overview of the median batholith, new zealand: a new interpretation of the geology of the median...

Pergamon Journal of African Earth Sciences, Vol. 29, No. 1, pp. 257-268, 1999

Pll:SO899-5382(99)00095-O g 1999 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0899.5362/99 $- see front matter

Overview of the Median Batholith, New Zealand: a new interpretation of the geology of the Median

Tectonic Zone and adjacent rocks

N. MORTIMER,‘** A.J. TULLOCH,’ R.N. SPARK,’ N.W. WALKER,’ E. LADLEY,2 A. ALLIf30NE3 and D.L. KIMBROUGH4

‘Institute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, New Zealand 2Department of Geology, Otago University, PO Box 56, Dunedin, New Zealand

3SRK Consulting, PO Box 250, Deakin West, ACT 2600, Australia 4Department of Geological Sciences, San Diego State University,

San Diego, CA, USA

ABSTRACT-This paper proposes an alternative high-order, non-genetic classification of the basement rocks of medial New Zealand. More than 90% of the rocks in the Median Tectonic Zone are plutonic and can be included in part of a newly defined Carboniferous to Early Cretaceous, ca 10,200 km2 composite regional batholith - the Median Batholith. The plutonic rocks of the batholith intrude the volcanic and sedimentary rocks of the Brook Street and Takaka Terranes (Eastern and Western Provinces, respectively). Emerging matches between the chronology of magmatism in the Median Batholith and batholiths in the Western Province also support probable in situ growth of most of the batholith. The internal and external contacts, and shape, of the batholith have been strongly modified by post-plutonic Cretaceous and Late Cenozoic tectonism, particularly within 50 km of the Alpine Fault. The Median Batholith represents a significant but previously little-recognised 250 Ma record of magmatism along the continental margin of South Gondwana, and invites comparison with other Cordilleran batholiths. @ 1999 Elsevier Science Limited. All rights reserved.

RESUME-Ce manuscrit propose une classification non-genetique des roches du socle de la Nouvelle Zelande centrale. Plus de 90% des roches de la Zone Tectonique Mediane sont plutoniques et peuvent dtre considerees en bonne partie comme composant un batholite regional (10.200 km21 composite Carbonifere a C&ace inferieur nouvellement defini que nous appelons le batholite median. Les roches plutoniques de ce batholite intrudent les roches volcaniques et sedimentaires des terranes de Brook Street et de Takaka (Provinces orientale et occidentale, respectivement). La concordance qui apparait entre la chronologie du magmatisme au sein du batholite median et des batholites de la Province occidentale suggere egalement une genese en place de la plus grande partie du batholite. Les contacts internes et externes, comme la forme du batholite ont Bte fortement modifies par les tectoniques post-plutoniques Cretacee et Cenozoi’que terminal, particulierement a moins de 50 km de la faille alpine. Le batholite median represente un enregistrement magmatique important, quoique peu reconnu, de 250 Ma le long de la marge continentale du Gondwana meridional, ce qui invite a le comparer avec d’autres batholites cordillerains. @ 1999 Elsevier Science Limited. All rights reserved.

(Received l/7/98: revised version received 30/l 199: accepted 912199)

*Corresponding author [email protected] (N. Mortimer)

Journal of African Earth Sciences 257

N. MORTIMER et al.

INTRODUCTION

The New Zealand basement consists of Cambrian to Early Cretaceous rocks that were deposited, intruded, erupted and/or deformed at plate boundaries near the southern edge of the Gondwana continent (Fig. la). Gondwana break-up in the New Zealand region in the Late Cretaceous means that the nearest Precambrian craton is now found in neighbouring Australia and East Antarctica. New Zealand basement geology is currently described in terms of batholiths, suites and tectonostratigraphical terranes grouped at the highest level into Western Province, Median Tectonic Zone and Eastern Province (central panel of Fig. 1 b; see also Landis and Coombs, 1967; Coombs eta/., 1976; Bradshaw, 1989; Mortimer and Campbell, 1996 for reviews). The terrane approach, which emphasises potential allochthoneity of crustal fragments, and continental growth through terrane accretion, has proved to be very successful in elucidating the geological evolution of New Zealand.

The Early Palseozoic Buller and Takaka Terranes of the Western Province are intruded by Devonian I- and S-type plutons, mostly in the Karamea Batholith (Fig. 2). All these rocks have broad counterparts in

EAST ANTARCTICA

MEDIAN MEDIAN MEDIAN TECTONIC LINE TECTONIC ZONE BATHOLITH

Figure 1. la) Middle Jurassic palaaogeographic reconstruction (after Mortimer et al., 1995) showing the location of the Eastern and Western Provinces of the South Island of New Zealand (Sl EP, SI WP, respectively) at the margin of continental Gondwana (dashed line). Eastern Province terranes may not have accreted at this time (Adams et al., 1998). Ferrar Dolerite rocks are in black. Ibl Evolution of the high level nomenclature of the basement of the South Island, New Zealand from the Median Tectonic Line (Landis and Coombs, 19671, to the Median Tectonic Zone (e.g. Frost and Coombs, 1989; Kimbrough et al., 1993; Bradshaw, 1993) to the Median Batholith (this paper).

