metamorphism and metamorphic k–ar ages of the mesozoic accretionary complex in northland, new...

The Island Arc

(2004)

13,

416–431

et al.

*Correspondence address: 1648-1 Miyano-shimo, Yamaguchi 753-0011, Japan.

Received 10 November 2003; accepted for publication 1 March 2004.© 2004 Blackwell Publishing Asia Pty Ltd

Research Article

Metamorphism and metamorphic K–Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

Y

UJIRO

N

ISHIMURA

,

1,

* P

HILIPPA

M. B

LACK

2

AND

T

ETSUMARU

I

TAYA

3

1

Department of Earth Sciences, Faculty of Science, Yamaguchi University, Yamaguchi, 753-8512 Japan (email: [email protected]),

2

Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand

and

3

Research Institute of Natural Sciences, Okayama University of Science, Okayama, 700-0005 Japan

Abstract

A southwest dipping Mesozoic accretionary complex, which consists of tectoni-cally imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, andbedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island,New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. Thegrade of subduction-related metamorphism increases from northeast to southwest, indi-cating an inverted metamorphic gradient dip. Three metamorphic facies are recognizedlargely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite,prehnite-pumpellyite and pumpellyite-actinolite. From the apparent interplanar spacing

d

002

data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest gradeof metamorphism is considered to have attained only the lowermost grade of the pumpel-lyite-actinolite facies for which the highest temperature may be approximately 300

∞

C.Metamorphic white mica K–Ar ages are reported for magnetic separates and

<

2

m

mhydraulic elutriation separates from 27 pelitic and semipelitic samples. The age dataobtained from elutriation separates are approximately 8 m.y. younger, on average, thanthose from magnetic separates. The age difference is attributed to the possible admixtureof nonequilibrated detrital white mica in the magnetic separates, and the age of theelutriation separates is considered to be the age of metamorphism. If the concept, basedon fossil evidence, of the subdivision of the Northland accretionary complex into north andsouth units is accepted, then the peak age of metamorphism in the north unit is likely tobe 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas thatin the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. Theage cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouthsection (uncertain terrane), while the age cluster for the south unit is older than that ofthe Younger Torlesse Subterrane in the Wellington area, and may be comparable with thatof the Nelson and Marlborough areas (Caples and Waipapa terranes).

Key words:

accretionary complex, Caples Terrane, graphitization, K–Ar age, Northland,peak age of metamorphism, subduction metamorphism, Torlesse Terrane, WaipapaTerrane.

INTRODUCTION

It is widely accepted that the pre-Cretaceousbasement rocks of the South Island of New

Zealand can be divided into the Western Provinceand the Eastern Province connected by a majorsuture, the Median Tectonic Zone (Fig. 1). TheWestern Province is a fragment of the Gond-wanan continent, whereas the Eastern Province isa collage of six tectono-stratigraphic terranes thatwas sutured to the Gondwana margin in Mesozoictimes (Bishop

et al

. 1985; Frost & Coombs 1989;Mortimer 1993).

Metamorphic history of Northland, New Zealand

417

The tectonic framework of the North Island isfundamentally interpreted to be an extension tothe northeast of the terranes established in theSouth Island (summarized in Fig. 1) (Spörli & Bal-lance 1989; Aita & Spörli 1992; Aita & Bragin1999). In the North Island, however, the Gond-wana Foreland, Median Tectonic Zone and BrookStreet Terrane mostly lie beneath young sedi-ments and volcanics just offshore along the westcoast, while the Murihiku, Waipapa, Torlesse andMata River terranes are relatively well exposedonshore. A terrane on the east of the JunctionMagnetic Anomaly (Hunt 1978), which is believedto represent the Dun Mountain–Maitai Terrane, isinferred to be the northeast extension of theCaples Terrane (Blake

et al

. 1974), but it isreferred to as the Waipapa Terrane in the NorthIsland (Spörli 1978). However, the subdivision andcorrelation of the Waipapa as well as of the CaplesTerrane have recently become a matter of debate(e.g. Black 1994; Begg & Johnston 2000; Campbell2000; Coombs

et al

. 2000).

