Magmatism during Gondwana break-up: new geochronological data from Westland, New Zealand

Figures & data

Table 1 Recalculation of K/Ar ages of lamprophyre dikes and a basalt sill reported by Adams & Nathan (1978) to decay constants of Steiger & Jäger (1977).

Sample No.	Dated material	Intrusion type	Published age (Ma)	Recalculated age	± 2σ
P40127	Total Rock	Dike	82.5	84.4	1.2
P40726	Hornblende	Dike	83.2	85.1	1.2
P40727	Hornblende	Dike	84.2	86.2	0.4
P40728	Hornblende	Dike	78.1	79.9	1.2
P38713	Total Rock	Dike	79.3	81.2	1.2
P38704	Total Rock	Sill	89.9	92.0	1.6
			89.1	91.2	2

Table 2 Results of ⁴⁰Ar/³⁹Ar analyses by laser heating at Universität Potsdam and Lund University (OU79915, OU79943, OU79956 and KST-2).

Laser output (W)	^40Ar/^39Ar	^37Ar/^39Ar	^36Ar/^39Ar	K/Ca	% ^40Ar*	fr.% ^39ArK	^40Ar*/^39ArK	Age (Ma)	2σ
Sample ID: OU79943		J = 0.002650 (against 28.02 Ma FCT age)				Laboratory ID: C12023
1	750.32	130.97	2.7719	0.00	−7.75	0.03	−64.48	−338.0	± −380.5
1.2	523.66	102.18	1.6400	0.01	9.04	0.04	51.28	229.9	± 293.2
1.5	321.09	53.48	0.9452	0.01	14.36	0.08	48.05	216.2	± 126.9
1.8	31.29	62.60	0.2098	0.01	−81.88	0.07	−26.89	−133.3	± −158.0
2.2	32.86	23.04	0.0701	0.03	42.71	0.19	14.28	67.0	± 42.7
2.5	14.86	16.68	0.0028	0.03	99.96	32.63	15.04	70.5	± 4.9
2.7	14.53	10.34	0.0022	0.06	99.96	31.94	14.64	68.7	± 3.5
2.9	14.68	15.72	0.0016	0.04	99.95	28.07	14.85	69.6	± 4.6
3.1	15.08	13.80	0.0021	0.04	99.81	6.95	15.21	71.3	± 23.2
				Plateau age (four steps from 5.0 to 6.2%):				69.4	± 2.4
Sample ID: OU79956		J = 0.002654 (against 28.02 Ma FCT age)				Laboratory ID: C12031
0.7	116.96	25.88	0.3652	0.02	9.52	0.16	11.35	53.6	± 45.3
0.9	23.22	0.60	0.0095	0.97	88.15	7.12	20.48	95.5	± 1.3
1.1	20.74	0.20	0.0016	2.92	97.81	21.33	20.29	94.6	± 1.2
1.3	20.15	0.18	0.0006	3.27	99.13	23.94	19.98	93.2	± 1.5
1.5	20.44	0.21	0.0005	2.81	99.30	20.53	20.30	94.7	± 1.2
1.7	20.69	0.35	0.0006	1.68	99.30	12.30	20.55	95.8	± 1.3
1.9	21.18	0.41	0.0007	1.44	99.24	10.54	21.03	98.0	± 1.0
2.1	20.83	1.06	0.0007	0.56	99.47	4.07	20.74	96.7	± 1.6
				Plateau age (seven steps from 1.8 to 4.2%):				95.7	± 0.4
Sample ID: OU79915		J = 0.002658 (against 28.02 Ma FCT age)				Laboratory ID: C12032
0.8	35.07	1.41	0.0683	0.42	42.81	2.46	15.03	70.7	± 3.7
1	20.06	0.19	0.0038	3.15	94.50	18.56	18.96	88.7	± 1.3
1.2	19.72	0.18	0.0014	3.19	97.94	18.81	19.31	90.3	± 1.4
1.4	20.36	0.26	0.0009	2.24	98.76	13.24	20.11	94.0	± 1.3
1.6	20.07	0.22	0.0007	2.65	99.00	15.64	19.87	92.9	± 1.2
1.8	20.04	0.27	0.0007	2.18	99.12	12.86	19.87	92.9	± 1.0
2	19.91	0.28	0.0006	2.11	99.27	12.47	19.77	92.4	± 0.9
2.2	20.42	0.58	0.0005	1.01	99.48	5.96	20.32	94.9	± 1.2
				Plateau age (four steps from 2.8 to 4.0%):				92.9	± 0.6
Sample ID: KST2		J = 0.005848 (against 28.34 Ma TCR age)
2.1	3206.159	2.811398	8.8423	0.1533	18.5	0.014051	594.65389	2704.4	± 339.9
2.3	837.8695	0.5765036	2.2863	0.7477	19.4	0.028102	162.37311	1204.4	± 303.5
2.5	501.8065	1.272675	1.4349	0.3387	15.5	0.049179	77.95771	677.7	± 93.3
2.7	268.2241	1.794722	0.7938	0.2402	12.6	0.06323	33.82935	325.7	± 56.2
2.8	164.2169	0.8259922	0.5037	0.5218	9.4	0.117093	15.44906	156.0	± 34.5
2.9	144.0491	0.8190386	0.4583	0.5263	6	0.128803	8.70149	89.5	± 31.2
3.1	146.9716	0.8983874	0.4683	0.4798	5.9	0.161589	8.65932	89.1	± 27.8
3.3	99.43718	0.9563877	0.3031	0.4507	10	0.199059	9.9623	102.2	± 18.4
3.5	62.63894	0.9151869	0.1829	0.471	13.8	0.273998	8.67339	89.3	± 14.1
4	37.43918	1.446422	0.0871	0.298	31.6	0.634646	11.83436	120.7	± 6.6
4.5	15.14282	2.572341	0.0174	0.1676	67.4	3.592422	10.22118	104.7	± 2.5
5	9.190742	2.966431	0.0031	0.1453	92.5	94.73783	8.52067	87.7	± 0.3
				Total gas age:				88.5	± 0.6