Australia and in Northern Victoria Land and Marie Byrd Land of Antarctica (Cooper and Tulloch, 1992; Muir e? a/., 1996a, b). Recent identifications of Per- mian and Triassic sedimentary Gondwana sequences, along with Jurassic Ferrar Dolerite (Mortimer et al., 1995) have confirmed that the Western Province was a part of stable, consolidated autochthonous South Gondwana (Fig. la) by at least the Late Palaeozoic. However, there is much less agree- ment on when the rocks of the Median Tectonic Zone (MTZI and several Eastern Province terranes became relatively immobile parts of Gondwana (e.g. Adams et al., 19981. This paper contributes to this debate by presenting a new interpretation of the geology of the MTZ.

SCOPE OF PAPER

Over the last five years various targeted co-operative studies have been undertaken in geochemistry, palaeomagnetism, isotope geology, geochronology and structural analysis of the Median Tectonic Zone, supported by detailed mapping of the contacts. This paper is an overview of much of that work, most of which has been published (e.g. Allibone and Tulloch, 1997; Kimbrough et al., 1993, 1994; Mot-timer and Tulloch, 1996, 1997; Mortimer et al., 1995, 1997, 1999; Tulloch et a/., 1999).

A significant result which is emphasised in this paper is that most of what has previously been called the MTZ can be viewed as the eastern half of a composite regional batholith that is called here the Median Batholith (right panel of Fig. lb). Data is presented suggesting that most of the batholith developed in situ along the edge of New Zealand’s Western Province (Gondwana) since the Carbonifer- ous. If true, this means that the combined plutonic

Figure 2. Simplified geological map of the Median Batholith and flanking rocks, South Island, New Zealand, subtracting the Alpine Fault displacement. Geology after Williams and Harper (19781, Johnston (198 II, King (1984). Gibson et al. (19881, Bradshaw (19901, Blattner (19911, Kimbrough et al. (19941, Allibone and Tulloch (1997). Muir et al. (1995, 1998), Tulloch et al. (1999) and Mortimer et al. (1999). Plutonic rock units are shown by patterns; metasedimentary and metavolcanic rocks which occur outside and inside the Median Batholith are white and black (with bold type), respectively. ‘U’ indicates an unconformable contact beneath sedimentary units and ‘I’ an intrusive contact of plutons into sedimentary units. Terrane abbreviations as in Fig. 3. Western Province batholiths and isolated plutons are K: Karamea Batholith; P: Paparoa Batholith; H: Hohonu Batholith; C: Crow Granite; R: Rocky Creek Granite; G: Mount George Gabbro; S: Supper Cove Orthogneiss; V: Revolver Pluton. TE: Te Anau; LR: Longwood Range; LH: Lake Hauroko. Compare the Median Batholith in this figure with the Median Tectonic Zone as shown in Fig. 1 of Kimbrough et al. (19941.

258 Journal of African Earth Sciences

Overview of the Median Batholith, New Zealand

_1 Separation Point Granite

Sams Creek dike

Kirwans (Ferrar) Dolerite

Platform Gneiss & Echinus Granite

Palisade Andesite & Drumduan Group (143-7208Ma) I & ?U

Big Bush Andesite &

(147-180Ma) I & U

uller Diorite & Rotoiti Gneiss MEDIAN BATHOLITH

AGES OF PLUTONS

(incl. Nurse, Glade)

- - - ___ ??Late Early Cret --- 125-105Ma -.-

H MJur-EE Cret 170-l 25Ma

lullI Mid-Late Tr 230-21 OMa

(incl. Mistake Diorite)

Largs ignimbrite / (140Ma) I EL ?U

Permian 290-250Ma

L. Dev-Carb 365-300Ma

___-- Lake Hankinson Complex _ Loch Bum Formation -

(195Ma) I & U

Lake Roxburgh Tonalite

III

-.::::* unknown, multiple .:.. . “: and/or composite

METAVOLCANIC & METASED.

/ ( ‘m Pomona Island granites \

~,~ ~

,

rw Electric Granite

LR Pourakino Trondhjemite

Sr md Hill Point meta- Se idiments (undated) 7

Paterson Group (146?‘-\ I a “lli’

Hekeia Gabbro

South West Arm Pluton -

STEWART IS1

Freds Camp Pluton

Escarpment Fault

bt (Pegasus Group)

batholiths and plutons in the Western Province

metavolcanic and metasedimentary rocks of the E & W Provinces

// Cenozoic faults

Penetrative fabrics (mainly Cretaceous)


N. MORTIMER et al.

record of the Western Province and Median Batholith can be used in preliminary interpretations of relative Palasozoic-Mesozoic plate motions between continental Gondwana and oceanic Panthalassa. This is a somewhat different interpretation from earlier and existing models of the MTZ as an amalgamated collage of small terranes, arcs and/or crustal fragments of uncertain status that were accreted to the Gondwanan margin as late as the late Early Creta- ceous (Bradshaw, 1993; Kimbrough et al., 1993, 1994; Muir et a/., 1995, 1998). The results presented in this paper focus on field relations and the igneous record, rather than structural and metamorphic events.

