Early Jurassic felsic and associated mafic meta-igneous rocks in Otago Schist, Central Otago, New Zealand

ABSTRACT

Metamorphosed felsic volcanic rocks occur interlayered with metasedimentary schist in close association with a 7 km long metabasaltic (greenschist) horizon in the Otago Schist belt of Central Otago. The felsic metavolcanic rocks are weakly foliated and locally segregated and consist principally of quartz and albite. Geochemical analyses suggest phonolitic compositions and a general within-plate affinity for the felsic metavolcanics rocks, and a MORB/back arc basalt affinity for the associated metabasalts. Zircons from two felsic metavolcanic samples are stubby, sharply faceted, prismatic grains with no visible inherited cores, and are considered to be of primary magmatic origin. Calculated weighted average ²⁰⁶Pb–²³⁸U ages for zircons from the two felsic metavolcanic samples are 190.5 ± 0.6 Ma and 190.1 ± 0.3 Ma. We interpret these ages (late Aratauran–early Ururoan; middle Early Jurassic) to date the deposition of the Torlesse Terrane metasediments at that locality, and also possibly the emplacement of the associated metabasalts.

KEYWORDS:

• Zircon
• Jurassic
• igneous
• schist
• greenschist
• felsic metavolcanic: Otago
• Torlesse Terrane

Introduction

Metavolcanic rocks comprise a volumetrically minor but widespread component of the mainly metasedimentary Torlesse Terrane in the South Island of New Zealand. These rocks are typically mafic in composition, and at low metamorphic grade have preserved basaltic features such as pillow structures and porphyritic textures (Pringle 1980). The bodies are irregular and podiform, and although contacts with the hosting metasediments are locally sheared, they commonly appear to be depositional (Pringle 1980). At higher metamorphic grades, up to greenschist facies, the metavolcanic rocks have been pervasively recrystallised and are generally well foliated with little preservation of primary features. These metavolcanic bodies, usually termed greenschists, are typically elongate parallel to the enclosing foliation, and many can be traced along strike for several kilometres (Turnbull 2001; Bierlein and Craw 2009).

Greenschist layers are mineralogically variable internally, on the scale of centimetres to metres, but are dominated by albite, chlorite, actinolite and epidote in varying proportions (Craw 1984; Bierlein and Craw 2009). In this study, we describe an unusual felsic rock type in Central Otago previously unreported in the literature, that is spatially associated with a greenschist layer. This felsic rock is interpreted to have a volcanic protolith (see later discussion); however, its genetic relationship to the adjacent greenschist unit is uncertain. The felsic metavolcanic rock has been metamorphosed along with the greenschist and the adjacent quartzofeldspathic metasedimentary schist. The field relationships and original textures, although obscured by deformation and metamorphism, suggest that the felsic volcanic rock may have had a tuffaceous protolith. If correct, this felsic unit provides an opportunity to further unravel the geological history of the enclosing schist, by providing new information on the compositional range and tectonic affinities of the metavolcanic layers in the Torlesse Terrane, as described herein. Further, we report results of U–Pb dating of zircons separated from the felsic metavolcanic unit, which are interpreted to be of primary magmatic origin. Hence, we report here the first crystallization age for formation of rocks within the schistose Torlesse Terrane of Central Otago, which we interpret to be a depositional age. We then evaluate these results in the context of the geological evolution of the Torlesse Terrane.

General setting

The Torlesse Terrane consists mainly of quartzofeldspathic metagreywackes and meta-argillites which range in age from Early Permian to middle Cretaceous (Mackinnon 1983; Mortimer 2004). The internal structure is complex, reflecting the origin of the terrane as an accretionary complex along the margin of Gondwana (Mortimer 2004). The terrane was variably metamorphosed and uplifted in the Jurassic and Cretaceous, with the highest metamorphic grades exposed as the Otago Schist belt (Figure 1; Mortimer 1993, 2000). The Otago Schist has a core of pervasively recrystallised upper greenschist facies Torlesse Terrane schists, which contain minor biotite and garnet (Mortimer 2000). There is a regional metamorphic and structural transition from these high grade rocks to prehnite–pumpellyite facies rocks, in which bedding and fossils can be recognised (Mackinnon 1983; Mortimer 1993, 2000). This transition has been extensively disrupted, and most metamorphic boundaries now occur at faults (Mortimer 2000; MacKenzie and Craw 2005; Henne et al. 2011).

