High-K adakitic granite in post-Gondwana collisional stage: example for the Vengen Granite, Sør Rondane Mountains, East Antarctica

Abstract

The Vengen Granite, one of early Paleozoic granitic rocks crops out of the southern end of the Vengen ridge, the Kanino-tsume Peak at the Main Shear Zone (MSZ) of the Sør Rondane Mountains, East Antarctica. This granite is composed of medium- to fine-grained mylonitic biotite granite and cuts the MSZ and Kanino-tsume Shear Zone. The fine-grained two-mica granitic dykes locally intrude the Vengen Granite. The two-mica granitic dykes have foliations parallel to mylonitic foliations of the Vengen Granite. The Vengen Granite is composed of plagioclase, quartz, K-feldspar, biotite, and muscovite with trace amounts of titanite, allanite, apatite, zircon and opaques as accessory minerals. The granite is geochemically characterized by a high-K content, which resembles adakitic affinity. These chemical data combined with rare earth elements and Sr-Nd isotope geochemistry suggest that the source magma of the Vengen Granite was derived from partial melting of the meta-tonalite with minor amounts of the pelitic gneisses in the SW-terrane subducted under the NE-terrane during collision of the West and East Gondwana continents.

INTRODUCTION

The petrogenesis of igneous rocks and their space-time distribution in a continent-continent collision zone would provide clarification of the magmatic processes with the transition of the source region for the collisional event. Dronning Maud Land, East Antarctica was situated within a collision zone between the West and East Gondwana continents (Shiraishi et al., 1994; Jacobs et al., 2003; Meert, 2003; Asami et al., 2005; Shiraishi et al., 2008; Osanai et al., 2013). The collisional event is regarded to have a geological time scale from the late Neoproterozoic to the early Paleozoic, known as the Pan-African orogeny. The Sør Rondane Mountains, located in eastern Dronning Maud Land, consist mainly of low- to high-grade metamorphic rocks that can be divided into the Northeast terrane (NE-terrane) and the Southwest terrane (SW-terrane) in terms of their protoliths and metamorphic processes (Osanai et al., 1992, 2013). The post-collisional mafic to felsic igneous rocks intrude the metamorphic rocks of both terranes as stocks and dykes (e.g., Shiraishi et al., 1997; Fig. 1).

Figure 1. Simplified geological map of the Sør Rondane Mountains (modified from Shiraishi et al., 1997; Osanai et al., 2013; Tsukada et al., 2017). MSZ, Main Shear Zone; KSZ, Kanino-tsume Shear Zone; MTB, Main Tectonic Boundary. AG, Austkampane Granite; DG, Dufek Granite; LG, Lunckryggen Granite; MG, Mefjell Granite; PG, Pingvinane Granite; VIG, Vikinghøgda Granite.

The mafic dykes that occur in the entire area of the Sør Rondane Mountains are characterized by high K and light rare earth elements (REEs). These mafic magmas were derived from partial melting of an enriched mantle source by the mixing of remnant subduction related materials in the Pan-African orogenic time (Ikeda et al., 1995; Owada et al., 2008, 2010, 2013). The late- to post-tectonic granitic rocks consist mainly of syenite and alkali granite (Tainosho et al., 1992; Arakawa et al., 1994) and they are divided into two groups. Li et al. (2001, 2003) geochemically classified the post-tectonic granitic rocks into type-1 and type-2, and discussed their petrogenesis with respect to different field occurrence, mineral composition, geochemical characteristics and isotopic composition. Elburg et al. (2016) performed zircon U-Pb dating and determined the Hf isotopic compositions of the granitic rocks. They suggested that the Sør Rondane Mountains may be a collage of several different terranes based on the ∼ 150 million years duration of magmatism and geographic trend in the Hf isotopic composition. In addition, an overview of the formation of granitic magma was reported based on geochemical studies (Arakawa et al., 1994; Owada et al., 2006).

Parts of the late- to post-tectonic granitic rocks show high K and Sr characteristics (Takahashi et al., 1990; Tainosho et al., 1992; Li et al., 2001, 2003), and geochemically resemble ‘high-K adakitic granite’ (Moyen, 2009). Such potassic rocks have been known as their sodic counter parts with their origin from deep melting of subducting oceanic crust (Chung et al., 2003; Hou et al., 2004) although their magma genesis has been under debate. Alternatives of their genesis include adakitic magma with shoshonitic melt (Wang et al., 2018; Shen et al., 2021), low-degree melting of metabasaltic rocks (Rapp and Watson, 1995), melting of K-rich metabasaltic source or crustal assimilation (Rapp et al., 2002), or melting of potassic basalt to tonalites at >2 GPa (Xiao and Clemens, 2007). Yi et al. (2022) recently reported that high-K adakitic granites can be generated directly from melting of medium-K intermediate to felsic arc rocks at intermediate pressures (>0.7 GPa). Therefore, before adopting the discrimination diagrams as the tectonic environments, it is necessary to determine the genesis of the high-K adakitic granite in the Sør Rondane Mountains.