MEDIAN TECTONIC ZONE

For the last 30 years, the Median Tectonic Line or Zone has been recognised as a feature of medial New Zealand across which the pre-Late Cretaceous geology changes in fundamental character (Fig. 1 b; Landis and Coombs, 1967; Bishop et al., 1985; Bradshaw, 1993; Mot-timer and Tulloch, 1996). The original concept of the MTL as a fault separating components of a paired metamorphic belt (Landis and Coombs, 1967 and references therein) was superceded by its interpretation as a major terrane boundary (Bishop et a/., 1985) and then as a zone of deformed or dismembered small crustal fragments and/or magmatic arc rocks (e.g. Frost and Coombs, 1989; Bradshaw, 1993; Kimbrough et al., 1993). Reviews of the MTZ are given by Bradshaw (1993), Kimbrough et a/. (1993, 1994) and Mortimer and Tulloch (1997).

Content and new observations The name Median Tectonic Zone is a testimony to the fact that the ca 5000 km2 MTZ has defied, and continues to defy, simple and unified geological classification; as described by Bradshaw (1993) it includes terranes, formations, plutons, igneous complexes, gneisses and faults. There has been much emphasis on the presence of metasedimentary and metavolcanic rocks within the MTZ and appli- cation of the term ‘terrane’ to the Drumduan and Largs units (Fig. 2) has served to convey an impression of allochthoneity and non-correlation both within the MTZ and with other parts of New Zealand. However work on the MTZ in the last five years has emphasised and/or revealed that:

il plutonic rocks make up most of the exposed area of the MTZ; only ca 400 km* of the MTZ is underlain by volcanic or sedimentary rocks. The number of discrete plutonic units is also much greater than the number of volcanosedimentary ones (Fig. 2);


iii the volcanosedimentary units are of Jurassic or earliest Cretaceous age and most rest unconformably on, or contain clasts of, older plutonic rocks; all the volcanosedimentary units are intruded by younger plutons (Fig. 2; Williams and Harper, 1978; Johnston, 1981; King, 1984; Beresford et al., 1996; Tulloch et al., 1999; Mortimer et a/., 1999);

iiil the MTZ plutons are of Carboniferous to early Early Cretaceous age, and mainly of talc-alkaline I- type petrological character (Kimbrough et al., 1994; Muir et al., 1998);

iw) within 50 km of the Alpine Fault, many contacts on the eastern margin of the MTZ are brittle faults of probable Late Cenozoic age. Thus the time of movement on the Median Tectonic Line of Landis and Coombs (1967), and on other faults which disrupt MTZ units in Nelson and Fiordland (Bradshaw, 1993), is probably some 100 Ma younger than the igneous rocks which comprise the MTZ. In contrast > 50 km away from the Alpine Fault, in the Foveaux Strait area, intrusive contacts dominate (Allibone and Tulloch, 1997; Mortimer et al., 1999); and

w) in northern Fiordland and Nelson, plutonic and orthogneissic units in the western part of the MTZ (as shown by Kimbrough et al., 1994) lie adjacent to plutonic and orthogneissic units hitherto considered part of the Western Province (Fig. 2; italicised and unitalicised labels, respectively). In some cases these Western Province units are slightly younger than, and intrude, the MTZ plutons (e.g. Separation Point Batholith).

Collectively, these results point to a more coherent, dominantly plutonic and less tectonised MTZ than hitherto recognised. Because of this it was felt desira- ble to explore alternative high-level names that would convey, in a more direct manner, the dominantly plutonic geology of the zone and adjacent areas, particularly as depicted at the scale of Figs 2 and 3.

Alternative name Recent small-scale maps and diagrams of the New Zealand basement show a subdivision into terranes (e.g. Bishop et al., 1985; Bradshaw, 1989; Frost and Coombs, 1989), batholiths (Tulloch, 1988) or sometimes both (Kimbrough et al., 1994; Muir et a/., 1996a, 1998). The MTZ is also depicted as a regional unit in most of these maps. The term ‘Pro- vince’ has been used as a grouping of geological units east and west of the MTZ with the Eastern Province comprising only terranes, and the Western Province comprising terranes and batholiths (Landis and Coombs, 1967; Bradshaw, 1993). In exploring commonly used regional classification schemes as an alternative to ‘MTZ’, ‘Complex’ was rejected as conveying too disorganised an impression, and ‘Belt’


0 SEDIMENTARY &

TERRANES

P Pahau r Rakaia c Caples & Waipapa d Maitai (incl DMOB) m Murihiku S Brook Street

170”E

VOLCANIC ROCKS

t Takaka 9s 00

b Buller 5.2 x(D 83

I

174”E

b \ Christchurch

t N

1 OOkm

42%

a PLUTONIC ROCKS

BATHOLITHS H Hohonu (Cretaceous) P Paparoa (Cretaceous) M Median (Carboniferous-Cretaceous) K Karamea (Devonian)

46%

RtCilUNAL ME Ir ------- - - ----AMORPHIC-TECTONIC OVERPRINTS - ““&c.C~C /PIP yl IGIppGo ,~&aceous & some Devonian) 7 Haast Schist (Jurassic-Cretaceus)