Figure 1. Location maps. A, Location of Otago Schist belt in South Island of New Zealand. B , Geological map of the Otago Schist belt, showing North Rough Ridge area (Figure 2) within upper greenschist facies (Textural Zone 4) Torlesse Terrane schists.

Figure 2. Geological map of a portion of North Rough Ridge, showing principal greenschist horizons and associated felsic metavolcanic rock localities. Grid is NZTM 2000.

The felsic metavolcanic rocks of interest in this study occur in the highest grade core of the Otago Schist belt, on North Rough Ridge (Figure 1B). On a regional scale, the ridge has a broad antiformal structure of folded flat-lying schist foliation, although zones of steeper foliation occur as well (Figure 2; MacKenzie and Craw 2005). The antiform was initiated in the Pleistocene, as part of a regionally extensive set of folds that dominate Central Otago (Bennett et al. 2005). Uplift of the ridge, which is ongoing, has exposed the schist basement after erosion of overlying Miocene and Pliocene sediments (Bennett et al. 2005).

Felsic metavolcanic localities on North Rough Ridge (Figure 2) were originally identified by Coote et al. (2011). These units occur in close association with a distinctive greenschist layer that is traceable for 7 km on the flanks of the ridge (Figure 2; MacKenzie and Craw 2005). Several felsic metavolcanic bodies have been identified along this greenschist layer (Figure 2). Another thinner greenschist unit has been mapped farther to the northeast (Figure 2), and airborne magnetic surveys (Martin et al. 2014) suggest that these bodies are likely much more extensive in the subsurface. However, no other occurrences of felsic metavolcanic rocks associated with these greenschists have been identified thus far.

Methods

Mapping of the greenschist horizons on North Rough Ridge was conducted by MacKenzie and Craw (2005) and as part of a regional prospecting project by Glass Earth Gold Ltd. One locality at the northernmost end of the greenschist layer was sampled for this study (Figure 2). Samples were collected from both the felsic metavolcanic rocks (Figure 3) and the nearby greenschist for petrographic and geochemical characterisation, and for U–Pb zircon dating. In addition, a sample of quartzofeldspathic metagreywacke that encloses the felsic metavolcanic and greenschist layers was collected for extraction and dating of detrital zircons. Major, trace and rare earth element (REE) analyses were completed for two samples of the felsic metavolcanic rocks and one sample of greenschist from the North Rough Ridge locality. Analyses were done at the ALS Chemex laboratory in North Vancouver, Canada, using inductively coupled plasma (ICP) atomic emission (ICP-AES) and mass spectrometry (ICP-MS) methods for major and trace/REE analyses, respectively.

Figure 3. Felsic schist horizon that is interlayered with greenschist and minor metagreywacke on North Rough Ridge in the vicinity of NZTM 2000 E1347177, N993518. A, General view of outcrop of felsic schist. B , Close view of outcrop of felsic schist. C , Thin section view (transmitted light, crossed polars) of the felsic schist (OU 85799) from outcrop in (A). D , Thin section view of the felsic schist (OU 85797) from outcrop in (A). Dark areas in (B) are lichen.

Two samples of felsic metavolcanic rock, each weighing c. 5 kg, were collected from separate surface outcrops for U–Pb zircon dating. Zircons were separated from each using conventional crushing, grinding, Wilfley table, heavy liquids and Frantz magnetic separator techniques. Isotopic analyses were done by laser ablation (LA) ICP-MS methods at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. The methodology for zircon selection, mounting, and analysis for the U–Pb age determinations by laser ablation ICP-MS, are described by Tafti et al. (2014) and Beranek and Mortensen (2011). A spot size of c. 35 μm was used, and line scans rather than spot analyses were utilised to minimise the extent of within-run elemental fractionation. A metagreywacke sample was processed and analysed in the same way for determination of ages of detrital zircons.

Field characteristics and petrography

The felsic metavolcanic units form blocky, fine grained, laminated outcrops with a well-defined planar foliation (Figure 3A,B). The felsic metavolcanic shown in outcrop in Figure 3(A) is c. 4 m thick, and has grey quartzofeldspathic schist structurally above and below the felsic layer. The boundaries between the felsic metavolcanics and enclosing schist are diffuse, partly because of the foliation overprint, and are poorly exposed, but occur over < 30 cm. The felsic metavolcanic is only weakly segregated (Figure 3B), in contrast to the adjacent quartzofeldspathic schist which has irregular micaceous foliation anastomosing around numerous lenses of quartz–albite metamorphic segregations and veins. The felsic metavolcanic units are conformable with the layering (at least some of which is depositional) in the host metagreywackes; however, we cannot preclude the possibility that the felsic units may have been emplaced discordant to layering in the metagreywackes and subsequently transposed into parallelism with that layering during deformation and metamorphism.