Decay constants after Steiger & Jäger (1977), 4 amu discrimination = 1.0574 ± 0.11%, ⁴⁰K/³⁹K = 0.022 ± 69.30%, ³⁶Ca/³⁷Ca = 0.000264 ± 3.44%, ³⁹Ca/³⁷Ca = 0.000672 ± 0.94%.

Figure 1 Simplified geological map of central Westland, modified after Nathan et al. (2002). Sample locations are indicated by the green dots, accompanied by sample number and obtained dike intrusion ages. B, Bonar Range; Bu, Buller Gorge; C, Mt Camelback; G, Granite Hill; H, Hohonu Range; K, Mount Te Kinga; L, Lake Kaniere; M, Lake Brunner; PC, Paparoa Core Complex with arrows indicating the direction of stretching lineations (Tulloch & Kimbrough 1989); T, Mount Turiwhate.

Figure 2 A, Photograph of the stream boulder sampled for Moa-16. Greenish grey lamprophyre contains abundant megacrystic black kaersutite and peridotite/pyroxenite xenoliths of several cm in size. Hammer shown for scale. B, Photograph of a thin section (plane polarized light) of sample KST-2, scale in figure. The thin section shows part of a megacrystic kaersutite grain (7 mm across) with a c. 0.2 mm thick rim and matrix made up primarily of smaller kaersutite and biotite with feldspar and opaque minerals and a fine-grained epidote aggregate (Ep). C, Photograph of thin section (plane polarized light) of sample Hoh-1, scale in figure, showing similar mineralogy to KST-2.

Figure 3 ⁴⁰Ar/³⁹Ar step heating age spectra of kaersutite from the four studied dikes. Boxes indicate 2σ errors on heating steps and on calculated averages. Ages displayed in figures are considered the best approximation of the age when no plateau or isochron age could be calculated.

Table 3 Results of ⁴⁰Ar/³⁹Ar analyses at UNLV (Hoh-1 and Moa-16).

