Chapter 13: Permian–Triassic felsic tuffs in South Island, New Zealand: significance for oceanic and active continental margin subduction
Published:May 08, 2019
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Alastair H. F. Robertson, Romesh Palamakumbura, Hamish J. Campbell, 2019. "Permian–Triassic felsic tuffs in South Island, New Zealand: significance for oceanic and active continental margin subduction", Paleozoic–Mesozoic Geology of South Island, New Zealand: Subduction-related Processes Adjacent to SE Gondwana, A. H. F. Robertson
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Felsic tuffs play an important role in the Permian–Triassic geology of the Eastern Province in South Island. In the Brook Street Terrane, primary felsic tuff is minor in the south (e.g. Takitimu Mountains) but abundant in the north (Grampian Formation, Nelson area). Felsic fallout tuff dominates one interval of the Maitai Group (Early Triassic Kiwi Burn Formation), south of the Alpine Fault, but is otherwise mainly redeposited by gravity flows. The Murihiku Terrane is characterized by two main intervals of felsic fallout tuff, the Middle Triassic Gavenwood Tuffs and the Late Triassic Bare Hill Tuff Zone, south of the Alpine Fault (e.g. Hokonui Hills and south Otago coast). Counterparts north of the Alpine Fault (Richmond Group) are mainly reworked, with terrigenous admixtures. Tuffaceous sediments are also abundant in the late Middle–early Late Triassic Willsher Group (south Otago coast). Based on combined field, petrographical, semi-quantitative X-ray diffraction (XRD) and chemical evidence, the felsic tuffs of the Brook Street Terrane in the south are interpreted as small-scale eruptions of fractionated oceanic-arc-type magmas. In contrast, the Triassic felsic tuffs of the Murihiku Terrane, Willsher Group and Maitai Group erupted violently and episodically in proximal to distal segments of the SE Gondwana continental margin.
Supplementary material: Supporting petrographic and geochemical data are available at https://doi.org/10.6084/m9.figshare.c.4407866
Felsic tuffaceous rocks occur within mainly volcaniclastic sequences of four different tectonic–stratigraphic units of overall Permian–Triassic age in South Island. These deposits are used here to help understand the origin and geological significance of both oceanic and continental-margin-type arc magmatism, which played a key role in the geological development of SE Gondwana (Fig. 13.1, inset). The geochemistry of basic extrusive igneous rocks has been used extensively to help infer the tectonic settings of eruption (e.g. Pearce et al. 1984; Pearce 1996). Felsic tuffs provide fewer direct chemical indications as to the tectonic setting of eruption. However, they encode much valuable information, especially concerning eruptive processes and provenance (Sigurdsson et al. 1980; Heiken 1991; Scudder et al. 2016).
There is current interest in the volcanological processes and eruptive settings of deep-marine volcanogenic sediments, both in the modern oceans and in land exposures (Cambray et al. 1993, 1995; Carey & Sigurdsson 2000; Straub 2003; Scudder et al. 2009, 2014; Carey et al. 2011; Schindlbeck et al. 2018). An excellent example is the Eocene–Recent Izu–Bonin arc in the NW Pacific region, where a combination of whole-rock chemical analysis of background tuffaceous sediments (Robertson et al. 2018) and tephra chemical analysis (Bryant et al. 2003; Kutterolf et al. 2018) allows the identification of both oceanic-arc and continental margin-arc settings. Comparable results have been obtained from several ancient continental margin settings on land, notably Central America (e.g. Hannah et al. 2002; Kutterolf et al. 2008a, 49_CH13C56b, 49_CH13C57c) and Japan (Yoshida 2001; Yoshida et al. 2005). Here we use a combination of stratigraphy, sedimentology, optical petrography, semi-quantitative X-ray diffraction (XRD) and geochemical analysis to help determine the eruptive setting, depositional processes and provenance of tuffaceous rocks in South Island. The tuffs occur within the Permian–Jurassic Brook Street Terrane, the Permian–Triassic Dun Mountain–Maitai Terrane, the Permian–Cretaceous Murihiku Terrane and also within a small, enigmatic Mid–Late Triassic outcrop known as the Willsher Group (see Mortimer et al. 2014) (Fig. 13.1).
The main lithologies considered here are: first, felsic (siliceous) tuff, which represents tephra fallout that was variably affected by seafloor reworking; and, secondly, tuffaceous sedimentary rocks, which typically contain variable proportions of felsic tephra and epiclastic (secondary) volcanic detritus. The felsic tuffaceous rocks are commonly interbedded with volcaniclastic sedimentary rock, of variable grain size, in which most of the material is of epiclastic origin, together with basic to felsic extrusive rocks. Unless otherwise indicated, all of the felsic tuff discussed here is fine grained (<0.062 mm), although where these rocks are strongly altered by diagenesis and metamorphism their primary grain size can be indeterminate.
Regional setting of tuffaceous sequences
We will first consider felsic tuffs within the Permian–Jurassic Brook Street Terrane, which comprises about half a dozen variably sized outcrops in South Island, both north and south of the Alpine Fault (Fig. 13.1). Prior to the 480 km of right-lateral displacement during the later Cenozoic (e.g. Sibson et al. 1981), these widely separated outcrops were contiguous and can be interpreted together. However, where the contacts between the different outcrops are faulted, more than one Permian arc unit is likely to be present. Based mostly on lithology (Houghton 1981, 1985; Houghton & Landis 1989), geochemistry (Spandler et al. 2005; Nebel et al. 2007) and isotopic data (Frost & Coombs 1989; Nebel et al. 2007), it is generally believed that the Brook Street Terrane, at least the outcrops south of the Alpine Fault, formed within the Panthalassa Ocean above a westwards (continentwards)-dipping subduction zone (see Robertson & Palamakumbura 2019a). Questions remain concerning the location of the arc while magmatically active, especially whether it was in a remote or nearby position relative to the SE continental margin of Gondwana. The outcrops to the north of the Alpine Fault differ in stratigraphy and possibly in age (see below). Interbedded volcaniclastic sedimentary rocks there have the chemical signatures of continental margin-arc volcanism (Robertson & Palamakumbura 2019a).
The second felsic tuffaceous rock occurrence considered here is within the Permian–Triassic Maitai Group, which is a very thick (c. 6000 m) deep-marine sequence of Middle Permian–late Early Triassic age (Landis 1969, 1974; Kimbrough et al. 1992; Owen 1995; Campbell & Owen 2003; Robertson & Palamakumbura 2019b, 49_CH13C81c); this unconformably overlies the late Early Permian–Middle Permian (c. 278–269 Ma) supra-subduction zone Dun Mountain ophiolite and associated oceanic-arc rocks (Davis et al. 1980; Sinton 1980; Jugum 2009; Jugum et al. 2019). The Permian–Triassic succession is dominantly volcaniclastic and is known to contain felsic tuffaceous rocks at various stratigraphic levels in several different areas (Landis 1969, 1974; Johnston 1981, 1982; Aitchison et al. 1988; Aitchison & Landis 1990; Owen 1995; Stratford et al. 2004). The Maitai Group is widely interpreted as a continental margin forearc basin (Carter et al. 1978; Owen 1995; Robertson & Palamakumbura 2019b, 49_CH13C81c). However, it has also been interpreted as an oceanic-arc terrane (Aitchison et al. 1988; Aitchison & Landis 1990) that was potentially separated from SE Gondwana by a back-arc marginal basin. The felsic tuffaceous rocks within the Maitai Group shed light on such alternatives, including the nature and timing of arc magmatism along the SE Gondwana active continental margin.
The third tuffaceous rock occurrence is within the Murihiku Terrane, which is located between the Brook Street Terrane to the west and the Dun Mountain–Maitai Terrane to the east (Fig. 13.1). The Murihiku Terrane, which is of Late Permian–Early Cretaceous age as a whole (H.J. Campbell et al. 2003), includes abundant felsic tuffaceous rocks at two main levels in the Triassic succession (Boles 1971, 1974; Roser et al. 2002). The Murihiku Terrane is alternatively interpreted as part of a forearc basin bordering SE Gondwana (Carter et al. 1978; Noda et al. 2002), or as the southerly part of an off-margin-arc terrane founded on oceanic crust (Adams et al. 2009).
The final tuffaceous occurrence discussed here occurs within a small fault-bounded sequence of mudrocks, sandstones and felsic tuffaceous rocks of late Middle–earliest Late Triassic age, which is exposed along the south coast. Known as the Willsher Group or the Kaka Point Structural Belt, this succession is sandwiched between the Dun Mountain–Maitai Terrane to the east and the Murihiku Terrane to the west (H.J. Campbell 1996; J.D. Campbell et al. 2003) (Fig. 13.1). This distinctive succession has been correlated with the Murihiku Terrane (Roser & Coombs 2005) or with the Dun Mountain–Maitai Terrane (Turnbull & Allibone 2003), and has also been interpreted as a possible exotic microterrane (J.D. Campbell et al. 2003), alternatives which are tested here with new geochemical data.
Stratigraphy and sedimentology
Brook Street Terrane
Three accessible outcrops were chosen for study: first, northwesterly and northeasterly exposures in the Takitimu Mountains; secondly, the East Eglinton–Hollyford outcrop (close to the Alpine Fault); and, thirdly, the type area of the Brook Street Terrane, near Nelson city.
Takitimu Mountains (locations 1 and 2)
The volcanogenic succession is estimated to be up to 15 000 m thick, of which c. 10 000 m are volcanic and related volcanogenic rocks, and the remainder is intrusive rocks, mainly diorite (Houghton 1977, 1981) (Figs 13.2 & 13.3a). In general, the lower part of the succession is mostly basaltic (Brunel Formation), whereas the mid parts (Chimney Peaks, Heartbreak and MacLean Peaks formations) have abundant basic and felsic volcanic rocks. The mid part of the succession in the high mountains was not accessible during his work. The upper part of the sequence (Elbow Formation) shows a return to more basic volcanic rocks and related volcanogenic sedimentary rocks (Houghton 1977, 1981). Seven discrete volcanogenic facies have previously been recognized in the Takitimu Mountains (Houghton 1977, 1981; Houghton & Landis 1989; see the Supplementary material text for further details). The main outcrop in the west is separated by a high-angle fault (Tin Hut Fault) from the highest levels of the volcanogenic succession in the east (Caravan Formation), which are dominated by basaltic rocks (Landis et al. 1999). Further east, in the Wairaki Hills (Fig. 13.3a), the basaltic rocks are depositionally overlain by redeposited shallow-water carbonates with volcaniclastic intercalations (Productus Creek Group: Fig. 13.2, location 1). ‘Tuffaceous’ intercalations (Force 1975; Landis et al. 1999,) within the Productus Creek Group were found to be dominated by epiclastic volcanic debris (Fig. 13.4c) and are, therefore, not considered further here (see Robertson & Palamakumbura 2019a).