At the microscopic scale, the abundant albite and quartz that make up the felsic metavolcanic are commonly medium-grained (100–200 µm) with coarser and finer grained laminae (Figure 3C,D). Near-planar quartz and albite laminations show up grain size variations on the 200 µm scale (Figure 3C,D). Subordinate fine-grained muscovite (<200 µm) enhances the foliation, especially in the finer grained laminae (Figure 3D). Most albite has been completely recrystallized and is untwinned, with grains elongated parallel to the foliation and intergrown with the quartz (Figure 3C). Rare, relatively coarse-grained (up to 1 mm) twinned albite grains occur in some coarser grained laminae, and may represent relict primary igneous crystals (c.f. Craw and Angus 1993).

Igneous geochemistry

Geochemical analyses for two samples of felsic metavolcanic and one sample of massive greenschist from the Rough Ridge locality are presented in Table 1, and are plotted together with available analyses of greenschists from elsewhere in the Otago Schist belt in Figure 4. Major element compositional plots (Le Bas et al. 1986; Figure 4A) suggest a trachytic composition for two samples of the felsic metavolcanic and a basaltic composition for the greenschist. In light of the high degree of strain and metamorphic recrystallization that the units have suffered, however, it is probable that at least some amount of major element mobility has affected the rock units. A plot of immobile element ratios (Zr–TiO₂ versus Nb–Y; Winchester and Floyd 1977; Figure 4B) confirms the basaltic composition of the greenschist, as well as the relatively alkaline affinity of the felsic metavolcanic rocks, which plot in the phonolite field. The greenschist sample plots with most other Torlesse Terrane greenschists in the mid-ocean ridge basalt/back arc basalt field on a Shervais (1982) plot of V versus Ti/1000, and shows slight light REE enrichment and no discernible Eu anomaly on a plot of REE/primitive mantle (Figure 4D). In contrast, the two felsic metavolcanic samples show stronger light REE enrichment and a pronounced negative Eu anomaly when plotted against primitive mantle (Figure 4D), indicating that substantial plagioclase fractionation likely affected the parent magma prior to eruption.

Figure 4. Geochemistry of North Rough Ridge felsic metavolcanic rocks (

) and greenschist (

), together with analyses of the Pember diorite near Christchurch (

; from Jongens et al. 2009), and Torlesse Terrane greenschist whole rock data (

) from Barber and Craw (2002), Bierlein and Craw (2009), Fagereng and Cooper (2010), Pitcairn et al. (2015) and Mortensen (unpublished data). Field boundaries are: A, Le Bas et al. (1986); B , Winchester and Floyd (1977); and C , Shervais (1982). REE analyses shown in Figure 4D are normalised to the values for primitive mantle from Sun and McDonough (1989).

Table 1. Geochemical analyses of felsic metavolcanic rocks and greenschist from North Rough Ridge.

Sample	OU85797	OU85799	OU85800	Sample	OU85797	OU85799	OU85800
SiO2	65.7	67.1	42.3	Ho	2.0	2.5	1.4
TiO2	0.07	0.06	2.52	La	145	197	12
Al2O3	17.30	17.60	15. 45	Lu	0.9	1.0	0.5
Fe2O3	3.74	3.31	12.25	Nb	269	273	15
MnO	0.10	0.07	0.23	Nd	73	94	18
MgO	0.07	0.11	6.00	Pr	24	31	3.9
CaO	0.04	0.08	10.90	Rb	100	0.6	1.5
Na2O	7.24	9.99	2.57	Sm	12.7	16.6	5.7
K2O	3.91	0.06	0.04	Sn	21	4	1
P2O5	0.01	0.04	0.27	Sr	70.5	85	215
LOI	1.30	1.10	7.26	Ta	24	24	1
Total	99.51	99.53	99.83	Tb	1.7	2.3	1.1
Ba	145	38	7.9	Th	47	49	1.0
Ce	293	331	28.3	Tl	<0.5	<0.5	<0.5
Cr	10	10	90	Tm	1.0	1.1	0.6
Cs	0.48	0.01	0.08	U	5.0	4.5	0.8
Dy	10.2	13.5	7.14	V	7	9	405
Er	6.17	7.2	4.1	W	2	3	1
Eu	0. 6	0. 8	2.2	Y	58	74	40
Ga	50	47	24	Yb	6.2	7.1	3.5
Gd	10.0	14.4	7.0	Zr	981	1020	138
Hf	34	35	3.8