The Vengen Granite was first named by Shiraishi et al. (1992) for the granitic body distributed in the Kanino-tsume Peak at the southern end of the Vengen ridge. Tainosho et al. (1992) named the Vikinghøgda Granite (VIG in Fig. 1), which is the body distributed at the eastern part of the Vikinghøgda. After that, both bodies were confused and called the Vikinghøgda Granite (Li et al., 2001, 2003), Vengen Granite (Shiraishi et al., 2008), and Vengen/Vikinghøgda Granite (Elburg et al., 2016). The names of both granites are thus used interchangeably, and the geochemical chracteristics of the Vengen Granite have not been clarified. In this paper, we use the term Vengen Granite for the body distributed in the Kanino-tsume Peak.

The 50th Japanese Antarctic Research Expedition (JARE) surveyed the western part of the Sør Rondane Mountains to reevaluate the igneous activities. We have performed a geochemical study of the high-K adakitic granite, the Vengen Granite at the Kanino-tsume Peak, and discussed the petrogenesis of the high-K adakitic granite. The results presented here may be applicable to other post-collisional settings in the Pan-African orogeny.

GEOLOGICAL OUTLINE

The Sør Rondane Mountains are underlain by low- to high-grade metamorphic rocks and various intrusive rocks (e.g., Shiraishi et al., 1997; Fig. 1). The metamorphic rocks are divided into the NE-terrane and SW-terrane by the Main Tectonic Boundary (MTB) (Osanai et al., 2013; Fig. 1). Both terranes present different constituent rock types, metamorphism and geochronological evidence (Osanai et al., 2013). The Main Shear Zone (MSZ) (Kojima and Shiraishi, 1986) is the boundary between older meta-tonalite (Nils Larsen Tonalite) and metamorphic rocks in the SW-terrane (Fig. 1).

The NE-terrane is subdivided into Unit A (amphibolite-facies rocks) and Unit B (granulite-facies rocks), and the SW-terrane is also subdivided into Unit C (granulite-facies rocks), Unit D (amphibolite- to greenschist-facies rocks) and Unit D’ (meta-tonalite metamorphosed under amphibolite- to greenschist-facies conditions: Nils Larsen Tonalite) (Osanai et al., 2013; Fig. 1). The geochemical chracteristics of the Nils Larsen Tonalite show tholeiitic and calc-alkaline suites (Kamei et al., 2013).

The late- to post-tectonic plutonic rocks are classified into type-I and type-II granitoids and Mefjell Plutonic Complex (Li et al., 2001, 2003). It is considered that the type-I granitoids, which consist of the Defek and Lunckryggen Granites, are derived from partial melting of the Nils Larsen Tonalite and minor mixing with host metamorphic rocks based on geochemical signatures (Arakawa et al., 1994; Li et al., 2001; Owada et al., 2006). On the other hand, the type-II granitoids, which consist of the Austkampane, Pingvinane, Vikinghøgda, and Rogestoppane Granites, are formed by the assimilation of crustal materials of host metamorphic rocks by the melt derived from partial melting of the Nils Larsen Tonalite and then fractional crystallization (Li et al., 2001). Furthermore, Li et al. (2001) also suggested that these rocks were possibly derived from different magma sources and had different petrogenetic processes.

U-Pb zircon ages indicated an igneous age of the Nils Larsen Tonalite in the range from 772 to 1015 Ma (Shiraishi et al., 2008; Kamei et al., 2013; Elburg et al., 2015). The collision of the NE- and SW-terranes occurred in the period from ∼ 650 to 600 Ma (Osanai et al., 2013). Intrusions of the late- to post-tectonic plutonic rocks are divided into at least three phases (Jacobs et al., 2015). The oldest is the Dufek Granite, dated at ∼ 620 Ma (Li et al., 2003, 2006), followed by 560-550 Ma granitic rocks (Shiraishi et al., 2008; Elburg et al., 2016). The youngest granitic rocks are dated at ∼ 530 Ma (Tainosho et al., 1992; Li et al., 2003, 2006; Shiraishi et al., 2008).

GEOLOGY AND PETROGRAPHY OF THE VENGEN GRANITE

Geology

The Vengen Granite forms the Kanino-tsume Peak at the southern end of the Vengen ridge of the Sør Rondane Mountains, East Antarctica (Figs. 2 and 3a), and extends to the eastern part of the Vikinghøgda located on the opposite bank across from the Ketelers glacier and is situated in the central part of the Vengen ridge (Shiraishi et al., 1992).

