Abstract
Detrital zircon U–Pb ages and Rb–Sr metamorphic ages from low-grade Torlesse Supergroup metasedimentary rocks from North Canterbury and Marlborough provide preliminary evidence for a continuation of the Kaweka Terrane of the central North Island into the Torlesse Composite Terrane of the South Island. This would extend from the south side of the Wairau Fault in the upper Wairau River valley southwards to the Lake Tennyson and Lake Sumner areas, and as far as Hawarden. Rb–Sr ages indicate Jurassic metamorphism, 170±24 Ma, with initial 87Sr/86Sr ratios at that time 0.7073±0.0007 i.e. similar to their North Island counterparts. These Kaweka Terrane rocks have detrital zircon ages that follow the distinctive pattern of the Torlesse rocks in general, i.e. substantial (>30%) Permian–Triassic and Precambrian–Early Palaeozoic groupings, but they also have minor youngest age components c. 175–165 Ma which constrain a maximum Early–Middle Jurassic depositional age. In detail, significant, older zircon components are Late Permian–Early Triassic (260–240 Ma) and Late Triassic (220–210 Ma), but the Precambrian–Early Palaeozoic detrital zircon ages are more scattered. The Kaweka Terrane thus forms a linear belt (c. 500 km) along the length of the Torlesse Composite Terrane in both North and South Islands. In the South Island it coincides with the Esk Head Belt, i.e. east of the Rakaia Terrane (Triassic–Late Permian) and west of the Pahau Terrane (Early Cretaceous–Late Jurassic).
Introduction
The Torlesse Supergroup forms a major part of the basement of the Eastern Province of New Zealand. Its extensive sandstone-dominated (Late Permian–Early Cretaceous) successions are placed entirely within a Torlesse Terrane (Bishop et al. Citation1985) comprising Late Permian–Late Triassic Rakaia Terrane, Early–Middle Jurassic Kaweka Terrane and Late Jurassic–Early Cretaceous Pahau Terrane (Bishop et al. Citation1985; Adams et al. Citation2009a) (). This is now referred to as the Torlesse Composite Terrane (Begg & Johnston Citation2000; Rattenbury et al. Citation2006). The Rakaia Terrane as a Torlesse subunit is long-established and well-studied in South and Mid-Canterbury, with important extensions southwards into the Otago Schist and northwards to the North Island Wellington region. Similarly, the Pahau Terrane is best known from North Canterbury and Marlborough, and extends northwards along the eastern part of the North Island axial ranges through Wairarapa, Hawkes Bay and Urewera districts.
Figure 1 Basement rocks of the northern South Island and southern North Island, New Zealand, showing the main study area with sample localities and numbers of this and previously published geochronological studies (locality numbers in italics in brackets) and other places mentioned in text. Rakaia, Kaweka and Pahau Terranes of the Torlesse Composite Terrane are shown separately. Solid dashed line is the Torlesse/Waipapa Terrane boundary. Esk Head Belt and other melange belts (RSR Random Spur Melange, South Island; Rimutaka Melange, North Island) are indicated by diagonal ruling.
In the Kaimanawa and Kaweka ranges of the North Island, Torlesse rocks have been placed in a Kaweka Terrane on the basis of their Jurassic detrital zircons, rare dinoflagellates and macrofossils (Adams et al. Citation2009a, Leonard et al. Citation2010). The terrane has a presumed fault boundary with Waipapa Terrane to the north and, along a zone of melange (Esk Head Belt) through the North Island, a fault boundary with Pahau Terrane to the southeast (). This raises the possibility that a similar relationship might exist within the Torlesse Terrane in the South Island; to investigate this, we report here Rb–Sr metamorphic ages and detrital zircon ages from tracts of largely unfossiliferous Torlesse rocks in North Canterbury and Marlborough areas.
Geological outline
Recent recompilations of all available regional geological data for the New Zealand QMAP 1:250 000 geological maps of North Canterbury, Marlborough and Wellington (Rattenbury et al. Citation1998, Citation2006; Begg & Johnston 2000; Lee & Begg Citation2002; Nathan et al. Citation2002; Cox & Barrell 2007; Forsyth et al. Citation2008) provide a modern summary of the age, structure and composition of Torlesse basement rocks, and only a brief summary is given here.