Note: Geochemical analyses were done at the ALS Chemex analytical laboratory in North Vancouver, Canada. Major element analyses employed the ME-ICP06 analytical package, in which samples were fused and then analysed using ICP-AES. Trace and rare earth element analyses were done using the ME-MS81 ultratrace level analytical package, using ICP-MS following fusion with a lithium metaborate flux. Locations of all samples are: NZTM 2000–Easting 1347177 m, Northing: 4993518 m; Latitude 45°09′43.70′′ S, Longitude 169°46′57.04′′ E.

U–Pb geochronology

Abundant zircons recovered from each of the two felsic metavolcanic samples consisted of stubby, sharply faceted, square prismatic grains with no visible inherited cores. Cathodoluminescence imaging of the zircons show weak concentric zonation typical of igneous zircons (Figure 5). Twenty-six zircon grains were analysed from sample OU85797 and all fall on or very near to concordia (Table 2; Figure 5A). A calculated weighted average of the ²⁰⁶Pb–²³⁸U ages for 20 of the concordant analyses gives an age of 190.5 ± 0.6 Ma; however, there is some scatter in the data, resulting in a calculated mean square weighted deviation (MSWD) of 2.5 and a probability of fit = 0.0 (Figure 5B). Six of the grains yielded slightly older ages, ranging from 204 to 195 Ma, and are interpreted to have been xenocrysts. A total of 30 zircon grains were analysed from the second sample (OU85799), and all but one of the analyses were concordant (Figure 5C). Twenty-six of the concordant analyses give a weighted average ²⁰⁶Pb–²³⁸U age of 190.1 ± 0.3 Ma, with a MSWD = 1.01 and a probability of fit of 0.44 (Figure 5D). This is interpreted as the crystallization age for the sample and is in good agreement with that determined for the previous sample. Two grains from this sample gave older ages of 206 and 197 Ma, and are interpreted to have been xenocrysts. One grain gave a slightly younger age, reflecting the effects of minor post-crystallization Pb loss.

Figure 5. U–Pb concordia and weighted average ²⁰⁶Pb/²³⁸U plot for zircons from North Rough Ridge felsic metavolcanic samples. Error ellipses and error boxes show errors at a 2σ level. Errors on calculated ages are also at a 2σ level. Analyses shown as blue boxes in Figure 5B and D were excluded from the weighted average age calculations. Cathodoluminescence images of representative zircon grains from each sample are also shown. Laser ablation tracks on the zircon images (white dashed outlines) are c. 120 μm long.

Table 2. U–Pb LA-ICP-MS analytical data for zircons from North Rough Ridge felsic metavolcanic rocks.