Figure 2. Geological map of the Kanino-tsume Peak (modified from Tsukada et al., 2017). Equal-area lower hemisphere projections of mylonitic foliation of the Vengen Granite are also shown. MSZ, Main Shear Zone; KSZ, Kanino-tsume Shear Zone.

Figure 3. Photographs showing field occurrence of the Vengen Granite. (a) Distant view of the Kanino-tsume Peak. (b) Vengen Granite with mylonitic texture. (c) Vengen Granite with weak foliation. (d) Melanocratic and leucocratic granitic dykes intruded into the Vengen Granite. (e) Contact of melanocratic granitic dyke and Vengen Granite. (f) Leucocratic granitic dyke intruded into the Vengen Granite. (g) Contact of leucocratic granitic dyke and Vengen Granite.

The Vengen Granite is composed of medium-grained mylonitic biotite granite (Figs. 3b and 3c), and is intruded into the MSZ and the Kanino-tsume Shear Zone (KSZ) (Tsukada et al., 2017). The mylonitic foliations of the granite indicate N85°E to N78°W strike and 38° to 68° southward dip similar to the MSZ trend (Fig. 2). This granite includes lenticular or elongated mafic microgranular enclaves (MME, Fig. 3b) that show mingling textures such as feldspar phenocrysts originated from the granite that are up to 55 cm in the major axis. The Vengen Granite includes xenoliths of gneisses, of which the margins are often diffused (Shiraishi et al., 1992). The Vengen Granite is intruded by melanocratic and leucocratic fine-grained two-mica granitic dykes (Figs. 3d-3g). Both granitic dykes are cutting mylonitic foliations of the Vengen Granite (Figs. 3d and 3e). The leucocratic granitic dyke has local foliations parallel to mylonitic foliations that are folded (Figs. 3f and 3g). Another low-angle cataclasite zone parallel to the KSZ is present in this granitic body (Tsukada et al., 2017, Fig. 2).

U-Pb SHRIMP dating from the Vikinghøgda Granite gave a magmatic age of 562 ± 7 Ma, and an inherited age of ∼ 1000 Ma (9091405A; Shiraishi et al., 2008), which is considered to be continuous with the Vengen Granite. Elburg et al. (2016) reported a similar U-Pb zircon age of 551 ± 8 Ma from the Vikinghøgda Granite (MESR30) using laser ablation inductively coupled plasma mass spectrometry (LA-ICP MS).

Petrography of analyzed samples

The Vengen Granite is composed of quartz, K-feldspar, plagioclase, and biotite with trace amounts of titanite, allanite, apatite, zircon and opaques as accessory minerals. Parts of the Vengen Granite contain muscovite. The Vengen Granite is plotted on the granite field in the normative An-Ab-Or diagram (Fig. 4), and shows mylonitic texture (Figs. 5a and 5b). Parts of the quartz grains are ribbon shaped and have a reduced grain size by dynamic recrystallization up to 0.2 mm in diameter. There are asymmetric pressure shadows (Figs. 5a and 5b). Parts of the brown to greenish-brown biotite are emplaced by chlorite or a combination of chlorite and epidote.

Figure 4. Normative An-Ab-Or diagram (Barker, 1979) of the Vengen Granite and related rocks. To calculate the normative mineralogy, the Fe₂O₃/FeO ratios of the samples are presumed to be 1.8 (Tainosho et al., 1992). Leuco., leucocratic; Melano., melanocratic.

Figure 5. Photomicrographs (cross-polarized light) of the Vengen Granite and related rocks. (a) Y09012501G, (b) Y08121102C, (c) Y08120902E, and (d) Y09012501B.

The melanocratic fine-grained two-mica granite dyke is comprised mainly of quartz, K-feldspar, plagioclase, biotite, and muscovite with trace amounts of allanite, titanite, apatite, zircon and opaque minerals (Fig. 5c). K-feldspar and plagioclase are porphyroclasts up to 3.2 mm in diameter. These porphyroclasts are embedded in smaller quartz, K-feldspar, plagioclase, biotite, muscovite and accessory minerals. The matrix minerals up to 0.8 mm exhibit a hypidiomorphic equigranular texture (Fig. 5c). There is little dynamic recrystallization of quartz. Brown biotite is partly emplaced by chlorite.

The leucocratic fine-grained two-mica granite dyke is comprised mainly of quartz, K-feldspar, plagioclase, muscovite, and biotite with trace amounts of allanite, titanite, apatite, zircon and opaques (Fig. 5d). K-feldspar and plagioclase are porphyroclasts up to 2.5 mm in diameter. These porphyroclasts are embedded in smaller quartz, K-feldspar, plagioclase, biotite, muscovite and accessory minerals. The matrix exhibits a hypidiomorphic equigranular texture (Fig. 5d) and matrix minerals are up to 0.25 mm in diameter. Very few quartz grains are ribbon shaped, which indicates little dynamic recrystallization of quartz. Most of the brown biotite is emplaced by chlorite.