Figure 3. Simplified, small-scale, MR-free map of the South Island basement showing subdivision into terranes, batholiths and metamorphic-tectonic overprints. DMOB: Dun Mountain Ophiolite Belt. Unlabelled areas are Late Cretaceous to Qua ternary cover.

and ‘Province’ (along with ‘Zone’) were rejected as being too neutral and devoid of concept. Unlike the above terms, ‘Plutonic Zone/Belt’ does convey the dominantly plutonic nature of the unit, but this terminology has not been applied to other batholithic bodies in New Zealand. ‘Median Terrane’ was offered by Kimbrough et al. (1993) and the MTZ has been termed a ‘superterrane’ by Bradshaw et al. (1997). As will be described more fully below, the MTZ plutons have been found to locally intrude both flanking terranes, so a terrane terminology is inappro- priate. Furthermore, many intra-MTZ contacts are intrusive and/or unconformable and the MTZ is, in many ways, actually less tectonised than other terranes and batholiths in New Zealand (e.g. Paparoa Batholith, Haast Schist, Fig. 3).

Tulloch (1988) produced a simple guide to the definition and nomenclature of some South Island plutonic rocks in terms of formal batholiths, plutons and suites. In view of the dominantly plutonic nature of the MTZ, the term Median Batholith is proposed here as both a practical and conceptually useful alternative way to describe the plutonic rocks of the MTZ and some of the adjacent Western Province plutons, at the scale of diagrams depicted in Figs 1 b, 2 and 3. Unlike the terms considered in the previous paragraph, use of the single, formalised word ‘Batholith’ is entirely compatible with the existing Tulloch (1988) scheme for New Zealand plutonic rocks. As Median Batholith is a new unit this paper will expand on its definition and significance. In doing so it is stressed that the authors are not necessarily

Journal of African Earth Sciences 26 1

N. MORTIMER et al.

recommending abandonment of the term MTZ; the motive is to suggest an alternative high-level classification scheme that conveys a more direct impression of rock types and that may be useful in regional geological work.

MEDIAN BATHOLITH Definition and geographic extent Batholiths are the largest of all intrusive bodies and have long been used to describe the voluminous, elongate belts of Phanerozoic circum-Pacific plutons such as the Coast Mountains Batholith, Sierra Neva- da Batholith, Peninsular Ranges Batholith, Coastal Batholith and New England Batholith. In most parts of the world, batholiths have been obvious and un- controversial features for decades; their internal complexity has been progressively revealed by increasingly detailed geochronlogical, structural and petrological studies. In contrast in New Zealand, many small areas of the proposed Median Batholith have been investigated in great detail, but the fact that these parts collectively constitute a contiguous batholith-sized whole has never previously been recognised. Reasons for this may include the thick New Zealand forest cover obscuring Sierran-quality and quantity exposures, offset of the batholith on the Alpine Fault, the obscuring of much of the batholith by Quaternary deposits and water, and a focus on Mesozoic tectonic models, geochemistry and geochronology rather than present day geometry and field relations.

In this paper, the term batholith is used entirely in accord with existing international (e.g. Pitcher, 1978; Bates and Jackson, 1987; Kearey, 1993) and New Zealand (Tulloch, 1988) definitions as being a large volume composite intrusion, of area > 100 km2 comprising many individual but contiguous plutons, and different suites. It is important to note that batholith is a non-genetic term that simply describes a large area of plutonic rock; there is no genetic connotation as to age or chemical similarity of constituent plutons. Furthermore, parts of batholiths can be deformed and metamorphosed (e.g. Hanson et al., 1988; Picket-t and Saleeby, 1993) and can contain some plutons that are allochthonous (e.g. Gehrels et al., 1991; Cowan et a/., 19971.

The Median Batholith is defined as all contiguous plutonic and metaplutonic rocks that underlie the ca 10,200 km* area between the volcaniclastic rocks of the Late Palsaozoic Brook Street Terrane and the siliciclastic and calcareous rocks of the Early Palaao- zoic Takaka Terrane (Figs 2, 3). In keeping with the convention for New Zealand’s terranes the same name for the batholith is used on both sides of the

262 Journal of African Eerrh Sciences

Alpine Fault (Fig. 2). For convenience the batholith can be divided into three geographic segments - Nelson, Fiordland and Foveaux (Fig. 2).

Figure 2 shows that in most parts of New Zealand there is a wide metasedimentary country rock region of Buller-Takaka Terrane rocks between the Median Batholith and the Western Province batholiths and plutons of Tulloch (1988). But, as defined above, the Median Batholith includes the previously-docu- mented ca 800 km2 Separation Point Batholith in Nelson (Fig. 2; Tulloch, 1988; Muir et a/., 1995) and also the ca 1200 km* Rakeahua Batholith of southern Stewart Island (All&one and Tulloch, 1997). These names should not necessarily be abandoned, but the Separation Point and Rakeahua Batholiths should be seen in a new context as portions of a much larger, composite, regional Median Batholith. Allowing for restoration of Alpine Fault displacement, the Median Batholith is the largest area of contiguous plutonic rock in New Zealand, greatly exceeding the combined areas of the Karamea (ca 3200 km*), Paparoa (ca 400 km*) and Hohonu (ca 300 km*) Batholiths (Fig. 3).

