Emplacement, metamorphism, deformation and affiliation of mid-Cretaceous orthogneiss from the Paparoa Metamorphic Core Complex lower-plate, Charleston, New Zealand

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating and whole-rock geochemistry of plutonic rocks from coastal exposures of the Paparoa Metamorphic Core Complex (PMCC) lower-plate has revealed their suite affinity and magmatic and metamorphic ages. Emplacement and metamorphism of the Charleston Orthogneiss occurred at 118±2 Ma and 107±2 Ma, respectively. Metamorphism of the Charleston Orthogneiss was coeval with Buckland Granite intrusion, suggesting a link between the two. Field relations between the post-metamorphic Doctor Bay dike (DBD) and Charleston Orthogneiss indicate intrusion of the dike at 105±2 Ma was syn-tectonic. Deformation of the Charleston Orthogneiss occurred during mid-crustal continental extension and PMCC formation in the New Zealand region of the Pacific Gondwana margin. Fabric development in the Charleston Orthogneiss may therefore be related to mid-crustal deformation associated with extension. The Charleston Orthogneiss and DBD are assigned to the I-/S-type Rahu Suite on the basis of their geochemistry, mid-Cretaceous ages, inherited zircon populations and inboard location.

Keywords:

• Charleston Metamorphic Group
• Charleston Orthogneiss
• continental extension
• geochemistry
• Gondwana
• LA-ICP-MS U–Pb zircon dating
• Paparoa Metamorphic Core Complex
• Rahu Suite

Introduction

Extensive outcrops of the Paparoa Metamorphic Core Complex (PMCC) lower-plate are exposed along the coast between Charleston and Belfast Creek, Buller, New Zealand (Kimbrough & Tulloch 1989; Nathan 1975; Tulloch & Kimbrough 1989; White 1994). The PMCC formed during mid-Cretaceous continental extension in the New Zealand segment of Gondwana, which preceded fragmentation of the supercontinent (Tulloch & Kimbrough 1989; Spell et al. 2000). A small granitic pluton and numerous dikes cut the PMCC lower-plate gneisses in the Charleston area (Nathan 1975; Kimbrough & Tulloch 1989; White 1994). The timing of magmatism, metamorphism and fabric development in the footwall gneisses of the PMCC in relation to the continental extension that occurred immediately prior to separation of New Zealand from Gondwana is poorly constrained. This study reports on zircon geochronological and whole-rock geochemical investigations of granitoid orthogneisses from the lower-plate in order to provide constraints on the timing of their emplacement, metamorphism and deformation and their affiliation with previously described Cretaceous plutonic suites in New Zealand (Tulloch 1983, 1988; Tulloch & Kimbrough 2003).

Geological background

New Zealand basement geology

The geology of New Zealand is subdivided into pre-Cenozoic basement rocks and the unconformably overlying Late Cretaceous–Cenozoic sedimentary plus volcanic rocks (Mortimer 2004; Wandres & Bradshaw 2005). The basement rocks of New Zealand are further subdivided into the Western and Eastern Provinces, which contain a number of tectonostratigraphic terranes, and the Median Batholith (Mortimer 2004; Wandres & Bradshaw 2005). The Cenozoic Alpine Fault, which is the on-land expression of the Pacific–Australian plate boundary, has dextrally offset the basement rocks by c. 480 km (Cooper et al. 1987).

Western Province

The Western Province contains the Takaka and Buller Terranes, which together formed the Pacific margin of Gondwana from the Devonian onwards (Cooper 1989; Cooper & Tulloch 1992; Wandres & Bradshaw 2005; Mortimer 2004; Tulloch et al. 2009a). The Takaka Terrane is structurally complex, comprising numerous fault-bound slices of rock which have experienced multiple episodes of metamorphism and deformation of varying degrees. It contains a wide variety of rock types and is dominated by Middle–Late Cambrian intra-oceanic arc volcanics and interbedded arc-derived sediments deposited in a back-arc basin (Mortimer 2004; Wandres & Bradshaw 2005). In contrast, the adjacent Buller Terrane (the most inboard of the New Zealand terranes) consists largely of Ordovician quartzose greywacke turbidites of continental Gondwana provenance (Greenland Group) and pelagic argillite deposited in continental margin submarine fans, affected by Late Ordovician lower greenschist facies metamorphism and folded into upright folds with axial planar cleavage (Cooper 1974, 1989; Adams et al. 1975; Roser & Korsch 1988; Cooper & Tulloch 1992; Ireland 1992; Roser et al. 1996; Adams 2004).

Median Batholith

Subduction along the Pacific margin of Gondwana from the Devonian through the Cretaceous produced the Median Batholith, some of which occupies a position between the Western and Eastern Provinces, and includes contiguous plutons which intrude both provinces (Allibone et al. 2009a). Plutons that make up the Median Batholith and intrude the Western Province are subdivided on the basis of their age and composition into nine suites: the Karamea, Paringa, Tobin, Ridge, Foulwind, Darran, Western Fiordland Orthogneiss (WFO), Separation Point (SPS) and Rahu Suites (Tulloch 1983, 1988; Cooper & Tulloch 1992; Tulloch & Kimbrough 2003; Allibone et al. 2009a, b; Tulloch et al. 2009a). Key characteristics of the relevant suites are summarised in Table 1.

Table 1  Key characteristics of relevant granitoid suites (Kimbrough et al. 1994; Muir et al. 1994, 1995, 1997, 1998; Waight et al. 1997, 1998; Ireland & Gibson 1998; Tulloch & Kimbrough 2003; Bolhar et al. 2008; Scott & Palin 2008). Bracketed ages indicate single plutons that are older or younger than most of those comprising a particular suite.

Suite	Age (Ma)	Geochemistry
Rahu Suite	(119) 116–106	SiO2≤78 wt%, moderate Sr/Y (<40–200), low Al2O3 and Na2O3, high K2O, flat HREE, Eu anomalies
Separation Point Suite	126–109 (105)	SiO2≤72 wt%, 40< Sr/Y< 700 >40 (HiSY), high Al2O3 and Na2O3, low K2O and Rb, depleted HREE, lacks Eu anomalies

Eastern Province

The Eastern Province contains six variably metamorphosed Permian–Cretaceous volcanic-sedimentary terranes, which were accreted to the Pacific Gondwana margin due to transform and convergent tectonics (Mortimer 2004; Wandres & Bradshaw 2005). The latest stage of magmatism in the Median Batholith (SPS) ‘stitched’ the Takaka Terrane to the Eastern Province (Mortimer et al. 1999).

Paparoa Metamorphic Core Complex

The NE–SW trending Paparoa Range (Fig. 1A) is situated in the Buller Terrane. The lower-plate exposed in the southern and central portions of the Paparoa Range consists of Pecksniff Metasedimentary Gneiss, Okari Granite-Gneiss and Awakiri Tonalite Gneiss of the Charleston Metamorphic Group (CMG) (Nathan 1978; Tulloch & Kimbrough 1989; White 1994). Geothermobarometry indicates metamorphism of the gneisses occurred under amphibolite facies conditions of 600±50 °C and 4±1 kb (White 1994). Deformed Carboniferous (Windy Point and Foulwind Granites) and mid-Cretaceous Rahu Suite (Buckland Granite, Railway Quartz Diorite, Steele Granodiorite and Blackwater Granite) granitoid plutons comprise the northern PMCC lower-plate (Nathan 1975, 1978; Tulloch & Kimbrough 1989; Graham & White 1990; Muir et al. 1994; White 1994). The largest of the lower-plate plutons is the c. 180 km² Buckland Granite, which is a leucocratic medium-grained weakly foliated biotite + muscovite±garnet monzogranite (Graham & White 1990; White 1994; Spell et al. 2000). Mid-Cretaceous crystallisation ages of 110±2 Ma have been obtained via sensitive high-resolution ion microprobe (SHRIMP) U–Th–Pb dating of zircon and monazite from the Buckland Granite (Muir et al. 1994; Ireland & Gibson 1998). Xenoliths of Pecksniff Metasedimentary Gneiss and adjacent granitoids, chilled margins and cross-cutting relations indicate the Buckland Granite is also the youngest lower-plate pluton (Nathan 1978; Graham & White 1990). Comparison of ⁴⁰Ar–³⁹Ar cooling ages for upper-plate and lower-plate rocks indicate relatively rapid cooling of the middle crust during the interval 111–90 Ma, interpreted to reflect extensional exhumation (Spell et al. 2000). Syn-tectonic emplacement of the Buckland Granite is implied by the weak tectonic foliation, clasts in the Hawks Crag Breccia member of the Pororari Group and overlap between radiometric dates of intrusion and PMCC unroofing (Tulloch & Kimbrough 1989; Spell et al. 2000; Klepeis et al. 2007).

Figure 1  A, Simplified geological map of the Charleston area. B, Photo and digitised field sketch illustrating the intrusive relations between the Doctor Bay dike and Charleston Orthogneiss (this study; Nathan 1975; Kimbrough & Tulloch 1989; Nathan et al. 2002). The diagonal dashed lines on the digitised sketch (B) represent the foliation.