The present work in the Takitimu Mountains–Wairaki Hills area focuses on scarce, fine-grained, pale felsic tuffaceous rocks that were studied in two sections.
First, samples were collected from near the exposed base of the Brunel Formation on Tower Peak Estate (Coal Creek) in the NW Takitimu Mountains (location 1, Fig. 13.3ai). This outcrop is >90% basalt and andesite, with occasional lenticular rhyolite layers (up to 10 m thick by several hundred metres long). The felsic intervals include thin layers of felsic tuff, up to tens of centimetres thick (Fig. 13.4a). Occasional, interbeds of green, laminated, normal-graded mudrock (<10 cm thick) include ‘ghosts’ of radiolarians. The relative abundance of felsic tuffaceous rocks on a much larger scale was estimated by a study of float boulders in nearby Whare Creek and Redcliff Creek, which exclusively drain the Brunel Formation. Both creeks contain <1% of very hard, indurated white to pale grey, fine-grained massive or parallel-laminated rhyolitic tuff, similar to the exposures at locality 1. There are also very rare clasts of sand-sized rhyolitic tuff. Felsic tuff therefore appears to represent only a very small fraction of the Brunel Formation.
Secondly, felsic tuffs were collected from the higher levels of the Elbow Formation, within the Princhester Creek drainage catchment in the NE Takitimu Mountains (location 2: Figs 13.3aii & 13.4b). Observations of float boulders suggest that felsic tuffs are again rare within this formation.
Overall, the Takitimu Mountains succession is interpreted, based mainly on petrological and geochemical evidence, as an Early Permian oceanic-type arc (Houghton 1977, 1981; Houghton & Landis 1989; Spandler et al. 2005; Nebel et al. 2007; see also Robertson & Palamakumbura 2019a).
East Eglinton–Hollyford outcrop (location 3)
The succession (Williams 1975, 1978; see Raine et al. 2015) (Figs 13.2 & 13.3b) begins with dominantly coarse-grained, poorly sorted, mostly volcaniclastic rocks of basaltic composition, including clinopyroxene-phyric facies (Gondor Formation: up to 1000 m thick). A persistent limestone intercalation (Melita Limestone Member: c. 100 m thick) contains the Permian bivalve Atomodesma sp. The succession passes upwards into well-indurated, greenish-grey feldspar-rich volcaniclastic sandstone, siltstone and mudrock (i.e. Consolation Formation: up to c. 1200 m thick); this includes an interval of well-bedded volcaniclastic siltstone (i.e. Kaka Creek Siltstone Member: up to 100 m thick). The sedimentary rocks become more greenish and feldspar-rich upwards, with occasional chlorite-rich tuffaceous sandstone and siltstone interbeds (i.e. Divide Formation: up to c. 1000 m thick) (Fig. 13.4d). Normal-graded beds with local sole marks and other sedimentary structures are indicative of turbidity current deposition. Greyish-green to greyish feldspathic sandstones that were similarly deposited by gravity flows form the highest levels of the succession (i.e. Fergus Formation: up to c. 350 m thick).
The lower part of the succession (Gondor Formation) is dominated by redeposited basaltic and basic pyroclastic material from a nearby eruptive centre (Williams 1975, 1978). The higher levels of the formation mostly represent erosive products of basaltic, to andesitic, to locally felsic volcanics, which accumulated in a deep-water arc-margin setting. Volumetrically minor reworked felsic tuff was studied here. Samples were collected from the easily accessible middle to upper part of the succession, east of Marion Creek Bridge (Consolation, Divide and Fergus formations), near the main road from Te Anau to Milford Sound. Interbedded fine-grained volcaniclastic sandstone (Kaka Creek Siltstone) was also sampled for comparison.
Nelson city (location 4)
To the north of the Alpine Fault, the succession in the type area of the Brook Street Terrane, as exposed in Nelson city (Figs 13.2 & 13.3c), begins with tuffaceous sandstone–siltstone, together with subordinate intercalations of felsic tuff and basalt-andesite (i.e. Grampian Formation: c. 1000 m thick). The succession passes upwards into predominantly basaltic lava flows and pyroclastic lava breccias (i.e. Kaka Formation: >1500 m thick). The upper part of the succession is well-bedded, greyish-coloured volcaniclastic sandstone, siltstone and mudrock (Groom Creek Formation: >1200 m thick) (Johnston 1981; see also Robertson & Palamakumbura 2019a).
The lowest levels of the succession are well exposed in Maitai River (near Gibbs Bridge, Jickell's Bridge and in an adjacent small quarry) (see Robertson & Palamakumbura 2019a for locality details), where they are dominated by normal-graded, medium- to thick-bedded volcaniclastic sandstone–siltstone, with planar and low-angle cross-lamination. Many of the beds have pale felsic tuffaceous tops. There are also discrete interbeds of pale felsic tuff-rich sandstone and shale, up to 0.6 m thick (Fig. 13.4e, f). Higher levels of the succession (exposed in nearby Grampian Reserve) begin with medium- to thick-bedded, coarse-grained volcaniclastic sandstones, pale felsic tuff and poorly exposed basalt flows, estimated as 3–4 m thick. These are followed by c. 20 m of soft white felsic tuff, grey semi-porcellanous tuff and dark grey vitric tuff, forming up to c. 1 m-thick interbeds. Higher in the succession, volcaniclastic sandstone–siltstone and mudrock are intercalated with a prominent, laterally persistent layer (c. 3–5 m thick) of white powdery felsic tuff and grey vitric tuff (porcellanite), which is exposed near (and along) the crestal ridge of the Grampian Reserve. Individual tuff layers range from massive units to multiple thin-bedded, parallel-laminated units. The higher levels of the formation, which are well exposed in Flaxmore/York Quarry, are dominated by well-bedded volcaniclastic sandstone turbidites, without homogeneous felsic tuff layers.
Felsic tuffs were collected for chemical analysis from the Grampian Formation along the Maitai River and in the adjacent Grampian Reserve (Fig. 13.3c). Reconnaissance of the overlying Kaka Formation (e.g. Brook Street motor camp area) revealed dominantly coarse fragmental basaltic rocks. In addition, study of the overlying Groom Creek Formation, as exposed in Maitai River and Hira Forest (Lud Valley) (see Robertson & Palamakumbura 2019a), indicated the presence of fine- to medium-grained felsic sandstone–siltstone turbidites, as <10 cm-thick interbeds. Many beds exhibit planar lamination, normal grading and small rip-up clasts. Samples were collected to determine whether these rocks contain significant amounts of felsic tuff.
Maitai Group (locations 5 and 6)
Felsic tuffaceous rocks of Early–Middle Triassic age were studied from the upper part of the succession of the Maitai Group (Stephens Subgroup), from both south and north of the Alpine Fault (Fig. 13.5a, b). Sparse tuffaceous rocks in the underlying Permian levels of the Maitai Group were mostly redeposited by gravity flows and are not considered here (see Robertson & Palamakumbura 2019b, 49_CH13C81c).
Bare Peak (location 5)
The basal part of the succession of the Stephens Subgroup (c. 150 m thick) was termed the Kiwi Burn Tuffs by Aitchison et al. (1988) and treated as one of the formations of the Early–Mid Triassic Stephens Subgroup. Although this interval is highly tuffaceous in the Livingstone Mountains-type area, volcaniclastic siltstone, siltstone and minor conglomerate are also present (Landis 1974). The correlative interval to the north of Alpine Fault in the Wairoa–Lee River area (Owen 1995) is dominated by volcaniclastic sandstone and includes exotic limestone blocks (see Robertson & Palamakumbura 2019b). For this reason, the tuffaceous interval in both regions is termed the Kiwi Burn Formation, in line with other formations of the Stephens Subgroup.
In the Countess Range, the Kiwi Burn Formation is dominated by pale, fine- to medium-grained vitric felsic tuff (Fig. 13.5a), interbedded with volcaniclastic sandstone, siltstone, mudstone and minor conglomerate. Lapilli tuffs are reported locally (Aitchison et al. 1988). Felsic tuffs are also widespread towards the top of the succession (Snowdon Formation): for example, along the western side of the Countess Range (Aitchison et al. 1988). During this study, pale, homogeneous, massive or crudely stratified, medium-grained felsic tuff was collected for chemical study from the eastern flank of Bare Peak, north of Mararoa River (Fig. 13.5a). Several samples collected by C. Landis from the Countess Range were included in the study (from the Otago University collection).
Wairoa–Lee River area (location 7)
North of the Alpine Fault, sandstone–siltstone turbidites of the Stephens Subgroup, which are widely exposed in the Wairoa–Lee River area, are commonly rich in reworked felsic tuff (Fig. 13.2). Andesitic crystal–vitric tuff has been reported locally (e.g. Delaware Bay), north of Nelson (Owen 1995). In the Wairoa River area, tuffaceous-looking sandstones–siltstones, which are correlated with the upper part of the Kiwi Burn Formation (Owen 1995), form the matrix of a sedimentary melange (olistostrome); this contains detached blocks of mostly Permian shallow-water limestone (Martins Olistoliths: Johnston 1982; Owen 1995; see Robertson & Palamakumbura 2019b). Samples were collected from the extensive Pig Valley Quarry (Figs 13.5b & 13.6e) to test the inference that the matrix of the exotic limestone blocks is made up of tuffaceous sandstone (Owen 1995).
Interbeds and partings of pale tuffaceous-looking sandstone–siltstone and shale also occur within the overlying sand-rich Acheron Lakes Formation (Figs 13.1 & 13.6b); this also shows spectacular evidence of soft-sediment deformation (Fig. 13.6c). The overlying highly distinctive reddish-coloured, relatively fine-grained Cerberus Formation (Fig. 13.2) includes felsic tuffaceous interbeds. A medium-thickness bed of pale tuffaceous sandstones was sampled downstream from the Lee River Valley Reserve (Fig. 13.5b), and also in a disused quarry along Eyles Road, east of Stoke–Richmond (Fig. 13.6d).