While there have been studies of the WaipapaTerrane Mesozoic accretionary complex of theNorthland Peninsula investigating its lithology(Mayer 1969; Kear 1971; Jennings 1989; Meshesha& Black 1989), paleontology (Hornibrook 1951;Spörli & Grant-Mackie 1976; Caridroit & Ferriére1988; Spörli

et al

. 1989; Aita & Spörli 1992; Take-mura

et al

. 1998, 1999; Aita & Bragin 1999; Aita

et al

. 2001; Takemura

et al

. 2001), and metamor-phism (Brothers 1956; Black 1989; Jennings 1989),there has been no radiometric age dating study.The age of the subduction-related metamorphismhas provided a powerful tool for constrainingterrane affinities and clarifying the geologicaldevelopment and geotectonic subdivision ofaccretionary complexes (e.g. Isozaki 1996; Nishi-mura 1998).

In this paper, we re-examine the metamorphismof the Mesozoic accretionary complex mainlyexposed in the Northland Peninsula, provide pre-liminary data for the Coromandel Peninsula, andpresent K–Ar ages for recrystallized white micaconcentrated by magnetic separation and hydrau-lic elutriation methods. The objective is to investi-gate the feasibility of subdivision of the complexand to compare the age data obtained with pub-lished data for similar rocks elsewhere in theNorth Island and in the South Island. The geolog-ical time scale used is that of Gradstein

et al

. (1994)for the Mesozoic and Odin (1994) for the Permian.Abbreviations for rock-forming minerals are thoseof Kretz (1983).

GEOLOGICAL SETTING

The Northland and Coromandel peninsulas, in thenorthern North Island, have exposures of the Wai-papa Terrane accretionary complex along theirnortheastern and western margins, respectively,but elsewhere, the basement is overlain by sedi-mentary and volcanic rocks of Cretaceous toCenozoic age (Fig. 2). The complex consists oftectonically imbricated trench fill, turbiditic mud-stone and sandstone of terrigenous origin; sili-ceous mudstone from a hemipelagic environment;and bedded chert and basaltic rocks of pelagic ori-gin. Interpillow limestone is sometimes associatedwith the basaltic rocks. The general strikes in thecomplex are roughly parallel to the trends of thepeninsulas, and dips are moderate to the south-west. Unfortunately, inland exposures of WaipapaTerrane are poor and the basement rocks aredeeply weathered, so studies are confined to

Fig. 1

Generalized tectono-stratigraphic terrane map of New Zealand(compiled from Bishop

et al

. 1985, Frost & Coombs 1989 and Mortimer1993 for the South Island; and Spörli & Ballance 1989, Aita & Spörli 1992and Aita & Bragin 1999 for the North Island). MTZ, Median Tectonic Zone.

418

Y. Nishimura

et al.

coastal sections and quarries which provide onlyoccasional windows into the sequence.

The Waipapa sequence is sparsely fossiliferous.Hornibrook (1951) described fusulinids and radio-larians of Late Permian to Late Jurassic ages fromseveral areas in Northland. Aita and Spörli (1992)provided the first stratigraphic sections with oceanfloor sequences of Tethyan affinity and terrigenoussequences with non-Tethyan features, based onsystematic descriptions of radiolarian fauna sup-plemented with non-radiolarian fossils, and sug-gested a possible division of the Waipapa Terraneinto two or three subterranes or units (north andsouth). Aita

et al

. (2001) have recently revised thesections on the basis of new radiolarian studies(e.g. Takemura

et al

. 1998, 1999), as shown inFigure 3. Although the boundary between the

units has not yet been recognized, the samplesexamined by us have all been provisionallyassigned to either the north or south units afterAita and Spörli (1992) and Aita

et al

. (2001), asshown in Table 1.

In contrast, Black (1994) classified the basementrocks of the northern North Island, including thewhole of this study area, into five types based ontheir lithologic features and age relationships(largely based on faunal evidence) as follows:Omahuta–Puketi, eastern Bay of Islands, westernBay of Islands, Helena Bay–Hunuas and Moehau–Morrinsville. Black also proposed that theOmahuta–Puketi area is correlated with the CaplesTerrane, the eastern and western Bay of Islandsareas with the Older Torlesse or Rakaia Subter-rane, and the Helena Bay–Hunuas and Moehau–

Fig. 2

Simplified geological map of Northlandshowing metamorphic mineral zones, samplelocalities and fossil-bearing sequences (compiledfrom New Zealand Geological Survey 1972, Black1989 and Aita

et al

. 2001). Refer to Figure 4 fordetails of mineral zones.