The Ohika and Pike Detachment Faults are oppositely dipping, low-angle (10–25° dip) normal faults displaying opposite senses of shear that separate the lower- and upper-plates in the northern and southern Paparoa Range, respectively (Tulloch & Kimbrough 1989; Tulloch & Palmer 1990). Shear zones accommodated extension in the middle crust beneath the detachment faults (Tulloch & Kimbrough 1989; Klepeis et al. 2007). Extension is thought to have first started in the lower crust (now exposed in Fiordland) at c. 114 Ma (Klepeis et al. 2007). Due to cooling and compositional contrasts the lower crust became strong relative to the mid-crust by c. 111 Ma. This led to partitioning of extension into narrow mid-crustal shear zones, some of which penetrated the lower crust to maintain kinematic compatibility between the deforming layers of crust (Klepeis et al. 2007). Motion along the Ohika and Pike Detachment Faults and their underlying shear zones accommodated mid-Cretaceous extensional exhumation of the PMCC mid-crustal lower-plate (Tulloch & Kimbrough 1989; Spell et al. 2000).

The Greenland Group and intruding granitic plutons (e.g. Berlins Porphyry and Punakaiki River pluton) constitute the upper-plate of the PMCC (Tulloch & Kimbrough 1989; Tulloch & Palmer 1990). Terrestrial Late Cretaceous (NZ Motuan stage, 100–103 Ma) Pororari Group sediments were derived from the PMCC upper- and lower-plates during unroofing of the metamorphic core. The Pororari Group unconformably overlies the basement units of both the upper- and lower-plates at the northern and southern extremities of the Paparoa Range (Nathan 1978; Raine 1984; Laird 1988; Tulloch & Palmer 1990; Muir et al. 1997; Tulloch et al. 2009b).

Charleston area

Two samples from coastal exposures of the PMCC lower-plate near Charleston are reported in this paper: (1) Charleston Orthogneiss granite (OU80006), and (2) a syn-tectonic granitic dike (OU80007) emplaced into the Charleston Orthogneiss near Doctor Bay. Location details of these samples are provided in Table 2.

Table 2  Details of the samples reported in this paper. Eastings and northings are in the NZTM coordinate system. Sample number prefix OU indicates the samples are held in the University of Otago, Department of Geology collection.

Sample #	Unit	Easting	Northing	Description of locality
OU80006	Charleston Orthogneiss (granite)	1471154	5361054	Outcrops in the surf zone at the north end of Little Beach
OU80007	Doctor Bay dike	1469681	5359527	Sea cliffs immediately south of Doctor Bay

Charleston Orthogneiss

The Charleston Orthogneiss intruded the Devonian (Ireland & Gibson 1998) Pecksniff Metasedimentary Gneiss, and is made up of dark-grey plagioclase-porphyroblastic biotite tonalite which in turn has been intruded by two-mica granite. A gneissic foliation, defined by aligned micas, traverses intrusive contacts between the two orthogneisses, indicating that metamorphism and deformation occurred subsequent to emplacement of both lithologies (Fig. 1B; Kimbrough & Tulloch 1989). Along most of the Charleston coastal section, the complex, small-scale intrusive relation of granitic into tonalite orthogneisses prevents the distribution of these two rocks to be shown on maps at reasonable scales (1:10 000 or smaller). Hence, neither the tonalite nor granitic orthogneiss should be awarded formation status. However, exposures consisting of both orthogneiss lithologies are mappable; it is therefore suggested the tonalite and granitic orthogneisses in the Charleston area be collected under a formal formation name: the Charleston Orthogneiss.

An outcrop at Little Beach of coarse-grained, inequigranular, weakly foliated granitic orthogneiss was sampled for zircon geochronology and geochemistry (Fig. 1A). The texturally equilibrated mineral assemblage of this specimen consists of plagioclase+K-feldspar+quartz+biotite+ muscovite±garnet(spessartine–almandine)±sillimanite, which indicates the Charleston Orthogneiss underwent upper amphibolite facies metamorphism at temperatures ≥500 °C. Biotite displays dark-brown pleochroism, shows evidence of minor retrogressive alteration to chlorite and, along with muscovite, is euhedral–subhedral. Myrmekite is common, especially at the margins of K-feldspar grains, which are subhedral–anhedral. Plagioclase is subhedral–anhedral, multiple twins are often deformed and grains occasionally exhibit magmatic zoning. Quartz is anhedral and displays undulose extinction.

Doctor Bay dike (DBD)

In the sea cliffs south of Doctor Bay, a granitic dike intrudes the Charleston Orthogneiss oblique to the host rock foliation (Figs. 1A, 1B). This dike here is informally named the Doctor Bay dike (DBD) after the small keyhole-shaped bay to the north known as Doctor Bay (Fig. 1A). The DBD has a weak tectonic foliation defined by aligned micas and quartz and feldspar crystals, which parallels that in the Charleston Orthogneiss (Fig. 1B). The field relations suggest syn-tectonic emplacement of the DBD into the Charleston Orthogneiss.

The DBD is an inequigranular, coarse-grained muscovite + biotite granite rooted in a sub-horizontal coarse-grained–pegmatitic two-mica granite parent dike (Fig. 1B). Metamorphic mineralogy developed in the Charleston Orthogneiss, such as garnet and sillimanite, are absent. The entire assemblage is therefore considered to be magmatic; the DBD is post-metamorphic. Biotite exhibits dark-brown pleochroism, is euhedral–subhedral and minor retrogression to chlorite is evident. Myrmekite is ubiquitous at the margins of K-feldspar, which along with plagioclase is subhedral–anhedral and forms the majority of the groundmass. Quartz is anhedral and displays undulose extinction. Magmatic muscovite (euhedral) indicates the DBD was emplaced at ≥ 3–4 kb (Speer 1984; Anderson 1996), consistent with metamorphic mineral assemblages of the Charleston Orthogneiss (described above; Kimbrough & Tulloch 1989) and geothermobarometry of the CMG in the Paparoa Range (White 1994).

U–Th–Pb zircon data and cathodoluminescence

Zircon was extracted by standard crushing and heavy liquid methods. Grains were hand-picked and mounted in epoxy resin, analysed under cathodoluminescence using the JEOL JXA-6800 Superprobe at the Department of Geology, University of Otago, and dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Research School of Earth Sciences (RSES), Australian National University (ANU) following the methods described in Scott & Palin (2008). TEMORA-2 zircon and NIST SRM 610 glass were used as reference standards (Pearce et al. 1997; Black et al. 2004). Processing of raw data was achieved with a series of offline Excel workbooks. See Appendix 1 for a comprehensive description of the methodology.

Charleston Orthogneiss

Cathodoluminescence (CL) images of Charleston Orthogneiss zircon crystals exhibit a wide range of morphologies including rounded-anhedral pale cores mantled by thin irregular overgrowths, subhedral stubby prismatic pale cores with dull overgrowths exhibiting weak oscillatory zoning and euhedral grains with well-developed terminations and oscillatory zoning (Fig. 2A). Cores yielded ages ranging from Neoproterozoic to Early Cretaceous, although the majority record Devonian–Carboniferous dates (Fig. 2A). Mid-Cretaceous ages are restricted to dull rims and oscillatory zoned sectors (Fig. 2A).

Figure 2  Charleston Orthogneiss granite (OU80006) zircons. A, CL images. B, C, Probability density plots. D, E, Tera-Wasserburg concordia age diagrams of ²³⁸U/²⁰⁶Pb* (* indicates correction for common Pb). The laser pits visible in the CL image are located by the white and black circles (I & G '98: Ireland & Gibson 1998).

Forty-five of 71 analyses of Charleston Orthogneiss zircons yielded Cretaceous ²³⁸U/²⁰⁶Pb* ages (where * indicates correction for common-Pb, see Appendix 1; Table 3). Distinguishable clusters of ages fall at c. 200 Ma, 300–400 Ma, 500–600 Ma, 700–900 Ma, 1000–1200 Ma and c. 2000 Ma (Fig. 2B; Table 3). On a probability density plot, the Cretaceous dates comprise two prominent peaks centred at 118 Ma (major peak) and 107 Ma (lesser peak) and a series of minor peaks between c. 90 Ma and c. 50 Ma (Fig. 2C). The analyses comprising the major and lesser Cretaceous peaks constitute two normally distributed populations, with identical Tera-Wasserburg concordia and error-weighted mean ages of 118±2 Ma (mean squared weighted deviates [MSWD]=1.42, n = 24) and 107±2 Ma (MSWD = 1.45, n = 11), respectively (Fig. 2C–E).

Table 3  U-Th-Pb isotope data for Charleston Orthogneiss (granite) zircon analyses. Pb*: radiogenic component only; italicised dates are those pooled for the metamorphic age; bold italicised dates are those giving the emplacement age.

