Geochronology and geochemistry of the Thirsty Creek Norite, Westland, New Zealand

ABSTRACT

Thirsty Creek Norite is a small, undeformed, mafic intrusion intruding gneisses of the Granite Hill Complex in the Buller Terrane. An age of 107.1 ± 4.1 Ma has been obtained using Rb-Sr geochronology. A relatively high MSWD of 4.5 is attributed to sericitization and alteration of plagioclase. This date is consistent with other ages for the timing of high-grade metamorphism in the Granite Hill Complex and may represent a metamorphic age, thus providing a minimum age for metamorphism and emplacement of the norite. However, the relatively undeformed nature of the Thirsty Creek Norite suggests it may postdate metamorphism and we speculate that the date could represent a crystallisation age which is contemporaneous with Rahu Suite granitic magmatism in the adjacent Hohonu Batholith. Several geochemical characteristics, including overlapping initial Sr and Nd isotope compositions, suggest the Thirsty Creek Norite could be a relatively mafic yet contaminated melt that is petrogenetically related to Rahu Suite magmatism. Overlaps in the isotopic compositions of the Thirsty Creek Norite and Rahu Suite granitoids therefore imply that any mafic components involved during granitic magmatism must have had relatively enriched isotopic compositions, and that evolution to more felsic compositions occurred without significant interactions with upper crustal components.

KEYWORDS:

• New Zealand
• Western Province
• Granite Hill Complex
• Thirsty Creek Norite
• granitic magmatism
• Cretaceous
• Rb-Sr dating
• Rahu Suite

Introduction

The Thirsty Creek Norite is a small mafic intrusion in the Buller Terrane, located inland from Greymouth near Lake Brunner (Figure 1). Mafic intrusions are relatively rare in the Western Province, yet important, as they can provide constraints on the sources and/or parental magmas of regional granitic magmatism. Mafic intrusions of various ages have been previously described from Westland (e.g. the Zetland Diorite in NW Nelson (Muir et al. 1996; Turnbull et al. 2013)), Fiordland (e.g. the Hollyford Gabbronorite (Williams and Smith 1983), the Beehive Diorite (Scott et al. 2009), mafic intrusions in the Darran Suite (Muir et al. 1998) and the Western Fiordland Orthogneiss (Muir et al. 1998; Allibone et al. 2010; Decker et al. 2017)), and the Eastern Province (the Greenhills Complex (Mossman 1973)). Mafic igneous rocks that occur in geographic proximity to the Thirsty Creek Norite are the late Cretaceous Hohonu Dike Swarm (Waight, Weaver, Muir et al. 1998; van der Meer et al. 2013, 2016), and relatively rare gabbroic and dioritic rocks in the adjacent Hohonu Batholith (Waight 1995).

Figure 1. A: Overview of the crystalline basement geology of the central Westland region (the figure is modified from van der Meer et al. (2016)), PMCC: Paparoa Metamorphic Core Complex. The black square shows the region expanded in B. B: Overview geology of the Lake Brunner/Granite Hill area (modified from Waight et al. 1997). Sample location of the Thirsty Creek Norite (samples QH03, RTC1, RTC2 and SDWRHN) is shown. Undeformed mafic rocks likely to be related to the Thirsty Creek Norite can be found at Crooked River and Rough and Tumble Creek (samples CR2 and GRT5, respectively, from Waight (1995)).

The Thirsty Creek Norite is exposed as several isolated outcrops over a distance of c. 0.5 km in the bed of Thirsty Creek on the NW side of a low (240 m a.s.l.) bush-covered hill immediately west of Mount Te Kinga near Lake Brunner in Westland (Figure 1). The Thirsty Creek Norite was first described briefly by Tulloch and Brathwaite (1986) and geochemical data have been presented by Mason and Taylor (1987), Pickett and Wasserburg (1989) and Waight (1995). Isolated outcrops of undeformed mafic rocks, exposed in the Granite Hill Complex in Crooked River and in Rough and Tumble Creek, are likely to be related to the Thirsty Creek Norite (Mason and Taylor 1987; Waight 1995) (Figure 1). Possible correlatives to the Thirsty Creek Norite also occur c. 50 km to the north in the southern Victoria Range. Mason (1990) describes similar rocks from Lake Ahaura, Mason and Taylor (1987) note rare pebbles of pyroxenite from Troulands Creek, and norite has also been observed near Mt. Beckham and as float in the Alexander River (A.J. Tulloch pers.comm.).

