Pyrometamorphosed Otago Schist xenoliths cause minor contamination of Dunedin Volcanic Group basanite

Figures & data

Figure 1. Metamorphic grades within the Otago Schist, with the studied sample site (Ram Rock) located in the highest-grade belt of upper greenschist facies. Diagram is modified from Wellnitz et al. (2019).

Figure 2. A, Ram Rock is a columnar-jointed basanite plug that hosts crust and mantle xenoliths. B, This schist xenolith in the basanite displays dark mafic domains and yellow felsic domains. Purple layers are cordierite. C, Crustal xenolith in outcrop of the basanite. Coin is 26.5 mm. D, Sample of the regional Otago Schist collected from near to the basanite outcrop. E, Outcrop of basanite with a small peridotite xenolith and small crustal xenolith. Coin is 26.5 mm.

Table 1. Representative analyses of mineral geochemistry from Otago Schist and crustal xenolith samples.

Sample #	OU85425_03	OU85425_05	OU85425_13	OU85425_09	OU85425_04.1	OU85425_04.3	OU85424_47.1	OU85424_47.2	OU85423_13	OU85421_09	OU85423_33.1	OU85422_44.3	OU85421_03	OU85423_14.2	OU85422_26.1
Mineral	Plagioclase	Muscovite	Epidote	Chlorite	Garnet	Garnet	Olivine	Olivine	Orthopyroxene	Cordierite	Olivine	Olivine	Spinel	Plagioclase	Plagioclase
					Rim	Core	Rim	Core			Symplectite	Cruciform		Prismatic	Anhedral
	Otago Schist	Otago Schist	Otago Schist	Otago Schist	Otago Schist	Otago Schist	Basanite	Basanite	Xenolith	Xenolith	Xenolith	Xenolith	Xenolith	Xenolith	Xenolith
SiO2	67.85	47.01	38.70	25.27	36.84	36.76	39.81	37.48	48.80	45.19	35.61	34.20	0.00	55.89	48.67
TiO2	0.00	0.31	0.00	0.00	0	0	0.00	0.06	0.70	0.00	0.00	0.00	1.11	0.05	0.00
Al2O3	20.27	30.50	27.99	21.98	21.21	21.07	0.24	0.16	3.59	34.95	0.39	3.03	63.30	28.09	33.17
FeO	0.00	3.42	–	26.52	13.84	12.33	15.67	26.20	29.83	9.21	35.72	41.09	24.28	0.40	0.52
Fe2O3	–	–	9.41	–	0.66	0.62			–	–	–	–	–	–	–
MnO	0.00	0.00	0.64	0.85	13.92	17.28	0.21	0.50	0.87	0.00	0.6	0.99	0.00	0.00	0.00
MgO	0.00	2.29	0.00	12.68	0	0	43.97	35.07	13.92	8.39	26.91	20.18	11.19	0.14	0.00
CaO	0.00	0.00	23.46	0.00	13.59	11.91	0.24	0.41	0.63	0.00	0.17	0.63	0.00	9.60	15.89
Na2O	11.36	0.33	0.00	0.00	0	0	0.12	0.05	0.55	0.00	0.34	0.43	0.00	5.54	2.25
K2O	0.00	14.47	0.00	0.00	0	0	0.09	0.05	0.62	1.78	0.00	0.00	0.00	0.63	0.27
Cr2O3	0.00	0.00	0.00	0.00	0	0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Total	99.48	93.33	100.20	87.30	100.07	99.97	100.35	99.98	99.51	99.52	99.74	100.55	99.88	100.34	100.77
Oxygen #	8.00	12.00	12.50	10.00	12	12	4.00	4.00	6.00	8.00	4.00	4.00	4.00	8.00	8.00
Si	2.97	3.43	3.02	1.93	2.95	2.96	1.00	1.00	1.91	2.08	1.00	0.98	0.00	2.51	2.21
Ti	0.00	0.02	0.00	0.00	0	0	0.00	0.00	0.02	0.00	0.00	0.00	0.02	0.00	0.00
Al	1.05	2.63	2.57	1.97	2.01	2	0.01	0.01	0.17	1.90	0.01	0.10	1.98	1.49	1.78
Fe^3+	–	–	0.61	–	0.04	0.04			–	–	–	–	–	–	–
Fe	0.00	0.21	–	0.00	0.93	0.83	0.33	0.58	0.98	0.36	0.84	0.98	0.54	0.02	0.02
Mn	0.00	0.00	0.04	1.69	0.95	1.18	0.01	0.01	0.03	0.00	0.01	0.02	0.00	0.00	0.00
Mg	0.00	0.25	0.00	0.05	0	0	1.65	1.39	0.81	0.58	1.12	0.86	0.44	0.01	0.00
Ca	0.00	0.00	1.96	1.44	1.17	1.03	0.01	0.01	0.03	0.00	0.01	0.02	0.00	0.46	0.77
Na	0.97	0.05	0.00	0.00	0	0	0.01	0.00	0.04	0.00	0.02	0.02	0.00	0.48	0.20
K	0.00	1.35	0.00	0.00	0	0	0.00	0.00	0.03	0.10	0.00	0.00	0.00	0.04	0.02
Cr	0.00	0.00	0.00	0.00	0	0	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Total	4.99	7.93	8.20	7.09	8.03	8.03	3.02	3.00	4.02	5.02	3.01	2.98	2.99	5.00	5.00

