated from one such layered gabbro.

CONCLUDING REMARKS

K-Cl-rich amphibole has often been considered as a product of metasomatism caused by infiltration of K-Cl-rich fluid (e.g., Giesting and Filiberto, 2016; Aranovich and Safonov, 2018). This paper shows that the formation of Cl-poor and relatively magnesian cummingtonite may have played an essential role for potassic-ferro-(chloro-)pargasite to become even more K- and Cl-rich. The local modification of amphibole composition during cooling is subdivided into three stages, but it may have taken place for a short span of time because Hiroi et al. (2023) inferred rapid cooling based on the occurrence of nanogranitoid inclusions in layer A. The higher Ba contents of potassic-ferro-(chloro-)pargasite and biotite in the biotite-rich part than those in the main part are suggestive of Ba derived from infiltrated fluid, but the enrichment of large-ion lithophile elements in layer B and related metasomatism by fluid or melt infiltration are to be solved with additional data (e.g., Higashino et al., 2015; Kawakami et al., 2016, 2017; Higashino et al., 2019).

ACKNOWLEDGMENTS

We thank many collaborative researchers, in particular N. Furukawa, Y. Suda, M. Satish-Kumar, F. Higashino, N. Tsuchiya, M. Ishikawa, Y. Osanai, M. Owada, T. Kawasaki, and M. Yoshida. Special thanks are due to E.S. Grew for his kind help and fruitful comments. We are thankful to T. Kawakami and T. Tsunogae for the critical and constructive comments that improved the manuscript substantially. This work was supported by JSPS KAKENHI Grant Numbers 18K03789 to Y.H., 17H02976 and 21H01182 to T.H. and National Institute of Polar Research (General Collaboration Project no. 29-36), Japan.

SUPPLEMENTARY MATERIALS

Supplementary Figure S1 and Table S1 are available online from https://doi.org/10.2465/jmps.230329.

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