Torlesse Terrane in North Canterbury and Marlborough
Torlesse Supergroup (Suggate Citation1978) rocks are widespread throughout New Zealand, but their monotonous character, complex structure and scarcity of fossils makes lithostratigraphic subdivision only locally possible. The tectonostratigraphic subvisions (Bishop et al. Citation1985) are more useful and followed here. Torlesse rocks comprise relatively quartzose- and sandstone-dominated turbidite successions. Sediments are derived from sources of predominantly granitoid compositions in the older Late Permian–Triassic (Rakaia Terrane) and have a more felsic-intermediate volcaniclastic character in the Late Jurassic–Early Cretaceous (Pahau Terrane) rocks (MacKinnon Citation1983; Andrews et al. Citation1976). Conglomerates are minor, but in the Pahau Terrane in particular there are several significant examples (Andrews et al. 1976; Smale Citation1978). Limestones and volcanic rocks are very rare except in melange zones such as the Esk Head Belt (Bradshaw Citation1972, Citation1973; Rattenbury et al. Citation1998, Citation2006), as described below.
The main clastic sequences are rarely fossiliferous but Late Permian, Middle–Late Triassic, and Late Jurassic–Early Cretaceous zones have been established (Campbell & Warren Citation1965; Webby Citation1967; Milne & Campbell Citation1969; Speden Citation1976; Campbell Citation1983). The sedimentary successions accumulated in an extensive submarine accretionary tectonic environment, typically mid-fan but sometimes delta-fan (Andrews Citation1974; Johnston Citation1990; Bassett & Orlowski Citation2004). Subsequent complex deformation has imbricated kilometre-scale packets of sedimentary rocks into an accretionary wedge (Korsch & Wellman Citation1988; Roser & Korsch Citation1999). Thus low-grade regional metamorphism, prehnite-pumpellyite to pumpellyite-actinolite mineral facies, is common in the Rakaia Terrane and zeolite facies in the Pahau Terrane (Mortimer Citation1993; Mortimer et al. Citation1993; Begg & Johnston 2000; Rattenbury et al. Citation2006). Geochronological studies of the Rakaia Terrane indicate Early Triassic, Early Jurassic and Early Cretaceous phases of metamorphism (Bradshaw et al. Citation1981; Adams & Graham Citation1996; Adams Citation2003; Adams & Maas 2004a) but no age studies have been attempted in the Pahau Terrane because of the low metamorphic grade. Detrital zircon ages from Torlesse Terrane constrain maximum depositional ages to Late Triassic in the Rakaia Terrane in the Nelson Lakes region (Pickard et al. Citation2000) and Early Cretaceous in the Pahau Terrane in North Canterbury (Pickard et al. Citation2000; Wandres et al. Citation2004a).
Rakaia and Pahau Terranes are separated by Esk Head Melange, Esk Head Subterrane (Bradshaw Citation1972, Citation1973; Silberling et al. Citation1988) or Esk Head Belt (Rattenbury et al. Citation2006). The latter, with zones of associated broken formation, continues through the North Island to include the former Rimutaka, Pohangina and Whakatane Melanges (Barnes & Korsch Citation1990, Citation1991; Campbell et al. Citation1993; Mortimer Citation1995; Begg & Mazengarb Citation1996; Leonard et al. Citation2010). The Esk Head Belt is almost continuous over its 200 km length (but offset by many Cenozoic faults), and is often up to 20 km in width ().