				Measured isotope ratios and 1-σ internal errors (%)								Corrected ages and 1-σ absolute internal errors (Ma)									Corrected ages and 1-σ absolute internal errors (Ma)
Spot #	Pb* (ppm)	U (ppm)	Atomic Th/U	^206Pb ^238U	±	^207Pb ^235U	±	^207Pb ^206Pb	±	^208Pb ^232Th	±	^206Pb* ^238U	±	^207Pb* ^235U	±	^207Pb* ^206U*	±	^206Pb*/^238U ^207Pb*/^235U concordance (%)	% common ^206Pb	Spot MSWD	Age	Error
49	43.09	3812	0.05	0.01047	2.6	0.2238	4.4	0.1551	3.6	0.09013	10.9	57.4	1.6	25.1	19.1	0.0	0.0	228	16.00	2.82	57	± 2
01	38.10	3394	0.06	0.01128	1.7	0.2225	4.3	0.1431	3.9	0.04497	5.5	65.0	1.1	79.0	11.7	536.1	63.9	82	10.58	4.49	65	± 2
27	58.92	4245	0.05	0.01354	1.6	0.4297	4.4	0.2301	4.1	0.11081	5.1	68.8	1.2	97.6	20.7	865.5	128.8	70	20.44	1.82	69	± 2
29	58.69	3806	0.11	0.01495	1.4	0.4211	3.7	0.2042	3.4	0.05378	3.4	79.0	1.2	106.8	16.6	780.7	88.7	74	17.45	2.73	79	± 2
23	7.95	592	0.09	0.01388	4.2	0.0997	35.9	0.0521	35.6	0.01144	18.3	88.2	3.8	85.0	37.2	9.2	4.1	104	0.75	5.00	88	± 4
10	13.57	988	0.19	0.01423	2.8	0.0972	6.2	0.0495	5.5	0.00534	5.9	90.4	2.6	84.8	12.0	1.5	0.2	107	0.60	1.94	90	± 3
49	35.57	2266	0.20	0.01553	1.8	0.1887	3.4	0.0881	2.9	0.01375	3.9	93.8	1.7	80.5	7.9	0.3	0.0	117	5.75	3.69	94	± 2
04	23.10	1566	0.05	0.01594	1.5	0.1204	5.2	0.0548	5.0	0.00721	13.4	101.7	1.5	111.7	6.0	330.7	15.4	91	0.21	2.01	102	± 3
43	27.58	1560	0.15	0.01673	2.8	0.1995	13.2	0.0865	12.9	0.02544	21.9	102.0	2.9	95.9	29.9	2.5	0.8	106	5.04	4.67	102	± 3
34	35.14	2121	0.12	0.01697	1.1	0.1816	4.4	0.0776	4.3	0.01685	6.3	103.8	1.2	96.7	9.7	1.4	0.1	107	4.04	1.94	104	± 2
10	2.81	144	0.97	0.01667	3.0	0.1423	10.4	0.0619	9.9	0.00537	5.0	104.9	3.4	111.9	25.4	256.6	53.8	94	1.30	1.53	105	± 4
35	7.60	458	0.27	0.01670	1.9	0.1399	7.9	0.0608	7.6	0.00654	6.3	106.0	2.1	115.8	11.7	326.6	29.5	92	0.96	1.21	106	± 3
16	11.09	666	0.23	0.01691	2.1	0.1263	7.7	0.0542	7.4	0.00702	5.6	107.1	2.3	106.8	11.9	96.8	10.7	100	0.76	1.61	107	± 3
41	15.62	969	0.16	0.01686	2.3	0.1379	9.3	0.0593	9.0	0.00666	8.2	107.2	2.5	121.2	12.6	405.1	36.1	88	0.55	5.00	107	± 3
42	11.89	724	0.20	0.01694	3.6	0.1408	13.6	0.0603	13.1	0.00665	9.2	107.6	3.9	122.4	18.7	419.2	54.3	88	0.63	2.43	108	± 4
12	14.06	852	0.19	0.01713	1.8	0.1236	8.6	0.0523	8.4	0.00579	6.4	109.5	1.9	117.2	10.3	278.3	22.2	93	0.06	1.61	109	± 3
39	25.10	1500	0.06	0.01756	1.2	0.1587	4.0	0.0655	3.8	0.02107	9.2	109.9	1.3	107.2	6.4	53.3	3.2	103	2.26	5.00	110	± 2
04	26.08	1563	0.15	0.01753	0.9	0.1133	3.0	0.0469	2.8	0.00617	4.1	111.7	1.1	104.5	3.7	1.9	0.1	107	0.24	1.42	112	± 2
30	27.44	1493	0.25	0.01821	1.4	0.1999	6.6	0.0796	6.4	0.01126	4.9	112.3	1.6	118.9	12.8	251.3	25.2	94	3.49	2.10	112	± 3
47	45.37	2368	0.16	0.01866	1.1	0.1976	5.2	0.0768	5.1	0.02059	10.6	114.4	1.3	104.7	12.7	1.0	0.1	109	4.00	2.07	114	± 2
18	6.82	362	0.49	0.01806	1.5	0.1291	6.6	0.0518	6.4	0.00571	3.3	115.1	1.8	120.3	9.2	222.5	16.0	96	0.15	5.00	115	± 3
03	12.05	652	0.44	0.01797	1.2	0.1255	5.2	0.0506	5.0	0.00534	2.9	115.2	1.4	127.5	7.1	362.4	17.6	90	−0.39	5.00	115	± 3
36	24.55	1375	0.23	0.01828	1.4	0.1373	5.0	0.0545	4.8	0.00672	4.0	116.1	1.6	119.3	7.5	183.0	11.0	97	0.58	1.95	116	± 3
27	37.40	2151	0.13	0.01827	1.8	0.1131	6.6	0.0449	6.3	0.00755	7.3	116.2	2.1	97.8	7.9	0.3	0.0	119	0.55	2.00	116	± 3
45	33.83	1809	0.21	0.01877	1.2	0.1613	4.3	0.0623	4.1	0.01184	5.1	116.2	1.5	89.9	9.3	0.1	0.0	129	3.07	1.16	116	± 3
37	33.21	1910	0.08	0.01852	1.0	0.1404	3.8	0.0550	3.6	0.01028	4.7	117.2	1.2	115.0	5.1	71.5	3.2	102	0.93	1.99	117	± 2
29	35.29	1967	0.10	0.01877	2.5	0.1696	6.1	0.0655	5.5	0.01444	6.5	117.4	2.9	117.1	9.9	113.4	9.2	100	2.11	2.88	117	± 4
41	18.42	988	0.41	0.01831	2.3	0.1155	8.9	0.0457	8.6	0.00526	7.0	117.7	2.7	123.8	11.9	242.6	21.4	95	−0.65	1.60	118	± 3
24	22.13	1183	0.22	0.01893	2.0	0.1614	6.4	0.0619	6.1	0.00995	5.7	118.2	2.4	106.2	11.5	0.7	0.1	111	2.27	1.57	118	± 3
44	23.79	1272	0.28	0.01876	1.6	0.1443	7.3	0.0558	7.2	0.00732	5.8	119.0	1.9	124.0	13.2	219.0	22.1	96	0.65	2.09	119	± 3
38	28.86	1543	0.23	0.01881	1.1	0.1650	3.6	0.0636	3.4	0.00905	7.1	119.3	1.4	140.3	7.4	513.2	22.3	85	0.75	1.87	119	± 3
35	11.13	590	0.30	0.01889	2.9	0.1369	7.1	0.0526	6.5	0.00661	8.4	119.8	3.5	118.5	12.7	91.2	9.5	101	0.58	1.48	120	± 4
42	22.10	1224	0.18	0.01881	2.3	0.1233	8.5	0.0475	8.2	0.00630	6.5	119.9	2.7	114.5	10.4	12.3	1.1	105	0.18	2.20	120	± 3
07	18.49	1007	0.32	0.01867	1.7	0.1777	5.2	0.0690	4.9	0.00509	7.0	120.4	2.1	179.3	9.8	1054.8	37.4	67	−0.69	1.47	120	± 3
48	7.27	367	0.50	0.01887	1.9	0.1305	7.8	0.0502	7.6	0.00606	4.9	120.6	2.3	118.7	12.8	90.0	9.7	102	0.29	1.41	121	± 3
21	23.54	1289	0.20	0.01895	1.5	0.1385	7.1	0.0530	7.0	0.00601	3.9	120.9	1.8	131.1	9.3	318.1	20.3	92	0.03	5.00	121	± 3
16	9.23	459	0.52	0.01909	2.0	0.1253	7.4	0.0476	7.2	0.00590	4.4	121.8	2.4	121.3	12.4	109.2	11.0	100	−0.07	1.84	122	± 3
01	21.51	1131	0.34	0.01903	1.1	0.1422	3.8	0.0542	3.7	0.00550	3.0	122.0	1.4	145.7	6.1	547.2	18.4	84	−0.53	1.65	122	± 3
26	13.35	697	0.23	0.01941	2.3	0.1534	13.4	0.0573	13.2	0.00885	8.9	122.1	2.8	112.6	21.3	1.4	0.3	108	1.56	0.99	122	± 4
46	8.89	443	0.40	0.01972	4.9	0.1165	32.2	0.0428	31.8	0.00594	13.6	126.6	6.2	112.8	40.1	0.7	0.2	112	−0.04	1.19	127	± 7
43	16.56	820	0.35	0.01996	1.7	0.1267	8.5	0.0460	8.3	0.00669	5.7	127.0	2.2	114.2	12.5	0.8	0.1	111	0.32	1.30	127	± 3
28	13.72	669	0.15	0.02100	4.1	0.1897	18.8	0.0655	18.4	0.01510	13.1	130.2	5.3	116.2	33.9	0.7	0.2	112	2.73	1.48	130	± 6
12	12.28	604	0.37	0.02035	1.4	0.1544	5.2	0.0550	5.0	0.00517	4.7	131.1	1.8	170.3	8.3	752.4	27.1	77	−1.16	1.98	131	± 3
23	2.14	89	0.71	0.02207	6.5	0.3523	18.1	0.1157	16.9	0.00825	14.3	136.6	9.1	243.2	63.3	1462.4	220.7	56	3.10	5.00	137	± 9
07	16.32	789	0.22	0.02143	1.3	0.1590	4.7	0.0538	4.5	0.00607	4.5	137.1	1.8	158.8	7.2	494.0	18.5	86	−0.40	2.09	137	± 3
06	13.30	602	0.48	0.02144	1.2	0.1723	5.2	0.0583	5.0	0.00615	4.0	137.1	1.9	172.2	14.8	681.2	45.8	80	−0.49	1.81	137	± 3
21	21.16	966	0.14	0.02308	2.0	0.1776	8.7	0.0558	8.4	0.00832	13.9	146.8	3.0	158.6	16.3	341.3	31.6	93	0.31	0.85	147	± 4
33	15.11	450	0.42	0.03334	1.7	0.2785	6.0	0.0606	5.7	0.00763	5.4	216.0	3.7	312.6	15.1	1117.4	36.2	69	−2.05	5.00	216	± 5
13	28.54	832	0.21	0.03490	2.6	0.2865	7.2	0.0595	6.8	0.01616	6.9	217.6	5.6	205.7	18.8	69.4	6.6	106	1.48	3.17	218	± 7
32	11.11	223	0.20	0.04983	2.5	0.5136	10.4	0.0748	10.1	0.03437	5.4	302.4	7.4	263.1	44.6	1.5	0.3	115	3.62	1.77	302	± 9
46	11.53	224	0.49	0.04896	2.4	0.4018	7.2	0.0595	6.8	0.01664	5.1	306.1	7.2	312.4	29.1	359.8	32.0	98	0.70	2.20	306	± 9
14	61.97	1160	0.48	0.05124	1.3	0.3714	3.8	0.0526	3.5	0.01540	2.8	322.3	4.2	333.2	15.8	404.9	18.0	97	−0.27	3.80	322	± 8
31	17.85	357	0.17	0.05180	1.8	0.4238	5.7	0.0593	5.4	0.02020	5.8	324.6	5.6	339.2	21.6	443.5	26.1	96	0.43	5.00	325	± 8
17	57.53	1162	0.02	0.05398	1.0	0.4271	3.5	0.0574	3.3	0.01405	7.6	339.1	3.4	363.7	10.6	523.7	13.6	93	−0.06	2.27	339	± 8
28	22.55	411	0.22	0.05556	2.5	0.3707	8.8	0.0484	8.4	0.02306	7.3	344.8	8.3	266.3	30.3	0.3	0.0	129	1.05	2.71	345	± 10
32	7.54	126	0.37	0.05726	2.6	0.5758	10.0	0.0729	9.6	0.02854	6.2	349.0	9.1	296.1	50.4	1.3	0.2	118	3.42	1.26	349	± 11
13	60.82	1066	0.37	0.05629	0.9	0.4256	1.9	0.0548	1.7	0.01667	2.2	353.6	3.1	377.0	9.8	519.3	12.1	94	−0.35	2.98	354	± 8
02	20.50	274	0.91	0.06227	1.9	0.5748	5.7	0.0670	5.4	0.02520	2.0	369.3	7.9	198.3	70.3	0.0	0.0	186	4.75	2.41	369	± 11
17	44.15	807	0.05	0.05916	1.5	0.4361	6.3	0.0535	6.1	0.01277	12.3	371.5	5.4	381.4	19.4	441.8	21.2	97	−0.28	1.90	371	± 9
11	36.01	629	0.06	0.06159	0.9	0.4669	2.8	0.0550	2.6	0.02002	4.5	385.1	3.3	387.4	9.1	401.0	8.9	99	0.03	2.41	385	± 8
08	48.41	795	0.24	0.06223	0.8	0.5030	2.0	0.0586	1.9	0.01951	2.3	388.9	3.1	413.9	8.2	553.9	9.7	94	0.00	2.90	389	± 8
15	11.04	170	0.37	0.06464	1.3	0.5275	5.3	0.0592	5.1	0.01715	3.6	406.8	5.3	474.5	20.5	814.5	29.4	86	−0.87	1.90	407	± 10
02	39.81	390	1.85	0.07174	1.3	0.7868	5.6	0.0795	5.4	0.02381	2.2	430.1	6.6	455.2	62.1	554.7	72.0	94	2.53	3.70	430	± 11
40	8.95	88	0.88	0.08555	2.3	0.8723	7.7	0.0740	7.4	0.03279	3.8	515.3	12.1	442.8	72.9	91.7	17.7	116	3.10	2.40	515	± 15
20	30.28	339	0.29	0.08777	5.6	1.0165	6.0	0.0840	2.3	0.04612	3.5	524.2	28.1	513.0	37.8	460.0	27.9	102	3.33	109.93	524	± 30
22	32.06	250	0.09	0.13122	2.3	1.3537	6.5	0.0748	6.1	0.12510	8.5	779.1	17.3	695.5	48.9	437.4	33.5	112	2.25	2.11	779	± 22
05	64.67	367	2.21	0.11877	1.3	1.1635	2.8	0.0710	2.5	0.02686	6.5	824.8	27.9	1502.4	111.6	2655.0	129.5	55	−14.90	7.47	825	± 32
50	70.34	544	0.16	0.13570	2.4	2.0163	3.5	0.1078	2.6	0.03286	6.4	824.9	18.5	1153.2	25.4	1837.1	23.8	72	−0.57	8.02	825	± 24
05	53.90	363	0.62	0.13674	1.0	1.3540	2.5	0.0718	2.3	0.03931	2.0	829.0	7.9	913.6	23.4	1118.9	25.5	91	−0.61	2.60	829	± 19
09	22.77	128	0.43	0.17115	1.3	1.7919	3.3	0.0759	3.0	0.04618	2.9	1024.1	12.3	1093.4	24.8	1233.2	24.8	94	−0.66	2.85	1024	± 24
22	145.99	647	0.54	0.20973	1.3	2.7839	2.2	0.0963	1.7	0.06306	2.5	1226.9	15.2	1346.2	23.5	1541.7	22.5	91	0.07	5.87	1227	± 27
19	205.60	645	0.41	0.30277	0.7	5.5938	1.0	0.1340	0.7	0.08869	1.3	1701.8	11.1	1912.1	10.0	2145.6	8.0	89	0.05	7.88	1702	± 36