Field relations suggest that the Thirsty Creek Norite intrudes into orthogneisses of the Granite Hill Complex, a belt of high-grade metamorphic rocks exposed as an uplifted sliver between the Fraser Fault (termed the Granite Hill Fault in Waight et al. 1997) and the Alpine Fault north of the Taramakau River (Kamp et al. 1992; Suggate and Waight 1999). The Granite Hill Complex is considered to be equivalent to the more extensive Fraser Complex further south (Rattenbury 1991) and both consist of an amphibolite facies assemblage of paragneiss, metabasite and orthogneiss (Rattenbury 1991; Waight et al. 1997), representing metamorphosed equivalents of Greenland Group sediments and Karamea and Rahu Suite granitoids of the Buller Terrane (Hiess et al. 2010, 2015). No actual intrusive contacts with the gneisses are exposed, however the norite contains inclusions of gneiss (Tulloch and Brathwaite 1986) which are consistent with it having an intrusive origin into the Granite Hill Complex. Petrographic observations suggest the Thirsty Creek Norite is unmetamorphosed and largely undeformed suggesting it could be younger than the metamorphic event that affected the Granite Hill Complex gneisses. Metamorphism of the Granite Hill Complex has been constrained to have a minimum age of c. 108–110 Ma based on U-Pb monazite ages from paragneiss in the Crooked River close to the Alpine Fault (Hiess et al. 2010). To the west of the Thirsty Creek Norite and the unexposed Fraser Fault, the geology is dominated by Rahu Suite granitoids of the Hohonu Batholith (114–109 Ma), emplaced into typical greenschist facies Ordovician Greenland Group metasediments, and cross-cut by mafic dikes of the late Cretaceous Hohonu Dike Swarm (Waight et al. 1997; Waight, Weaver, Maas, et al. 1998; Waight, Weaver, Muir et al. 1998, Waight, Weaver, Muir, Maas et al. 1998; van der Meer et al. 2013, 2016).

Methods

We present major and trace elements derived from several sources for various samples of the Thirsty Creek Norite as well as new data obtained for this study. Details of the various methods used to obtain this data and sample locations are given in Table 1. Sr and Nd isotope data for whole rock sample RTC1 were determined at La Trobe University, Melbourne using methods described by Waight, Weaver, Muir, Maas et al. (1998). The new Rb-Sr data presented here were determined on a whole rock powder and plagioclase and biotite separates hand-picked from sample QH03 collected in Thirsty Creek (Figures 1 and 2). Analyses were carried out by thermal ionisation mass spectrometry (TIMS) isotope dilution at the University of Copenhagen using dissolution procedures and analytical methods described by Scott et al. (2014). During the analyses, the SRM987 standard gave ⁸⁷Sr/⁸⁶Sr = 0.710236 ± 4 (2SE abs), in good agreement with long-term reproducibility in the lab of ⁸⁷Sr/⁸⁶Sr = 0.710240 ± 17 (2SD abs, n = 23). Analytical blanks were insignificant compared to the amounts of Sr available for analysis. Unmixing of the spike and calculations of Rb and Sr concentrations were carried out offline using in-house Excel spreadsheets. Isochrons were constructed using the Excel add-in Isoplot 3.71 (Ludwig 2008) assuming an external reproducibility of 0.5% for ⁸⁷Rb/⁸⁶Sr and 0.003% for ⁸⁷Sr/⁸⁶Sr (e.g. Waight et al. 2002). All errors on geochronological data are presented as 2SE. Mineral chemistry was determined by electron microprobe, JEOL JXA-8200 SuperProbe, in Copenhagen using methods described in Waight and Tørnqvist (2018); representative mineral analyses are presented in Table 2.

Table 1. Major and trace element analyses of the Thirsty Creek Norite and related rocks, and a sample of the Eastern Hohonu River Gabbro (EHR8) from the Hohonu Ranges.