The uppermost levels of the Stephens Subgroup (Chrome Creek Formation) include abundant vitric, crystal and lithic tuff (Owen 1995), which is mostly reworked within gravity-flow deposits. Samples were collected, first, from the Wairoa River (off Irvine Road), where the local succession comprises grey, thin- to medium-bedded, fine- to medium-grained, very hard tuffaceous sandstone–siltstone, and, secondly, from Wairoa Gorge, where regularly bedded, fine-medium grained, normal-graded tuffaceous sandstones and siltstones predominate (Fig. 13.5b). Other interbeds include calciturbidites (up to 25 cm thick) and dark organic-rich calcareous siltstones, both with parallel lamination and micro-cross-lamination.
Four sections with tuffaceous sediments were investigated in the Murihiku Terrane (Murihiku Supergroup), three to the south and one to the north of the Alpine Fault.
Gavenwood Tuffs, Hokonui Hills (location 8)
The Gavenwood Tuffs unit (64–100 m thick), of probable Middle Triassic (Anisian) age (H.J. Campbell et al. 2003), crops out in the northern part of the Hokonui Hills (Fig. 13.7a), within the Early–Middle Triassic North Range Group. The Gavenwood Tuffs unit was sampled from a steeply-dipping, east-younging sequence in a road cutting at Ram Hill (Fig. 13.8b). The tuffaceous interval is underlain by thick-bedded, medium- to coarse-grained sandstone turbidites and mass-flow deposits. Several sandstone beds contain shale rip-up clasts, up to several centimetres in size. The main tuffaceous interval (Fig. 13.6f) culminates in thin- to medium-bedded alternations of tuffaceous sandstone–siltstone, followed by a return to mostly medium- to thick-bedded gravity-flow deposits, typical of the North Range Group.
Late Triassic Bare Hill Tuff Zone, Hokonui Hills (location 9)
Prominent tuffaceous rocks, known as the Bare Hill Tuff Zone (80–160 m thick), are well exposed further SE in the Hokonui Hills (Boles 1971, 1974), where they are dated as Late Triassic (Early Norian (=Oretian)) (H.J. Campbell et al. 2003). The tuffs were sampled along a dirt road over Bare Hill to the west of Gore (Fig. 13.7a). The exposure (Fig. 13.8b) is inadequate to allow bed-by-bed logging. However, the succession begins with thick-bedded sandstone turbidites, followed by c. 35 m of pale-coloured felsic tuffs, which include interbeds of vitric tuff and occasional poorly exposed sandstone turbidites. The tuffaceous interval is overlain by dark-coloured, fine- to medium-grained, medium-bedded sandstone turbidites.
Mid–Late Triassic tuffaceous rocks, Roaring Bay (location 10)
Middle Triassic (Ladinian–Carnian = Kaihikuan) tuffaceous sediments are also well exposed along the south coast, near Nugget Point (Campbell 1996). Lapilli and crystal tuff are interbedded with volcaniclastic sandstone turbidites on the coastal platform and in sea stacks. Variably altered tuffaceous interbeds (up to c. 1 m thick) were sampled near the top of the succession in a road cutting (Parks Cutting) above the beach (Fig. 13.9). This outcrop is strongly deformed, with small duplexes, shear structures and small-scale high-angle faults. Andesitic crystal tuff (tuffite) from this locality yielded a U–Pb zircon age of c. 237 Ma (Kaihikuan) (D. Kimbrough in Campbell 1996). The base of the Late Triassic early Norian (Oretian) sequence (Taringatura Group) is recognized higher on the coastal slope (Campbell 1996).
The Late Triassic (early–middle Norian = Oretian–Otamitan) (H.J. Campbell et al. 2003) is well exposed in Roaring Bay, c. 500 m SW of Parks Cutting (Campbell 1996) (Figs 13.7a & 13.9). Several tuffaceous intervals are interbedded with a dominantly siltstone–sandstone turbiditic sequence, as exposed along the beach and in the adjacent undercliff (Figs 13.6g–j & 13.8, location 10). Near the base of the tuffaceous interval, a prominent white felsic tuff (c. 0. 9 m thick) exhibits normal grading, parallel lamination, small (<2 cm) rip-up clasts, scour structures, convolute lamination and small-scale soft-sediment deformation structures. There is evidence of dewatering and local sediment injection. A stratigraphically higher, thicker tuffaceous interval (c. 3.5 m thick), which is well exposed in the adjacent undercliff, is underlain by medium- to coarse-grained tuffaceous sandstone and overlain by pebblestone. This tuffaceous bed has a coarse-grained sandy base and is mainly parallel laminated. The uppermost 1 m of the bed exhibits wavy and disturbed bedding. Up-section (c. 60 m) another prominent tuffaceous interval (c. 6 thick) encompasses up to 12 normal-graded depositional units, each <1.8 m thick. Paler and coarser tuff layers alternate with darker and finer-grained partings of tuffaceous mudstone. The Oretian–Otamitan-aged succession exposed in Roaring Bay includes very thick (up to c. 20 m) conglomeratic mass-flow intervals with rounded boulders and cobbles of granitic rocks (Campbell 1996).
Richmond area (location 11)
North of the Alpine Fault, the Murihiku Terrane is exposed in a narrow (c. 2 km), fault-bounded, north–south-trending strip, known as the Richmond Group (equivalent to the Taringatura Group) (Campbell & Coombs 1966) (Fig. 13.7b). The succession encompasses Middle–Late Triassic zeolite-facies volcaniclastic conglomerates, sandstones, siltstones, mudstones and tuffs. The sandstones and shales are commonly tuffaceous, although primary fallout tuff is rare. Where locally present, conglomerates include basaltic to felsic volcanic clasts and some granitic clasts (Johnston 1982; Owen 1995). Chemically, the basaltic clasts are indicative of a volcanic-arc origin (Robertson & Palamakumbura 2019c). The felsic tuffaceous rocks mainly occur in the middle and upper levels of the succession, which is subdivided into eight stratigraphic units (Johnston 1982; Owen 1995; see the Supplementary material text). The most tuffaceous of the exposures are strongly weathered. However, relatively unaltered felsic tuffaceous sandstone–siltstone (Garden Formation) was collected from near Champion Road (along Eyles Road), especially behind a prominent water tank (Fig. 13.7, location 11). Pale tuffaceous intervals are typically <1 m thick, individually in road cuttings. A 20 m-thick interval of pale tuffaceous sediment (exposed behind the water tank) is thickened by high-angle faulting (Fig. 13.6k; see also Supplementary Fig. S3). The interbedded background sediments are grey-weathering, thin- to medium-bedded sandstone turbidites and silty mudstones, rich in plant debris. Macrofossils in the vicinity, along with the stratigraphic position, suggest a Late Triassic (middle Norian = late Otamitan age).
Taking the tuffaceous deposits of the Murihiku Terrane as a whole, three main types of ash occur: vitric tuff, vitric–crystal tuff and less abundant crystal tuff (Coombs 1950; Wood 1956; Boles 1971, 1974; Owen 1995; this study). Primary fallout tephra is variably replaced by secondary minerals, notably laumontite, analcime, feldspar, heulandite and smectite (Boles & Coombs 1975; see the Supplementary material text). The ash layers range from repeated thin beds (0.5–5 cm thick) to isolated very thick beds (up to 5 m thick). Normal grading is locally well-developed. There is local evidence of soft-sediment deformation and slumping. Where present, parallel lamination is picked out by shards, opaque grains or plant fragments (including small leaves), especially in the North Range Group (Coombs 1950). Cross-lamination structures are occasionally observed (Boles 1971, 1974).
Willsher Group (location 12)
Felsic tuffs are well exposed within an east-younging, up to 2000 m-thick succession (Willsher Group) on the south coast, NW of Nugget Point (Jeans et al. 1997, 2003; J.D. Campbell et al. 2003) (Figs 13.7a & 13.9). The felsic tuffs occur within the relatively deformed rocks of the Kaka Point Structural Belt (J.D. Campbell et al. 2003), which is separated by steep faults from the Dun Mountain–Maitai Terrane to the north and from the Murihiku Terrane to the south (Fig. 13.9). The succession is dated as late Early Triassic – early Late Triassic (Olenekian–Anisian–Carnian = Nelsonian–Kaihikuan), based on diverse assemblages of ammonoids, brachiopods, bivalves, gastropods, radiolarians, agglutinated foraminifera and plant spores (H.J Campbell 1996; J.D. Campbell et al. 2003; H.J. Campbell & Owen 2003; Hori et al. 2003). The tuffaceous deposits range from millimetre-thick partings to several-metre-thick interbeds (Fig. 13.6l), within a succession of dominantly volcaniclastic mudrocks; there are also several intercalations of mostly fine-grained sandstones (Fig. 13.9). More than 300 tuffaceous beds (bentonites) have been recorded within the c. 500 m-thick section (Jeans et al. 2003). Most of the tuffaceous beds exhibit normal grading, and some are represented by amalgamations of repeated coarser and finer-grained layers within overall fining-upwards packages. The tuffaceous deposits range from soft, diagenetically altered claystones to hard porcellanites. The porcellanites variably contain analcime, quartz and heulandite, together with occasional pale albite-rich beds and pale calcitized ash layers, based on XRD analysis (Jeans et al. 2003).
The poorly exposed lower part of the succession (Potiki Siltstone: Fig. 13.9), dated as late Early Triassic Olenekian–Middle Triassic Anisian (J.D. Campbell et al. 2003), contains only a few tuffaceous layers. Above this, sporadic beds of grey, medium-grained massive tuff (c. 0.6 m thick) and also dark grey medium-grained, normal-graded and planar-laminated tuff are well exposed, just north of Kaka Point (Fig. 13.9). Thin (<5 cm) white layers, interpreted as altered fine-grained ash, can be traced individually for >10 m on the wave-cut platform, extending westwards for up to c. 200 m. Further north (c. 0.7 km north of Bates Beach), a prominent, 2.2 m-thick fine-grained tuff layer (near an outlet pipe) exhibits a massive base and vague parallel lamination, together with six additional felsic tuff layers. Overlying buff-grey shales, c. 180 m thick, contain sporadic tuffs, mostly <10 cm thick. Further north (Tilson Beach), an interval of uniform mudstone mostly lacks tuff. Above this, relatively massive muddy siltstones (Port Molyneux Siltstone) contain additional clay-rich tuffs. The background sediments of these uppermost sediments include sparse brachiopods and bivalves, indicative of a Middle Triassic Ladinian–Carnian (=Kaihikuan) age (H.J. Campbell et al. 2003; J.D. Campbell et al. 2003). During this study, representative samples of mainly medium- to fine-grained, pale, relatively homogeneous tuff were collected from the lower and upper levels of the overall succession.