The two mid-Cretaceous populations of ages from Charleston Orthogneiss zircons are not distinguishable by their Th/U ratios (Fig. 3A). However, some dates contained in the major population are associated with higher Th/U values relative to those comprising the lesser population (Fig. 3A). Zones with exceptionally high U concentrations (>3000 ppm) yielded the youngest (<90 Ma) dates (Fig. 3B).

Figure 3  Atomic Th/U, present-day U, and Th plotted against ²³⁸U/²⁰⁶Pb* age of zircon from the Charleston Orthogneiss granite (OU80006). A, Atomic Th/U. B, Present-day U. C, Th (* indicates correction for common Pb).

Doctor Bay dike

Most DBD zircons are stubby prismatic subhedral–euhedral grains with pale un-zoned cores enveloped by overgrowths with well-developed oscillatory zoning (Fig. 4A). However, some grains lack cores while others are anhedral (Fig. 4A). Late Cretaceous ages were obtained from oscillatory zoned overgrowths, dull rims and grains that lacked inherited cores (Fig. 4A).

Figure 4  Doctor Bay dike (OU80007) zircons. A, CL images. B, D, Probability density plots. C, Tera-Wasserburg concordia age diagram (* indicates correction for common Pb). The laser pits visible in the CL image are located by the white and black circles.

Thirty-three of the 64 ²³⁸U/²⁰⁶Pb* ages obtained from DBD zircons comprise a slightly left-skewed mid-Cretaceous distribution, with a peak at c. 105 Ma (Fig. 4B; Table 4). Of these, 15 form an approximately normally distributed population with an error-weighted mean age of 106±2 Ma (MSWD = 1.60) (Fig. 4B). Concordant analyses within this population yield a Tera-Wasserburg concordia age of 105±2 Ma (Fig. 4C). The older dates in the mid-Cretaceous population were excluded in this age determination (Fig. 4B) on the basis they are probably from zircon inherited from the Charleston Orthogneiss, while Pb-loss likely accounts for the few younger dates. The remaining ages are distributed between smaller Jurassic, Permotriassic, Devonian–Carboniferous, Cambrian–Ordovician and Proterozoic groupings (Fig. 4D; Table 4). Cretaceous ages show a range in Th/U ratios from c. 0.85–0.02, but most are less than the ‘typical’ magmatic value of 0.5 (Fig. 5A) (Hoskin & Schaltegger 2003). There is also a large scatter in U concentrations from c. 250–4200 ppm (Fig. 5B).

Figure 5  Atomic Th/U and present-day U content against ²³⁸U/²⁰⁶Pb* age of DBD (OU80007) zircon. A, Atomic Th/U. B, present-day U. There is a large range in both Th/U ratios and U content (* indicates correction for common Pb).

Table 4  U-Th-Pb isotope data for DBD zircon analyses. Pb*: radiogenic component only; bold italicised dates are those considered for the age.