T (°C)	^36Ar	^37Ar	^38Ar	^39Ar	^40Ar	%^40Ar*	% ^39Ar rlsd	Ca/K	^40Ar*/^39ArK	Age (Ma)	± 2σ
Sample ID: Moa-16			J = 0.001732 (against 28.02 Ma FCT age)
850	1.07	3.82	0.62	8.13	619.17	52.40	1.19	2.04	39.92	120.62	± 0.93
950	0.15	1.53	0.15	6.97	192.48	84.40	1.02	0.95	22.21	68.11	± 0.59
1050	0.10	2.51	0.15	5.79	172.24	90.20	0.85	1.89	25.38	77.61	± 0.59
1090	0.11	6.73	0.24	4.95	179.58	89.90	0.73	5.91	30.95	94.22	± 0.79
1100	0.07	8.11	0.24	5.34	169.84	96.20	0.78	6.60	28.97	88.33	± 0.66
1110	0.11	26.43	0.74	17.12	517.14	97.10	2.51	6.71	29.10	88.73	± 0.65
1120	0.24	105.20	2.91	68.27	1988.85	98.00	10.00	6.70	28.79	87.78	± 0.56
1130	0.31	227.88	6.09	149.80	4251.61	99.00	21.95	6.61	28.43	86.72	± 0.55
1140	0.26	269.85	7.10	175.43	4878.06	99.50	25.71	6.69	28.01	85.45	± 0.59
1160	0.23	295.16	7.67	194.23	5383.98	99.80	28.46	6.61	28.00	85.45	± 0.57
1190	0.09	34.20	0.91	22.71	655.37	98.80	3.33	6.55	28.34	86.44	± 0.71
1400	0.21	36.28	1.02	23.74	725.32	95.90	3.48	6.64	28.71	87.56	± 0.56
									Total gas age	86.41	± 0.53
Sample ID: Hoh-1			J = 0.001769 (against 28.02 Ma FCT age)
950	0.73	35.46	1.06	48.57	704.84	72.90	11.23	3.19	10.56	33.38	± 0.29
1050	0.14	19.76	0.35	8.37	206.97	87.10	1.93	10.33	20.65	64.72	± 0.56
1100	0.10	19.13	0.44	10.04	270.05	94.60	2.32	8.33	24.73	77.25	± 0.65
1110	0.12	32.46	0.76	15.87	438.42	96.20	3.67	8.94	26.29	82.01	± 0.64
1120	0.22	106.14	2.41	50.96	1451.12	97.60	11.78	9.11	27.95	87.08	± 0.68
1130	0.27	131.38	3.01	63.82	1799.76	97.50	14.75	9.00	27.71	86.32	± 0.69
1140	0.14	72.96	1.68	35.54	997.19	98.20	8.22	8.98	27.61	86.04	± 0.68
1150	0.12	37.03	0.84	17.65	511.78	97.10	4.08	9.17	27.83	86.69	± 0.74
1160	0.16	67.99	1.53	32.39	935.18	97.50	7.49	9.18	28.19	87.80	± 0.69
1190	0.31	162.06	3.58	76.84	2174.28	97.60	17.76	9.22	27.88	86.84	± 0.75
1250	0.30	126.16	2.78	60.07	1721.04	69.90	13.89	9.19	27.99	87.18	± 0.70
1400	0.19	25.97	0.63	12.47	392.45	93.00	2.88	9.11	27.79	86.57	± 0.77
									Total gas age	80.00	± 0.53

Isotope beams in mV, rlsd, released; error in age includes J error. ³⁶Ar to ⁴⁰Ar are measured beam intensities, corrected for decay for the age calculations. Decay constants after Steiger & Jäger (1977), 4 amu discrimination = 1.0574 ± 0.11%, ⁴⁰K/³⁹K = 0.022 ± 69.30%, ³⁶Ca/³⁷Ca = 0.000264 ± 3.44%, ³⁹Ca/³⁷Ca = 0.000672 ± 0.94%.

Figure 4 ⁴⁰Ar/³⁹Ar step heating age spectra and isochrons of biotite from the Bonar Orthogneiss. Includes plateau ages and isochrons, errors are 2σ.

Figure 5 Backscatter electron image of a zircon grain previously targeted for LA-ICP-MS analysis. LA-ICP-MS spots and the age of the specific spot are shown in pink. Bright areas as a result of secondary electron charging indicated by ‘C’. The image shows a coarse grain with inclusion-rich regions, examples of thorite (bright phases) and presumed biotite (dark phases) inclusions are indicated. Scale in figure.

Figure 6 Concorida plot including analysed grains up to 400 Ma, error ellipsoids indicate 2σ error.

Table 4 Full LA-ICP-MS results for sample OU55684.