In the Foveaux Strait area, away from the trans- pressive influence of the Alpine Fault and late Early Cretaceous tectonic complexity of Fiordland, the Me- dian Batholith is at least 100 km wide (Figs 2, 3). This outcrop width is similar to that of other Cordil- leran batholiths, e.g. the Lima segment of the Coastal Batholith of Peru (70 km, e.g. Pitcher, 1978) and the Sierra Nevada Batholith (100 km, e.g. Saleeby, 1981). Plutonic rocks, correlative with those in the 400 km long on-land batholith, have been found in wells and dredges in the adjacent offshore region (e.g. Mor- timer et a/., 19971, suggesting a contiguous length of at least 600 km and local width of up to 120 km.

Constituent plutons and suites Like most batholiths, the Median Batholith comprises a variety of plutons of different ages and compositions. The grouping of the nearly 70 rock units in a single batholith emphasises the large area of contiguous plutonic and metaplutonic rocks in medial New Zealand for which, until now, there has been no collective name. Calling these rocks the Median Batholith does not reveal the wide range of constituent protoliths, crystallisation ages, and complexity of internal deformation and metamorphism that is present within it. In this sense the term is no different to existing high-level groupings of New Zealand rocks into provinces, terranes and other batholiths (e.g. Fig. 3) in which internal geological complexity is traded for the convenience of a short name.

The provisional division of the batholith into six age groupings of plutons (Fig. 2) reveals that about


35% of the exposed basement area of the batholith is composed of Triassic-early Early Cretaceous gab- broids, dioritoids and granitoids; this percentage would probably be greater if sub-basin and submarine parts of the batholith near Nelson and Te Anau, and under Foveaux Strait were included. Another 35% of the exposed Median Batholith consists of alkali- calcic and adakitic Separation Point Suite granitoids, mainly of 105-l 25 Ma age in the former Separation Point Batholith and Western Fiordland Orthogneiss (e.g. Tulloch, 1988; Tulloch and Rabone, 1993; Muir et a/., 1995). An overall westward younging trend is apparent within the Permian-Cretaceous rocks of the batholith.

Some 10% of the Median Batholith consists of a variety of Palaeozoic suites including:

i) Carboniferous l-type plutons, e.g. Lake Roxburgh Tonalite, older Pomona Island monzogranite and Echinus Granite (Kimbrough et al., 1994; Muir et al., 1998);

iii one Carboniferous A-type granite, Freds Camp pluton, on Stewart Island (All&one and Tulloch, 19971;

iii) Permian gabbro and trondhjemite in the Long- wood Range on the east side of the Median Batholith (Mot-timer et al., 19991; and

iv) the Riwaka Complex of Nelson (Muir et al., 1996bI. About 20% of the batholith consists of plutons and orthogneiss complexes, mainly in eastern Fiordland, of composite, unknown, or poorly determined age (Fig. 2).

Field relationships Continental margin batholiths can hide or contain ancient suture zones and terrane boundaries (e.g, Saleeby, 198 1; Cowan et a/., 1997); roof pendants and older plutons can be allochthonous, even if younger parts are clearly post-tectonic. Despite significant tectonism in the Nelson and Fiordland segments, many internal and external contacts of the Median Batholith contacts are demonstrably intrusive. Intrusive contacts between different age units within the batholith demonstrate significant internal integrity, and intrusive contacts with the surrounding terranes have important implications for the timing of terrane accretion.

Takaka Terrane con tat t Rocks of the Takaka Terrane in Nelson comprise thick siliciclastic and carbonate sequences; in Fiordland and Stewart Island these are represented by paragneisses and marbles. An intrusive contact is observed between the Takaka Terrane and plutons of the Median Batholith in at least five places (Fig. 2):

il a long contact of Separation Point Granite in Nelson (e.g. Bradshaw, 1993; Kimbrough et a/., 1994);

ii) between the Lake Hankinson Complex and the Lake Wapiti Paragneiss (King, 1984; Spark, pers. obs. at NZ map grid reference C42/728590);

iiil between the Western Fiordlan;d Orthogneiss and rafts of the George Sound Paragneiss (Bradshaw, 1990);

ivl a granodioritic orthogneiss unit that intrudes both the Albert Edward Gneiss and paragneisses on the shores of Lake Hauroko (e.g. Kimbrough et al., 1994; Ladley, pers. obs. at C45/6656151; and

v) between various Palaeozoic and Cretaceous plutons and Pegasus Group metasediments on Stewart Island (Allibone and Tulloch, 1997); notable among these is a l-l 0 m size quartzofeldspathic paragneiss xenolith in the Jurassic South West Arm Pluton (Fig. 2; Allibone, pers. obs. at E491354472).

Late Early Cretaceous plutons dominate the western edge of the Median Batholith and it has long been known that these relatively young plutons effectively stitch older plutons of the batholith to the Takaka Terrane (e.g. Bradshaw, 1983; Kimbrough et al., 1994). Apart from the South West Arm Pluton on Stewart Island, there is no example of where a pre-Cretaceous pluton of the Median Batholith intrudes the Takaka Terrane, but indirect evidence for a probable pre-Cretaceous in situ origin for the Median Batholith is described below.