				Measured isotope ratios and 1-σ internal errors (%)								Corrected ages and 1-σ absolute internal errors (Ma)									Corrected ages and 1-σ absolute internal errors (Ma)
Spot #	Pb* (ppm)	U (ppm)	Atomic Th/U	^206Pb ^238U	±	^207Pb ^235U	±	^207Pb ^206Pb	±	^208Pb ^232Th	±	^206Pb* ^238U	±	^207Pb* ^235U	±	^207Pb* ^206Pb*	±	^206Pb*/^238U ^207Pb*/^235U concordance (%)	% common ^206Pb	Spot MSWD	Age	Error
22	6.21	413	0.21	0.01463	6.1	0.1238	27.3	0.0614	26.6	0.01140	19.9	91.7	5.7	77.8	39.7	0.3	0.1	118	2.54	5.00	92	±6
21	4.07	240	0.25	0.01612	5.4	0.2972	17.8	0.1337	17.0	0.01970	12.9	91.9	5.9	83.9	73.1	0.8	0.7	110	11.01	5.00	92	±6
49	59.26	4145	0.19	0.01476	2.5	0.1211	6.4	0.0595	5.9	0.00611	3.8	93.6	2.4	100.3	7.8	262.8	18.0	93	0.99	5.19	94	±3
32	6.15	437	0.04	0.01519	2.2	0.1070	9.3	0.0511	9.1	0.01037	12.5	96.6	2.1	91.8	9.6	3.5	0.4	105	0.69	5.00	97	±3
34	50.79	3417	0.06	0.01558	1.3	0.1564	3.0	0.0728	2.7	0.02311	7.1	97.1	1.3	103.7	4.8	258.9	10.7	94	2.63	4.16	97	±2
28	7.81	489	0.20	0.01601	3.8	0.2270	15.5	0.1028	15.0	0.01322	8.8	97.5	3.7	125.9	32.7	706.3	139.0	77	4.98	1.82	97	±4
40	46.33	3074	0.02	0.01617	1.2	0.1526	4.0	0.0685	3.8	0.05146	8.4	101.3	1.2	106.0	6.4	217.9	12.2	95	2.21	2.51	101	±2
04	12.90	812	0.28	0.01606	1.7	0.1155	4.8	0.0521	4.5	0.00554	5.1	102.3	1.8	104.1	8.9	143.6	11.8	98	0.40	1.56	102	±3
33	14.25	936	0.03	0.01630	1.5	0.1314	5.5	0.0585	5.3	0.02720	6.7	102.4	1.5	96.0	7.0	1.8	0.1	107	1.66	5.00	102	±2
31	14.53	954	0.11	0.01611	1.1	0.1040	5.9	0.0468	5.8	0.00634	6.8	102.7	1.1	94.1	6.3	0.9	0.1	109	0.36	5.00	103	±2
18	16.15	1093	0.02	0.01619	2.8	0.1110	7.1	0.0497	6.5	0.00715	19.6	103.4	2.9	104.6	7.3	131.3	8.4	99	0.13	5.00	103	±4
44	39.49	2653	0.02	0.01627	0.9	0.1061	3.2	0.0473	3.0	0.00553	10.2	104.0	1.0	102.0	3.2	57.9	1.8	102	0.02	5.00	104	±2
27	12.97	765	0.25	0.01699	2.3	0.1839	7.9	0.0785	7.6	0.00918	5.6	105.5	2.4	119.0	15.5	398.4	44.9	89	2.95	1.88	106	±3
29	25.49	1632	0.04	0.01672	1.0	0.1348	3.8	0.0584	3.7	0.01559	6.9	105.9	1.1	110.3	4.9	206.5	8.6	96	1.01	5.00	106	±2
24	54.93	3428	0.08	0.01707	1.4	0.1362	3.4	0.0579	3.1	0.01037	3.5	107.9	1.5	109.3	4.3	140.2	5.2	99	1.11	3.96	108	±2
08	59.85	3848	0.02	0.01697	0.7	0.1191	2.6	0.0509	2.5	0.01282	4.8	108.0	0.7	107.3	2.9	91.8	2.4	101	0.38	1.69	108	±2
14	12.44	788	0.04	0.01705	1.8	0.1222	9.2	0.0520	9.1	0.01060	10.8	108.3	1.9	104.9	10.6	33.4	3.4	103	0.66	5.00	108	±3
42	13.88	842	0.21	0.01709	2.1	0.1348	8.8	0.0572	8.6	0.00544	7.6	109.2	2.3	128.2	12.2	496.8	39.0	85	0.01	1.16	109	±3
21	4.91	211	0.84	0.01989	4.1	0.4140	13.0	0.1509	12.3	0.01156	5.8	109.8	5.3	94.8	65.3	0.3	0.2	116	13.34	0.77	110	±6
45	50.51	3135	0.04	0.01742	1.6	0.1398	7.8	0.0582	7.6	0.01113	10.6	110.6	1.7	121.7	9.9	343.8	24.9	91	0.60	1.19	111	±3
28	6.27	331	0.08	0.01902	2.4	0.2981	7.0	0.1136	6.6	0.04616	7.7	112.1	2.7	117.8	20.7	233.8	38.6	95	7.72	1.07	112	±3
41	32.18	1997	0.02	0.01762	1.4	0.1108	5.5	0.0456	5.3	0.00876	13.5	112.5	1.6	104.7	5.6	1.6	0.1	107	0.10	5.00	112	±3
04	12.06	653	0.50	0.01766	1.2	0.1165	6.9	0.0478	6.8	0.00525	3.1	113.3	1.4	122.6	9.0	304.0	20.1	92	-0.57	5.00	113	±3
10	5.01	240	0.48	0.01914	2.4	0.3246	8.5	0.1230	8.2	0.01228	9.3	113.7	2.9	162.7	29.0	944.2	117.6	70	6.61	5.00	114	±4
18	16.42	1001	0.01	0.01792	1.6	0.1088	5.9	0.0440	5.7	0.01876	12.1	114.1	1.8	98.8	6.0	0.4	0.0	116	0.31	5.00	114	±3
40	54.33	3190	0.02	0.01833	1.4	0.1339	4.1	0.0530	3.9	0.03141	8.9	115.4	1.6	97.9	5.9	0.4	0.0	118	1.49	1.79	115	±3
22	18.39	951	0.40	0.01853	2.2	0.1503	9.7	0.0588	9.5	0.00811	6.6	115.6	2.6	95.0	17.2	0.2	0.0	122	2.36	1.32	116	±3
47	55.30	3185	0.07	0.01856	1.2	0.1354	3.5	0.0529	3.3	0.01010	8.4	117.9	1.5	117.5	5.2	110.5	4.8	100	0.57	2.37	118	±3
39	21.73	1178	0.03	0.01926	1.7	0.2061	5.1	0.0776	4.8	0.07690	9.8	118.2	2.1	104.3	12.3	0.5	0.1	113	4.26	1.91	118	±3
14	10.81	597	0.08	0.01936	1.3	0.1406	5.1	0.0527	5.0	0.00923	6.1	122.9	1.6	120.5	6.9	75.9	4.3	102	0.63	1.43	123	±3
46	42.07	2314	0.06	0.01955	2.0	0.1251	10.5	0.0464	10.3	0.00927	16.8	124.4	2.5	113.4	12.2	1.0	0.1	110	0.30	1.76	124	±3
36	43.06	2237	0.02	0.02067	1.4	0.1296	5.5	0.0455	5.3	0.03601	17.1	130.2	1.8	89.0	10.1	0.1	0.0	146	1.54	0.49	130	±3
22	5.19	223	0.62	0.02166	2.9	0.2044	11.8	0.0685	11.4	0.00709	6.7	137.0	4.2	170.2	28.9	661.0	88.4	81	0.85	5.00	137	±5
50	26.60	1026	0.04	0.02796	5.5	0.2276	9.7	0.0591	8.0	0.02636	16.1	175.8	9.5	174.6	19.5	160.6	16.1	101	1.20	5.00	176	±10
39	24.22	925	0.03	0.02810	2.3	0.2177	7.0	0.0562	6.6	0.04159	5.2	175.8	4.0	154.0	13.4	0.6	0.1	114	1.60	5.00	176	±5
37	22.78	712	0.03	0.03451	3.6	0.2750	5.4	0.0578	4.0	0.02907	9.5	217.3	7.7	222.4	11.8	277.7	11.4	98	0.73	5.00	217	±9
36	48.46	1211	0.03	0.04298	1.4	0.3558	3.5	0.0600	3.2	0.04746	4.8	268.2	3.7	262.8	10.0	215.3	7.9	102	1.17	4.60	268	±6
16	13.35	303	0.34	0.04353	1.5	0.3274	5.5	0.0545	5.3	0.01512	4.1	273.0	4.1	267.5	23.3	217.0	19.1	102	0.50	1.83	273	±7
23	43.99	1000	0.16	0.04480	2.2	0.4601	10.4	0.0745	10.2	0.03160	7.8	274.1	5.8	263.4	39.4	169.7	26.3	104	3.04	0.93	274	±8
48	26.85	607	0.14	0.04621	1.8	0.3217	6.3	0.0505	6.0	0.01790	5.3	289.7	5.1	260.3	17.1	12.7	0.9	111	0.53	2.25	290	±7
13	8.66	181	0.29	0.04787	1.3	0.3534	5.0	0.0535	4.8	0.01634	3.6	300.1	3.9	290.2	16.3	210.5	12.0	103	0.39	1.63	300	±7
35	35.07	684	0.03	0.05507	1.0	0.4615	3.2	0.0608	3.0	0.05250	5.1	342.6	3.3	342.4	10.9	342.0	10.5	100	0.91	2.30	343	±7
03	11.66	211	0.35	0.05509	1.5	0.4383	5.2	0.0577	5.0	0.01601	3.5	346.8	5.1	386.7	19.2	632.2	27.2	90	-0.38	5.00	347	±9
50	47.61	919	0.02	0.05619	1.3	0.4121	3.9	0.0532	3.7	0.03326	9.4	351.2	4.4	333.2	12.2	210.0	7.8	105	0.35	1.94	351	±8
26	44.64	766	0.36	0.05699	1.2	0.5364	2.9	0.0683	2.6	0.02239	3.0	352.5	4.1	362.5	15.0	431.4	16.8	97	1.55	3.01	352	±7
25	27.63	490	0.14	0.05749	1.4	0.6045	4.8	0.0763	4.6	0.04187	9.5	352.8	4.9	372.8	24.3	502.4	30.4	95	2.31	1.57	353	±8
23	56.56	930	0.43	0.05845	2.0	0.4873	4.9	0.0605	4.4	0.02093	3.7	362.7	7.1	353.6	25.4	294.8	21.2	103	1.00	3.02	363	±10
02	16.14	275	0.33	0.05833	0.9	0.4415	3.7	0.0549	3.6	0.01875	2.7	364.5	3.3	363.4	13.6	354.2	13.0	100	0.16	1.38	364	±8
43	6.81	104	0.65	0.05936	2.2	0.4951	9.2	0.0605	8.9	0.02177	4.5	364.9	8.2	283.3	50.9	0.3	0.0	129	2.40	1.82	365	±10
30	26.71	470	0.17	0.05887	1.4	0.5268	4.7	0.0649	4.5	0.02251	5.5	366.9	5.1	408.0	19.3	646.4	26.6	90	0.45	5.00	367	±8
25	39.02	650	0.25	0.06027	1.8	0.5456	4.7	0.0656	4.4	0.02594	5.2	371.9	6.6	366.7	24.6	335.5	22.3	101	1.51	2.90	372	±9
19	17.49	270	0.51	0.06187	1.6	0.4881	4.3	0.0572	4.0	0.01711	2.6	389.8	6.1	448.6	19.5	759.4	27.4	87	−0.89	3.94	390	±10
07	47.52	783	0.19	0.06249	1.5	0.4790	2.7	0.0556	2.2	0.02063	7.3	391.3	5.6	409.5	11.7	511.6	12.6	96	−0.23	9.67	391	±10
15	34.48	559	0.26	0.06292	1.5	0.5037	4.8	0.0581	4.6	0.01744	4.8	394.6	5.7	439.7	22.0	679.1	29.6	90	−0.50	2.20	395	±10
01	8.78	132	0.50	0.06354	1.3	0.5038	5.5	0.0575	5.4	0.01774	3.0	400.6	5.1	457.5	22.3	755.4	31.5	88	−0.85	1.36	401	±10
09	24.66	334	0.33	0.07519	1.0	0.6265	3.4	0.0604	3.3	0.02304	2.9	466.7	4.5	500.6	18.8	653.1	22.4	93	−0.12	1.72	467	±10
20	22.17	280	0.20	0.08050	2.2	0.6305	6.2	0.0568	5.9	0.03433	3.6	493.8	10.3	422.0	28.8	50.4	4.0	117	1.17	3.21	494	±14
15	61.16	766	0.24	0.08082	2.0	0.7074	3.6	0.0635	3.0	0.03042	3.7	496.9	9.6	497.1	28.4	496.0	27.2	100	0.77	8.32	497	±14
38	6.06	69	0.22	0.08917	2.2	0.8629	8.0	0.0702	7.7	0.03470	6.6	546.1	11.7	574.6	43.0	690.6	47.8	95	0.93	1.40	546	±15
11	10.93	112	0.54	0.09197	1.4	0.7597	5.6	0.0599	5.4	0.02524	2.4	571.6	7.8	632.0	28.1	852.4	33.4	90	−0.91	5.00	572	±14
05	34.77	378	0.30	0.09457	0.9	0.8716	3.0	0.0668	2.8	0.03455	2.6	577.1	5.0	580.6	19.1	591.9	18.9	99	0.86	1.23	577	±13
06	28.62	282	0.28	0.10215	1.0	0.8579	2.7	0.0609	2.6	0.03076	2.6	627.3	6.1	633.9	15.7	656.9	15.3	99	−0.07	2.72	627	±14
17	105.20	653	0.30	0.16043	0.9	1.6909	1.4	0.0764	1.1	0.04378	1.8	962.9	8.1	1042.8	14.7	1212.5	14.7	92	−0.50	6.96	963	±21
12	46.32	212	1.17	0.17471	1.2	1.9078	2.5	0.0792	2.2	0.05433	2.2	1031.6	14.4	1057.5	70.3	1104.6	71.3	98	0.33	4.36	1032	±25