Rattenbury et al. (Citation2006) inferred fault boundaries for much of the belt through North Canterbury and Marlborough, but clear fault boundaries are present in the North Island. The melange zones contain blocks of sandstone, minor conglomerate, Late Triassic–Early Jurassic limestone, spilitic volcanics, dolerite and Triassic–Jurassic radiolarian chert (Bradshaw Citation1973; Silberling et al. Citation1988). There are also rare Late Jurassic macrofossil occurrences (Campbell & Warren Citation1965) which occur in a matrix of sedimentary origin, often as broken formation but occasionally as massive sandstone enclaves (Rattenbury et al. Citation2006). Several minor melange belts also occur within the Pahua Terrane proper, e.g. the Random Spur Melange (Bandel et al. Citation2000) in the Kaiwara River valley near Cheviot. Silberling et al. (Citation1988) suggested that the development of the Esk Head Melange occurred during subduction of a Triassic–Jurassic oceanic plate and simultaneous deposition of trench-slope Pahau Terrane clastic rocks. However, it is clear from the size, disposition and attenuation of many blocks that a true tectonic melange must have developed subsequently (Bradshaw Citation1973).
Kaweka Terrane in the North Island
The Torlesse Composite Terrane of northern South Island continues into the North Island (), where there is an excellent cross-section of Rakaia and Pahau terranes along the southern Wellington and Wairarapa south coasts of the North Island (George Citation1988; Barnes & Korsch Citation1990, Citation1991; Suneson Citation1993; Adams & Graham Citation1996; Begg & Mazengarb Citation1996). The Pahau Terrane continues northwards along the east coast of the North Island, across the Hawkes Bay region to the Raukumara Peninsula. Along its western boundary there are fault-bounded blocks of the Esk Head Belt. The Rakaia Terrane is 30 km broad at the Wellington coast. To the north along the western flanks of the Tararua Range it becomes much attenuated against the Esk Head Belt, and disappears at the southern end of the Ruahine Range. Further north, on the western side of the Ruahine and Kaweka Ranges and throughout the Kaimanawa Range, there are extensive sandstone-dominated turbiditic rocks in which fossils are very rare; only two Late Jurassic macrofossils (Speden Citation1976) and some broad-ranging Early Cretaceous dinoflagellates (Leonard et al. Citation2010) have been recorded. These have now been defined as a Kaweka Terrane (Adams et al. Citation2009a) on the basis of (1) inferred fault boundaries with Rakaia and Waipapa terranes to the north and west and clear fault boundaries with Pahau Terrane to the southeast, (2) Jurassic depositional ages, especially in the Early–Middle Jurassic interval, and (3) detrital zircon age and initial Sr-isotope patterns which are intermediate between those of Rakaia, Pahau and Waipapa terranes. In their original definition, Adams et al. (Citation2009a) did not consider the inclusion of the Esk Head Belt in the Kaweka Terrane.
Near the boundary with Waipapa Terrane (Spörli Citation1978), the rocks pass from prehnite-pumpellyite facies into higher grade pumpellyite-actinolite facies Kaimanawa Schist (Beetham & Watters Citation1985) which yield Early Cretaceous, c. 135 Ma, Rb–Sr metamorphic ages (Graham Citation1985; Adams et al. Citation2009a). The Rb–Sr age and initial 87Sr/86Sr characteristics of these Kaweka Terrane metasediments are intermediate between (but slightly overlap) those of the Waipapa and Rakaia terranes (Adams et al. Citation2009a). The youngest U–Pb detrital zircon ages in ten Kaweka sandstone samples constrain maximum depositional ages variously to the Early and Middle Jurassic. However, the overall Kaweka zircon age patterns are intermediate between those of undoubted Late Triassic Rakaia Terrane and undoubted late Early Cretaceous Pahau Terrane sandstones. Using this trend, Adams et al. (Citation2009a) suggested that the youngest zircon age components could record contemporary volcanism (and deposition) in Early and Middle Jurassic time. These similarities affirm the position and status of the Kaweka Terrane as a distinct subunit within a Torlesse Composite Terrane.
Sample collection and experimental techniques
Samples for Rb–Sr isochron and U–Pb detrital zircon dating were collected from small areas (1–10 m) of Torlesse metasediments at the localities numbered in . Also included here are sampling localities for other Torlesse detrital zircon studies mentioned in the text. Details of sample localities (both new and previously published) are listed in .