Previous petrographical studies of tuffaceous sedimentary rocks focused on their composition and low-grade metamorphism: that is, the Brook Street Terrane (Williams 1975; Houghton 1977), the Maitai Group (Landis 1969, 1974; Owen 1995), the Murihiku Terrane (Boles 1971, 1974; Owen 1995) and the Willsher Group (J.D. Campbell et al. 2003; Jeans et al. 1997). During this work, 62 thin sections of tuffaceous rocks were studied under the optical microscope, particularly to distinguish between fallout tuff and redeposited (epiclastic) gravity-flow deposits.
Petrography: Brook Street Terrane
Felsic tuffaceous rocks from the lower part of the succession (Brunel Formation) in the NW Takitimu Mountains (Tower Peak Estate: location 1) are composed of very fine-grained felsic tuff with scattered small, angular, colourless quartz grains (mostly shards) and rare plagioclase crystals, set in a very fine-grained quartzose matrix. This sediment is interpreted as primary fallout tuff (Fig. 13.10a, b). Other felsic samples are epiclastic, with well-sorted grains of quartz, feldspar and scattered biotite (Fig. 13.10c). Compositionally basic samples are also epiclastic, with basalt, devitrified basic volcanic glass, plagioclase, clinopyroxene, scattered angular quartz grains and opaque grains (e.g. magnetite). Occasional associated thin (<15 cm) siliceous interbeds contain numerous radiolarian tests that are infilled with finely crystalline drusy quartz in a very fine siliceous matrix (Fig. 13.10d). Radiolarian spines and siliceous sponge spicules are occasionally preserved.
Tuffaceous samples from the upper part of the succession (Elbow Formation) in the NE Takitimu Mountains (Princhester Creek: location 2) are dominated by greenish chloritic, volcanogenic material (Fig. 13.10e). Typical fine-grained, parallel-laminated tuffaceous siltstones contain mostly sub-angular to subrounded grains of basalt, andesite (with flow-banded plagioclase), hyaloclastite (altered basic volcanic glass) and altered plagioclase, together with rare quartz and clinopyroxene. Grains range from mostly sub-angular to mostly subrounded in different thin sections. Pale laminae and partings are commonly plagioclase-rich. Rare felsic tuffs are dominated by tiny angular quartz and scattered feldspar grains.
A sample (Otago University OA 35341, collected by J. Williams) from the Gondor Formation in the East Eglinton–Hollyford outcrop (Fig. 13.3b) includes two thin layers. The first (Fig. 13.10f) is reworked crystal tuff with abundant relatively unaltered clinopyroxene and scattered highly altered feldspars. There are also numerous subrounded grains of slightly recrystallized aphyric or sparsely feldspar-phyric basalt, set in a fine- to medium-grained mafic volcaniclastic matrix (partly recrystallized). The second layer is bioclastic micritic limestone with abundant, randomly orientated atomodesmatinid (bivalve) prisms and occasional larger shell fragments, together with scattered small grains of clinopyroxene and plagioclase. Felsic fallout tuffs are not reported from this lower formation (Williams 1975, 1978).
Tuffaceous rocks from the overlying Consolation Formation (Fig. 13.3b, location 3; near Marion Creek) are fine-grained, well-sorted epiclastic sandstone (micro-grainstone), with variable mixtures of mostly feldspar and quartz, together with rare clinopyroxene and abundant feldspathic extrusive igneous rock (basaltic andesite) (Fig. 13.10i). Some grains are subrounded, indicating reworking. The Kaka Creek Member (<100 m thick) is made up of a mixture of moderately well-sorted, angular to sub-angular grains of plagioclase, clinopyroxene, hornblende (as scattered crystals) and opaque grains (magnetite) (Fig. 13.10i). There are also highly angular grains of quartz, interpreted as shards (tephra).
Normal-graded, massive or parallel-laminated siltstone and fine-grained sandstone form the overlying Divide and Fergus formations (Fig. 13.2, location 3). Some samples are basaltic with basic volcanic glass (hyaloclastite), variable amounts of plagioclase, and scattered grains of orthopyroxene, clinopyroxene and hornblende (Fig. 13.10g), whereas others are more andesitic with abundant flow-banded feldspar microphenocrysts. Felsic lithologies (Fig. 13.10h) contain abundant angular quartz, plagioclase and altered alkali feldspar, together with tiny rounded grains of altered volcanic glass, common biotite and sericite (white mica). One sample contains intraclasts of fine-grained tuff,
North of the Alpine Fault, pale layers that are exposed low in the succession (Grampian Formation) in the Maitai River (near Jickells bridge: Fig. 13.2, location 4) are represented by felsic tuff with abundant elongate shard-like quartz grains, mixed with abundant broken and disaggregated atomodesmatinid prisms (i.e. mixed tuff–carbonate rock). Subtle normal grading and lamination indicate that the tuffs have been reworked by low-density turbidity currents.
Dark vitreous or semi-porcellanous felsic tuff from the overlying, mid part of the succession (near the summit of the Grampians) is granular-textured, with ‘ghosts’ of elongate, randomly orientated glass shards (tephra), together with scattered crystals including volcanic quartz and plagioclase. Preferential grain alignment parallel to bedding, observed in one sample, is suggestive of current reworking (Fig. 13.10j).
Well-bedded sandstones from the upper part of the succession (York/Flaxmore Quarry) are, in contrast, epiclastic throughout (Fig. 13.10k). The overlying Kaka Formation is dominated by locally erupted basalt, basaltic breccia and hyaloclastite (Fig. 13.10l). Tuffaceous sandstone from the overlying mainly medium- to fine-grained Groom Creek Formation is made up of a variable mixture of reworked quartz, plagioclase clinopyroxene and highly altered fine-grained felsic (glassy) material (Fig. 13.11a). This lithology is comparable to some of the redeposited felsic tuffaceous sedimentary rocks of the Divide and Fergus formations in the East Eglinton–Hollyford outcrop.
Petrography: Maitai Group
Felsic tuff from Bare Peak (Kiwi Burn Formation: Fig. 13.5b, location 5) is characterized by a nearly random orientation of volcanic glass shards, suggesting fallout through the water column, with minimal reworking. Identifiable felsic tephra includes elongate (needle-like), cuspate and bubble-wall shards (Fig. 13.11b–e). There are also scattered grains of clinopyroxene, hornblende, volcanic quartz (euhedral grains), microcrystalline quartz (felsic volcanic material), altered plagioclase, orthoclase, muscovite and biotite.
Pale-coloured fine-, medium- and coarse-grained sandstones from Pig Valley Quarry (Richmond area: Fig. 13.5b) (Kiwi Burn Formation) are highly plagioclase-rich (arkosic), together with hornblende, quartz and intermediate to felsic volcanic grains (Fig. 13.11f).
Tuffaceous sediments from the overlying Cerberus, Acheron Lakes and Chrome Creek formations in the Wairoa–Lee River area (Fig. 13.5b) are mostly reworked, and commonly include muscovite laths of inferred terrigenous origin. A sample from the Wairoa River (off Irvine Road) has small, rounded, to elliptical grains of felsic tuff (enclosing tiny angular quartz grains), together with larger grains that include plagioclase, microcrystalline and monocrystalline quartz, mica and carbonate (largely replaced by secondary calcite spar) (Fig. 13.11g). A tuffaceous sandstone from the Chrome Creek Formation in Wairoa Gorge is very feldspathic, and includes abundant quartz shards, common biotite and scattered clinopyroxene grains, suggesting a mixed andesite–rhyolite composition.
Petrography: Murihiku Terrane
Typical fine-grained vitric tuff of the Gavenwood Tuffs from Ram Hill (Fig. 13.7a, location 8) is composed of abundant quartz, together with relict plagioclase and minor pyroxene, biotite, flow-banded rhyolite and other detrital grains, including muscovite. Several samples retain well-preserved tephra morphologies, including bubble-wall and flow-aligned pumiceous shards (Fig. 13.11k, l).
Interbedded tuffaceous sandstone comprises parallel-aligned, reworked volcanic grains, also common muscovite and biotite. Medium-grained, normal-graded pebblestone–sandstone beneath the tuffaceous interval is dominated by basaltic andesite with abundant flow-orientated feldspar microphenocrysts. Many grains are rounded or subrounded. There are also abundant angular grains of volcanic glass (zeolitized tephra). Small rip-up clasts of siltstone–mudstone are present as tiny angular grains.
The tuffaceous sediments from the overlying Bare Hill Tuff Zone (Fig. 13.7a, location 9) range from homogeneous fallout felsic tuff to variably reworked felsic tuff, interbedded with volcaniclastic gravity flow-deposits. Highly altered tephra includes bubble-wall and needle-like shards (Fig. 13.12c, f, g, h), together with angular streaky grains of flow-banded, altered rhyolite (pumice), scattered volcanic quartz and relict plagioclase. Other interbeds are epiclastic, with parallel-aligned grains of volcanic quartz, plagioclase, abundant sericite and secondary minerals (Fig. 13.12a, b).
A sample of tuffaceous sandstone from Parks Cutting on the south coast, of Ladinian (Kaihikuan) age (Fig. 13.9), is composed of vitric tuff, with platy and cuspate shards and some volcanic quartz grains (Fig. 13.12i). Samples of white felsic tuff from the overlying Late Triassic (Oretian–Otamitan) succession in the adjacent Roaring Bay (Fig, 13.7a, location 10) include abundant platy shards (Fig. 13.12j, k). Several samples comprise a mixture of felsic and andesitic debris and pumice (Fig. 13.11j), indicating a mixed epiclastic–pyroclastic origin.
The palest-coloured, most tuffaceous-looking samples in the (early Norian = Oretian) succession in the Richmond area (e.g. Fig. 13.7b, location 11) were found to be highly altered, reworked felsic tuff (Fig. 13.12l). No undisturbed felsic fallout tuff was observed in the Richmond Group generally.