Spot #	Pb* (ppm)	U (ppm)	Atomic Th/U	Uncorrected ^206Pb/^238U	± % (1σ)	Uncorrected ^207Pb/^235U	± % (1σ)	Uncorrected ^207Pb/^206Pb	± % (1σ)	Uncorrected ^208Pb/^232Th	± % (1σ)	^208Pb corrected ^206Pb*/^238U age (Ma)	± (1σ)	^208Pb corrected ^207Pb*/^235U age (Ma)	± (1σ)	^208Pb corrected ^207Pb*/^206Pb* age (Ma)	± (1σ)	Spot MSWD	Selected age (Ma)	± (1σ)
30	92.59	6219	2.13	0.00996	1	0.1212	3.2	0.0883	3.1	0.00347	4.9	61.9	1	66.3	30.8	261.9	111	14.52
21	3.54	347	0.07	0.0105	1.2	0.1531	3.7	0.1058	3.5	0.02183	4.6	63.1	0.8	72	6.6	381.1	29.7	1.5	63.1	1.9
23	114.2	5139	4.63	0.01002	0.9	0.085	3.5	0.0615	3.3	0.00304	1.9	65.3	2.5	193	54.6	2192.4	274	54.8
31	62.26	3833	2.22	0.01047	1.2	0.0899	2.7	0.0623	2.4	0.00365	3.5	65.5	1.6	32.4	23.7	0	0	3.2	65.5	2.4
27	123.4	6930	3.04	0.01015	0.8	0.0789	2.2	0.0564	2	0.00309	1.7	65.7	1	115.4	22.7	1272.2	150	9.16	65.7	2
28	115.1	5882	3.66	0.01009	0.6	0.0702	1.5	0.0505	1.4	0.00311	1	67.4	0.7	108.6	17.1	1155.5	114	30.07
29	73.3	3845	3.31	0.01041	0.7	0.0921	18.7	0.0642	18.7	0.0034	3.7	68.4	0.9	56.2	48.6	0.3	0.3	8.81	68.4	2.1
0	112.4	4458	4.78	0.0112	1.3	0.0785	6.6	0.0508	6.5	0.00362	2.1	70.4	4.2	13	74.2	0	0	3.27	70.4	4.6
15	3.03	239	0.4	0.01196	1.3	0.1614	5.6	0.0979	5.5	0.00729	3.8	72.5	1	82.1	10	370.7	39.3	1.5	72.5	2.2
18	4.8	426	0.03	0.01196	1.8	0.1103	10.4	0.0669	10.2	0.0311	13.5	74.7	1.4	71.5	12.1	3.2	0.6	0.57	74.7	2.4
19	7.62	471	0.26	0.01643	0.8	0.1212	4	0.0535	3.9	0.00593	3.2	104.6	0.9	106.1	5.3	143.3	7	1.5	104.6	3
20	6.73	323	0.87	0.018	1.2	0.1982	6.3	0.0798	6.1	0.00648	3.2	113	1.4	144.9	14.8	711.9	55.6	1.59	113	3.4
12	5.41	253	0.85	0.01871	1.7	0.2139	7	0.0829	6.7	0.00658	4	117.8	2.1	166.2	17.1	929.5	67	2.05	117.8	3.8
8	3.7	170	0.98	0.01843	1.3	0.1527	7.1	0.0601	7	0.00587	3.3	118	1.7	140.1	16.2	542.3	51.5	1.25	118	3.6
17	17.25	951	0.04	0.01942	0.7	0.1482	5.4	0.0553	5.4	0.02177	5.1	122.2	0.9	108.5	7.6	0.6	0	1.08	122.2	3.4
2	13.41	739	0.07	0.01955	1.4	0.1387	4.9	0.0515	4.7	0.00707	10.5	124.6	1.8	128.8	6.6	206.6	9.9	1.61	124.6	3.8
24	16.26	841	0.11	0.0205	1.6	0.1735	6.7	0.0614	6.5	0.00809	12.2	130.5	2.1	156.9	12.5	576.7	37.2	1.85	130.5	4.2
11	5.92	220	0.96	0.02333	1.6	0.2242	8.1	0.0697	7.9	0.00642	2.6	152.4	2.5	255.4	16.4	1359.9	53.2	1.91	152.4	4.8
10	33.27	1115	0.02	0.03212	2.3	0.3974	6	0.0897	5.5	0.10149	11.3	199.4	4.5	277.5	19	1000.7	47.6	3.1	199.4	7
13	3.95	81	0.64	0.04335	3	0.433	9.2	0.0724	8.7	0.01871	11	261.6	8.7	210.8	65.2	0.3	0.1	0.81	261.6	11.2
1	78.19	1904	0.04	0.04449	0.8	0.3531	1.9	0.0576	1.7	0.02234	15.2	279.8	2.1	295.8	5.8	423.8	7.3	1.71	279.8	7.9
26	56.15	1338	0.01	0.0459	0.8	0.3481	2.1	0.055	1.9	0.04586	12.4	288.9	2.4	298	5.6	369.7	6.2	2.93	288.9	8.2
3	28.35	571	0.25	0.05071	1.1	0.3708	3.2	0.053	3	0.01484	3.3	319.7	3.5	331.6	10.6	416.3	12.3	1.99	319.7	9.4
14	17.85	342	0.34	0.05184	1	0.3983	4.1	0.0557	4	0.0166	4.1	324.9	3.4	333.1	22.3	387.8	25.1	1.55	324.9	9.5
25	235.1	4964	0.01	0.05172	0.8	0.3673	1.7	0.0515	1.5	0.0158	11.9	325.1	2.4	318	4.6	267.2	3.6	3.09	325.1	9.2
4	13.94	252	0.38	0.05412	1.1	0.4305	4.9	0.0577	4.8	0.01771	3.3	338.9	3.6	350.3	19	427.1	22	1.39	338.9	9.9
7	33.94	590	0.42	0.05568	0.6	0.4413	2.2	0.0575	2.1	0.01824	2.8	348.3	2.1	358.5	13.9	424.2	15.9	1.13	348.3	9.7
6	28.96	241	0.33	0.11857	1.2	1.4068	4	0.086	3.8	0.04295	11	717.9	9.3	833.5	46.9	1160.5	56.3	4.16	717.9	21.7
5	30.34	247	0.2	0.12557	0.8	1.2965	4	0.0749	3.9	0.04091	5.5	761.9	5.8	831.6	26.4	1024.4	29.6	1.04	761.9	21.5
16	65.97	346	0.35	0.18667	0.8	2.0012	1.8	0.0777	1.6	0.05083	2.4	1105.6	7.8	1154.1	15.6	1241.3	15.2	0.72	1241.3	51.9
9	200	603	0.3	0.31927	1.3	6.5798	2.1	0.1495	1.6	0.12216	4	1773.7	20	1978.4	22	2208.3	19.5	1.19	2208.3	90.3