Timing of magmatism, metamorphism and extension

Charleston Orthogneiss

The probability density plot of ages from the Charleston Orthogneiss granite, particularly the Cretaceous portion, bears a striking resemblance to that of SHRIMP U–Pb zircon ages from the tonalite member of the Charleston Orthogneiss (Fig. 2C) (fig. 7 of Ireland & Gibson 1998). Most importantly, the mid-Cretaceous ages on both these plots exhibit two populations with pooled mean ages of c. 120 Ma and c. 110 Ma (Fig. 2C), previously interpreted to record Separation Point Suite granitoid protolith emplacement and metamorphism coeval with syn-tectonic intrusion of the Buckland Granite (110±2 Ma), respectively (Ireland & Gibson 1998). Furthermore, the ages of Charleston Orthogneiss intrusion (119±2 Ma) and metamorphism (109±5 Ma) of Ireland and Gibson (1998) are the same within error as those presented here (118±2 Ma and 107±2 Ma). Due to this similarity, Ireland and Gibson's (1998) interpretation of the major and less Cretaceous peaks on the probability density plot (Fig. 2C) representing emplacement and metamorphism of the Charleston Orthogneiss, respectively, is retained here. Conventional multi-grain thermal ionisation mass spectrometry (TIMS) U–Pb zircon dating of the Charleston Orthogneiss yielded discordant data with a lower intercept of 114±18 Ma interpreted as the magmatic crystallisation age (Kimbrough & Tulloch 1989), which also encompasses the ages reported here. The timing of metamorphism is further supported by SHRIMP U–Pb (103–113 Ma) and Th–Pb (105–114 Ma) monazite ages from the Charleston Orthogneiss tonalite (Ireland & Gibson 1998; Hiess et al. 2010), which are the same within error as our age of metamorphism (105–109 Ma) (Figs. 2C–2E). Overlap of the metamorphic age of the Charleston Orthogneiss and the age of Buckland Granite suggests heat from intrusion of the latter was responsible for metamorphism of the Charleston Orthogneiss.

Ages obtained from dull, featureless rims characteristic of metamorphic zircon comprise the lesser Cretaceous population, proposed to record metamorphism, while ages obtained from oscillatory zoned sectors (typical of igneous zircon; Corfu et al. 2003) make up the major Cretaceous population, considered to record emplacement of the Charleston Orthogneiss granite (Fig. 2A). Thus, cathodoluminescence of the Charleston Orthogneiss granite zircons also supports the interpretation that they record ages of both intrusion and metamorphism.

Metamorphic zircon often possesses low Th/U values relative to igneous zircon (Hoskin & Schaltegger 2003). The two Cretaceous populations of dates cannot be distinguished by their Th/U ratios alone, although the older population possesses a higher average Th content (Figs. 3A, 3C). SHRIMP analyses of zircon from the tonalite member of the Charleston Orthogneiss also reveal Cretaceous populations cannot be identified solely from their Th/U ratios (Ireland & Gibson 1998). However, the composition of metamorphic zircon is determined by whether it forms via re-equilibration of pre-existing zircon (closed or partially closed system behaviour) or crystallisation or precipitation of new zircon overgrowths (open system behaviour; Geisler et al. 2007; Martin et al. 2008). Metamorphic zircon formed by re-equilibration of pre-existing zircon retains textural and geochemical characteristics of the parent zircon (Martin et al. 2008). Conversely, new zircon crystallised in an open system does not preserve textural or geochemical characteristics of its precursor, but the composition of the rims is controlled by that of the fluid phase/melt present and surrounding minerals (Martin et al. 2008). The Th/U ratios of zircon therefore do not conclusively elucidate its origin. Hence, despite not possessing uniformly low Th/U values (Fig. 3A), a metamorphic origin for zircon rims from the Charleston Orthogneiss granite cannot be ruled out.

The high U content of the youngest dates yielded by Charleston Orthogneiss zircons (Fig. 3B) opens the possibility that they may not record metamorphic or igneous events; rather, they are the result of Pb loss from Cretaceous grains facilitated by radiation damage of the crystal lattice (Palenik et al. 2003). Alternatively, Pb loss may have been aided by crystal-plastic deformation of zircon, which has been suggested to create high diffusivity pathways for migration of elements through the zircon lattice (Reddy et al. 2006). Regardless of the mechanism, the progressive trailing of young dates from the Cretaceous populations (Fig. 2C) supports Pb loss as the cause. Groups of ages falling at c. 200 Ma, 400–300 Ma, 600–500 Ma, 900–700 Ma, 1000–1200 Ma and c. 2000 Ma (Fig. 2B) are interpreted to be from xenocrystic zircon.

Doctor Bay dike

The ages of magmatic zircon populations are thought to be characteristically normally distributed, with MSWD values approaching 1 (Bryan et al. 2004). An intrusive age of 106±2 Ma (MSWD = 1.60) for the DBD (Fig. 4B) is justified for this reason. Younger ages were excluded on the basis they are the result of Pb loss. Despite mostly showing Th/U ratios lower than typical magmatic zircon (Fig. 5A) (Hoskin & Schaltegger 2003), the DBD zircon ages used in the age determination are considered to be magmatic in origin. This is because the composition of zircon is dependent on that of melt from which it crystallised (Geisler et al. 2007; Martin et al. 2008), as outlined in more detail for the Charleston Orthogneiss above. Small clusters of Jurassic, Permo-Triassic, Devonian–Carboniferous, Cambrian–Ordovician and Proterozoic ages are interpreted as inherited (Fig. 4D). However, the slightly younger Tera-Wasserburg concordia age of 105±2 Ma (Fig. 4C) is preferred because only grains that are concordant and mutually overlapping within error are considered.