Petrography: Willsher Group
Typical pale, normal-graded beds from the Willsher Group (Figs 13.7, location 12 & 13.9) consist of fine- to medium-grained felsic tuff, showing preferential alignment of platy and cuspate shards, and small clinopyroxene crystals (e.g. Tilson Siltstone from Bates Point) (Fig. 13.11h). Other samples are variably reworked, with abundant angular quartz grains, common plagioclase, altered tephra, altered basic- or intermediate-composition glass, plagioclase-rich flow-orientated andesite, biotite, opaque grains (pyrite), and also grains of relatively unaltered felsic extrusive rock (mostly rhyolite).
Semi-quantitative X-ray diffraction
To indicate the mineral content, semi-quantitative XRD analysis was carried out on representative felsic tuffaceous lithologies, using the method of McCusker et al. (1999) (see the Supplementary material). Values <1% are not considered to be significant.
The XRD data (see Supplementary material Table S1) highlight the relative abundances of the minerals in each of the units studied. Quartz occurs in significant abundance in all of the units, ranging from 11% in the Brook Street Terrane at Hollyford (location 3) to 50% in the Willsher Group (location 12). Feldspar minerals (albite, anorthite, microcline (maximum) and orthoclase) are significant constituents. Albite ranges from 2 to 56%, being highest in a sample from the Murihiku Terrane (Bare Hill Tuff Zone, location 9). Anorthite reaches 17% in another sample from the same unit, which also contains the highest recorded value of microcline (11%). Orthoclase reaches c. 4% in several samples (locations 4, 8, 9 and 11).
The clay minerals, illite_90144, kaolinite and chlorite llb occur in all samples. Illite_90144 is most abundant (14%) in the Grampian Formation (location 4), Brook Street Terrane. Kaolonite reaches 4% in the Gavenwood Tuffs (location 8), Murihiku Terrane. Chlorite llb is most abundant (5%) in two of the Brook Street Terrane samples (locations 3 and 4). Muscovite occurs in all samples, reaching levels of 6–7% in the Murihiku Terrane samples from the Gavenwood Tuffs (location 8) and the Richmond Group (location 11).
The zeolites, laumontite and analcime, are important constituents of some samples. Laumontite ranges from not detected to >37% in samples from the Maitai Group (locations 5 and 7), Murihiku Terrane (location 10) and Willsher Group (location 12). Analcime forms a key component (39%) of one sample, from the Taringatura Group, Murihiku Terrane in Roaring Bay (location 10).
Epidote is abundant in only one sample (33%), from the Brook Street Terrane at Hollyford (location 3).
Of the carbonates, calcite, dolomite and siderite are present at <1% levels in a few samples. Other minerals, rutile, anatase, pyrite, gypsum and ankerite, also occur rarely at <1% levels.
The relative abundances of the main minerals present are highlighted in Figure 13.13. Notable features are: the presence of epidote in one of the Brook Street Terrane samples (location 3); the occurrence of albite in two of the Brook Street Terrane samples (locations 3 and 4) and one from the Murihiku Terrane (location 9); laumontite in both of the Maitai Group samples (locations 5 and 7); and analcime in one sample from the Murihiku Terrane (location 11).
Seventy-three samples of tuffaceous rocks were chemically analysed for major, trace and rare earth elements (REEs), that is: the Maitai Group, 19 samples; the Murihiku Terrane, 22 samples; the Brook Street Terrane, 24 samples; and the Willsher Group, eight samples. The analysis was carried out at ACME Laboratories in Vancouver, Canada. Major element contents were determined from a LiBO2 fusion by ICP-ES (inductively coupled plasma emission spectrometry) using 5 g of sample pulp. Trace elements (including REEs) were also determined from a LiBO2 fusion, by ICP-MS (inductively coupled plasma mass spectrometry). The data are recorded, together with lithology and location details, in the Supplementary material Tables S2–S5. The available chemical data were plotted on a range of well-established geochemical diagrams to help interpret the lithology, volcanic affinities and provenance.
Despite metamorphism and variable alteration, some interesting correlations of major element oxides emerged which are detailed in Supplementary material Figures S1 and S2. Trace elements and REEs that are relatively immobile under the prevailing conditions are, however, a better guide to provenance and tectonic setting. For example, the compositions of the tuffaceous rocks are usefully compared with source materials such as chondrite, mid-ocean ridge basalt (MORB), volcanic-arc granite, upper continental crust (UCC) and two shale composites.
The samples were first plotted on the well-known total alkali-silica (TAS) diagram (Le Bas et al. 1986) (Fig. 13.14a–d). This suggests that many of the Brook Street Terrane samples are relatively basic compared to the Maitai Group. The Maitai Group and the Murihiku Terrane become generally more felsic stratigraphically upwards. Plots using alkalis may, however, not be reliable for rocks that have undergone low-grade regional metamorphism, potentially resulting in major-element mobility. For this reason, an igneous rock classification diagram was also used which utilizes elements that are known to be relatively stable during low- to intermediate-grade metamorphism (Winchester & Floyd 1977) (Fig. 13.15). The classification is applicable to homogeneous tuff but not to epiclastic rocks, which may include compositionally variable material.
For the Brook Street Terrane (Fig. 13.15a), samples from the lower part of the succession in the Nelson area (Grampian Formation: location 4) are bimodal: basaltic and rhyodacitic. Samples from the lower part of the Takitimu Mountain succession from location 1 are andesitic to rhyodacitic. Samples from East Eglinton–Hollyford (Gondor, Consolation, Divide and Fergus formations: location 3) are all basaltic, as are those from the upper part of the succession in the Takitimu Mountains (Elbow Formation: location 2). One sample from the upper part of the succession in the Nelson area (Groom Creek Formation) has an andesitic composition, although this is an epiclastic rock.
For the Maitai Group (Fig. 13.15b), the felsic tuff from Bare Peak, south of the Alpine Fault (Kiwi Burn Formation: Fig. 13.5a), plots in the rhyodacitic field. Samples from the Countess Range further north (Stephens Subgroup) range from basaltic to andesitic to rhyolitic. Samples from Wairoa–Lee River area (Pig Valley Quarry), north of the Alpine Fault (Kiwi Burn Formation: Fig. 13.5b, location 7), are basaltic to andesitic. Samples from the Acheron Lakes Formation in the Wairoa–Lee River area are andesitic to rhyodacitic, whereas those from the overlying Cerberus Formation are andesitic, as is the overlying Chrome Creek Formation.
For the Murihiku Terrane (Fig. 13.15c), the felsic Gavenwood Tuffs unit at Ram Hill (Fig. 13.7a, location 8) is andesitic to marginally rhyodacitic to dacitic. Felsic tuff from the regionally overlying Bare Hill Tuff Zone (location 9) and from Parks Cutting on the south Otago coast (location 10) is mostly rhyodacitic to dacitic. The reworked tuff from the Richmond–Stoke area (Richmond Group: location 11) plots near the andesite to rhyodacite–dacite boundary.
For the Willsher Group (Fig. 13.15d, location 12) all of the samples analysed are rhyodacitic to dacitic.
To assess the effects of alteration and weathering, the samples were plotted on a ternary Al2O2 v. Cao + Na2O v. K2O diagram (A–CN–K) plot, which illustrates the relative degree of alteration and any trends in alteration (Bock et al. 1994; Fedo et al. 1995). The Brook Street Terrane data (Fig. 13.16a) mostly plot off the reference line and away from average continental crust, indicating plagioclase enrichment, with the exception of several Al-rich samples from low in the Takitimu Group succession and from the Nelson city area. An unusually high CaO content from the Gondor Formation at East Eglinton–Hollyford reflects the presence of bivalve shell fragments and a micritic matrix. One sample from the lower part of the Grampian Formation in the Nelson area is carbonate-rich for the same reason. The more felsic samples are typically more altered.
For the Maitai Group (Fig. 13.16b), the samples again mostly plot off the reference line, other than for the most Al-rich samples that are mainly from the Acheron Lakes and Cerberus formations (Stephens Subgroup) in the Waioroa–Lee River area (east of Richmond–Stoke). One sample is unusually Ca-rich owing to an abundance of microspar-sized carbonate grains.
For the Murihiku Terrane (Fig. 13.16c), the samples generally plot near the clay-rich end of the weathering line. The tephra-rich samples from the Gavenwood Tuffs, the Bare Hill Tuff Zone, and their approximate time-equivalents from Parks Cutting and Roaring Bay all plot well off the predicted weathering line, probably owing to an abundance of alteration minerals (e.g. laumontite).
For the Willsher Group (Fig. 13.16d), the samples either plot near, or well off, the predicted weathering line, similar to the samples from the Murihiku Terrane.
A plot of the chemical index of alteration (CIA) (Al2O3/Al2O3 + K2O+CaO*) × 100 v. the index of chemical variation (ICV) is indicative of alteration trend. CIA values of 70–85 represent average shale and clay minerals (e.g. muscovite and illite) (Nesbitt & Young 1982). ICV values (CaO + K2O + Na2O + Fe2O3 + MgO + MnO + TiO2)/Al2O3) reflect the variation of alumina to other cations (Cox et al. 1995), and therefore variations between mafic minerals, clay minerals and the effects of compositional maturity.
The samples from the Brook Street Terrane (Fig. 13.17a) lie along two trends, on either side of the fresh basalt line. The felsic tuffaceous rocks (e.g. Maitai River, Nelson area; Brunel Formation, Murihiku Terrane) line up towards the fresh granite trend, whereas more basic rock samples (e.g. East Eglinton–Hollyford) mostly lie close to the inferred fresh basalt trend, with the exception of the calcareous sample from East Eglinton–Hollyford. The felsic samples are typically amongst the most altered. Samples from the Maitai Group (Fig. 13.17b) are generally more altered than those of the Brook Street Terrane and mainly plot midway between the basalt and granite trends. The mixed tuffaceous–volcaniclastic samples from the Acheron Lakes and Cerberus formations generally plot near the fresh basalt trend. For the Murihiku Terrane (Fig. 13.17c), the samples mostly plot between the fresh basalt and fresh granite trends, similar to the Maitai Group. The Gavenwood Tuffs samples lie closer to the basaltic trend than those from the overlying Bare Hill Tuff Zone. Samples from the Willsher Group (Fig. 13.17d) are generally similar to those of the Maitai Group but are less altered than some of those from the Murihiku Terrane.