	Isotopic ratios							Isotopic ages						Background subtracted mean counts/s
	^207Pb/^206Pb	1s	^207Pb/^235U	1s	^206Pb/^238U	1s	Rho	^207Pb/^206Pb	1s	^207Pb/^235U	1s	^206Pb/^238U	1s	202	204	206	207	208	232	235	238
Felsic metavolcanic sample OU85797
1	0.05067	0.00071	0.20826	0.00323	0.02957	0.00016	0.35	225.6	32.04	192.1	2.71	187.9	0.98	0	0	41954	2080	13169	205417	5052	351828
2	0.05182	0.0007	0.21242	0.00318	0.02853	0.00014	0.33	277.6	30.81	195.6	2.67	181.3	0.9	0	8	20811	1055	5094	81098	2513	181356
3	0.04949	0.00048	0.20667	0.00224	0.02987	0.00011	0.34	171.1	22.68	190.8	1.89	189.7	0.68	29	7	42576	2062	12504	207717	5046	355247
4	0.04934	0.00051	0.20833	0.00239	0.02981	0.00011	0.32	164.2	24	192.2	2.01	189.3	0.71	18	0	47153	2277	14533	235188	5527	395273
6	0.04949	0.00056	0.20417	0.00253	0.03007	0.00012	0.32	171.1	26.02	188.6	2.13	191	0.77	0	2	25670	1243	5326	88324	3079	214378
7	0.05123	0.00072	0.21879	0.00342	0.02978	0.00016	0.34	251.2	31.96	200.9	2.85	189.2	0.99	49	0	34909	1750	8944	154220	4044	295171
8	0.05002	0.00058	0.20713	0.00264	0.02987	0.00013	0.34	195.8	26.57	191.1	2.22	189.7	0.8	38	5	33976	1663	9742	156875	4059	287183
9	0.04997	0.00073	0.20285	0.00329	0.02982	0.00016	0.33	193.8	33.73	187.5	2.78	189.4	1.03	0	0	44538	2178	10815	171650	5428	378052
10	0.05036	0.00067	0.21481	0.00319	0.03132	0.00016	0.34	211.8	30.65	197.6	2.67	198.8	0.98	41	13	29809	1469	6996	109277	3457	241498
11	0.0496	0.00053	0.20409	0.00241	0.0298	0.00012	0.34	176.1	24.68	188.6	2.03	189.3	0.73	40	0	30555	1483	6995	118641	3673	261552
12	0.05162	0.00059	0.2101	0.00268	0.02976	0.00013	0.34	268.7	26.18	193.6	2.25	189.1	0.81	0	17	43766	2211	13421	213811	5319	376082
13	0.05024	0.00061	0.20893	0.00281	0.03039	0.00014	0.34	205.9	27.84	192.7	2.36	193	0.86	0	4	29374	1444	7340	117138	3493	247865
14	0.05071	0.00089	0.20204	0.00392	0.02983	0.0002	0.35	227.6	40.19	186.8	3.31	189.5	1.23	47	27	30814	1529	7167	126758	3825	265533
15	0.04932	0.00046	0.20388	0.00211	0.03016	0.0001	0.32	163	21.6	188.4	1.78	191.6	0.66	0	4	63783	3080	28145	482115	7631	545074
16	0.04967	0.00063	0.20631	0.0029	0.02983	0.00014	0.33	179.7	29.31	190.5	2.45	189.5	0.88	22	9	32356	1573	7888	135826	3852	281816
17	0.05039	0.00059	0.20425	0.00265	0.03	0.00013	0.33	213.1	26.92	188.7	2.24	190.5	0.83	35	7	35274	1740	9789	158619	4303	306289
18	0.05075	0.00087	0.21938	0.0042	0.03176	0.00021	0.35	229.7	39.08	201.4	3.49	201.6	1.28	0	15	30912	1536	6694	115309	3536	254177
19	0.05018	0.00055	0.19396	0.00233	0.02997	0.00012	0.33	203.1	25.11	180	1.99	190.4	0.77	0	11	44224	2172	13082	219903	5657	386437
20	0.05114	0.00059	0.20706	0.00266	0.03022	0.00013	0.33	247	26.51	191.1	2.24	191.9	0.82	28	11	32739	1639	9496	149434	3997	284443
21	0.05024	0.00088	0.20976	0.00409	0.03081	0.0002	0.33	206.3	40.26	193.3	3.44	195.6	1.27	28	0	25093	1234	5944	94129	2971	214981
22	0.05047	0.00065	0.20874	0.003	0.03081	0.00015	0.34	216.8	29.65	192.5	2.52	195.6	0.94	1	0	57009	2817	20103	352556	6814	489859
23	0.04983	0.00046	0.20733	0.00214	0.0315	0.00011	0.34	186.9	21.37	191.3	1.8	200	0.68	0	14	43565	2125	13372	220349	5176	367040
24	0.04947	0.00099	0.2007	0.00445	0.03043	0.00023	0.34	170.2	46.24	185.7	3.77	193.3	1.43	0	0	26268	1272	5838	95363	3200	229729
25	0.04922	0.00048	0.20092	0.00216	0.02998	0.00011	0.34	158.2	22.49	185.9	1.83	190.4	0.68	17	40	41402	1995	11782	213118	5013	368563
26	0.04978	0.00071	0.20357	0.00322	0.03031	0.00016	0.33	184.6	32.92	188.1	2.72	192.5	1.01	8	0	29201	1423	6199	108986	3529	258543
27	0.05048	0.00056	0.20989	0.00261	0.03023	0.00013	0.35	217.3	25.55	193.5	2.19	192	0.8	0	0	57453	2840	20609	372092	6829	511383
28	0.05088	0.00103	0.21128	0.00475	0.03145	0.00024	0.34	235.3	46	194.6	3.98	199.6	1.5	11	0	25294	1260	5235	91167	3010	217008
29	0.04906	0.00053	0.20416	0.00247	0.03007	0.00012	0.33	150.8	25.23	188.6	2.09	191	0.77	50	5	70569	3391	27482	516145	8381	634953
30	0.04992	0.00063	0.20899	0.00294	0.02984	0.00014	0.33	191.4	29.11	192.7	2.47	189.6	0.89	35	0	40425	1976	9132	162409	4772	367496