Figure 3. Total alkali versus silica (TAS) classification diagram of Le Maitre et al. (2005). The host lava plots as a basanite (Reay et al. 1991; Scott et al. 2020; this paper). Glass compositions from the different segregations of the xenoliths plots in three clusters that display little overlap. Glasses within the felsic domains have sub-alkaline compositions while glasses from the mafic domains and xenolith-basalt interface display alkaline compositions. The Dunedin Volcanic Group field is based on compiled data from Price et al. (2003), Hoernle et al. (2006), Sprung et al. (2007), Timm et al. (2010) and Scott et al. (2020).

Figure 4. Multi-element plot of measured Ram Rock basanite and an Otago Schist xenolith with two samples of basanite at varying distances from an Otago Schist xenolith.

Table 2. Whole rock geochemistry and Sr-Nd-Pb isotopic analyses for Ram Rock basanite samples and an Otago Schist xenolith.

	Otago Schist	± 2s	Basanite	± 2s	Basanite	± 2s	Basanite
	1A		2B		3C		RAM-1^a
SiO2	68.6		43.7		43.4		42.9
TiO2	0.5		2.52		2.42		2.35
Al2O3	14.2		13.95		14		13.6
Fe2O3t	3.54		13.4		12.9		12.35
MnO	0.08		0.2		0.2		0.19
MgO	2.14		9.93		9.85		9.31
CaO	2.09		10.6		10.25		9.92
Na2O	3.45		3.72		3.9		4.11
K2O	3.1		1.36		1.58		1.83
P2O5	0.09		0.64		0.62		0.61
L.O.I.	2.31		0.65		0.91		0.17
Total	100.1		100.67		100.03		97.34
Ba	348		481		518		502
Rb	77		45		45		51
Sr	461		827		799		845
Th	11.5		7.83		7.85		8.24
U	2.4		2.08		2.03		2.05
Nb	8.9		100		103		95.3
Ta	0.7		5.8		6.5		6.7
La	31.2		56.9		57.2		62.3
Ce	62.7		109		108		115
Pr	7.19		12.15		12.05		12.2
Nd	25.7		45.2		44.9		45.8
Zr	234		317		310		285
Hf	6.1		6.9		6.4		6.2
Sm	4.64		8.51		8.19		8.66
Eu	1.05		2.75		2.64		2.85
Gd	3.63		7.4		6.95		6.56
Tb	0.56		1.08		0.99		1.02
Ga	16.1		20.4		19.8		19.4
V	75		222		220		214
Dy	3.23		5.85		5.59		5.35
Ho	0.6		1.04		1.06		0.97
Er	1.85		2.75		2.64		2.59
Tm	0.29		0.39		0.38		0.37
Y	18.4		29.3		28.6		27.4
Yb	1.77		2.27		2.22		2.13
Lu	0.26		0.37		0.33		0.32
Cr	30		350		320		360
87Rb/86Sr^b	0.48		0.12		0.14		0.18
86Sr/87Sr	0.70848	0.000011	0.7031	0.00001	0.70294	0.000012	0.70289
86Sr/87Sr(16 Ma)	0.70837		0.70308		0.7029		0.70285
147Sm/144Nd^b	0.115		0.112		0.112		0.115
143Nd/144Nd	0.51237	0.000011	0.51292	0.000013	0.51292	0.000011	0.51292
143Nd/144Nd(16 Ma)	0.51236		0.51291		0.51291		0.51291
206Pb/204Pb	18.98	0.0011	19.559	0.0012	19.741	0.001	19.831
207Pb/204Pb	15.664	0.001	15.656	0.0013	15.647	0.0011	15.644
208Pb/204Pb	38.992	0.0034	39.243	0.0038	39.3212	0.0034	39.376