Metamorphic history of Northland, New Zealand

419

Morrinsville area with the Younger Torlesse orPahua Subterrane, respectively. As shown inTable 1, the terms Caples Terrane, Older TorlesseSubterrane and Younger Torlesse Subterrane arealso assigned to the samples examined.

METAMORPHISM

MINERAL ZONES

On the basis of the appearance and disappearanceof some metamorphic minerals, the basementrocks of the Northland area were mapped into fourmineral zones by Black (1989): zeolite, prehnite,prehnite-pumpellyite and pumpellyite-epidote-actinolite. The four mineral zones parallel thenorthwest–southeast trend of Northland and thegeneralized regional strike of the basement rocks,and the metamorphic grade increases from north-east to southwest, indicating an inverted meta-

morphic gradient (Fig. 2). Jennings (1989) alsodivided the Omahuta–Puketi area into prehnite-pumpellyite and pumpellyite-epidote-actinolitezones, and estimated the temperatures to rangebetween 230 and 350

∞

C at pressures between 2 and5 kbar.

In this study, the mineral zonations in Northlandhave been re-investigated with the examination ofapproximately 150 additional thin sections, andthey have been confirmed as reliable in outline.However, because of the scarcity of basaltic rocksand the lack of continuous outcrops, the zoneboundaries are not precisely located. The fourzones are termed hereafter Zones A, B, C and Din Figure 2, and they are defined by the mineralparageneses as outlined in Table 2 and Figure 4.Preliminary examination has also revealed thatthe rocks of the western side of the CoromandelPeninsula have been metamorphosed into the pre-hnite zone grade; that is, Zone B.

Zone A contains zeolites such as laumontite,analcime, heulandite and mordenite in the matri-ces of sedimentary and basaltic lithologies. InZone B, prehnite commonly appears in the matri-ces and veins of sedimentary rocks, but pumpelly-ite is not found in sedimentary rocks and is rare inbasaltic rocks. Zone C is characterized by theassemblage prehnite

+

pumpellyite, and this zoneis the most extensive in Northland. In Zone D,prehnite completely disappears, and needles ofactinolite appear in basaltic rocks as well as sedi-mentary rocks. The rocks of Zones A and B areunfoliated and would be assigned to textural zoneI, most of those in Zones C and D are foliated andare classified as textural zone IIA, but some rocksin Zone D are phyllitic and have reached IIB in the

Fig. 3

Stratigraphic sequences for the Mesozoic accretionary complexof the Northland and Auckland areas (modified from Aita

et al

. 2001).Localities of fossil-bearing sequences are shown in Figure 2, except forWaiheke Island and Kawakawa Bay, which are to the east of Auckland andare shown in Figure 1.

Fig. 4

Schematic mineral parageneses with increasing grade of meta-morphism in basaltic, pelitic and psammitic rocks of the Northland area,including some of the Coromandel area. The broken line indicates themineral is uncommon or minor in amount.

420

Y. Nishimura

et al.

Table 1

Descriptions of examined samples, carbonaceous material, photomicrographs and terrane affinity after the schemeof Aita

et al

. (2001) and Black (1994) for pelitic and semipelitic metamorphic rocks from the Northland and Coromandelareas, New Zealand

Sampleno.

Gridreference

Mineralzone

Siltsize

Rocktexture

Carbonaceous material

Photomicrograph Terrane affinity

d

002

(Å)