Felsic metavolcanic sample OU85799
1	0.04999	0.00053	0.20456	0.00241	0.0299	0.00012	0.34	194.4	24.32	189	2.03	189.9	0.74	61	32	39144	1916	10666	193915	4726	360165
2	0.05011	0.00051	0.20501	0.00236	0.02978	0.00012	0.35	200.1	23.66	189.4	1.99	189.2	0.73	0	13	75421	3702	32003	613125	9107	698614
3	0.04918	0.00062	0.19763	0.00276	0.02929	0.00014	0.34	156.3	29.2	183.1	2.34	186.1	0.86	33	0	28531	1374	6073	117467	3507	269523
4	0.04979	0.00054	0.20597	0.00252	0.02973	0.00012	0.33	185.2	25.26	190.2	2.12	188.8	0.77	0	4	40070	1954	11847	218989	4785	373965
5	0.04808	0.0008	0.19917	0.00368	0.03003	0.00018	0.32	103.1	38.97	184.4	3.11	190.7	1.15	0	0	19474	917	3698	68012	2322	180443
6	0.04939	0.00063	0.20322	0.00287	0.03005	0.00014	0.33	166.3	29.33	187.8	2.42	190.9	0.9	0	18	30900	1495	8863	164718	3709	287687
7	0.04989	0.00049	0.20466	0.00224	0.02991	0.00011	0.34	189.8	22.52	189.1	1.89	190	0.7	18	0	45925	2244	15667	289589	5529	430834
8	0.05026	0.00057	0.20679	0.00261	0.02994	0.00013	0.34	207.2	25.96	190.9	2.2	190.2	0.8	27	0	31398	1546	7942	144593	3769	295091
9	0.04886	0.00075	0.19754	0.00338	0.03002	0.00017	0.33	141.2	35.84	183	2.87	190.7	1.09	0	0	34064	1630	9263	171356	4161	320257
10	0.04989	0.00047	0.20919	0.00224	0.02968	0.00011	0.35	189.8	21.84	192.9	1.88	188.6	0.68	70	17	88183	4310	42250	761995	10385	840852
11	0.04906	0.00084	0.20221	0.00384	0.03018	0.00019	0.33	150.5	39.58	187	3.24	191.7	1.2	37	12	23303	1120	5149	96499	2791	219790
12	0.04867	0.00058	0.20252	0.00272	0.02994	0.00014	0.35	131.9	27.99	187.3	2.3	190.2	0.85	0	9	31118	1484	7828	145806	3692	296792
13	0.0495	0.00058	0.20595	0.0027	0.03004	0.00013	0.33	171.7	27.09	190.1	2.28	190.8	0.84	5	0	32980	1599	9576	176129	3913	314329
14	0.05198	0.00236	0.20597	0.01016	0.03008	0.00049	0.33	284.4	100.5	190.2	8.55	191.1	3.06	48	24	2994	152	412	7443	373	28587
15	0.04906	0.00051	0.20311	0.00238	0.03002	0.00012	0.34	150.6	24.22	187.8	2.01	190.7	0.75	14	0	47784	2297	11766	233396	5697	458483
16	0.05025	0.00054	0.20707	0.00249	0.02991	0.00012	0.33	206.7	24.51	191.1	2.09	190	0.78	0	0	40084	1974	12807	248626	4801	389394
17	0.05136	0.00072	0.21562	0.0034	0.02981	0.00016	0.34	257.2	31.91	198.3	2.84	189.4	1.01	0	56	30624	1541	12179	237651	3600	299415
18	0.0513	0.0006	0.21413	0.00283	0.02995	0.00014	0.35	254.3	26.66	197	2.36	190.2	0.85	49	0	34265	1722	11314	211936	4051	334449
19	0.04943	0.00079	0.20517	0.00366	0.03025	0.00018	0.33	168.3	36.84	189.5	3.08	192.1	1.15	0	19	30454	1475	9334	186030	3621	295150
20	0.04995	0.00054	0.2073	0.00252	0.0298	0.00013	0.36	192.8	24.74	191.3	2.12	189.3	0.78	53	19	62758	3072	23406	457956	7463	619254
21	0.0499	0.00052	0.2093	0.00247	0.03002	0.00012	0.34	190.1	24	193	2.07	190.7	0.77	43	0	40222	1967	13223	252030	4731	396376
22	0.0502	0.00085	0.20922	0.00398	0.02997	0.00019	0.33	204.2	38.93	192.9	3.34	190.3	1.22	0	0	36253	1783	8538	169992	4292	359020
23	0.0498	0.00062	0.21506	0.00301	0.03118	0.00015	0.34	185.7	28.56	197.8	2.51	198	0.93	6	0	25834	1261	5535	111073	2951	246587
24	0.04978	0.00063	0.21068	0.00299	0.02995	0.00015	0.35	184.9	29.07	194.1	2.51	190.3	0.91	67	0	26279	1282	5815	114364	3063	261941
25	0.04907	0.00057	0.20287	0.00265	0.03016	0.00014	0.36	151.1	26.85	187.6	2.23	191.5	0.85	26	0	40106	1929	15447	308750	4785	398274
26	0.0509	0.00072	0.20454	0.00324	0.02979	0.00016	0.34	236.1	32.22	189	2.73	189.2	1.03	37	0	39619	1976	8838	173502	4863	400777
27	0.04894	0.00062	0.2031	0.00289	0.02978	0.00014	0.33	144.7	29.37	187.7	2.44	189.2	0.91	58	0	28459	1365	7258	144754	3382	288818
28	0.05156	0.00076	0.20859	0.00347	0.03022	0.00018	0.36	265.9	33.58	192.4	2.91	191.9	1.1	34	32	31643	1599	11185	222747	3858	317449
29	0.05182	0.00129	0.22029	0.00613	0.03254	0.00031	0.34	277.4	55.82	202.2	5.1	206.4	1.94	91	0	22564	1146	4920	94403	2618	210908
30	0.05025	0.00093	0.20742	0.00431	0.02995	0.00021	0.34	206.6	42.56	191.4	3.62	190.2	1.33	26	14	21174	1043	4359	89559	2530	215694