Trace element and REE trends
On chondrite-normalized plots (Fig. 13.18ai), the samples from the Brook Street Terrane show relatively flat patterns with, or without, a negative Eu anomaly, related to feldspar fractionation (Rollinson 1993). An anomalous sample with very low REE values is from the Elbow Formation that is seen in thin section to be partially replaced by quartz. The samples from the Maitai Group (Fig. 13.18aii) are mostly enriched in light REEs (LREEs) relative to heavy REEs (HREEs), and mostly have a marked negative Eu anomaly. In contrast, plagioclase-rich arkosic sandstone from Pig Valley Quarry (Kiwi Burn Formation) has a relatively flat, depleted REE pattern with a marked positive Eu anomaly, consistent with its feldspar-rich composition.
The samples from the Murihiku Terrane (Fig. 13.18aiii) are mostly similar to those of the Maitai Group, commonly with well-defined negative Eu anomalies. One sample with a high epiclastic content from Parks Cutting is unusually rich in REEs. A sample from Roaring Bay, higher in the succession, has an unusually pronounced negative Eu anomaly, indicative of strong plagioclase fractionation. Overall, the Murihiku Terrane samples are slightly enriched in REEs compared to the Maitai Group (see below). The samples from the Willsher Group (Fig. 13.18aiv) lie within the range of those from both the Murihiku Terrane and the Maitai Group. However, their slight average REE enrichment and very well-defined negative cerium anomalies are more similar to the Maitai Group samples than those from the Murihiku Terrane.
Normalized against upper continental crust (UCC) (Fig. 13.19ai–aiv), the patterns of all four units are broadly similar. Several elements (e.g. Ba, Cs, Rb and Th) behave erratically, suggesting chemical mobility. Variable Sr largely relates to an admixture of biogenic carbonate. The slope in normalized HREEs from La to Lu is steepest for the Brook Street Terrane (Fig. 13.19ai), intermediate for the Maitai Group (Fig. 13.19aii) and lowest for the Murihiku Group (Fig. 13.19aiii). There is a sharp downwards step in normalized values between Y and La for the Brook Street Terrane, a moderate drop for the Maitai Group, but only a small drop for the Murihiku Terrane samples. In the Maitai Group, the epiclastic sandstone samples from Pig Valley Quarry (Kiwi Burn Formation) are relatively depleted in REEs (e.g. Nb, U and La). The Willsher Group plots (Fig. 13.19aiv) are similar to those of the Maitai Group and the Murihiku Terrane.
Normalized against volcanic-arc granite (Fig. 13.19bi), the Brook Street Terrane tuffaceous sediments are markedly depleted in Nb. The Maitai Group tuffaceous samples (Fig. 13.19bi) are generally less depleted in Nb. The Murihiku Terrane patterns (Fig. 13.19biii) have open U-shaped patterns with less marked Nb negative anomalies compared to the Brook Street Terrane and Maitai Group samples. For the Willsher Group (Fig. 13.19biv), the patterns are similar to both those of the Maitai Group and the Murihiku Terrane.
The tuffaceous rocks can also be compared with sandstones derived from different arc-related tectonic–magmatic settings using well-known discrimination diagrams. The compositions can also be compared, specifically, with the intrusive arc rocks of the Median Batholith and with the Late Paleozoic metasedimentary rocks of the Western Province. Recently, traditional discrimination diagrams (e.g. Bhatia & Crook 1986) have been criticized mainly on the grounds that the analytical data used to make the plots from the Tasman Basin, eastern Australia were small and unrepresentative of the full range of tectonic settings worldwide (e.g. rift, passive margin, oceanic arc, continental margin arc, active margin) (Armstrong-Altrin & Verma 2005). However, all of our samples are from arc-related settings and the diagrams are used to highlight compositional differences rather than to identify unique tectonic settings. Tectonic settings are best interpreted using a range of geochemical criteria, together with field, petrographical and mineralogical evidence.
On the Th/Sc v. Zr/Sc diagram, most of the Brook Street Terrane samples (Fig. 13.20a) have a relatively non-evolved mantle composition. Exceptions are those from the type Nelson area (Grampian and Groom Creek formations) and to a lesser extent from the base of the succession in the Takitimu Mountains (Brunel Formation). On the Ti/Zr v. La/Sc diagram (Fig. 13.20b), the samples mostly plot in the area of Median Batholith mafic intrusive rocks. In contrast, the samples from the Nelson area plot in the range of evolved Median Batholith intrusive rocks and the dominant field of most of the Western Province metasedimentary rocks. On the La/Th v. Hf diagram (Fig. 13.20c), the samples mostly plot in the andesitic island-arc field, with some in the felsic field (especially those from the Nelson area), with others plotting in the mixed mafic and felsic sources space. On the Hf/3 v. Th v. Nb/16 diagram, the Brook Street Terrane samples (Fig. 13.20d) are split between the island-arc tholeiite (IAT) and calc-alkali basalt (CAB) fields, with the felsic tuffs from the Grampian Formation and one sample from East Eglinton–Hollyford plotting in the CAB field. On the La v. Th v. Sc diagram, the samples from the Takitimu Mountains (Fig. 13.20e) range from more depleted than the ocean island-arc field (OIA) to within the OIA field. Three tephra-rich samples from the Nelson city area (Grampian Formation) plot within the active continental margin (ACM) + passive margin (PM) field, and also lie within the continental island-arc (CIA) field. On the La v. Th plot (Fig. 13.20f), the samples mainly lie within the OIA field, with the Nelson area ones plotting in, or near, the CIA field. Compared with the modern Japanese continental margin volcanic arc, on the Rb/Hf v. Th/Nb plot (Fig. 13.20g), many of the samples plot off the recognized fields, indicating a relatively depleted nature. In contrast, the samples from the Nelson area (Grampian Formation), in particular, plot in the N-Honshu field. On the same plot (Fig. 13.20h), compared to Eocene–Recent Izu tephra (NW Pacific), most of the samples plot in the rear-arc field, whereas some, especially from the Nelson area, plot in the arc field.
For the Maitai Group, on the Ti/Zr v. Zr/Sc plot (Fig. 13.21a), most of the samples plot away from the upper mantle trend; several samples, especially from the Nelson area (Grampian Formation), are suggestive of sediment recycling and zircon addition. On the Ti/Zr v. La/Sc plot (Fig. 13.21b), a greater proportion of samples, compared to the Brook Street Terrane, plot in the fields of evolved Median Batholith intrusive rocks and the main field of Western Province metasedimentary rocks. Similarly, on the La/Th v. Hf plot (Fig. 13.21c), a high proportion of the samples plot within, or near, the felsic island-arc source field. On the Hf/3 v. Th v. Nb/16 plot (Fig. 13.21d), the samples all plot in the CAB field (calc-alkali basalt). On the La v. Th v. Sc plot (Fig. 13.21e), the samples are spread across the OIA, CIA and active margin (ACM) + PM (passive margin) fields. Similarly, on the La v. Th plot (Fig. 13.21f), the samples plot within, or near, the OIA and CIA fields. Compared to the Japanese continental margin arc (Fig. 13.22g), the samples are relatively Hf-rich, consistent with a highly evolved felsic composition. Many samples plot near the field of the Izu arc tephra (Fig. 13.21h), although some are relatively Th-rich.
For the Murihiku Terrane, on the Th/Sc v. Zr/Sc plot (Fig. 13.22a), the samples are relatively evolved, similar to those from the Maitai Group. On the Ti/Zr v. La/Sc plot (Fig. 13.22b), the samples are all in the field of the dominant Western Province metasedimentary rocks, except for one (Parks Cutting), which lies in the subordinate Western Province metasedimentary rock field, which contains alkaline igneous material. Otherwise, the samples also lie within compositional range of evolved Median Batholith intrusive rocks. On the La/Th v. Hf plot (Fig. 13.22c), the samples mostly plot in the felsic island-arc source, trending towards a passive margin source with an increased sediment component. The samples lie in the CAB field in the Hf/3 v. Th v. Nb/16 plot (Fig. 13.22d). On the La v. Th v. Sc plot (Fig. 13.22e), the samples are mainly in the CIA field. On the Th v. La diagram (Fig. 13.22f), they commonly plot in the CIA field but are relatively scattered, especially the Gavenwood Tuffs (location 8), probably as the result of the regional low-grade metamorphism. On the Rb/Hf v. Th/Nb plots (Fig. 13.22g, h), the samples mostly plot in the N-Honshu arc field and in the Izu tephra arc field, indicating that these two diagrams do not effectively separate continental v. oceanic-arc provenance for this dataset.
For the Willsher Group (location 12), on the Th/Sc v. Zr/Sc plot (Fig. 13.23a), the samples plot similarly to those from the Murihiku Terrane, suggesting a high degree of fractionation. Similarly, on the Ti/Zr v. La/Sc plot (Fig. 13.23b), the samples plot similarly to the Murihiku Terrane ones. The La/Th v. Hf plot (Fig. 13.23c) emphasizes the felsic nature of the tuffs. On both the La v. Th plot (Fig. 13.23d) and the La v. Th. v. Sc diagram (not shown), the samples mainly plot in the CIA field.
The REE patterns are highlighted by a plot of Eu/Eu* v. LaCN/YbCN (chondrite-normalized La/Yb ratio) (McLennan 1989) compared to NASC (North Atlantic Shale Composite), PAAS (post-Archaean Australian Average Shale) and UCC. The Brook Street Terrane samples have relatively high Eu/Eu* but low LaCN/YbCN (Fig. 13.24a), far removed from a continental source composition. The Maitai Group samples (Fig. 13.24b) mostly have Eu/Eu* values close to a continental source composition. The low values and limited ranges of LaCN/YbCN ratios, relative to UCC, reflect the presence of little-evolved source rock. The three plagioclase-rich epiclastic samples from the Pig Valley Quarry (Kiwi Burn Formation, location 7) are exceptions owing to their high plagioclase content. The Murihiku Terrane samples (Fig. 13.24c) have Eu/Eu* ratios similar to, or lower than, continental source materials, reflecting an overall relatively evolved source. The LaCN/YbCN ratios are generally higher than in the Brook Street Terrane and the Maitai Group, reflecting a relatively more-evolved source. The Willsher Group samples (Fig. 13.24c) plot with the more enriched of the Maitai Group samples and also, generally, with the less-enriched Murihiku Terrane samples. As noted above, using other geochemical plots (Figs 13.18 & 13.19), there appears to be a closer similarity with the Maitai Group samples than with the Murihiku Terrane sample analysed.