Table 1 Kaweka Terrane South Island study: details of Torlesse sandstone samples.
Rb–Sr samples were collected within graded beds to represent a lithological range from fine-grained sandstone, through siltstone, to mudstone. Whole-rock powders (<5 micron) were prepared in a tungsten-carbide swingmill for 3–5 minutes. Rb–Sr technical procedures and age calculations then followed those of Adams & Maas (2004a). The analytical data are tabulated in , and presented in isochron form in .
Figure 2 Isochron diagrams. A, Rb–Sr whole-rock isochron diagram for Torlesse Composite Terrane metasediments at locality (4), Lake Sumner Rd, North Canterbury (data in ). B, Rb–Sr isochron age (t) and initial 87Sr/86Sr ratio (i) whole-rock isochron data for Torlesse and Waipapa metasediments throughout New Zealand (Graham & Mortimer Citation1992; Adams & Graham Citation1996, Citation1997; Adams et al. Citation1999, Citation2009a; Adams & Maas Citation2004a, Citationb; this work). Waipapa Terrane: black; Torlesse Composite Terrane subunits Rakaia Terrane: blue; Kaweka Terrane: red; Pahau Terrane: green; Kaweka Terrane locality (4): bold. Decay constants used are from Steiger & Jaeger 1977.
Table 2 Rb–Sr whole-rock isochron analytical and age data for Torlesse metasediments at locality (4) in North Canterbury.
For detrital zircon age studies, coarse- to medium-grained sandstones were sampled from graded beds. General procedures for zircon separation are given in Adams et al. (Citation2009a, Citationb). U–Pb ages were determined on an Agilent 7500 LA-ICPMS instrument at GEMOC laboratory, Macquarie University, Sydney using the general analytical and calibration techniques of Jackson et al. (Citation2004). Further technical details relevant to the present study are given in Adams et al. (Citation2009a, Citationb). Full isotopic and age data are tabulated in Appendix 1, and probability density/histogram diagrams generated with ISOPLOT 3 software (Ludwig Citation2003) are shown in .
Figure 3 Combined cumulative probability/histogram diagrams of detrital zircon age data from Torlesse sandstones in the Kaweka Terrane (middle column) compared with those from neighbouring Rakaia Terrane (left column) to the west and Pahau Terrane to the east (right column). Dataset for locality (1) is a representative Kaweka sandstone from the upper Rangitikei River area, central North Island (Adams et al. Citation2009a). Below this, datasets for localities (2)–(5) are from suggested equivalent rocks in Marlborough and North Canterbury, South Island (this work). Rakaia Terrane datasets are from northern South Island: dataset at locality (6) is from Rainbow River, Nelson (Pickard et al. Citation2000) and at locality (7) from Mt Pember, Canterbury (this work). Pahau terrane datasets for locality (8) are from Hundalee, North Canterbury (Pickard et al. Citation2000) and for locality (9) from Ethelton (Wandres et al. Citation2004a). Significant age components (expressed in millions of years) are shown in bold italics; other age components are shown in normal italics (from Appendix 1). Ages >500 Ma are stacked at the right-hand side. Ages < 1000 Ma are 238U/206Pb data, ages > 1000 Ma are 207Pb/206Pb data.
Where the probability density plots show clear components, their ages are computed using a weighted average. Their ages, errors (95% confidence) and statistical MSWD (goodness-of-fit) parameters are shown in Appendix 1. Where some overlap of age components is apparent, the ISPLOT ‘Unmix Ages’ algorithms are employed and the ages, errors and proportions of deconvoluted components are shown in Appendix 1. Rb–Sr and U–Pb age values are expressed as geological ages using the New Zealand Geological Timescale of Cooper (Citation2005).