Detrital zircons were separated from a sample of metagreywacke (sample OU85801) that is structurally beneath the felsic metavolcanic and U–Pb ages for a total of 65 grains were determined using LA-ICP-MS methods. Because the goal of this part of the study was to attempt to provide a maximum possible depositional age for the sample, euhedral zircons that had a higher probability of being first-cycle were preferentially selected for analysis. Analytical data are listed in Table S1 and are shown on a conventional concordia plot and a cumulative probability plot in Figure 6(A) and (B), respectively. Despite the sampling bias imposed by preferentially selecting only euhedral igneous zircons, the range of ages obtained from the sample is very similar to those reported from many other samples of Torlesse Terrane metaclastic rocks (Adams et al. 2007). There is a cluster of eight zircon grains with concordant ages of 258–236 Ma (Figure 6). However, the youngest zircon age that was obtained is 236 ± 3 Ma (latest Middle Triassic), providing a maximum depositional age for the unit.

Figure 6. U–Pb ages for single detrital zircons from North Rough Ridge metagreywacke.

Discussion

The regional distribution of greenschists within the Otago Schist belt has been well documented both from surface mapping and airborne geophysical studies, and a substantial body of lithogeochemical data has been generated for many of these units. The North Rough Ridge locality is the only area recognised to date where there is evidence for spatially closely associated mafic and felsic magmatism within the Otago Schist belt. Although the greenschist bodies are compositionally clearly distinct from the enclosing quartzofeldspathic metasediments, the difference in appearance between the felsic metavolcanic and the host metasediments in the field is more subtle. As indicated in Figure 2, we have identified several other occurrences of possible felsic metavolcanic in the North Rough Ridge area, and it is very possible that numerous other felsic metavolcanic localities have gone unrecognised, or been mapped as metacherts, outside of our immediate study area.