WHOLE-ROCK CHEMICAL COMPOSITIONS

Major and trace elements

The major and trace elements concentrations of whole-rock samples of the Vengen Granite and related rocks were analyzed using X-ray fluorescence spectrometer (XRF; Rigaku ZSX100e) at Fukuoka University, after the methods reported by Yuhara and Taguchi (2003a, 2003b), Yuhara et al. (2004), and Takamoto et al. (2005) using 1:5 glass beads and 1:1 powder pellets; Spectromelt A12 (Merck) and LiNO₂ (Suprapur, Merck) were used for preparation of the glass beads, and Spectromelt A10 (Merck) was used for preparation of the powder pellets. The results of XRF analyses are listed in Table 1, where total iron is shown as Fe₂O₃*.

Table 1. Whole-rock chemical compositions of the Vengen Granite and related rocks

	Vengen Granite
Sample No.	Y09012501G	Y08121102B	Y08120902A	Y08120902G	Y09012501C	Y08121102H	Y09012501H
SiO2 (wt%)	66.49	66.55	68.27	68.87	68.92	69.10	69.48
TiO2	0.70	0.55	0.53	0.55	0.56	0.54	0.52
Al2O3	15.28	15.84	14.73	14.21	14.32	14.18	14.39
Fe2O3*	2.87	2.65	2.56	2.71	2.49	2.45	2.38
MnO	0.04	0.04	0.03	0.03	0.04	0.03	0.03
MgO	0.73	0.60	0.60	0.63	0.72	0.61	0.56
CaO	1.86	1.18	1.82	1.83	1.75	1.56	1.67
Na2O	4.08	4.01	3.84	3.83	3.88	3.83	3.88
K2O	5.82	6.49	5.51	5.13	5.30	5.32	5.32
P2O5	0.22	0.11	0.13	0.15	0.18	0.16	0.14
L.O.I.	0.39	0.67	0.85	0.78	0.46	0.93	0.36
Total	98.48	98.69	98.87	98.72	98.62	98.71	98.73
As (ppm)	<4	<4	<4	<4	<4	<4	<4
Ba	2983	3288	2796	2645	2891	2511	2704
Cr	8	7	5	10	8	6	5
Cu	<4	n.d.	n.d.	<4	<4	7	<4
Ga	23	23	22	22	22	23	22
Nb	15	15	12	14	12	14	12
Ni	6	6	6	6	11	8	8
Pb	54	27	35	39	50	38	52
Rb	178	271	158	142	241	201	160
S	188	n.d.	14	87	32	198	245
Sr	1192	995	1138	1123	1140	1052	1118
Th	43	31	41	25	24	36	46
V	24	19	18	21	20	19	20
Y	23	20	20	19	22	18	19
Zn	73	41	49	53	64	49	57
Zr	448	382	340	369	337	366	346

*total iron as Fe₂O₃.

L.O.I., loss on ignition; n.d., not detected.

Y081211T02	Y09012501F	Y09012501A	Y08120902B	Y081208T01	Y090125T01	Y081208T03	Y09012501E
69.50	69.53	69.56	70.19	70.19	70.31	70.41	70.61
0.48	0.51	0.52	0.51	0.46	0.52	0.46	0.49
14.44	14.28	14.06	14.03	13.96	13.92	13.95	13.57
2.38	2.38	2.45	2.46	2.42	2.45	2.28	2.40
0.02	0.03	0.03	0.03	0.03	0.03	0.03	0.03
0.54	0.51	0.66	0.59	0.54	0.57	0.53	0.55
1.62	1.57	1.71	1.58	1.61	1.62	1.62	1.65
3.89	4.14	3.71	3.74	3.70	3.75	3.76	3.63
5.49	5.04	5.40	5.02	5.42	5.14	5.31	5.21
0.17	0.10	0.19	0.14	0.15	0.14	0.14	0.19
0.63	0.37	0.61	0.52	0.47	0.35	0.53	0.45
99.16	98.46	98.90	98.81	98.95	98.80	99.02	98.78
<4	<4	<4	<4	<4	<4	<4	<4
2433	2267	2769	2642	2362	2480	2367	2342
8	6	8	8	8	6	9	7
n.d.	n.d.	n.d.	n.d.	n.d.	<4	<4	n.d.
22	23	21	22	21	21	22	21
13	13	12	13	12	12	12	13
5	6	9	7	7	6	6	7
36	50	36	54	36	52	46	37
186	177	157	205	172	162	190	173
30	27	55	72	20	222	33	71
983	992	1081	1080	961	1033	981	921
24	58	31	37	43	46	30	31
20	19	22	23	17	20	20	19
20	19	19	20	18	19	19	19
42	54	51	64	47	59	41	49
334	332	329	342	325	337	316	352