Results
Geochronological studies were concentrated in areas that might represent a South Island continuation of the Kaweka Terrane and could be readily compared with the large database of nearly 30 zircon datasets in the Rakaia/Pahau terranes. They were therefore selected east of known Rakaia Terrane rocks with rare Late Triassic fossils in Nelson Lakes region (Campbell Citation1983; Rattenbury et al. Citation2006) and at Arthurs Pass (Cave Citation1987; Cox & Barrell Citation2007), and west of areas of known Pahau Terrane rocks with Late Jurassic–Early Cretaceous fossils (Campbell & Warren Citation1965; Rattenbury et al. Citation2006) in Marlborough and North Canterbury. Four such areas, Wairau Valley, Lake Tennyson, Lake Sumner and MacDonald Downs all fall within the Esk Head Belt as presently mapped (Rattenbury et al. Citation2006; Forsyth et al. Citation2008). A sandstone was also analysed from Mt Pember, North Canterbury where Torlesse rocks are locally intruded by an unusual Early Jurassic diorite (Jongens et al. Citation2009), in order to ascertain whether it showed Rakaia or Kaweka terrane age characteristics.
Petrography
Thin section examination of sandstones from Lake Tennyson (3) and Lake Sumner (4) (italicised numbers in brackets refer to locality number) shows both to be subangular, poorly sorted, fine-grained feldsarenites, typical of much of the Rakaia, Kaweka and Pahau terranes. Detrital quartz, unalbitised feldspar, muscovite, biotite, titanite and lithic grains are visible. Metamorphic pumpellyite occurs in some feldspars in both samples, along with rare prehnite at locality (4).
Rb–Sr age of metamorphism and initial Sr-isotope ratios
At locality (4) on Lake Sumner Road (), Torlesse metasediments within prehnite-pumpellyite facies yield a Rb–Sr whole-rock isochron age (t) 170±24 Ma and corresponding initial 87Sr/86Sr ratio (i) 0.7073±0.0007 (A). Following the interpretation of Torlesse and Waipapa Terrane (t)-(i) data in Wellington and Marlborough (Adams & Graham Citation1996; Adams et al. Citation1999) in metamorphic profiles within a similar prehnite-pumpellyite to lower greenschist facies range, the Lake Sumner age is treated as a cooling age, and indicates a minimum Jurassic age for metamorphism. The (t)-(i) values fall within a Kaweka Terrane data field acquired from eight similar Rb–Sr datasets in North Island localities (B; data compiled from Graham & Mortimer Citation1992; Adams & Graham Citation1996, Citation1997; Adams et al. Citation1999, Citation2009a; Adams & Maas 2004b). It should be noted that four of these are within slightly higher metamorphic grade, pumpellyite-actinolite facies, Kaimanawa Schist. Although the Kaweka Terrane data cannot be uniquely discriminated from those of the adjacent Rakaia and Waipapa terrane datasets, they certainly occupy an intermediate position between them.
Detrital zircon age patterns
New detrital zircon data are reported here for sandstones from five localities (2–5, 6) shown in and presented as combined probability density and histogram diagrams. In this diagram they are compared with some representative Torlesse data from localities in the Triassic Rakaia (7) and Cretaceous Pahau terranes (8, 9) in the South Island and typical Jurassic Kaweka Terrane (1) in the North Island, as previously reported by Pickard et al. Citation2000), Wandres et al. (Citation2004a) and Adams et al. (Citation2009a). All the data demonstrate the two large age groupings characteristic of the Torlesse Terrane, namely (1) a Permian–Triassic group, at least 30% of the total set and commonly 40–55%, and (2) a Precambrian–Early Palaeozoic group, at least 20% of the total and commonly 30–40%. Within these broad groups there are several significant age components (; Appendix 1), usually with those in the Permian–Triassic group of mostly Late Permian to Early Triassic. Otherwise, in the intervening Silurian–Carboniferous interval, there is only one at (4), namely 313±3 Ma.
The new detrital zircon age pattern from Mt Pember (6), with a youngest zircon age component of 209±2 Ma, imposes a maximum late Late Triassic depositional age. The nearby intrusive Mt Pember Diorite, c. 189 Ma (Jongens et al. Citation2009), provides an Early Jurassic minimum age. This pattern is similar to other Late Triassic, Rakaia Terrane localities in the South Island (Pickard et al. Citation2000; Wandres et al. Citation2004b; Adams et al. Citation2007).