Notes: MSWD, internal spot MSWD < 10 for used ages. Data sorted by corrected ²⁰⁸Pb age. Ages were calculated from ²⁰⁶Pb*/²³⁸U ratios, where * indicates radiogenic Pb, total Pb minus common Pb. Common Pb corrections were applied based on the difference between measured and expected ²⁰⁸Pb/²⁰⁶Pb ratios for the measured ²³²Th/²³⁸U value, according to methods described by Compston et al. (1984). Concordance was calculated on the basis of agreement between ²⁰⁷Pb*/²³⁵U and ²⁰⁶Pb*/²³⁸U ages within their reported 1σ internal errors. Analyses > 10% discordant were not included in the age calculation. Concordance was calculated on the basis of agreement between ²⁰⁷Pb*/²³⁵U and ²⁰⁶Pb*/²³⁸U ages, and has a typical uncertainty of 5–10%.

Table 5 Summary of new age results and their locations.

Sample	Grid reference (NZMS260)	Lithology	Age (Ma)	Method
Lake Brunner
Hoh-1	K32 755 323	Camptonitic lamprophyre	87 ± 1.0	Weighted mean
Moa-16	K32 847 386	Camptonitic lamprophyre	87 ± 2.7	Weighted mean
KST-2	K32 844 388	Camptonitic lamprophyre	88 ± 0.3	Final heating step
Bonar Range
OU79943	I34 201 907	Camptonitic lamprophyre	69 ± 2.4	Plateau age
OU79915	I34 199 894	Bonar orthogneiss	93 ± 0.6	Plateau age
OU79956	I34 236 916	Bonar orthogneiss	96 ± 1.2	Isochron age
Fraser complex
OU55684	J34 368 952	Quartz trachyte (deformed)	68 ± 4	Youngest Zr population

Ages are ⁴⁰Ar/³⁹Ar ages considered most accurate, as explained in the text, errors are 2σ. Except OU55684 where a zircon age is reported and the reported error indicates the spread in the youngest zircon age population (N = 6).

Figure 7 Central–north Westland magmatic events between 100 and 50 Ma, occurrences sorted from south to north. Ranges indicate the spread in data from the same locality (Mt Camelback and Arahura-1 drillcore, Hohonu and Paparoa) and uncertainty on the single measurements for the Bonar Range, Fraser Complex and Southern Paparoa – Victoria ages. Dotted lines indicate the approximate ages for the beginning and end of Tasman Sea spreading (Weissel & Hayes 1977; Veevers et al. 1991; Gaina et al. 1998).

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