The mineral content, as revealed by combined optical petrography and semi-quantitative XRD, reflects a combination of primary source material, diagenesis, metamorphism and alteration. The quartz content mainly reflects derivation from felsic tuffaceous material, with which it is associated, together, potentially, with a minor terrigenous component. Some terrigenous input is suggested by the presence of muscovite and trace amounts of rutile. Rutile is indicative of a relatively high-grade metamorphic source, assuming a primary origin (Zack et al. 2004). On the basis of these two minerals, all of the units studied contain minor amounts of terrigenous material. This suggests that none of the units are likely to have formed in a fully open-ocean setting, including the Grampian Formation in the Nelson city area (location 4) and the Fergus Formation in the East Eglinton–Hollyford area (location 3).
The feldspar composition partially represents the retention of some primary material (anorthite component), together with widespread albitization under the prevailing relatively low-grade metamorphic conditions (Surdam & Boles 1979). Potash feldspar, consisting of microcline and orthoclase, is likely to be primary, derived from evolved arc-related igneous rocks.
The chlorite llb and epidote mainly formed by the alteration of basic- to intermediate-composition igneous rocks, as in the Brook Street Terrane (East Eglinton–Hollyford, location 3; and Grampian Formation, location 4). The illite and kaolinite could be detrital or diagenetic in origin.
Laumontite and analcime are most abundant in some of the purest felsic fallout tuff samples (e.g. locations 5 and 10), which formed during the regional low-grade metamorphism that affected the Murihiku Terrane (Boles 1971; Boles & Coombs 1975) and the Maitai Group (Landis 1974). Analcime in the Murihiku Terrane occurs in the upper part of the succession (upper 5 km) and laumontite in the lower part (lower 5 km) (Surdam & Boles 1979). In the Murihiku Terrane, these zeolites are thought to have developed at a late stage (after maximum burial) related to fluid flow during post-Jurassic folding and fracturing that accompanied formation of the Southland Syncline (Boles 1991). In addition, the small amounts of pyrite, siderite and ankerite relate to burial diagenesis or alteration of minerals generally formed under low-oxygen conditions.
Arc volcanism of the Brook Street Terrane
The geochemical results suggest that the Brook Street Terrane, the Maitai Group and the Murihiku Terrane represent different settings of arc volcanism, but with some compositional overlaps. For the Brook Street Terrane, many of the samples from the lower part of the sequence in the NW Takitimu Mountains (Brunel Formation, location 1) are relatively evolved, and plot in the CIA and the felsic island-arc fields. In contrast, some tuffaceous rocks from higher in the sequence are more mantle-influenced, including the Elbow Formation (NE Takitimu Mountains, location 2) and the East Eglinton–Hollyford upper formations (Consolation, Divide and Fergus formations: location 3). These sediments are partially epiclastic and represent erosion of volcanics from different levels of the arc stratigraphy. Available Hf–Nd–Pb isotopic evidence for the Brook Street Terrane, from outcrops south of the Alpine Fault, is comparable to that of oceanic crust (including seamounts) and does not indicate any continental involvement in magmatism (Frost & Coombs 1989; Nebel et al. 2007).
The highly evolved nature of the tuffs in the lower part of the volcanic succession to the north of the Alpine Fault (Grampian Formation: location 4) is surprising because the Brook Street Terrane as a whole has been interpreted as an oceanic arc (Houghton 1985; Spandler et al. 2005; Nebel et al. 2007). One explanation is that the felsic tuffs represent fractionates from relatively primitive arc magmas, potentially involving the assimilation of hydrated and otherwise altered arc crust. However, felsic rocks that formed in this way in the Izu–Bonin arc exhibit much less depleted REE patterns than those of the Grampian Formation (Tamura et al. 2009). A second option takes account of the widely proposed correlation of the Brook Street Terrane with the Gympie Terrane of coastal central Queensland (Sivell & Waterhouse 1987, 1988; Sivell & McCulloch 2001; Korsch et al. 2009). Recent work has revealed the presence of Gondwana-derived detrital zircons in the Gympie Terrane, suggesting that the arc-related volcanism took place along the SE Gondwana active continental margin (Li et al. 2015). Sandstones of the Grampian Formation (Nelson city area) contain sparse Carboniferous, as well as Permian, detrital zircons (Adams et al. 2007). One option is that the magmatic arc represented by the Brook Street Terrane south of the Alpine Fault (e.g. Takitimu Mountains) was separated by a back-arc basin from the SE Gondwana continental margin. The arc possibly extended for several thousand kilometres north–south (in present coordinates), with part straddling the Gondwana margin (Gympie Terrane and Brook Street Terrane outcrops north of the Alpine Fault) and part lying further outboard (Brook Street Terrane south of the Alpine Fault). However, the enormous thickness of the succession in the Takitimu Mountains would be difficult to accommodate in a young (Early Permian) back-arc basin. Another option is that the Brook Street Terrane south of the Alpine Fault was constructed on accreted or trapped oceanic lithosphere along the SE Gondwana active margin, explaining its lack of continental influence (Robertson & Palamakumbura 2019a). In addition, it is likely that the Brook Street Terrane is composite, with the outcrops south of the Alpine Fault representing an oceanic arc near SE Gondwana, whereas those to the north of the Alpine Fault formed in a continental margin-arc setting, which would explain the chemical features of the voluminous felsic tuffs in the Grampian Formation (see Robertson et al. 2019 for a discussion).
The felsic tuffs in the Takitimu Mountains (locations 1 and 2) are relatively thin and localized, and, by comparison with modern oceanic island-arcs settings, can be interpreted as the products of relatively small, localized submarine eruptive centres (Yuasaka et al. 1991; Tamura & Tatsumi 2002; Tamura et al. 2005). Some island-arc volcanoes have a bimodal basalt-rhyolite composition, with lesser volumes of mainly felsic products. Prior to any caldera formation, such eruptions can produce pumice-rich material which is widely dispersed by gravity flows and ocean currents. A modern example is Myojin Knoll volcano in the Izu–Bonin arc south of Japan (Fiske et al. 2001). Such subaqueous eruptions produce only small volumes of non-welded material, compared to pumice. In contrast, the relatively thick and fine-grained felsic tuffs of the Grampian Formation (Nelson city area) are more likely to have been derived from Vesuvian- or Plinian-type eruptions on islands or an adjacent continent. The abundant felsic ash fallout was mostly reworked by subaqueous gravity flows, building up the thick sequence of associated volcaniclastic turbidites and mass-flow deposits (Robertson & Palamakumbura 2019a). Vesuvian or Plinian eruptions created fallout deposits, in the Grampian Formation coupled with some gravity reworking. The mostly epiclastic sediments of the upper formations of the East Eglinton–Hollyford succession (location 3) record felsic tuff of probably subaerial origin, variably mixed with more mafic to andesitic arc-derived material.
Arc volcanism of the Maitai Group
Throughout the Maitai Group, the vast majority of the volcanogenic sediments are epiclastic, involving redeposition of volcanic and terrigenous material (e.g. Humboldt petrofacies of the Tramway Formation). However, there is some evidence of coeval, albeit reworked tuffaceous sediments, notably the pyroclastic-derived, feldspar-rich McKellar petrofacies (Landis 1974; Owen 1995; see Robertson & Palamakumbura 2019b). The Maitai Group is interpreted as the distal part of a continental margin forearc basin (Robertson & Palamakumbura 2019b, 49_CH13C81c). Assuming this setting, the thick felsic fallout tuffs of the Kiwi Burn Tuffs south of the Alpine Fault (location 5) are likely to record powerful Plinian eruptions on land. Continental reconstructions of Pangaea are suggestive of monsoonal-type conditions, with offshore winds along the SE Gondwana continental margin (Parrish & Peterson 1988; Parrish 1993). Similar eruptions contributed numerous ash layers to the overlying Stephens Subgroup. The equivalent stratigraphic interval of the Kiwi Burn Formation, north of the Alpine Fault (Wairoa–Lee River Valley area) is dominated by feldspathic (arkosic) turbidites (e.g. Pig Valley Quarry: location 7). Major ash eruptions were apparently localized along the SE Gondwana active continental margin. The felsic tuffaceous material in the upper part of sequence north of the Alpine Fault (e.g. Cerberus and Chrome Creek formations) is mostly redeposited and is interbedded with volcaniclastic sandstones of variable basic to felsic composition (Robertson & Palamakumbura 2019c). The felsic tuffs of the Maitai Group as a whole are interpreted as fractionates from arc magmas in coastal areas, islands or shallow seas, where volcanic material, including felsic ash fallout, was reworked as turbidites and mass-flow deposits. The setting could be similar to the later Miocene (13–8 Ma) of the Japanese arc in northern Japan, where subaqueous basaltic to rhyolitic volcanism took place along a chain of islands (Yoshida 2001; Yoshida et al. 2005; Acocella et al. 2008).
Arc volcanism of the Murihiku Terrane
The thickest and most widespread felsic tuffs are represented by the Gavenwood Tuffs and the Bare Hill Tuff Zone, south of the Alpine Fault (location 8–10). There are also numerous, thinner tuffaceous intercalations throughout the Triassic succession as a whole. The tuffs can be compared with subaerial Plinian, phreatoplinian and ignimbrite-forming eruptions of continental margin-arc volcanoes, such as those of southern Japan (Honshu) (Yoshida 2001; Kimura et al. 2015), the Cascades (Kuehn & Foit 2006) and Central America (Kutterolf et al. 2008a, 49_CH13C56b, 49_CH13C57c). Petrographical and geochemical data indicate that interbedded volcaniclastic sandstones and conglomerates are mostly of andesitic to rhyolitic composition (Boles 1971, 1974; Roser et al. 2002; Robertson & Palamakumbura 2019c). The source volcanoes could have been located up to 250 km continentwards of the subduction zone and might, therefore, differ strongly from the composition of detrital sediments that reached the deep-marine basin through river transport, and, finally, by turbidity currents and mass flow. Much of the sand-sized and coarser sediment is likely to have been derived from small river catchments near the coast, whereas much of the felsic tuff originated from major eruptions further inland.