In , the four remaining new datasets (2–5) are set in the central column to allow a visual comparison with the representative North Island Kaweka Terrane pattern (1) noting that this is one of ten obtained (Adams et al. Citation2009a). Their age patterns are similar to those for older, Permian–Triassic Rakaia samples, e.g. (7). Crucially, however, there is in each a smaller Jurassic component, 7–10% of total, which is important in constraining a maximum depositional age for the sandstones, namely (2) 169±5 Ma (8%, n = 4), (3) 167±6 Ma (5%, n = 4), (4) 171±9 Ma (5%, n = 4) and (5) 176±2 (5%, n = 3).
Discussion
The Rb–Sr age and initial Sr ratio values at the Lake Sumner locality (4) fall within the datafield for Kaweka Terrane in the North Island and support a continuation of the latter into the South Island.
In the detrital zircon U–Pb age data, the major Permian–Triassic group described above is common in sandstones from all Eastern Province terranes (Cawood et al. Citation1999; Pickard et al. Citation2000) and is particularly important in Torlesse examples (Pickard et al. Citation2000; Wandres et al. Citation2004a, Citationb; Adams et al. Citation2007, Citation2009a) (). In addition, the major groups of Precambrian–Early Palaeozoic ages are very characteristic of the Torlesse Terrane. The present detrital zircon age results with the Rb–Sr (t)-(i) data (above) therefore strongly support the placement of these rocks within the Torlesse Composite Terrane, as expected. Furthermore, the smaller, Middle Jurassic age components (c. 170 Ma) support a South Island continuation for the Kaweka Terrane and impose maximum depositional age limits similar to those deduced for their counterparts in the central North Island.
In , age data are presented for the significant component peaks noted in the individual probability density curves of together with those from the previously published Torlesse Terrane examples cited above. These data are stacked on the vertical axis in assumed stratigraphic age order. This is well established from fossil occurrences nearby in some examples, but in unfossiliferous Kaweka Terrane examples only the youngest significant zircon component age can be used to provide a maximum value for this. However, on the basis of data from Torlesse sandstones of known stratigraphic age (Pickard et al. Citation2000; Adams et al. Citation2007), this almost invariably coincides with the stratigraphic age.
Figure 4 Detrital zircon 238U/206Pb age components derived from cumulative probability diagrams ( and Appendix 1) for metasediments of the Torlesse Composite Terrane, stacked vertically from top to bottom in ascending order of maximum stratigraphic age. Where the stratigraphic age is uncertain, a maximum age estimate is taken from the youngest zircon ages and/or minimum metamorphic age data (where available). Each data box represents a significant zircon age component of n ≥ 4 analyses and ≥ 4% of total dataset (usually N = 50–100), whose position and width on the horizontal axis represent the component age and error and whose height on the vertical axis shows the proportion of that component as a percentage of the total dataset (for comparison see scale bar at right, representing 25%). Data boxes from the present study are bold; those from previously published work are light. The dot-dash diagonal line represents a stratigraphic age limit, the minimum value for detrital zircons. Locality numbers (in italics) are at right margin.
In almost all Torlesse samples (), the Permian–Triassic components form the largest group irrespective of stratigraphic age. In detail, these fall mainly in the interval 270–230 Ma. However, Late Carboniferous–Early Permian zircons (310–280 Ma), commonly found in the Rakaia Terrane, are rarer in sandstones from the North Island Kaweka Terrane and the South Island extension suggested here. In the locality (2–5) samples, there are only two pre-Permian zircon significant age components, namely (4) 313±3 Ma and (5) 497±6 Ma. The Jurassic age components at these localities follow the pattern of those in the North Island Kaweka Terrane; from north to south they constitute a diminishing proportion as a percentage of total, from 7–21% (and all but one 16–21%) in localities in the North Island (Adams et al. Citation2009a) down to 7–8% in the South Island (this work). This suggests either a possible southwards-waning of contemporary volcaniclastic sediment inputs or an overwhelming increase in older, continent-derived sediment.