				Melano. Gr. D.		Leuco. Gr. D.
Y08120902F	Y08121102E	Y08121102C	Y081208T02	Y08120902E	Y08120902D	Y09012501B	Y08120902C
70.86	71.03	71.52	71.65	71.80	72.01	74.43	74.46
0.43	0.46	0.38	0.32	0.26	0.24	0.08	0.05
13.64	13.44	13.93	14.04	13.74	14.02	13.41	14.50
2.13	2.30	1.20	1.69	1.81	1.35	0.79	0.42
0.03	0.02	0.01	0.02	0.02	0.02	0.01	0.05
0.51	0.52	0.46	0.35	0.33	0.34	0.09	0.05
1.45	1.55	0.72	1.50	1.42	1.55	0.95	0.66
3.58	3.57	2.54	3.99	4.05	4.06	4.17	5.41
5.38	5.31	7.61	5.19	5.00	5.04	4.74	3.93
0.10	0.17	0.09	0.12	0.03	0.07	0.00	0.00
0.57	0.44	0.50	0.60	0.88	0.89	0.53	0.38
98.68	98.81	98.96	98.47	99.34	99.59	99.20	99.91
<4	<4	<4	<4	<4	<4	<4	<4
2484	2331	2775	1937	1275	1403	459	191
5	9	5	<4	<4	6	7	<4
n.d.	n.d.	8	<4	<4	<4	n.d.	n.d.
21	21	22	22	26	25	28	50
11	12	11	7	11	13	19	56
6	8	5	5	6	8	8	7
38	36	29	37	42	37	62	68
171	180	292	196	196	188	297	301
12	69	108	28	n.d.	44	10	4
993	942	722	799	477	573	169	142
42	44	44	33	51	50	38	24
17	19	15	16	13	12	4	7
17	19	20	15	17	23	22	39
43	42	27	29	38	36	13	34
280	319	271	282	260	227	107	45

Melano. Gr. D., Melanocratic Granitic Dyke; Leuco. Gr. D., Leucocratic Granitic Dyke.

SiO₂ contents of the Vengen Granite range from 66.5 to 73.0 wt% (Table 1). All major elements decrease with an increase of the SiO₂ content (Fig. 6). The abundances of Ba, Nb, Sr, V, Y, Zn, and Zr decrease, whereas the Cr, Ga, Ni, Pb, Rb, and Th contents are almost constant with an increase of the SiO₂ content (Fig. 7). Most of the major element compositions of the Vengen Granite resemble those of the type-I granitoids, whereas the ranges of the type-I and type-II granitoids are overlapped (Fig. 6). Furthermore, the Vengen Granite is characterized by a high K₂O content (5.0-7.6 wt%) as with the type-I and type-II granitoids. In comparison with Li et al. (2001), where limited major and trace element compositions were reported, Rb, Ba, and Sr contents of the Vengen Granite are similar to that of the type-I granitoids (Figs. 7 and 8). The Vengen Granite geochemically shows both of the volcanic arc and syn-collisional granite fields in the Rb- (Y + Nb) and Nb-Y tectonic discrimination diagrams (Pearce et al., 1984; Fig. 9) similar to the range of the type-I granitoids. The geochemical features suggest that the Vengen Granite is classified as the type-1 granitoids reported by Li et al. (2001). The Vengen Granite shows a high Sr/Y ratio (36.1-71.5), and is plotted on the adakite and Archean high-Al tonalite-trondhjemite-granodiorite (TTG) field in the (Sr/Y)-Y diagram (Fig. 10).

Figure 6. Major element variation diagrams for the Vengen Granite and related rocks. Detailed data are listed in Table 1. Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

Figure 7. Selected trace element variation diagrams for the Vengen Granite and related rocks. Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

Figure 8. Rb versus Sr concentrations, Sr versus Ba concentrations, and Rb/Sr ratios versus Sr/Ba ratios diagrams for the Vengen Granite and related rocks. Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

Figure 9. Rb versus (Y + Nb) and Nb versus Y concentrations diagrams for the Vengen Granite and related rocks. Granite fields are after Pearce et al. (1984). WPG, within plate granite; ORG, ocean ridge granite; VAG, volcanic arc granite; Syn-COLG, syn-collisional granite. Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

Figure 10. Sr/Y ratios versus Y concentrations diagram for the Vengen Granite and related rocks. Adakite and Archean high-Al TTG, and island arc ADR field are after Defant et al. (1991). Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

The SiO₂ contents of the melanocratic and leucocratic fine-grained two-mica granitic dykes range from 71.8 to 72.0 wt% and from 74.4 to 74.5 wt%, respectively (Table 1). Most of the melanocratic granitic dyke compositionally resembles that of the Vengen Granite (Figs. 6, 7, and 9), while the Ba (1275 and 1403 ppm) and Sr contents (477 and 573 ppm) are lower than that of the Vengen Granite (Figs. 7 and 9). The SiO₂ contents of the leucocratic granitic dyke are rather higher than those of the Vengen Granite, but the dyke seems to be plotted with the same chemical trend (Figs. 6 and 7). The leucocratic granitic dyke is plotted on the syn-collisional and within the plate granite fields (Fig. 9). Both of the granitic dykes are geochemically similar to the island arc andesite-dacite-rhyolite (ADR) field (Fig. 10).