Much of the felsic ash settled out over the sea, followed by variable gravity reworking. Some layers, up to 8 m thick, are interpreted as single eruptive events, pointing to intense, sustained Plinian-type eruption. Felsic ash was also variably reworked within more mafic gravity-flow deposits above and below major felsic tuffaceous intervals. The sparsity or absence of thick gravity-flow deposits within the main tuffaceous intervals suggests that the Gavenwood Tuffs and the Bare Hill Tuff Zone, in particular, represent short-lived pulses of very intense felsic volcanism. In contrast, the tuffaceous rocks north of the Alpine Fault (Richmond Group: location 11) are mostly variable mixtures of felsic fallout tuff and epiclastic material. These tuffaceous layers are interpreted as numerous less-intense felsic eruptions, with fallout over the ocean, followed by variable reworking, together with andesitic to rhyodacitic material.
Arc volcanism of the Willsher Group
The numerous (>300) millimetre- to c. 2.5 m-thick beds of volcanic ash are strongly altered to porcellanite, claystone (bentonite) and albite-rich siltstone (Coombs 1954; Jeans et al. 1997, 2003). The background sediments are mostly dark-coloured, laminated siltstone (c. 80% by volume), together with mudstone and several fine-grained sandstone intervals (c. 80 m Kaka Point Volcanic Sandstone and c. 6 m Pilot Point Sandstone). The available sedimentological evidence points to deposition of the Willsher Group in a relatively proximal-marine setting, possibly in a partially filled to overfilled forearc slope basin (see Robertson et al. 2019). Possible counterparts elsewhere include Plio-Pleistocene tuffaceous sediments in the Boso Peninsula, Japan (Ito 1992).
The volcaniclastic sandstones of the Willsher Group have been correlated, alternatively with the Murihiku Terrane (Roser & Coombs 2005), or with the Maitai Group (Turnbull & Allibone 2003), or could represent a relatively exotic unit (Campbell & Coombs 1966). The new chemical data show that similar felsic tuffs of comparable age occur in both the Murihiku Terrane and the Maitai Group. The Willsher Group felsic tuffs have a clear continental island-arc provenance that is generally intermediate between the average compositions of tuff from the Maitai Group and tuff from the Murihiku Terrane (but tending towards the compositions of some Maitai Group tuffs). Robertson et al. (2019) suggest that the Willsher Group could represent a relatively proximal equivalent of the Maitai forearc basin which is otherwise not preserved.
Implications for SE Gondwana and South Island
The felsic tuffs are plotted against known age in Figure 13.25, which indicates potentially five periods of felsic tuff eruption: (1) Early Permian (Sakmarian–Kungurian) for the Brook Street Terrane south of the Alpine Fault; (2) Late Permian for the Brook Street Terrane north of the Alpine Fault, assuming that Late Permian detrital zircon ages for the Grampian Formation (Nelson city area) are valid (Adams et al. 2007); (3) Early–Middle Triassic (Gavenwood Tuffs) for the Murihiku Terrane and the Maitai Group (Stephens Subgroup: locations 6 and 7); (4) Late Middle Triassic–early Late Triassic for the Willsher Group (location 12); and (5) Late Triassic (Norian) for the Murihiku Terrane (Bare Hill Tuff Zone; Roaring Bay) and counterparts north of the Alpine Fault (Richmond Group). There appears to have been a clear break in felsic tuff deposition between the Early Permian of the Brook Street Terrane oceanic arc south of the Alpine Fault and all of the other occurrences of continental margin-arc type (Fig. 13.26). The felsic tuffs of the Maitai Group, the Murihiku Terrane and the Willsher Group all represent explosive felsic volcanism from continental margin-arc volcanoes, although probably along different segments of the SE Gondwana active continental margin. The thick Early–Middle Triassic tuffs of the Maitai Group (Kiwi Burn Formation and Gavenwood Tuffs) include major Plinian-type eruptions along the continental margin arc, some probably tens to hundreds of kilometres inland.
The felsic tuffs shed light on the magmatic record of the Median Batholith and/or potential non-exposed equivalents to the north or south (assumed to be submarine). The Dun Mountain–Maitai Terrane and the Murihiku Terrane are interpreted as exotic terranes relative to the Median Batholith and the Western Province (Adams et al. 2007). The felsic tuffs of the Grampian Formation (location 4) could represent the initiation of continental margin-arc volcanism during latest Permian time, approximately coeval with the Longwood Suite of the Median Batholith in southern Southland (McCoy-West et al. 2014), although the two units were not necessarily contiguous. The felsic volcanism possibly relates to the resumption of arc magmatism followed by the docking of the Brook Street Terrane oceanic arc with SE Gondwana, during Mid–Late Permian time (see Robertson & Palamakumbura 2019a).
Felsic tuffs are absent from the oldest exposed Late Permian volcaniclastic succession of the Murihiku Terrane (Mataura Island–Titiroa area, eastern Southland) (Campbell et al. 2001; Robertson & Palamakumbura 2019c), suggesting that such volcanism only became important along this segment of the active margin during the Triassic (Fig. 13.26). Evolved quartz diorites, quartz monzodiorites and, rarely, granites appear within the Median Batholith during the Mid-Triassic (Mortimer et al. 1999); these are potentially coeval with felsic tuff in the Murihiku Terrane (Gavenwood Tuffs) and the Maitai Group (Kiwi Burn Formation). The felsic tuffs are indicative of eruptions that are not preserved in the Median Batholith owing to deep erosion or eruption along some other segment of the active margin. The Maitai Group is proposed to have originated further north and in a more distal location than the Murihiku Group, with the Willsher Group in a relatively proximal setting, either directly inboard of the Maitai Group or somewhere between the Maitai and Murihiku forearc basins. On a regional scale, the spasmodic appearance of the thick felsic tuff intervals could reflect several factors, including changes in the rate, angle or obliquity of subduction of the Panthalassa Ocean beneath SE Gondwana.
Felsic tuffaceous rocks of Early Permian–Late Triassic age, studied here, record explosive arc volcanism within and adjacent to the SE Gondwana continental margin.
Sparse felsic tuffs within the Early Permian Brook Street Terrane, south of the Alpine Fault (e.g. Takitimu Mountains), represent volumetrically minor fractionates from compositionally basic oceanic arc-type magmatism.
The occurrence of thick fallout tephra in the lower part of the succession (Grampian Formation) in the northerly located Nelson city outcrop of the Brook Street Terrane points to Plinian-type eruption, probably in terrestrial or shallow-marine setting.
Thick felsic tephra-rich intervals within the Early–Middle Triassic Maitai Group (e.g. Kiwi Burn Formation), south of the Alpine Fault, record major Plinian-type eruptions, probably on land.
Chemical data for the Early–Middle Triassic Maitai Group felsic tuffs suggest a continental margin-arc origin, although this was relatively primitive, similar to the Median Batholith (Darran Suite).
The two main intervals of felsic tuff in the Murihiku Terrane, south of the Alpine Fault, represent powerful Plinian-type eruptions, with little time for any accumulation of ‘background’ volcaniclastic turbidites.
Approximately contemporaneous, chemically similar, felsic tuff accumulated in the Triassic Maitai Group formations, the Murihiku Terrane and the Willsher Group (south Otago coast) during Middle–Late Triassic time. These deposits are likely to represent major eruptive pulses along different segments of the SE Gondwana active continental margin.
Early–Middle Triassic felsic eruptions recorded in the Maitai Group are likely to have mainly occurred in coastal areas, islands or shallow seas, whereas the sources of two main phases of Triassic explosive felsic eruption in the Murihiku Terrane were probably located up to hundreds of kilometres inland, on thick continental crust.
The distribution of the Early–Middle Triassic Maitai Group felsic volcanism is suggestive of variations in the composition and intensity of continental margin-arc volcanism along different segments of the SE Gondwana active continental margin, on a scale of tens to hundreds of kilometres.
The felsic tuffs of the late Early Triassic–early Late Triassic Willsher Group (south coast) are chemically intermediate between those of the Maitai Group and the Murihiku Terrane (although tending more towards the Maitai Group), and may represent a proximal equivalent of the Maitai forearc basin.
Fieldwork was carried out by the first author during the austral summer of 2017. The fieldwork in the Hokonui Mountains, Murihiku Terrane was carried out jointly with Nick Mortimer. Mike Johnston kindly provided advice on fieldwork in the Nelson area. Gillian Robertson generously acted as field assistant throughout the time in New Zealand. The fieldwork and subsequent laboratory work were partially funded by the John Dixon Memorial Fund. The manuscript benefited from comments by Christopher Jeans and an anonymous reviewer.
Figures & Tables
Paleozoic–Mesozoic Geology of South Island, New Zealand: Subduction-related Processes Adjacent to SE Gondwana
CONTAINS OPEN ACCESS
This volume presents a set of research papers that provide new data and interpretations of the Permian–Triassic terranes of SE Gondwana, now exposed in South Island, New Zealand. Following an introduction for general readers, a historical summary and a review of biostratigraphy, the individual papers primarily focus on the Permian magmatic arc of the Brook Street Terrane, the classic Permian Dun Mountain ophiolite and the Permian–Triassic Maitai Group sedimentary succession. The new results emphasize the role of subduction and terrane displacement adjacent to the Permo-Triassic Gondwana margin, and present fundamental insights into three crustal processes: subduction initiation, supra-subduction zone oceanic crust genesis and forearc basin evolution. The volume concludes with a wide-ranging summary and synthesis of the regional Cambrian to Early Cretaceous tectonostratigraphy of New Zealand's South Island in relation to the wider areas of Zealandia, East Australia and West Antarctica. The volume will interest geoscientists, including stratigraphers, sedimentologists, palaeontologists, igneous petrologists, geochemists, geochronologists and economic geologists, and is aimed at professional geologists and advanced students of geology.
- active margins
- chemical composition
- continental margin
- felsic composition
- igneous rocks
- Lower Triassic
- mineral composition
- New Zealand
- Permian-Triassic boundary
- plate tectonics
- rare earths
- South Island
- stratigraphic boundary
- trace elements
- Upper Permian
- volcanic rocks
- X-ray diffraction data
- Maitai Group
- Brook Street Terrane
- Murihiku Terrane
- Willsher Group