^aData from Scott et al. (2020).

^bEstimated using trace element data and isotopic weights and fractions.

Figure 5. A, Photomicrograph in plane polarised light (PPL) of a garnet-bearing sample of Otago Schist collected near Ram Rock. B, Photomicrograph in PPL of a felsic domain in one of the collected Ram Rock crustal xenoliths. Quartz has lobate boundaries and a dark corona. Glass is heterogeneous in colour, ranging from colourless to a dark brown. C, Backscattered electron (BSE) image of skeletal orthopyroxene grains within glass in a felsic layer. D, Photomicrograph in PPL of a mafic domain in one of the collected Ram Rock crustal xenoliths. The mafic segregations are heterogeneous with a variety of minerals that are too small for optical identification. Glass varies from a medium brown to a dark brown/black colour. E, BSE image of plagioclase grains with a rectangular prismatic crystal shape. This morphology of plagioclase is always found within glass in the mafic layers. F, Inverted BSE image of a zoned anhedral plagioclase grain. Zoned plagioclase grains in the studied xenoliths consistently have a Ca-rich core and Na-rich rim. mus = muscovite, chl = chlorite, grt = garnet, qtz = quartz, opx = orthopyroxene, plag = plagioclase, dv glass = devitrified glass.

Table 3. Representative geochemical analyses of glasses within the crustal xenoliths. Glass was measured in both the felsic and mafic domains and from the basanite-xenolith interface.

Sample #	OU85422_03.2	OU85422_09.2	OU85423_16.2	OU85420_14	OU85421_04	OU85421_09	OU85421_08.1	OU85423_10.3
Segregation	Felsic	Felsic	Felsic	Felsic	Mafic	Mafic	Mafic	Interface
SiO2	66.31	73.04	68.12	71.12	60.72	65.90	58.79	57.10
TiO2	0.89	0.42	0.62	0.44	0.00	0.99	1.08	0.92
Al2O3	16.69	14.10	15.44	15.77	20.62	15.89	18.67	20.92
FeO	4.77	3.72	5.66	2.63	1.92	3.62	3.01	3.48
MnO	0.00	0.00	0.03	0.00	0.00	0.00	0.00	0.00
MgO	0.92	0.00	0.56	0.19	0.63	0.00	0.53	0.24
CaO	2.13	0.67	0.51	0.86	2.75	0.71	2.30	0.50
Na2O	4.86	1.89	2.79	2.49	5.78	2.41	5.71	6.45
K2O	2.10	3.77	5.08	4.81	6.44	7.71	5.84	9.04
Cr2O3	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Total	98.67	97.61	98.81	98.31	98.86	97.23	95.93	98.65

Figure 6. Ternary feldspar diagram showing the wide variety of plagioclase compositions in the crustal xenoliths in comparison to the pure albite of the Otago Schist protolith. Plagioclase with a prismatic morphology tends to have an andesine composition while anhedral plagioclase is comparatively enriched in Ca. Considerable overlap in the plagioclase chemistry is present between the different plagioclase morphologies. Feldspar isotherms are from Benisek et al. (2010).