Sample	RTC1	RTC2	SDWRHN	QH03	CR2	GRT5	EHR8
Location	Thirsty Creek	Thirsty Creek	Thirsty Creek	Thirsty Creek	Crooked River	Rough and Tumble Creek	Eastern Hohonu River (float)
Grid Ref.	BU20 812 758	BU20 812 758	BU20 812 758	BU20 812 758	BU20 849 793	BU20 865 809	BU19 658 759
SiO2	47.98	48.31	47.35	47.21	50.98	51.76	42.04
TiO2	1.29	1.20	1.18	1.43	0.44	0.42	4.25
Al2O3	15.50	15.82	17.59	16.32	15.86	6.11	16.14
Fe2O3T	13.51	13.17	12.45	14.18	8.15	14.15	14.9
MnO	0.16	0.16	0.15	0.18	0.13	0.23	0.16
MgO	10.33	9.62	9.20	9.64	10.85	20.09	6.10
CaO	8.30	8.68	9.06	9.09	11.29	6.36	10.09
Na2O	1.45	1.61	1.67	1.44	1.23	0.58	2.79
K2O	0.47	0.53	0.40	0.86	0.41	0.22	0.66
P2O5	0.23	0.25	0.23	0.36	0.05	0.05	0.09
LOl	0.84	0.70	1.20	0.02	0.82	−0.37	1.35
Total	99.22	99.35	99.28	100.71	99.39	99.97	98.57
Ba	128	158.67	72	152.35	102	59	68
Ce	28	23.33	19	30.81	18	10	10.2
Co	–	59.92	–	43.46	–	–	56
Cr	282	315.59	217	273.63	189	1513	42
Cs	–	1.33	–	2.03	–	–	1
Cu	–	82.95	–	56.76	–	–	–
Dy	–	1.60	–	3.29	–	–	–
Er	–	0.92	–	1.72	–	–	–
Eu	–	0.71	–	1.15	–	–	0.86
Ga	23	20.37	20	20.98	20	18	15
Gd	–	1.96	–	4.06	–	–	2.30
Hf	–	0.83	–	0.81	–	–	1.28
Ho	–	0.33	–	0.64	–	–	–
La	16	10.80	14	12.72	10	13	5
Lu	–	0.15	–	0.22	–	–	0.08
Nb	6	3.96	6	4.82	2	2	10
Nd	34	11.31	–	18.95	15	10	6.5
Ni	137	127.89	111	105.41	62	364	44
Pb	12	7.27	13	6.42	4	6	7
Pr	–	2.86	–	4.23	–	–	–
Rb	19	20.69	15	36.40	12	7	31
Sc	–	28.71	–	30.73	–	–	21
Sm	–	2.14	–	4.20	–	–	1.76
Sr	621	615.62	771	534.48	422	108	645
Ta	–	0.29	–	0.35	–	–	0.95
Tb	–	0.29	–	0.60	–	–	0.35
Th	7	1.55	3	1.47	1	–	1
Tm	–	0.15	–	0.26	–	–	0.1
U	–	0.41	–	0.47	–	–	0.14
V	309	330.99	279	325.73	191	207	461
Y	9	9.25	11	17.80	16	12	7
Yb	–	0.96	–	1.52	–	–	0.6
Zn	131	133.37	128	144.84	64	139	120
Zr	60	17.92	48	16.73	38	34	58

Note: Sample QH03 is from this study, all other samples from Waight (1995). All major and trace element data from X-ray fluorescence (XRF) using methods described by Weaver et al. (1990), except QH03 (major elements determined by XRF methods described by van der Meer et al. (2016)), QH03 and RTC2 (trace element data analysed using Inductively coupled plasma mass spectrometry (ICP-MS) using methods described by van der Meer et al. (2016)), and EHR8 (trace elements by instrumental neutron activation analysis (INAA) using methods described by Weaver et al. (1990)). The symbol ‘–‘ is used for elements not analysed. Grid references are from the to NZ Topo50 map series.

Table 2. Representative electron microprobe major element data for minerals in Thirsty Creek Norite sample QH03.

Mineral	Bt	Bt	Bt	Hbl	Hbl	Hbl	Opx	Opx	Opx	Pl	Pl	Pl
No.	111	114	174	158	197	32	62	90	59	198	21	48
SiO2	38.04	37.74	37.63	45.13	45.76	45.55	51.43	51.4	51.53	48.63	51.21	48.77
TiO2	3.04	3.25	3.38	0.78	1.01	0.81	0.08	0.07	0.1	0	0	0
Al2O3	15.8	15.74	15.87	11.96	10.64	11.18	2.17	1.9	2.89	32.16	29.82	31.9
FeO	15.1	15.17	14.72	13.45	13.88	13.94	22.08	22.97	21.74	0.2	0.12	0.08
MnO	0.08	0.06	0.08	0.18	0.2	0.2	0.5	0.48	0.5	0.03	0.01	0.01
MgO	14.16	14.7	14.93	12.67	13.74	13.05	22.55	22.43	21.95	0	0	0
CaO	0.01	0	0	10.45	10.01	10.01	0.21	0.23	0.71	15.4	13.03	15.16
Na2O	0.23	0.26	0.32	1.26	1.18	1.27	0.03	0.03	0.12	2.47	4.18	2.81
K2O	9.27	9.12	9.26	0.39	0.39	0.35	0.01	0	0.02	0.03	0.04	0.03
NiO	0	0	0.02	0.01	0	–	–	–	–	0.02	–	–
Cr2O3	0.05	0.05	0.03	0.04	0.06	–	–	–	–	0	–	–
Total	95.78	96.09	96.24	96.32	96.87	96.36	99.06	99.51	99.56	98.94	98.41	98.76

Note: Bt = biotite, Hbl = hornblende, Opx = orthopyroxene, Pl = plagioclase, –  = not analysed.