Rare earth elements

The REEs and U were determined from LA-ICP-MS (SII SPQ9000) measurements at Rissho University after the method reported by Shindo et al. (2009), using 1:2 glass beads. The Y content measured using XRF was adopted for internal standard correction. Spectromelt A12 (Merck) and LiNO₂ (Suprapur, Merck) were used for preparation of the glass beads. REE and U data are given in Table 2.

Table 2. REE and U compositions of the Vengen Granite and related rocks

	Vengen Granite				Melano. Gr. D.	Leuco. Gr. D.
Sample No.	Y09012501C	Y08120902B	Y09012501E	Y08121102E	Y08120902E	Y09012501B	Y08120902C
La (ppm)	101.8	145.6	101.0	124.8	107.6	12.78	23.49
Ce	189.1	249.0	188.0	220.3	170.8	21.85	48.21
Pr	23.36	28.77	22.32	25.89	19.87	2.80	6.23
Nd	69.25	81.10	69.28	75.42	53.75	9.68	20.93
Sm	14.00	14.34	12.50	13.69	8.71	2.00	5.07
Eu	2.64	2.42	2.14	2.53	1.04	0.35	0.20
Gd	5.47	6.60	5.06	5.18	3.86	2.01	4.92
Tb	0.79	0.97	0.73	0.75	0.47	0.39	1.33
Dy	3.66	3.12	2.96	3.13	2.12	1.83	9.04
Ho	0.50	0.58	0.45	0.53	0.37	0.37	1.64
Er	1.70	1.62	1.35	1.53	1.27	1.38	5.97
Tm	0.22	0.19	0.15	0.21	0.18	0.18	0.97
Yb	1.88	1.81	1.32	1.26	1.47	1.92	9.26
Lu	0.21	0.37	0.27	0.31	0.21	0.32	1.50
U	6.77	5.84	6.53	7.62	5.87	10.53	7.05

Melano. Gr. D., Melanocratic Granitic Dyke; Leuco. Gr. D., Leucocratic Granitic Dyke.

The REE patterns were normalized with respect to C1 chondrite for the Vengen Granite and show light REE (LREE) enrichment and heavy REE (HREE) depression with little Eu anomalies similar to those of the type-I granitoids reported by Li et al. (2001) (Fig. 11). The melanocratic granitic dyke has a similar REE pattern to the Vengen Granite, whereas the leucocratic granite dyke shows weak LREE enrichment and flat or enrichment of HREE patterns that relatively resemble the REE patterns of the type-II granitoids reported reported by Li et al. (2001) (Fig. 11). Both of the granitic dykes show negative Eu anomalies (Fig. 11).

Figure 11. Chondrite-normalized REE abundances of the Vengen Granite and related rocks. Normalization values are from Anders and Grevesse (1989). Range of chemical compositions of the type-I and type-II granitoids are after Li et al. (2001). Leuco., leucocratic; Melano., melanocratic.

Sr and Nd isotope geochemistry

The procedure employed for the extraction of Sr and Nd from rock powders for isotopic analysis was after Kagami et al. (1986) and Iizumi (1996). Isotopic analyses were performed on a thermal ionization mass spectrometer (MAT262) equipped with five dynamic faraday cups at Shimane University after the method reported by Iizumi (1996). The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios were normalized with respect to ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. The NIST SRM987 Sr standard yielded ⁸⁷Sr/⁸⁶Sr ratios between 0.710214 and 0.710246. The normalized ⁸⁷Sr/⁸⁶Sr ratios were corrected using the NIST SRM987 standard of ⁸⁷Sr/⁸⁶Sr = 0.710241. The ¹⁴³Nd/¹⁴⁴Nd ratios were corrected using the Japanese standard JNdi-1 (Nd isotopic reference of the Geological Survey of Japan) of ¹⁴³Nd/¹⁴⁴Nd = 0.512106, which has been well documented using the international standard LaJolla of ¹⁴³Nd/¹⁴⁴Nd = 0.511849 (Tanaka et al., 2000). Initial Sr and Nd isotope ratios of the Vengen Granite were calculated using the decay constants of λ⁸⁷Rb = 1.42 × 10⁻¹¹/y (Steiger and Jäger, 1977) and λ¹⁴⁷Sm = 6.54 × 10⁻¹²/y (Lugmair and Marti, 1978), and corrected to 551 Ma (Elburg et al., 2016). For comparison with the Vengen Granite, the same age data were adopted in the calculation for the granitic dykes. The Rb-Sr and Sm-Nd isotopic data are given in Table 3.