Figure 7. A, BSE image of cordierite in a mafic segregation with angular grain boundaries. Cordierite is typically found with a rectangular to square crystal morphology and hosted in glass. B, BSE image of an olivine-spinel symplectite. C, BSE image of cruciform and laddered olivine grains within devitrified glass. D, BSE image of a large symplectite composed of intergrown anorthite, ilmenite, olivine, and spinel with a rim of larger subhedral pigeonite grains. E, BSE image of an unknown mineral within the xenoliths. This mineral has an acicular morphology and can also be found as dendrites. F, BSE image of acicular SiO₂ grains that have been identified at the xenolith-basanite interface. crd = cordierite, ol = olivine, spl = spinel, dv glass = devitrified glass, pig = pigeonite, ilm = ilmenite, an = anorthite, pl = plagioclase, un-id = unidentified mineral, qtz = quartz, cpx = clinopyroxene.

Table 4. Average values and the standard deviation for geothermometry of selected liquid-mineral pairs.

Geothermometer		n	Average T	2SD	Minimum T	Maximum T
Opx-Liquid	Beattie (1993)	9	1086°C	73°C	1037°C	1134°C
Opx-Liquid	Putirka (2008)	9	940°C	112°C	853°C	996°C
Plag-Liquid	Putirka (2005)	8	978°C	96°C	923°C	1049°C
Plag-Liquid	Putirka (2008)	8	942°C	111°C	886°C	1038°C
Ol-Liquid	Beattie (1993)	4	1044°C	86°C	986°C	1087°C
Ol-Liquid	Beattie (1993)	4	1044°C	86°C	986°C	1087°C
Ol-Liquid	Putirka et al. (2007)	4	856°C	167°C	752°C	927°C
Ol-Liquid	Putirka et al. (2007)	4	987°C	112°C	933°C	1036°C

n = number of mineral-glass pairs used. The number of mineral-glass pairs used was limited by those that displayed equilibrium. Opx = orthopyroxene, Plag = plagioclase, Ol = olivine.

Figure 8. Distribution of misorientation angles of analysed quartz in an Otago Schist sample (A) and a crustal xenolith (B). The peak of ∼60° represents the dauphine twins. A higher relative frequency of dauphine twins is present in the crustal xenolith (0.26) in comparison to the Otago Schist (0.18).

Figure 9. A, Schematic diagram of the mineralogical and textural changes that the regionally metamorphosed Otago Schist underwent during pyrometamorphism. Not to scale. B, P-T-t diagram of regional metamorphism of the Otago Schist around 160–140 Ma, followed by the pyrometamorphic event at 16 Ma (Coombs et al. 2008). Pressure and temperature range of Otago Schist metamorphism from Mortimer (2000). Entrainment of the crustal xenoliths occurred at a shallow depth, with peak metamorphism reached quickly before the xenoliths were erupted and cooled.

Table 5. Comparison of major element mineral chemistry of garnets from the analysed Otago Schist of a large anorthite-ilmenite-olivine-spinel-pigeonite symplectite in one of the crustal xenoliths.

	Garnet	Symplectite
	Otago Schist	Xenolith
SiO2	37	40
Al2O3	21	25
TiO2	0.0–0.3	1.3
MgO	0	5
FeO	13–19	16
MnO	8–15	0.12
CaO	12–14	12

Figure 10. A, Plot of ¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr shows little isotopic change in the basanite samples within several cm from a crustal xenolith (2B, 3C). B, ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb reveals variation in the basanite adjacent to the crustal xenolith, with the highest isotopic change occurring in the sample taken from closest to the xenolith. C, A similar trend to 10B is visible on a plot of ²⁰⁸Pb/²⁰⁴Pb against ²⁰⁶Pb/²⁰⁴Pb. The arrays represent the modelled fraction of melt remaining as the isotopic compositions evolve. The DVG basalts are from Scott et al. (2020).

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Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.