Results

Field geology and petrography

In outcrop, the Thirsty Creek Norite is mesocratic, equigranular and coarse grained with no apparent foliation. No intrusive contacts with the Granite Hill Complex are exposed, however inclusions of biotite-muscovite orthogneiss occur within the norite (Figure 2). The sample dated in this study (QH03) has similar petrographic characteristics to other previously described Thirsty Creek Norite samples (Tulloch and Brathwaite 1986; Mason and Taylor 1987; Waight 1995). It comprises about 50% modal plagioclase (An 55–79%; Table 2) either as subhedral crystals 3–5 mm in length and showing deformed polysynthetic twinning or as smaller euhedral to subhedral crystals, some displaying compositional zoning. Many plagioclase crystals show evidence for cataclastic deformation, as well as alteration to sericite (Figure 3). The dominant mafic mineral in QH03, representing 30% of the mode, is a green to pale green/brown pleochroic magnesio-hornblende to magnesio-ferri-hornblende (Mg# (molar Mg/(Mg + Fe)*100) = 58–65), c. 4 mm in size, anhedral to subhedral and displaying simple twinning. Optical observations show several hornblende crystals containing cores with weak green pleochroism, possibly representing relict clinopyroxene. Electron microprobe investigations show that hornblende is often accompanied by <0.5 mm subhedral apatite, ilmenite and magnetite. Orthopyroxene (hypersthene to bronzite En = 59–65%) occurs as subhedral crystals <3 mm in size, representing c. 5% of the rock. Rare anhedral quartz grains (<1 mm) with undulose extinction are also observed. Brown-yellow pleochroic biotite (Mg# = 62–67) makes up 10% of the rock as anhedral to subhedral crystals <3 mm in length. Biotite shows no petrographic evidence for recrystallization, deformation or alteration (Figure 3). Mason and Taylor (1987) and Waight (1995) also describe occasional euhedral olivine altered to serpentinite in some samples although none were observed in QH03. No zircon was identified in our samples of Thirsty Creek Norite, and none was obtained in an attempt to separate zircon from QH03. The mineral assemblage suggests that sample QH03 is more correctly described as a biotite-hornblende gabbronorite, whereas other samples described in the literature (including likely related rocks at Crooked River and on Granite Hill) contain higher abundances of orthopyroxene, in agreement with the original classification as a norite (Mason and Taylor 1987; Waight 1995). Sample GRT5 from Rough and Tumble Creek, differs from the previously described samples by being finer grained, and in having a higher proportion of orthopyroxene, approximately sub-equal in amount to amphibole (Waight 1995). Importantly, we note that none of the mafic rocks found in the Granite Hill area show signs of high temperature deformation.

Figure 2. Field photo of the Thirsty Creek Norite at Thirsty Creek. Sample locality for QH03, RTC1, RTC2 and SDWRHN (−42.6581880, 171.553701). Note the large inclusion of felsic orthogneiss under the hammer.

Figure 3. Thin section image of sample QH03 showing hornblende (HBL), sericitized and partially cataclased plagioclase (PL), ilmenite (ILM), clinopyroxene (PX) and fresh unaltered biotite (BT). PPL = plane polarised light. CPL = cross polarised light.

Major and trace element geochemistry

Geochemical data have been obtained from multiple samples of the Thirsty Creek Norite over the last 30 years. The data available from the earlier studies varies and as the methods, detection limits and technology used for acquiring data have since improved, only the most recent data from Waight (1995) and data collected for this study are presented and discussed here (Table 1). Previously collected data can be found in Mason and Taylor (1987) and Pickett and Wasserburg (1989). The samples from Thirsty Creek have SiO₂ contents of 47–48 wt% and all classify as gabbro in the plutonic TAS diagram of Middlemost (1994) (Figure 4A). These samples have Al₂O₃ between 15.5 and 17.6 wt%, MgO between 9.2 and 10.3 wt%, Mg# 57–60 (Figure 4), and Cr and Ni varying from 217 to 316 ppm and 105–137 ppm, respectively. All samples plot within the sub-alkaline field with K₂O ranging 0.4–0.9 wt% and fall within the calc-alkaline series bordering the tholeiitic series, on a K₂O versus SiO₂ plot (Figure 4E). The samples from Granite Hill (CR2 and GRT5) are characterised by somewhat higher SiO₂ contents (51–52%), and in particular, sample GRT5 has low Al₂O₃ and elevated MgO, Cr and Ni indicative of accumulation of orthopyroxene and consistent with petrographic observations.

Figure 4. Selected geochemical plots. A: Total alkali-silica classification diagram (fields from Middlemost 1994); B-E: Selected Harker diagrams, boundaries in E from Le Maitre et al. (1989); F: Y vs Sr/Y with the boundary between HiSY and LoSY compositions (Sr/Y > 40) from Tulloch and Kimbrough (2003). Comparative data are primarily downloaded from PETLAB (Strong et al. (2016) and include data from Spandler et al. (2003), Mossman et al. (2000), Williams and Harper (1978), Williams and Smith (1983), Tulloch and Kimbrough (2003), Bradshaw (1985), Blattner (2006), Tulloch et al. (2009), Burgess (2004), Muir et al. (1995, 1998), Wandres et al. (1998), Tulloch (1979), Turnbull et al. (2013) and Scott et al. (2009). Rahu Suite data are from the Hohonu Batholith, and Mafic Deutgam represents gabbroic, dioritic and mafic enclave samples from the Deutgam Granodiorite (data from Waight (1995), and Waight, Weaver, Muir, Maas et al. (1998)).