Table 3. Isotopic compositions of the Vengen Granite and related rocks

Sample No.	87Rb/86Sr	87Sr/86Sr(2σ)	SrI	147Sm/144Nd	143Nd/144Nd(2σ)	NdI
Vengen Granite
Y09012501C	0.6117	0.70905(1)	0.70425	0.1222	0.512267(13)	0.511826
Y08121102H	0.5528	0.70862(1)	0.70428
Y09012501H	0.4141	0.70771(1)	0.70446
Y09012501F	0.5163	0.70843(1)	0.70437
Y09012501A	0.4202	0.70768(1)	0.70438
Y08120902B	0.5492	0.70863(1)	0.70432	0.1069	0.512235(13)	0.511849
Y09012501E	0.5435	0.70856(1)	0.70429	0.1091	0.512204(13)	0.511810
Y08120902F	0.4981	0.70821(1)	0.70430
Y08121102E	0.5529	0.70869(1)	0.70435	0.1097	0.512269(13)	0.511873
Melano. Granitic Dyke
Y08120902E	1.189	0.71314(1)	0.70380	0.09796	0.512212(13)	0.511858
Leuco. Granitic Dyke
Y09012501B	5.102	0.74276(1)	0.70268	0.1249	0.512435(15)	0.511984
Y08120902C	6.158	0.74994(1)	0.70157	0.1464	0.512472(16)	0.511943

Melano., Melanocratic; Leuco., Leucocratic.

Initial Sr and Nd isotopic ratios (SrI, NdI) of the Vengen Granite range from 0.70425 to 0.70446 and from 0.511810 to 0.511873, respectively (Table 3), which are similar to that of tholeiitic meta-tonalite of the Nils Larsen Tonalite calculated with 551 Ma, and differ from that of calc-alkaline meta-tonalite (Fig. 12). Initial isotopic ratios of the melanocratic granitic dyke are similar to those of the Vengen Granite and plotted outside the tholeiitic meta-tonalite field (Fig.12). The leucocratic granitic dyke shows lower SrI (0.70157-0.70268) and higher NdI (0.511943-0.511984) than that of the Vengen Granite and the melanocratic granitic dyke.

Figure 12. Epsilon diagram for the Vengen Granite and related rocks. Data of the Nils Larsen Tonalite are after Kamei et al. (2013). Chondritic Uniform Reservoir (CHUR) parameters for calculation of the initial εSr and εNd values are as follows: ⁸⁷Sr/⁸⁶Sr(present) = 0.7045, ⁸⁷Rb/⁸⁶Sr(present) = 0.0827, ¹⁴³Nd/¹⁴⁴Nd(present) = 0.512638, and ¹⁴⁷Sm/¹⁴⁴Nd(present) = 0.1966. Leuco., leucocratic; Melano., melanocratic.

DISCUSSION

The Vengen Granite is characterized by high Sr contents (>700 ppm) and a high Sr/Y ratio (>40), and corresponds with adakite in the Sr/Y-Y diagram (Fig. 10). However, this granite has high K₂O (5.0-7.6 wt%) values, K₂O/Na₂O ratios (1.22-1.62 and 3.00), and high Ba, Rb, and Zr contents that are higher than those of common adakite derived from oceanic slab melting (Martin et al., 2005). The Vengen Granite is thus regarded as high-K adakitic granite or pseudo-adakite (Chung et al., 2003; Hou et al., 2004; Kamei et al., 2009). The K₂O contents of adakitic melts are dependent on the source compositions and the degree of partial melting of basaltic materials (Rapp and Watson, 1995; Rapp et al., 2002). Kamei et al. (2009) reported that the high-K adakitic granites were produced by the partial melting of intermediate to felsic continental materials, which left mainly hornblende with small amounts of plagioclase as residual phases. Yi et al. (2022) also reported that high-K adakitic granitic magma are derived from the melting of intermediate to felsic arc rocks at middle to lower crustal depths without any involvement of the high pressure melting of metabasaltic rocks. The Vengen Granite has high K₂O/Na₂O ratios belonging to the adakite derived from lower crust melting in the discrimination K₂O/Na₂O ratio versus Al₂O₃ diagram (Kamei et al., 2009; Fig. 13), which suggests that magma of the Vengen Granite was derived from the melting of crustal materials in the middle to lower crust.

Figure 13. K₂O/Na₂O ratio versus Al₂O₃ diagram for the Vengen Granite. Discrimination for oceanic slab-derived adakites and lower crust-derived adakites is after Kamei et al. (2009). Melano., melanocratic.