This age is the same within error as that of an undeformed muscovite pegmatite from the nearby Nile River dated at 108±3 Ma by Rb–Sr (recalculated from Aronson 1965), but marginally younger than a syn-tectonic granitic dike intruding biotite schist of the CMG in the central Paparoa Range that was dated at 113±4 Ma via the LA-ICP-MS U-Pb zircon method (Klepeis et al. 2007); this suggests lower-plate deformation was diachronous. Given the intrusive relations between the DBD and Charleston Orthogneiss (Fig. 1B), the age of the dike is also consistent with the newly reported and existing U–Pb zircon and monazite dating of the host rock (see above; Kimbrough & Tulloch 1989; Ireland & Gibson 1998).

Due to its syn-tectonic relations with the host rock and lack of metamorphic mineralogy, the age of the DBD constrains the timing of deformation of the Charleston Orthogneiss to 105±2 Ma. Although this age is indistinguishable from the metamorphic age of the Charleston Orthogneiss (107±2 Ma), the lack of metamorphic minerals in the DBD implies that fabric development post-dated metamorphism of the Charleston Orthogneiss by up to 4 Ma. The Charleston Orthogneiss was therefore metamorphosed at c. 107 Ma, and deformed and intruded by the DBD at c. 105 Ma.

Possible link between Charleston Orthogneiss deformation and continental extension

Syn-tectonic emplacement of the Buckland Granite occurred at 110±2 Ma, which may have facilitated the onset of extensional exhumation of the PMCC lower-plate (Tulloch & Kimbrough 1989; Muir et al. 1994; Ireland & Gibson 1998; Klepeis et al. 2007). Whole-rock, feldspar and muscovite Rb–Sr dating of ultramylonite from the Paparoa Shear Zone (PSZ) at White Horse Creek define an isochron age of 116±6 Ma, which is interpreted to date early-stage ductile deformation associated with continental extension and development of the PMCC (Ring et al. 2006). ⁴⁰Ar–³⁹Ar, K–Ar and Rb–Sr dating of the PMCC indicates extensional exhumation of the middle crust in the interval 111–90 Ma (Kimbrough & Tulloch 1989; Spell et al. 2000). Continental extension was therefore underway by 122–111 Ma. Development of the PMCC between 111 Ma and 90 Ma during continental extension, which may have started as early as 122 Ma, was therefore coeval with deformation of the Charleston Orthogneiss at c. 105 Ma.

Lineation measurements taken on Charleston Orthogneiss foliation surfaces trend c. NE–SE and dip at shallow angles to the north (Fig. 1A). These are sub-parallel to those from the PSZ, which accommodated most of the extension in the middle crust (Tulloch & Kimbrough 1989; Tulloch & Palmer 1990; White 1994; Spell et al. 2000; Klepeis et al. 2007). It is therefore suggested that fabric development in the Charleston Orthogneiss at c. 105 Ma was related to mid-crustal deformation associated with extension that led to unroofing of the PMCC lower-plate.

Whole-rock geochemistry and granitoid suite affiliation

Previously, the Charleston Orthogneiss has been correlated with either the Cretaceous Rahu or Separation Point Suites on the basis of geochemistry (Kimbrough & Tulloch 1989) and on the basis of emplacement age alone with the SPS (Ireland & Gibson 1998). More recently, the Charleston Orthogneiss has been mapped as Rahu Suite at the regional scale (Nathan et al. 2002).

Like the Rahu Suite (Tulloch 1983; Graham & White 1990; Muir et al. 1997; Waight et al. 1998; Tulloch & Kimbrough 2003), the Charleston Orthogneiss granite and DBD contain low Na₂O and Al₂O₃ and high SiO₂ and K₂O (intermediate between I- and S-type granitoid compositions; Chappell & White 1992) relative to the Separation Point Suite (Table 1; Fig. 6A, 6B). The peraluminous composition (alumina saturation index or ASI >1.0) of the Charleston Orthogneiss granite and DBD is also typical of the Rahu Suite (Table 1; Table 5) (Tulloch 1983; Waight et al. 1998). The moderate-SY (Sr/Y ratio) character of the two Cretaceous rocks also typifies the Rahu Suite; the Separation Point Suite is distinctly high Sr/Y (HiSY) (Table 1; Fig. 6C) (Muir et al. 1995; Tulloch & Kimbrough 2003). Furthermore, the rare earth element (REE) patterns of the Charleston Orthogneiss granite and DBD exhibit Eu anomalies and heavy rare earth elements (HREE) abundances akin to the Rahu Suite; the Separation Point Suite is relatively depleted in HREE and lacks Eu anomalies (Table 1; Fig. 6D) (Muir et al. 1995).

Figure 6  Whole-rock Harker and REE diagrams for the Charleston Orthogneiss (OU80006) and DBD (OU80007) compared to Cretaceous granitoids west of the Alpine Fault. A, Whole-rock Al₂O₃-SiO₂. B, Whole-rock Na₂O-K₂O. C, Whole-rock Sr/Y-Y. D, REE diagram (K & T '89: Kimbrough & Tulloch 1989). (Data sources: White 1987; Kimbrough & Tulloch 1989; Graham & White 1990; Muir et al. 1995, 1997; Waight et al. 1998; Tulloch & Kimbrough 2003).

Table 5  Typical major (wt%) and trace (ppm) element composition of the Charleston Orthogneiss (granite) at Little Beach (OU80006) and DBD (OU80007) (ND: not determined). Subscript average indicates the value given is an average of multiple isotopes of that species.

Wt%	Charleston Orthogneiss granite (OU80006)	Doctor Bay dike (OU80007)
SiO2	71.97	74.24
TiO2	0.26	0.08
Al2O3	15.11	14.55
Fe2O3 ^t	2.56	0.80
MnO	0.06	0.02
MgO	0.87	0.27
CaO	2.96	1.66
Na2O	3.14	2.36
K2O	2.02	5.65
P2O5	0.13	0.11
LOI	ND	ND
Total	99.07	99.74
ASI	1.19	1.12
ppm	Charleston Orthogneiss granite (OU80006)	Doctor Bay dike (OU80007)
Sc	4.72	1.87
V	37.56	16.78
Craverage	27.77	24.56
Co	36.90	55.34
Ni	37	847
Cu	52.92	28.23
Zn	92.41	82.88
Ga	26.60	26.16
Ge	2.93	3.54
As	3.34	3.38
Rb	68	116
Sr	595	509
Y	15.80	7.44
Zraverage	136	48
Nb	8.71	2.28
Mo	0.63	0.93
Sb	1.51	0.42
Cd	0.30	3.36
Cs	2.39	1.49
Ba	465	1394
La	47.30	18.24
Ce	82	27
Pr	9.27	3.25
Nd	33.74	10.86
Sm	6.32	2.44
Eu	1.33	1.19
Gd	4.47	1.52
Tb	0.67	0.38
Dy	3.25	1.40
Ho	0.59	0.37
Er	1.42	0.76
Tm	0.28	0.20
Yb	1.43	0.67
Lu	0.32	0.24
Hf	3.87	1.31
Ta	0.55	0.19
W	89	124
^208Pb	26	79
Th	17.69	6.78
U	2.79	1.64

Additionally, the presence of inherited zircon, their mid-Cretaceous ages (Fig. 2C–2E, 4B, 4C) and inboard location are typical of the Rahu Suite (Tulloch 1983; Muir et al. 1994, 1995, 1997; Waight et al. 1997, 1998; Tulloch & Kimbrough 2003). In combination, therefore, their geochemistry, zircon inheritance, age and inboard location indicate the Charleston Orthogneiss and DBD are of Rahu Suite affinity.

Conclusion

•	Intrusion and amphibolite facies metamorphism (≥ 500 °C) of the granitic Charleston Orthogneiss protolith occurred at 118±2 Ma and 107±2 Ma, respectively (Fig. 2C–2E). Metamorphism of the Charleston Orthogneiss occurred at the same time within error as intrusion of the Buckland Granite, which suggests they may be related.
•	Emplacement of the post-metamorphic, syn-tectonic Doctor Bay dike constrains deformation of the Charleston Orthogneiss (the host rock) to 105±2 Ma (Fig. 4C).
•	Fabric development (105±2 Ma) in the Charleston Orthogneiss was related to deformation of the middle crust associated with extension in the New Zealand portion of Gondwana.
•	In combination, the whole-rock major and trace element geochemistry (Fig. 6), age, presence of inherited zircon (Figs. 2C–2E, 4B, 4C) and inboard location of the Charleston Orthogneiss granite and DBD suggest correlation with the Rahu Suite, rather than the Separation Point Suite.

Acknowledgements

The majority of original content presented here formed the honours dissertation of MWS written in 2008. Brent Pooley, Kat Lilly, Damian Walls, Charlotte Allen, Katherine Cambridge and Dushan Jugum are thanked for their help with sample preparation and analyses. Liam Heath and David Sagar accompanied MWS during reconnaissance fieldwork, for which the author is very grateful. Trevor Ireland generously provided raw U–Pb–Th zircon data from the Charleston Orthogneiss. Gratitude is also extended to Andy Tulloch for insightful discussions during this project. James Scott critically reviewed the manuscript on several occasions, which greatly improved the document. The comments and suggestions of reviewers Uwe Ring and Joseph Kula and editor Nick Mortimer were most helpful in improving the final manuscript. The Department of Geology Benson Fund provided MWS with financial aid for fieldwork. A University of Otago Research Grant to JMP supported LA-ICP-MS U–Pb zircon dating at the Research School of Earth Sciences (RSES), Australian National University (ANU), Canberra.