In a multi-element variation diagram (Figure 5A), the Thirsty Creek Norite displays a subduction-like signature with depletion in Nb, enrichment in large-ion lithophile elements (LILE) depletion in high-field-strength elements (HFSE) and a positive Sr anomaly. The samples are light rare-earth element (LREE) enriched (La_N/Lu_N = 6.0–7.6) with relatively flat heavy rare-earth elements (HREE) (Gd_N/Yb_N = 1.0–1.7) and negligible Eu anomalies (Eu/Eu* = 0.9–1.1) (Figure 5B). We note that our new analyses have relatively low contents of Zr (ca. 17 ppm cf 50–60 ppm in XRF analyses of samples from the same locality (Table 1) and Hf. This could represent incomplete breakdown of zircon during dissolution – although as noted previously our attempts to separate zircon from the sample were unsuccessful.

Figure 5. A: Multi-element variation diagram (data normalised to primitive mantle values of McDonough et al. 1992) and B: REE plot (data normalised to chondrite values from Anders and Grevesse 1989) for the Thirsty Creek Norite and comparative rocks. Note a typical subduction-like signature with enriched LILE and a negative Nb anomaly, relatively small Eu anomaly and enrichment in LREE. Comparitive data (see caption for Figure 4 for data sources) are plotted as averages for samples with SiO₂ <53% for best comparison. The grey shaded area represents Rahu Suite granitoids from the Hohonu Batholith.

Rb-Sr and Sm-Nd isotopes

Isotopic results for samples RTC1 and QH03 are presented in Tables 3 and 4. A three-point errorchron for the Thirsty Creek Norite was constructed using biotite, plagioclase and whole rock powder from QH03 and yields an errorchron age of 107.1 ± 4.1 Ma (MSWD of 4.5, ⁸⁷Sr/⁸⁶Sr_(i) = 0.70740 ± 0.00021) (Figure 6). A biotite-plagioclase two-point isochron yields an age of 107.21 ± 0.56 Ma (⁸⁷Sr/⁸⁶Sr_(i) = 0.70739 ± 0.00002) and a biotite-whole rock two-point isochron gives an identical age of 107.01 ± 0.57 Ma (⁸⁷Sr/⁸⁶Sr_(i) = 0.70740 ± 0.00002). Using an age of 107 Ma, the whole rock analysis of RTC1 yields ⁸⁷Sr/⁸⁶Sr_(i)= 0.70743 and εNd = −5.9, correlating well with the previously published isotopic results for the Thirsty Creek Norite sample (WL-2) presented by Pickett and Wasserburg (1989), which yields ⁸⁷Sr/⁸⁶Sr_(i)= 0.70731 and εNd = −6.0 when age corrected to 107 Ma.

Figure 6. Errorchron for sample QH03 whole rock, plagioclase and biotite. The age is calculated assuming an external reproducibility of 0.5% for ⁸⁷Rb/⁸⁶Sr and 0.003% for ⁸⁷Sr/⁸⁶Sr. The errors on the individual data points are smaller than the symbol size used in the figure.

Table 3. Rb-Sr data used to calculate Rb-Sr whole-rock errorchrons and Rb-Sr ages for the Thirsty Creek Norite.

Sample	Rb	2SE%	Sr	2SE%	87Rb/86Sr	2SE%	2SE(abs)	87Sr/87Sr	2SE%	2SE(abs)
QH03WR	34	0.016	598	0.009	0.164	0.025	0.00004	0.707669	0.0012	0.000008
QH03BT	267	1.131	70	0.011	11.092	1.142	0.127	0.724287	0.002	0.000015
QH03PL	45	0.244	1499	0.036	0.086	0.28	0.0002	0.707518	0.0018	0.000013
RTC1WR	19	–	621	–	0.089	0.025	0.00002	0.707560	0.0012	0.000008

Note: WR = whole rock, BT = biotite, PL = plagioclase. RTC1 data from Waight (1995), RTC1 errors assumed identical to QH03 data that was produced for this study using the Sector 54-IT thermal ionisation mass spectrometer situated at University of Copenhagen.

Table 4. Sr and Nd isotope data for the Thirsty Creek Norite. Initial ratios and epsilon values calculated for an emplacement age of 107 Ma.

Sample	Rb (ppm)	Sr (ppm)	87Rb/86Sr	87Sr/86Sr(m)	87Sr/86Sr(i)	Sm (ppm)	Nd (ppm)	147Sm/144Nd	143Nd/144Nd(m)	143Nd/144Nd(i)	εNd(t)
QH03	34	598	0.164	0.707669	0.70739	–	–	–	–	–	–
RTC1	19	621	0.089	0.707560	0.70743	2.15	12.3	0.1041	0.51227	0.51220	−5.9

Note: εNd calculated using ¹⁴³Nd/¹⁴⁴Nd_(CHUR) = 0.512638 and ¹⁴⁷Sm/¹⁴⁴Nd_(CHUR) = 0.1967 with λ¹⁴⁷Sm = 6.54 × 10⁻¹². ⁸⁷Rb decay constant of λ = 1.42 × 10⁻¹¹. QH03 data produced for this study; RTC1 data from Waight (1995).