Brook Street Terrane contact Permian plutons in the Foveaux Strait area (Pourakino Trondhjemite, Hekeia Gabbro, Bluff Norite; Fig. 21 clearly intrude and contact metamorphose the volcaniclastic rocks of the Brook Street Terrane (e.g. Mortimer et al., 1999; Mortimer, pers. obs. at e.g. D46/147355). The Permian plutons have isotopically primitive compositions and are regarded as an inte- gral, subvolcanic part of the exotic and intraoceanic Brook St Terrane Arc by Mot-timer et a/. (1999). As such, they represent an allochthonous suite of rocks within the Median Batholith similar to some plutons in the Coast Mountains Batholiths of British Co- lumbia-Alaska (Gehrels et a/., 1991; Cowan et a/., 1997). The Permian plutons are intruded by 210- 230 Ma diorite dikes and plutons (Mortimer et a/., 1999; Mortimer pers. obs. at e.g. D46/106352), a relationship that suggests a Middle-Late Triassic age for Brook Street Terrane accretion to Gondwana. The Brook Street-Takaka Terrane contact is nowhere exposed in New Zealand, having been obliterated by intrusion of the Triassic-Cretaceouis plutons that now make up most of the Median Batholith. In eastern Fiordland and Nelson the Median Batholith-

Journal of African Barth Sciences 263

N. MORTIMER et al.

Brook Street Terrane contact is marked by Cenozoic faults (Mortimer et a/., 19991, although the Triassic Mistake Diorite (Fig. 2) has been interpreted by Williams and Harper (1978) to intrude the volcani- elastic rocks of the Brook Street Terrane.

Internal contacts All known Triassic and Carboniferous plutons and gneisses are either unconformably overlain by Jur- assic to earliest Cretaceous metasedimentary units (Tulloch et a/., 1999; Mot-timer et a/. 1999; Spark, pers. obs. of Lake Roxburgh Tonalite-Loch Burn Formation contact at D41/932635) or intruded by Jurassic and/or early Early Cretaceous plutons (Wil- liams and Harper, 1978; Johnston, 1981 with Plat- form Gneiss-Echinus Granite correlation of Beresford et a/., 1996; Allibone and Tulloch, 1997; Mot-timer et a/., 1999). Except at Sand Hill Point (Fig. 2) all metasedimentary-metavolcanic enclaves enveloped by the Median Batholith are themselves intruded by early Early Cretaceous plutons (Williams and Harper, 1978; Johnston, 1981; King, 1984; Allibone and Tulloch, 1997; Tulloch et a/., 1999). Most Jurassic to early Early Cretaceous plutons are intruded by late Early Cretaceous plutons of the Separation Point Suite (Kimbrough eta/., 1994; Alli-bone and Tulloch, 1997; Muir et a/., 19981.

This abundance of intrusive and unconformable contacts between different aged plutons and enclaves affirm the batholith’s pre-Late Cretaceous internal integrity and suggest a pattern of younger magmatic arcs developing in situ on or through older arcs. This is not to say that tectonic contacts are absent from the Median Batholith: as stated above, the Takaka-Brook Street Terrane boundary must occur within the batholith but has been entirely obliterated by Mesozoic intrusions. Recog- nisable regional structures within the batholith include:

i) the Rotoiti Gneiss: a narrow but > 50 km long Middle-Late Triassic ductile shear zone in the eastern part of the batholith (Mortimer et a/., 1999);

iii fabrics and a thermal overprint of post-intrusive, possibly extension-related, late Early Cretaceous granulite-amphibolite facies metamorphism and deformation, widespread in Fiordland (King, 1984; Gibson et a/., 1988; Bradshaw, 1990);

I!/ the Escarpment Fault and related orthogneisses on Stewart Island (Allibone and Tulloch, 1997). Currently there are no known intra-batholith structures along which Cretaceous ocean basin closure could have taken place (cf. model of Muir et a/., 19951, but this cannot rule out the possibility that future mapping within the Median Batholith may document this.


Indirect links with the Western Province Muir et a/. (1997a) used the presence of distinctive Palaeozoic igneous suites in the Buller and Takaka Terranes to infer the docking or close proximity of these terranes by the time of intrusion (see also Muir et a/., 1997b). Similar reasoning is used here, based on the presence of one Cretaceous and two Carboniferous suites to support the field evidence and argue that, prior to 125 Ma, most plutons within the Median Batholith intruded in a position adjacent to the Western Province. The evidence is:

i) l-type granites of Carboniferous age occur in the Buller Terrane (Paringa Suite of Cooper and Tulloch, 1992), as well as the Median Batholith (Echinus Granite, Lake Roxburgh Tonalite, Pomona Island; Kimbrough et a/., 1994; Muir et a/., 1998).

iTi) Distinctive Carboniferous A-type granites are known to occur only in two places in the Buller Terrane (Foulwind, Toropuihi; Cooper and Tulloch, 1992; Muir et a/., 1996a; Mot-timer et a/., 1997). A complex Carboniferous pluton which includes the range of A-type characteristics of the Foulwind and Toropuihi Granites is also present in the Median Batholith on Stewart Island (Freds Camp; Allibone and Tulloch, 1997).

iii) Five talc-alkaline plutons of Early Cretaceous age intrude the Western Province outside the Median Batholith. These are the Crow Granite (137 Ma SHRIMP age, Muir et a/., 1997b), the Rocky Creek and Revolver Plutons (both 132 Ma), and the Supper Cove and Mount George bodies (125-l 30 Ma) (Fig. 2; Tulloch, unpubl. data). Calc-alkaline magmatism of this age is widespread in the Median Batholith (Kimbrough eta/., 1994; Muir eta/., 1998). Plutons of the 105-l 25 Ma Separation Point Suite are numer- ous throughout the Western Province and Median Batholith (Fig. 4).