Age and extent of the Kaweka Terrane
shows the position of the Kaweka Terrane as first established from the Kaimanawa Ranges (Adams et al. Citation2009a). The sparse sampling does not allow any further speculation about the location of its terrane boundaries in the North Island. However, even with limited sampling, a continuation of the terrane into the South Island is indicated. In the North Island, Kaweka Terrane rocks are mostly within sandstone-dominated tracts outside the Esk Head Belt forming its southeast margin, but in the South Island they are (so far) only from within the Esk Head Belt. Adams et al. (Citation2009a) did not analyse rocks from within the Esk Head Belt in the North Island, and therefore did not consider its inclusion in the terrane. Since this is the case for the possible South Island continuation, then the Esk Head Belt must contain a significant ‘native’ content of clastic rocks rather than just deformed, older Rakaia (Triassic) and younger, Pahau (Early Cretaceous) terrane rocks. Since fossils as young as Late Jurassic do occur within Esk Head Belt matrix (Campbell & Warren Citation1965), then the Kaweka Terrane must extend through the Late Jurassic. A younger age limit (early Early Cretaceous) is imposed by 130–135 Ma metamorphic ages in the Kaimanawa Shist (Graham Citation1985; Adams et al. Citation2009a). This placement is further supported if the Esk Head Belt through the North Island is retrospectively included in the Kaweka Terrane. As in the South Island, fossils as young as Late Jurassic occur in both melange blocks and matrix (Speden Citation1976; Begg & Johnston 2000; Leonard et al. Citation2010). As a consequence of the above, the Pahau Terrane then becomes overwhelmingly (if not entirely) of late Early Cretaceous (Barremian–Albian) age.
In , all Kaweka Terrane detrital zircon age data fall between those of Late Permian–Triassic and Late Jurassic–Early Cretaceous Pahau Terrane. These samples, although unfossiliferous, are the same in most respects (petrography, sedimentation) to those fossiliferous Torlesse Terrane samples cited in case studies above. We therefore make the important assumption that a similar behaviour applies to Kaweka Terrane samples; their youngest zircon age components are therefore close to their stratigraphic age, Early–Middle Jurassic. An apparent stratigraphic gap, suggested by the absence of Early–Middle Jurassic fossils in the Torlesse Composite Terrane (Campbell & Warren Citation1965; Fleming Citation1970), is thus closed. If the assumption is not accepted, then this Kaweka Terrane age assignment can only be regarded as a maximum estimate and a Late Jurassic–early Early Cretaceous depositional age for Kaweka Terrane becomes equally possible.
Summary of conclusions
Early–Middle Jurassic detrital zircon U–Pb ages indicate the presence of a distinct belt of Torlesse metasediments of maximum Early and Middle Jurassic depositional age in North Canterbury and Marlborough. The detrital zircon age patterns have Late Permian–Triassic and Precambrian–Early Palaeozoic age groups that are characteristic of the Torlesse Composite Terrane as a whole. As part of this, the Kaweka Terrane of the North Island continues into the South Island where it coincides with the Esk Head Belt, i.e. between Rakaia and Pahau Terranes. A Jurassic Rb–Sr metamorphic age and associated initial Sr isotope characteristics of these metasediments support this placement. If the Kaweka Terrane now includes the Esk Head Belt, then the age range of rocks within it must be extended through the Late Jurassic. A younger limit, early Early Cretaceous, is provided by 130–135 Ma metamorphic ages.
Acknowledgements
Mark Rattenbury and John Begg are thanked for their recommendations on sampling localities and advice about regional geology. Graeme Luther, VIEPS, Department of Geology, La Trobe University, Melbourne and Suzy Elhlou, GEMOC, Department of Earth and Planetary Sciences, Macquarie University are thanked for their technical assistance with Rb–Sr and U–Pb analyses, respectively. This is Australian Research Council Key Centre Publication Number 682. Mike Isaac and Mark Rattenbury are also thanked for early critical reviews. Comments from two anonymous reviewers also led to improvements in the manuscript.
Related Research Data
References
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