Discussion

The analysed mineral phases from Thirsty Creek Norite fail to form a statistically viable isochron (MSWD = 4.5). The similarity of the plagioclase and whole rock data points on the calculated three-point errorchron indicate that the whole rock Sr budget is dominated by the plagioclase. Thin section observations show cataclastic deformation, with associated sericitization and alteration of plagioclase likely reflecting proximity to, and deformation associated with, the Fraser and Alpine Faults. These features are considered to account for the elevated MSWD. However, this disturbance is minor as biotite-whole rock and biotite-plagioclase two-point isochrons are identical within error. As the whole rock Sr budget is dominated by plagioclase, biotite is the dominant control on the age determined.

Our age for the Thirsty Creek Norite is in agreement with an initial suggestion of a Cretaceous age by Tulloch and Brathwaite (1986). Our age also coincides with monazite U-Pb ages of c. 107–110 Ma from paragneiss in Crooked River established by Hiess et al. (2010) which is interpreted to date an anatectic event in the Granite Hill Complex associated with metamorphism and representing the youngest age from these rocks. Given the relatively low closure temperature for Sr isotopes in biotite (Dodson 1973), our age could thus represent a complete metamorphic resetting of the Thirsty Creek Norite during this anatectic event. Alternatively, the age could be interpreted to represent cooling during regional mid-Cretaceous uplift. The lack of deformation in the Thirsty Creek Norite may then reflect its low quartz contents and thus a large ductility contrast with the surrounding quartz-rich orthogneisses. We cannot exclude that this age represents a metamorphic age, and if so it then provides a minimum constraint on the age of the norite and for metamorphism in the Granite Hill Complex.

Alternatively, the age could be interpreted to represent a primary crystallisation age. With the exception of some cataclastic features, the Thirsty Creek Norite shows magmatic textures and there is no petrographic evidence for it having been affected by an anatectic or high-temperature metamorphic event. In particular, biotite in the Thirsty Creek Norite appears primary and shows no obvious sign of deformation, alteration or disturbance. Therefore, we speculate that our age could be a primary crystallisation age. If correct, this would suggest that the norite was emplaced shortly after the monazite-forming metamorphic event, or even contemporaneously with anatexis. Furthermore, we consider that the overlap in ages and excellent agreement in both initial Sr and Nd isotope composition between the Thirsty Creek Norite with the adjacent Rahu Suite granitoids of the Hohonu Batholith is unlikely to be coincidental and this potentially has important implications for the petrogenesis of the Rahu Suite. The overlap in initial Sr and Nd isotope compositions of the Thirsty Creek Norite and Rahu Suite granites is particularly noteworthy as while whole rock and mineral Rb-Sr systematics could be potentially disturbed or reset by regional metamorphism and/or deformation, the Sm-Nd isotope system has a much higher closure temperature and is a more robust system. The age of the Thirsty Creek Norite precludes correlation with the younger Hohonu Dike Swarm emplaced between c. 92-68 Ma, which is also characterised by more alkaline geochemical signatures (van der Meer et al. 2016, 2018) and more primitive isotopic compositions (Waight, Weaver, Maas, et al. 1998; van der Meer et al. 2017; Figure 7). Below we speculate further on the possible implications of a mid-Cretaceous crystallisation age for the Thirsty Creek Norite.

Figure 7. Whole rock Sr and Nd isotope data for the Thirsty Creek Norite compared to selected lithologies from the literature. All analyses age-corrected to 107 Ma with the exception of the Hohonu Dike Swarm (90 Ma). Also shown is a simplistic bulk mixing curve between a mafic end-member with Separation Point Suite isotopic compositions (⁸⁷Sr/⁸⁶Sr_(107Ma) = 0.7042, εNd_(107Ma) = +1.37, Sr = 425.7 ppm, Nd = 14.88 (average continental arc basalt from Keleman et al. 2014)) and average Greenland Group (⁸⁷Sr/⁸⁶Sr_(107Ma) = 0.74409, εNd_(107Ma) = −11, Sr = 70 ppm, Nd = 36 (Waight, Weaver, Muir, Maas et al. 1998)). Tick marks represent 10% increments of mixing. Data sources: Hohonu Dike Swarm = van der Meer et al. (2017), Hohonu Granites = Waight, Weaver, Muir, Maas et al. (1998), Separation Point Suite = Muir et al. (1995), Western Fiordland Orthogneiss = McCulloch et al. (1987), Muir et al. (1998). Darran Suite = Muir et al. 1998.