The current interpretation is that field, geochrono- logical and petrological relationships indicate that the Carboniferous, Late Triassic, Jurassic and Creta- ceous plutons of the Median Batholith probably intruded into and/or alongside New Zealand’s West- ern Province. The isotopically primitive Permian plutons along the east side of the batholith represent the subvolcanic intrusions of the exotic Brook Street Terrane that accreted to the Western Province in the Early-Middle Triassic.

REVISED NEW ZEALAND PLUTONIC RECORD

Figure 4 shows summary histograms of pluton crystallisation ages and petrological types in the Western Province batholiths and Median Batholith. Except where noted in the figure caption, all ages are con- strained only by U-Pb zircon data, obtained by ther-


KARAMEA, PAPAROA, HOHONU BATHOLITHS 8, MINOR WESTERN PROVINCE INTRUSIONS a

100 150 200 250 300 350 400 I I I I I II 11 0 II 11 11 11 10 11 1 ” 0 11 10 11

Triassic 1 I

Cretaceous Jurassic Permian 1 Carboniferous I Devonian I I I I I I III I I I I I I I

PETROLOGIC CHARACTER: rm l-type ??TTD-adakitic ??I!Stype I S-type ??A-typehntraplate

PROBABLE IN SITU N ZEALAND - GONDWANA MARGIN b

-Fe++ 7 co)?c-CM--- ? f--?

POSSIBLE GONDWANA-PANTHALASSA PLATE MARGIN: R=rift/passive, C=mainly convergent, S=mainly strike&p

Figure 4. Summary of age andpetrological character of pre-break-up New Zealand igneous activity. Based mainly on U-Pb TIMS ages (Kimbrough et al., 1994; Tulloch et al., 1997, 1999, unpub. data; Mortimer et al., 1999) but with additional data from Tulloch (1992/, Mortimer et al. (19951 and Muir et al. 119976). v: volcanic rocks; e: Electric Granite; s: Sams Creek Dyke.

mal ionisation mass spectrometer (TIMSI methods (e.g. Kimbrough eta/., 1994). As discussed by Muir et a/. (19981, the SHRIMP 206Pb/236U ages of many Median Batholith plutons are uncomplicated by inheritance and are within error of those previously obtained by the TIMS method. Notable features of autochthonous Gondwana margin magmatism revealed by Figs 2, 3 and 4 include:

i) A database of nearly 100 U-Pb ages, which is beginning to give an accurate picture of pulses and gaps in the plutonic history; the dominance of intrusive and unconformable contacts within the Median Batholith suggests (but does not prove) a lack of significant pre-Cenozoic tectonic excision of plutonic units. Although the exercise of normalising frequency of age against area of suites has not been done, it is believed that Fig. 4 conveys a reasonably true impression of relative magma volume with time. The major exception to this would be dates on scattered Early Mesozoic dykes in the Western Province, whose volume is trivial.

ii) Carboniferous plutonism in New Zealand which is of both I- and A-type, with pulses broadly coeval in the Median Batholith and Western Province.

iiil The Median Batholith containing a >250 Ma record of magmatism. Significantly, for most of this time the locus of magmatism in New Zealand did

not move outside the limits of the batholith. A pronounced magmatic lull occurs from the Early Permian to the Middle Triassic, and there is a possible short lull at ca 135 Ma. The latter may be a sampling gap or represent a brief interruption in the subduction regime as a result of which the 134 Ma peralkaline Electric Granite was emplaced. The Jurassic gap in subduction-related magmatism, postulated by Kim- brough et a/. (1994, see their fig. 51, has been partially bridged with some new data (see also Muir et al., 19981, though the gap at ca 180 Ma, corresponding to the passive margin-related Ferrar Dolerite intrusion in the Western Province (Mortimer et a/., 19951, may still be real. The Median Batholith ap- pears to entirely lack 85-l 00 Ma Gondwana break- up igneous rocks that occur, albeit sparsely, in the Eastern and Western Provinces (Waight et a/., 1999 and references therein).

iv) ln situ adakitic magmatism, which commenced in the Median Batholith at ca 160 Ma, started to dominate volumetrically over talc-alikaline activity from 130 Ma and cut off abruptly at ca 105 Ma. Reasons for the abundance of Jurassic-Cretaceous New Zealand adakites are model deipendent: heat input to melt eclogite may result from the subduction of young oceanic crust (Tulloch and Rabone, 19931, arc-thickening by collision (e.g. Muir et a/., 19951,