Arakawa et al. (1994) reported that the source material of post-tectonic granite from the Sør Rondane Mountains significantly involved meta-tonalite in terms of the Nd isotopic compositions. Owada et al. (2006) reported that the high-K adakitic granite located in the central part of the Sør Rondane Mountains was produced by partial melting of the meta-tonalite based on the Sr isotopic compositions and whole-rock trace element geochemistry. The inherited zircon ages of the Vengen Granite range from 1100 to 620 Ma, and are concentrated around 1000 Ma (Shiraishi et al., 2008; Elburg et al., 2016). These populations are identical with the magmatic ages of the tholeiitic meta-tonalite; i.e., the Nils Larsen Tonalite (Kamei et al., 2013; Elburg et al., 2015). The isotopic compositions of the Vengen Granite are similar but slightly outside the compositional range of the tholeiitic meta-tonalite with an age correction to 551 Ma (Fig. 12), which indicates that there are other possibilities for the origin.

These alternative processes include the hybridization of adakite magma with shoshonitic melt (Wang et al., 2018; Shen et al., 2021). The Vengen Granite has rare MMEs (Fig. 3b). This occurrence suggests the possibility of hybridization with mafic magma. In addition, the εNdI values of the high-K mafic magma (minette) acting at the same time as the Vengen Granite are almost zero (Owada et al., 2013). Therefore, hybridization with the minette should be ruled out. The Vengen Granite includes xenoliths of gneisses (Shiraishi et al., 1992). Furthermore, some of the inherited zircon ages of the Vengen Granite are similar to those of pelitic gneisses from the central to western part of the Sør Rondane Mountains (Owada et al., 2013). Therefore, the genesis of Vengen Granite would be slightly influenced by the pelitic gneisses in addition to the granitic melt derived from melting of the meta-tonalite. The εSrI and εNdI values of the pelitic gneisses corrected to 551 Ma are +60 to +500 and +0.7 to −5.1, respectively (results are not shown, Table 7 in Owada et al., 2013). The Sr-Nd isotopic compositions of the Vengen Granite appear to be homogeneous, the process of contamination probably occurred in the source region, which then produced a homogenized high-K adakitic magma prior to ascent through the crust.

The NE-terrane thrust over the SW-terrane along the MTB during 600-650 Ma (Fig. 14; Osanai et al., 2013). At that time, the lower crust portion (Unit C, D, and D’) of the SW-terrane was subducted beneath the NE-terrane, and suffered isothermal compression caused by the load pressure of the overthrusted NE-terrane. After that, the geothermal gradient in this region would be increased due to heat conduction from the lower portion. The source magma of the Vengen Granite was thus produced at a deep seated portion of the SW-terrane (Fig. 14). Therefore, the type-I granitoids defined by Li et al. (2001) are probably produced by the partial melting of the tholeiitic meta-tonalite with minor pelitic gneisses.

Figure 14. Schematic geological cross section of the Sør Rondane Mountains (modified from Osanai et al., 2013). MSZ, Main Shear Zone; MTB, Main Tectonic Boundary.

The melanocratic granitic dyke intruded into the Vengen Granite has chemical compositions similar to the Vengen Granite and type-I granitoids (Figs. 6, 7, 9, 11, and 13). The Sr and Nd isotopic compositions of the melanocratic granitic dyke are slightly lower than those of the tholeiitic meta-tonalite, and are different from that of the Vengen Granite (Fig. 12). The genesis of this dyke invokes a mechanism of source mixing similar to that of the Vengen Granite, although the isotopic compositions of the meta-tonalite as a source are slightly different from those of the Vengen Granite. The chemical compositions of the leucocratic granite dyke are significantly different from those of the Vengen Granite (Figs. 6, 7, 9, 11, and 13), and are similar to the type-II granitoids, which suggests a different genesis for the origin of source magma. Investigation of the petrogenesis of these granitic dykes for comparison with other granitic bodies and dykes is thus necessary.

CONCLUSIONS

The Vengen Granite that forms the Kanino-tsume Peak is composed of medium-grained mylonitic biotite granite. This granite has high-K adakitic chemical compositions. The Vengen Granite was thus produced by partial melting of the meta-tonalite with minor amounts of the pelitic gneisses in the subducted SW-terrane under the NE-terrane after collision of the West and East Gondwana continents.

ACKNOWLEDGMENTS

This study was part of the Science Program of the JARE supported by the National Institute of Polar Research (NIPR) under MEXT. We are grateful to M. Abe and the 50th JARE led by T. Odate, and the Belgian Antarctic Research Expedition led by A. Hubert for support of our field work. Courteous and constructive anonymous reviews and editorial comments by T. Hokada improved the manuscript significantly and are gratefully acknowledged. This work was supported by JSPS KAKENHI Grant Number JP25400521.

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