The Thirsty Creek Norite shows broad scale geochemical similarities to other mafic rocks of various ages in New Zealand, and in particular overlaps chemically with the mafic compositions in the Western Fiordland Orthogneiss, Separation Point Suite and Darran Suite. Somewhat lower Na₂O + K₂O and higher MgO in the Thirsty Creek Norite may reflect a partly cumulative origin, however the lack of petrographic evidence for mineral accumulation and a lack of significant Eu anomalies suggests any mineral accumulation is minor (with the exception of sample GRT5). Notably, the Thirsty Creek Norite samples are clearly geochemically distinct from the cumulate rocks of the Greenhills Complex (Spandler et al. 2003) (Figures 4 and 5). The Thirsty Creek Norite is geochemically characterised by a relatively high Sr/Y (>60) in most samples (Figure 4F), although QH03 is somewhat lower (Sr/Y = 30). In addition, all samples have elevated Al₂O₃ (>15 wt%) and Na₂O/K₂O (>1), features tending towards adakitic compositions (Defant and Drummond 1990). Comparable geochemical trends are recognised in the Separation Point Suite (Muir et al. 1995), and sources and/or magmas similar in compositions to the Separation Point Suite have been invoked as source and/or end-member components involved in petrogenesis of the mid-Cretaceous Rahu Suite granitoids of the Hohonu Batholith (Waight, Weaver, Muir, Maas et al. 1998; van der Meer et al. 2018). Although there is considerable scatter in trace element compositions of potentially comparative mafic compositions (Figure 5), we note that the general trends of elevated LILE, depleted Nb and positive Sr are also present in the Separation Point Suite, Beehive Diorite, Tobin Suite and Darran Suite.

Our age for the Thirsty Creek Norite overlaps with the youngest portions of the Separation Point Suite as well as mid-Cretaceous Rahu Suite granitoid magmatism in the Hohonu Batholith. Silica-poor representatives of the Separation Point Suite show generally overlapping trace element signatures with the Thirsty Creek Norite, as well as overlapping peaks and troughs for Sr, Nb and Y, though these are more extreme in the Separation Point Suite (Figure 4). However, the Separation Point Suite SiO₂ content is generally higher than in the Thirsty Creek Norite (>53 wt%) and it is characterised by more primitive isotopic compositions of ⁸⁷Sr/⁸⁶Sr_(i)= 0.70415 and εNd = +1.7 (e.g. Pearse Granodiorite (Muir et al. 1995), age corrected to 107 Ma) (Figure 7). Therefore, it is not plausible to link magmatism in the Separation Point Suite directly to the Thirsty Creek Norite. Similarly, the Sr-Nd isotope compositions of the Thirsty Creek Norite are clearly distinct from the relatively primitive isotopic signatures of intrusives from the Western Fiordland Orthogneiss and Darran Suite (Figure 7).

The isotopic and chronological overlaps between the Thirsty Creek Norite and the Rahu Suite granitoids of the Hohonu Batholith (⁸⁷Sr/⁸⁶Sr_(i) = 0.70835–0.70939 and εNd = −5.8 to –4.5, age corrected to 107 Ma, Figure 7), coupled with their geographic proximity suggests a genetic link and has interesting implications for the origin of the Rahu Suite. The contrast in SiO₂ contents between the most Rahu Suite granitoids of the Hohonu Batholith and the Thirsty Creek Norite make ascertaining direct links between the two complicated. One option is that the Thirsty Creek Norite could represent a cumulate rock formed during Rahu Suite granitic magmatism in the Hohonu Batholith, which would be consistent with the overlap in age and isotopic compositions. Rare mafic rocks dominated by plagioclase and amphibole with clinopyroxene are described from the Hohonu Ranges by Waight (1995) (e.g. the Eastern Hohonu River Gabbro (sample EHR8)), some of which occur as mafic enclaves in the granitoids, while others show some cumulate features. Examples of these rocks from the Deutgam Granodiorite show some geochemical overlap with the Thirsty Creek Norite in terms of major elements and Sr/Y (see Figure 4) although they are characterised by relatively high K₂O – potentially reflecting diffusive exchange between inclusions and host granitoid magmas during mingling (e.g. Waight et al. 2001). The chemical and geological relationship (if any) between these mafic rocks and the Thirsty Creek Norite remains unclear. The Thirsty Creek Norite does not have the characteristic trace element signatures of other rocks identified as cumulates (e.g. the Greenhills Complex Figure 5). Furthermore, a lack of a positive Eu anomaly and no clear petrographic evidence for cumulate textures appears to exclude an origin of the Thirsty Creek Norite as a cumulate from contemporaneous mid-Cretaceous Rahu Suite granitic magmas.