N. MORTIMER et al.

mafic underplating, development of a slab window, crustal delamination or possibly other tectonic situa- tions. Permian trondhjemites with adakitic chemistry are present in the eastern Median Batholith in the Longwood Range; these were generated in the allochthonous Brook Street Terrane arc (Mortimer et al., 1999).

v) An overall westward migration of the main axis of Mesozoic magmatism in the Median Batholith, culminating in the voluminous 125-I 05 Ma pulse of adakitic Separation Point Suite plutonism along the west side of the batholith and in the Western Province. The Triassic Sams Creek dyke (Tulloch, 1992) may be extension-related and back-arc with respect to the Median Batholith.

vi) A compilation of the New Zealand magmatic record shown in Fig. 4a to make a highly speculative interpretation of the changing nature of the South Gondwana-Panthalassa continent-ocean plate regime at different times (Fig. 4b, cf. Armstrong, 1988, fig. 221. Given the overall accretionary style of tectonics, and the need to move terranes with low latitude Tethyan faunas to New Zealand, the authors agree with Bradshaw (I 989) that the New Zealand margin of Gondwana (Fig. 1) has, prior to 105 Ma always been ‘active’ in some sense. Thus, in an overall trans- pressive framework, the magmatic gaps could be interpreted as resulting from periods of dominantly strike-slip terrane movement (e.g. Adams et a/., 19981, and the magmatic pulses as resulting from significant plate convergence (Fig. 4b). Future itera- tions of this figure will include an analysis of structural and metamorphic events in New Zealand.

Comparison with Australia and Antarctica The marked similarities in chronology and chemistry of magmatism between the MTZ and West Antarc- tica (especially Thurston Island) have been pointed out by Kimbrough et a/. (19941, Bradshaw et al. (I 997) and Pankhurst et al. (I 998) and are not discussed further here. On standard Tasman Sea closures, Queensland reconstructs to a position alongside New Zealand (Fig. I). Note the corres- pondence of the following between the Median Bath- olith and Queensland:

il A-type granites at ca 300 Ma (accompanied by extension in Queensland; Holcombe ef a/., 1998);

ii) accretion of the (very similar) Gympie and Brook Street Terranes in the Early Triassic, with subsequent intrusion by Middle Triassic l-type stitching plutons (Mot-timer et a/., 1999); and

a major Early Cretaceous volcanism in Queensland related to either or both of subduction and extension (Bryan et al., 1997). Mesozoic adakitic plutonism seems to be a special feature of the New Zealand,


but not the Antarctic and Australian, parts of the Gondwana margin (Muir et a/., 1995; Bryan et a/., 1997; Bradshaw et al., 1997 and references therein).

CONCLUSIONS

An increasingly clearer picture of the Phanerozoic plutonic record of New Zealand is being obtained through detailed fieldwork, backed by acquiring a large database of U-Pb TIMS ages and petrological data. Until now there has been no collective name for the 10,200 km2 belt of contiguous plutonic and metaplutonic rocks that intrude and lie between the volcanic and sedimentary rocks of the Takaka and Brook Street Terranes. It is proposed here that the new, non-genetic, model-independent term ‘Median Bath- olith’ be used to describe these rocks (Figs 1 and 2). The term Median Batholith more clearly commu- nicates the dominant plutonic content and regional nature of the units formerly grouped into the Median Tectonic Zone and eastern parts of the Western Pro- vince. It also provides an alternative and simple way to depict New Zealand basement geology at a small scale - in terms of terranes, batholiths and tectonic overprints - and without the enigmatic MTZ (Fig. 3).

The Median Batholith is a previously undescribed composite regional batholith of Cordilleran propor- tions, longevity and complexity (Fig. 2). It represents the main locus of magmatism in the New Zealand sector of Gondwana from the Carboniferous to the early Early Cretaceous. With the exception of east- ernmost Permian plutons which are part of the accreted Brook Street Terrane, the Median Batholith probably developed in situ, adjacent to New Zea- land’s Western Province. The Fiordland and Nelson segments of the batholith were strongly deformed in the late Early Cretaceous and again in the Late Cenozoic. The principal control on magmatic activity in the Median Batholith has been the interaction of Panthalassan oceanic plates with the South Gon- dwana continental plate. Phanerozoic igneous pulses and gaps (Fig. 41, which match some events in Antarctica and Australia, can be tentatively interpreted in terms of local relative plate motions, and probably merit consideration in terms of still-wider Gondwana geology and palseogeography. The recog- nition of the Median Batholith may provide a useful new perspective on studies of New Zealand continental growth, crustal structure, geological history, mineral exploration and palsaogeography.

ACKNOWLEDGEMENTS

The authors thank Chuck Landis, Keith Klepeis, Gary Ernst, Barry Roser, Charlotte Allen, Mark Rattenbury,


David Gust, David Thomas and Stephen White for their helpful comments. Reviews of earlier versions of the manuscript by Doug Coombs, Ian Turnbull, Hamish Campbell, Steve Weaver and Tod Waight led the authors to make significant improvements. The study was partially supported by the New Zealand Public Good Science Fund. Institute of Geological and Nuclear Sciences contribution 1559. This paper is a contribution to IGCP436.

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