The difficulty in determining absolute depositional ages for both the metasedimentary rocks and the greenschists in the Torlesse Terrane has greatly hindered development of a comprehensive model for evolution of the terrane. Fossils are scarce in the metasedimentary units, especially in the higher grade and more strongly deformed portions, and to date no material amenable to precise and accurate isotopic dating methods had been identified. Detrital zircon ages from the metasediments provide maximum depositional ages (which cannot be older than the youngest concordant zircon age). Recent studies by Cooper and Ireland (2013) and Jugum et al. (2013) have demonstrated that protolith ages for significant portions of the schist belt are considerably younger than had originally been thought. We tentatively interpret the felsic metavolcanic rocks in the North Rock Ridge area to have had a tuffaceous protolith. The felsic metavolcanics are closely spatially associated with the greenschists, and original contact relationships are not completely clear; hence, we cannot be certain whether the felsic metavolcanics were originally concordant with primary layering in the enclosing metasediments or were in part discordant to layering and have been transposed into parallelism with it. The fine-grained character of the rock supports, but does not prove that it had an extrusive origin, or at least a very high-level of emplacement. Compositionally the felsic metavolcanic rocks yield a within-plate tectonic signature, which differs from the mid-ocean ridge basalt/back arc basalt signature of the greenschists, and suggests that the felsic and mafic rock units are not directly cogenetic. We interpret our U–Pb zircon age of 190–191 Ma (late Aratauran–early Ururoan; middle Early Jurassic) for the felsic metavolcanic rocks at North Rough Ridge to date the deposition of the metagreywackes at that locality, and also possibly the emplacement of the associated greenschists (metabasalts).

Jongens et al. (2009) describe a number of mafic dikes and small stocks of microdiorite in the vicinity of Mt. Pember, c. 70 km northwest of Christchurch (Figure 1A). These bodies cut low-grade sandstones of the Rakaia Terrane, which have locally given Late Triassic fossil ages. The intrusive rocks are generally massive and unfoliated. However, petrographic studies indicate that they have experienced prehnite–pumpellyite facies metamorphism along with the enclosing clastic rocks. Geochemical analyses of two samples of microdiorite (included in Figure 4) indicate intermediate compositions and within-plate or back-arc basin signatures (Jongens et al. 2009), with some influence from subduction-related processes (possibly through crustal contamination of the magmas). An ⁴⁰Ar–³⁹Ar crystallization age of 185.6 ± 3.3 Ma was obtained for hornblende from one of the diorite bodies (Jongens et al. 2009). This age is only slightly younger than the U–Pb zircon ages that we determined for the North Rough Ridge felsic metavolcanic rocks. Although the intrusions in the Mt. Pember area have not experienced the same high degree of strain as the felsic metavolcanic rocks, they show the same relative age with respect to regional deformation and metamorphism, and we speculate that the diorites (and associated mafic dikes) at Mt. Pember, the felsic metavolcanic rocks and at least some of the greenschists may be broadly comagmatic and result from the same general tectonic processes.

The new U–Pb zircon age results reported here provide the first direct evidence for the age(s) of greenschists in the Otago Schist belt. The regional significance of these data is uncertain as yet, although the possible correlation with nearly coeval intrusive rocks in low grade Torlesse Terrane (Pember Diorite; Figure 1A) suggest that the middle Early Jurassic magmatic event might have been regional in extent.

Conclusions

We present middle Early Jurassic (191–190 Ma) U–Pb zircon crystallization ages for two samples of felsic metavolcanic rock from the North Rough Ridge area of Central Otago. We interpret these units to have been derived from a tuffaceous protolith; hence their age also provides the first direct depositional age for the metagreywackes of the Torlesse Terrane in this part of South Island. The felsic metavolcanic rocks yield phonolitic compositions with a within-plate tectonic affinity. Greenschists that are spatially closely associated with the felsic metavolcanics at North Rough Ridge are basaltic in composition and have a mid-ocean ridge or back-arc basin basalt petrotectonic affinity. Despite the close spatial association of the felsic metavolcanics and greenschists, it is still uncertain what the original relationship, if any, was between these rock units. We speculate that mafic to intermediate intrusive rocks from the Torlesse Terrane northwest of Christchurch (the Pember Diorite of Jongens et al. 2009), which has been dated at 185.6 Ma, may also be part of a previously unrecognised regionally developed Early Jurassic magmatic event in the Torlesse Terrane. Many other felsic metavolcanic rock units may be present within the Torlesse Terrane but have not been recognised or have been mapped as metacherts.

Acknowledgments

We thank the staff of the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia for assistance in producing the U–Pb analyses reported here. This research was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to J. K. Mortensen. Additional support was provided from a research subcontract from NZ Ministry for Business Innovation and Employment. We thank Glass Earth Gold Ltd. for logistical support for the field work. The manuscript was improved by critical reviews by Adam Martin, Rodney Grapes, and Associate Editor Timothy Little.

Disclosure statement

No potential conflict of interest was reported by the authors.