The overlap in Sr and Nd isotope compositions between Thirsty Creek Norite and Rahu Suite granitoids of the Hohonu Batholith also suggests that the former may represent a less evolved, mafic equivalent to the Rahu Suite granitoids of the Hohonu Batholith. However, these isotopic characteristics, coupled with relatively low Ni and Cr contents, also indicate that the Thirsty Creek Norite is clearly not a primary melt derived from a typical depleted mantle composition. Even when the isotope data are sequentially age corrected back to ages up to 600 Ma, the Thirsty Creek Norite does not have compositions that overlap with age equivalent depleted mantle. The Rahu Suite granitoids show limited variations in Sr, Nd, Hf and O isotopic composition despite a wide range in silica contents (see also van der Meer et al. 2018) and this is inconsistent with geochemical diversity resulting from upper crustal contamination processes. van der Meer et al. (2018) suggest that the granitic magmas were instead derived by melting of a hybridised lower crustal source formed by mixing of primitive melts and melts of Greenland Group metasediment. The mafic nature of the Thirsty Creek Norite precludes an origin as a crustal melt, and therefore we suggest it represents a contaminated mafic melt emplaced during, and associated with, Rahu Suite granitic magmatism – indicative of mantle-derived melts providing both thermal energy and chemical components during melting in a deep crustal hot zone (e.g. Annen et al. 2006). The overlap in isotopic compositions of the Thirsty Creek Norite and the Rahu Suite granites would then suggest that the isotopic compositions of the entire suite were established at an early stage and the development of more evolved granitic compositions must have occurred as a closed system with limited interaction with the upper crust (e.g. Greenland Group metasediment). The Thirsty Creek Norite therefore provides constraints on the compositions of any primitive mafic components that may have been involved during Rahu Suite granitic magmatism. Mixing between a typical depleted MORB-mantle like mafic end member (i.e. ⁸⁷Sr/⁸⁶Sr = 0.703, εNd = +12) and Greenland Group crust (not shown) requires large percentages (c. 50%) of the crustal component to reproduce Thirsty Creek isotopic compositions. If we assume relatively low SiO₂ contents of 45% for the mafic component (a primitive basalt) and 61% for the crustal components (e.g. average bulk continental crust of Rudnick and Gao 2014) – such a bulk mix would have c. 53% SiO₂, which is considerably higher than observed. Instead, Sr and Nd isotopic compositions of Rahu Suite granitoids of the Hohonu Batholith and the Thirsty Creek Norite can be simplistically modelled as a c. 70:30 mix between a primitive mafic magma with more enriched isotopic compositions similar to the Separation Suite and a Greenland Group metasediment crustal component (see Waight, Weaver, Muir, Maas et al. 1998 and Figure 7). Assuming the same SiO₂ contents in the end-members – such a bulk mix would have c. 49% SiO₂, which is broadly similar to that observed in the Thirsty Creek Norite. Therefore, any mafic component involved in Rahu Suite magmatism must have had relatively enriched Sr and Nd isotope signatures, similar to or potentially even more enriched than on primitive Separation Point Suite compositions.

Conclusions

The Thirsty Creek Norite is a calc-alkaline biotite hornblende gabbronorite with evolved trace element patterns and isotopic compositions (⁸⁷Sr/⁸⁶Sr_(i) = 0.70739–0.70743, εNd_(i) = −5.9). Rb-Sr geochronology suggests a minimum age of 107.1 ± 4.1 Ma. While this age may represent a metamorphic and/or regional cooling age, lack of alteration and deformation in primary biotite in the Thirsty Creek Norite could also suggest that this age represents magmatic crystallization. If so, the age suggests emplacement shortly after or in direct relation to the contemporaneous anatectic event in the Granite Hill Complex (Hiess et al. 2010). The potentially similar crystallization age, coupled with overlapping initial radiogenic isotopic compositions, suggests that the Thirsty Creek Norite can be correlated to the mid-Cretaceous Rahu Suite granitoids of the Hohonu Batholith. Any petrogenetic relationship between these granitoids and the Thirsty Creek Norite is complicated by the wide offset in silica contents coupled with overlapping isotopic compositions. A cumulate origin is considered unlikely based on a lack of clear cumulate textures or geochemical signatures. It is concluded that the Thirsty Creek Norite could represent a contaminated mafic equivalent to the mid-Cretaceous Rahu Suite granitoids of the Hohonu Batholith. This would suggest that any mafic components involved were derived from relatively enriched mantle sources with evolved Sr and Nd isotope signatures, and that subsequent evolution from mafic to more evolved granitic compositions occurred as a closed system with limited interaction with upper crustal compositions.

Acknowledgements

We thank Siw Amanda Falk Egdalen for providing mineral chemistry data. Steve Weaver kindly provided the geochemical data for sample SDWRHN. The assistance of Toni Larsen in the chemistry lab and Toby Leeper in the TIMS lab was greatly appreciated. This study is part of the Bachelor’s Thesis of SEK at University of Copenhagen. Rose Turnbull, Paul Martin Holm, Andy Tulloch and an anonymous reviewer are thanked for their comments on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Sarah Estela Kylborg http://orcid.org/0000-0002-4474-7910

Tod Waight http://orcid.org/0000-0003-2601-1202

Quinten van der Meer http://orcid.org/0000-0002-6077-9496