Appendix

LA-ICP-MS U-Pb Zircon Dating

Standard techniques of rock crushing (to 250 µm), plus heavy liquid (lithium-polytungstate or LST ρ = 2.8 g/cm³ & methylene-iodide or MEI ρ = 3.3 g/cm³) and magnetic separation (using the Frantz L-1 Isodynamic Separator at GNS Science, Dunedin) were employed to liberate and concentrate zircons. Subsequently, zircon separates were dry hand-picked using a probe and mounted on double-sided tape. These zircon mounts were encased in epoxy resin. The upper surface of the briquettes containing the zircon samples was first ground with fine (1000 µm) sandpaper to expose the grains, and then polished with an automated polisher using a combination of aluminium-oxide solution, diamond paste and diamond mats.

LA-ICP-MS U-Pb zircon dating was carried out at the Research School of Earth Sciences (RSES), Australian National University (ANU), Canberra. Samples were housed in a HelEx sample cell. Laser ablation was performed with a LambdaPhysik LaserTechnik LPX 120I UV ArF pulsed excimer laser operated at 23 kV producing a 70 mJ 5 Hz beam. Laser spot diameter remained constant throughout an analytical session at either 22 µm or 28 µm. An Agilent 7500 s ICP-MS received ablated matter directed through a flow homogeniser from the sample chamber with He-Ar gas. Abundances of 18 isotopes were recorded with varying integration times according to their importance in U-Pb dating and relative abundances in zircon: 40 ms for ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb; 25 ms for ²³²Th, ²³⁵U and ²³⁸U; 20 ms for ⁴⁹Ti; 10 ms for ³¹P, ⁸⁹Y, ¹³⁹La, ¹⁴⁰Ce, ¹⁴⁷Sm, ¹⁵³Eu ¹⁶³Dy, ¹⁷⁵Lu and ¹⁷⁷Hf; 5 ms for ²⁹Si and ⁹¹Zr giving ∼396 ms per analysis. Background levels of the selected isotopes were collected for 20 s immediately prior to each spot analysis, which lasted 40 s allowing ∼100 mass scans (40 s/0.396 s) and the laser to penetrate ∼20 µm.

The TEMORA 2 zircon (413 ± 3 Ma; Black et al. 2004) and NIST SRM 610 glass (Pearce et al. 1997) standards were analysed at the beginning and conclusion, plus at regular intervals throughout each analytical session. The sequence in which the samples were analysed remained constant within an analytical session. Thus, corrections made with reference to the standards are applicable to all analyses within an analytical session. If the laser bored through a grain into the underlying epoxy, as recognised via real-time monitoring of isotope readings, ablation was halted for the remainder of the spot analysis.

Raw data was processed using a series of offline Microsoft Office Excel spreadsheets. The first 10 s of all spot analyses were discarded, because following activation of the laser a number of seconds passed before the signal stabilised. In addition, background levels of the measured isotopes were deducted, and outlier data >3σ from mean values were excluded from all analyses. Instrument drift and penetration depth fractionation were corrected via reference to data collected for the standards averaged for each analytical session. Standard ²⁰⁷Pb/²⁰⁶Pb or ²⁰⁶Pb/²³⁸U ratios that differed by greater than ∼10% from mean values were excluded. Direct measurement of common Pb was not undertaken because ²⁰⁴Hg, which interferes with the measurement of stable ²⁰⁴Pb, is present in background concentrations well above those of non-radiogenic Pb in zircons. Consequently, a ²⁰⁸Pb-based common Pb correction was implemented, which estimated the amount of non-radiogenic Pb through comparison of expected and measured ²⁰⁸Pb/²⁰⁶Pb ratios according to actual ²³²Th/²³⁸U values (Cumming & Richards 1975).

Uncertainties implicit in individual spot analyses were calculated as the standard error of the mean (SEM) for the corrected U, Pb and Th isotopic ratios across the interval selected for dating. Comparison of these internal errors with those estimated via statistics for each mass scan allowed calculation of the mean squared weighted deviates (MSWD) values. When calculation of the MSWD for common Pb-corrected ages failed, a conservative value was assumed for subsequent processing. Selection of the appropriate interval of unknown zircon analyses was based on age zonation (see below), achieving MSWD values approaching one if reasonably possible, and the presence or absence of inclusions indicated by spikes in trace element content. Cathodoluminescence images of the analysed zircons, captured with the JEOL JXA-6800 Superprobe at the University of Otago, aided in recognition of age zones indicated by trends in Pb/U ratios and trace element abundances with penetration depth in spot analyses. The unique advantage of LA-ICP-MS U-Pb zircon dating over alternative dating methods is the depth profiling of individual grains; multiple growth zones may be analysed within a single grain with each spot analysis (e.g., Fig. A1). Total uncertainties in the ratios of U, Pb and Th for each spot analysis were calculated as a combination of their internal uncertainty and the standard deviation in the same ratios measured in TEMORA 2 for the analytical session.

All dates <1000 Ma were calculated using the ratio of radiogenic (*) ²⁰⁶Pb to ²³⁸U, because uncertainty in the averaged ²⁰⁷Pb/²⁰⁶Pb ratio of TEMORA 2 was relatively poor. Pooled ages are error-weighted averages, whose uncertainty represents two SEM, do not include dates greater than 10% discordant or with MSWD values >10, and exhibit Gaussian distributions on probability density plots (generated with IsoPlot 2 add-in for Excel), which was assumed indicative of a single natural zircon population (Bryan et al. 2004). Ages were also calculated using a Tera-Wasserburg (²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb) concordia generated with IsoPlot 3.

XRF Whole-rock Major Element Geochemistry

Representative sub-samples of each rock were placed in an oven maintained at a constant temperature of 105 °C overnight to remove any free H₂O. Once dry, ∼0.64 g of each sub-sample was combined with ∼6.8 g of lithium borate flux and ∼1.0 g of NH₄NO₃ in a plastic jar of known weight. Utilising an Automated Fusion Technology Ltd. Phoenix 4000 unit, these cocktails were heated to 1100 °C in Pt-Au crucibles, mixed thoroughly once molten to ensure homogeneity, and cast into silicate glass discs of 40 mm diameter in pre-heated Pt-Au plates cooled by compressed oxygen. Discs of two sub-samples of each rock were prepared and analysed twice to assess the reproducibility of XRF whole-rock major element geochemistry results and compositional variation within each sample. Loss on ignition (LOI) values were not determined because the required furnace was nonoperational at the time of sample preparation. However, the total oxide weight percent for all analyses were within 1% of 100% indicating LOI was minimal, typical of granitic rocks (e.g., Kimbrough & Tulloch 1989). The Philips PW2400 sequential wavelength dispersive x-ray fluorescence spectrometer fitted with a 3 kW Rh tube, in the Department of Geology, University of Otago was employed to determine major element compositions.

LA-ICP-MS Whole-rock Trace Element Geochemistry

The same silicate glass discs prepared for XRF whole-rock geochemistry were analysed via LA-ICP-MS at the Otago Community Trust Centre for Trace Element Analysis, University of Otago, and RSES, ANU, Canberra. To reduce the duration of the analytical session, the need to exchange samples was eliminated by cutting small (∼0.5 cm long) sections from each disc, which were then mounted in a single epoxy resin briquette. The briquette was polished with a 1 µm diamond mat.

At the University of Otago, a NewWave 213 nm pulsed laser operated at a half-power and 10 Hz was used for laser ablation. Ablated material was transported by He gas from the sample cell, and mixed with gaseous Ar before being passed into an Agilent 7500 ICP-MS. Data for 45 mass peaks were collected in time-resolved mode with one point per peak. Integration times were 10 ms for all the isotopes analysed (²⁴Mg, ²⁷Al, ²⁹Si, ⁴³Ca, ³⁹K, ⁴³Ca, ⁴⁵Sc, ⁴⁷Ti, ⁴⁸Ti, ⁴⁹Ti, ⁵¹V, ⁵²Cr, ⁵³Cr, ⁵⁷Fe, ⁶⁹Co, ⁶⁰Ni, ⁷¹Ga, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³³Cs, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷⁴Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, ²³⁸U), giving a total scan time of 539 ms. Background levels were collected for the first 20 s of each analysis prior to triggering of the laser, followed by 40 s of data acquisition for each sub-sample. The laser was set to migrate at 10 µm/s and a spot diameter of 100 µm. This gave ∼100 mass scans from a 400 µm long ∼5 µm deep track across each disc. The effects of depth fractionation are negligible when employing laser-migration, because laser penetration is relatively shallow. A stationary spot, of 78 µm diameter was utilised at the RSES. However, depth fractionation was minimal due to the relatively large spot diameter employed. At the beginning and conclusion of the analytical session, two analyses of NIST SRM 610 glass standard were taken. Between each spot analysis, a purge time ≥120 s was allowed for a return to background levels.

Reduction of the raw trace element data utilised an offline Excel spreadsheet. The raw mass peak data were corrected for background levels and mass bias drift with reference to the NIST SRM 610 silicate glass standard. Trace element concentrations (ppm) in the sub-samples were obtained by normalising the corrected count rates to the known Si and trace element content of the standard and Si concentration in the unknowns, which was determined previously as whole-rock SiO₂ via XRF. The two NIST SRM 610 standard analyses directly bounding the analyses of the samples were used data for correction and conversion (i.e., the second and first NIST SRM 610 standard analyses taken at the beginning and end of the analytical session, respectively). The interval selection of data from the sub-samples excluded the first 10–15 s acquired following activation of the laser, because the signal only stabilised after this amount of time following the commencement of ablation.

Figure A1  Depth profile of Charleston Orthogneiss spot #16 illustrating correlation between changes in geochemistry and age within a single grain, which is interpreted to indicate zoning.