D

2

q

(

∞

) At Bk

N-01 Q05/28555688 A F D (3.515) 11.8 Fig. 5a N OTN-02 Q05/12826098 B C D (3.564) 10.3 Fig. 5g N OTN-03 Q06/42503430 B F D (3.523) 11.4 Fig. 5b S YTN-04 P04/82998853 C C D/P (3.556) 10.7 – N OTN-05 P04/81287914 C C D/P – – – N OTN-06 P05/00275485 C F D/P 3.604 9.1 Fig. 5c N OTN-07 Q06/18452890 C C D/P (3.534) 11.2 – N OTN-08 Q06/49901895 C F D/P (3.488) 11.7 – S YTN-09 Q07/49100866 C F D/P 3.589 9.0 – S YTN-10 Q07/51240081 C F D/P (3.542) 11.6 – S YTN-11 Q07/45290418 C C/F D/P 3.576 9.7 – S YTN-12 Q07/37660155 C F D/P (3.539) 11.0 Fig. 5d S YTN-13 Q07/28000318 C F D/P (3.545) 10.2 – S YTN-14 Q08/38507886 C C D/P 3.562 9.1 – S YTN-15 R08/59285842 C F D/P 3.642 9.5 – S YTN-16 O05/65406512 D C D/P 3.627 9.8 Fig. 5h N CpN-17 Q07/21100915 D C/F P 3.613 8.8 – S YTN-18 Q07/21050910 D C/F D/P (3.550) 10.6 – S YTN-19 Q08/29207765 D C P 3.566 9.3 – S YTN-20 R09/73164405 D F P 3.564 7.7 – S YTN-21 R09/72554329 D F P 3.593 8.9 – S YTN-22 R09/71854272 D F P 3.587 8.5 Fig. 5e S YTN-23 R09/74623420 D F P 3.613 8.3 Fig. 5f S YTN-24 R09/73593410 D C/F D/P 3.598 8.5 – S YTN-25 R09/71503410 D F P 3.597 8.6 – S YTC-01 T11/33708477 B F D (3.542) 11.1 – S YTC-02 S11/26677557 B F D – – – S YT

N-, Northland Peninsula; C-, Coromandel Peninsula; C, coarse grained; F, fine grained; D, detrital texture; P, phyllitic texture; At, Aita

et al

. (2001); Bk, Black (1994); N, north unit; S, south unit; Cp, Caples Terrane; OT, Older Torlesse Subterrane; YT, Younger TorlesseSubterrane. Values in parentheses were not useful for evaluating metamorphic grade. Refer to Figure 4 for details of mineral zones.

Table 2

Representative main mineral assemblages of Zones A to D from the Northland and Coromandel areas, NewZealand

Mineralzone

Sedimentary rocks Basaltic rocks

A Lmt

+

Anl

+

Whm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

Cal Mrd

+

Chl

+

Ab

+

Qtz

±

Whm

±

CalWhm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

Hul

±

CalB Lmt

+

Prh

+

Whm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

Cal Chl

+

Ab

+

Qtz

±

Whm

±

CalPrh

+

Whm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

Cal Pmp

+

Chl

+

Ab

+

Qtz

±

Whm

±

CalWhm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

CalC Prh

+

Whm

+

Chl

+

Qtz

+

Ab

+

Cbm

±

Cal Prh

+

Chl

+

Ab

+

Qtz

±

Whm

±

CalPrh

+

Pmp

+

Whm

+

Chl

+

Qtz

+ Ab + Cbm ± Cal Prh + Pmp + Chl + Ab + Qtz ± Whm ± CalWhm + Chl + Qtz + Ab + Cbm ± Cal Pmp + Chl + Ab + Qtz ± Ep ± Stp ± Whm ± Cal

D Pmp + Whm + Chl + Qtz + Ab + Cbm ± Cal Pmp + Act + Chl + Ab + Qtz ± Ep ± Stp ± Whm ± CalPmp + Act + Whm + Chl + Qtz + Ab + Cbm ± Cal Pmp + Chl + Ab + Qtz ± Ep ± Stp ± Whm ± CalWhm + Chl + Qtz + Ab + Cbm ± Cal Chl + Ab + Qtz ± Whm ± Cal

Cbm, carbonaceous material; Mrd, mordenite; Whm, white mica. Others after Kretz (1983).

Metamorphic history of Northland, New Zealand 421

scheme of Bishop (1972). Zone A corresponds tothe zeolite facies, Zones B and C to the prehnite-pumpellyite facies, and Zone D to the pumpellyite-actinolite facies (Fig. 4).

DESCRIPTION OF EXAMINED SAMPLES

Twenty-five fresh samples from Zones A to D inthe Northland area and two samples from Zone Bin the west coast of the Coromandel Peninsula(Fig. 2) were selected to examine the graphitiza-tion of carbonaceous material and the K–Ar agesof micaceous material contained in them. The sam-ples examined are all micaceous siltstones, whichare broadly classified into fine-grained and coarse-grained groups, and they display detrital, phylliticor composite detrital–phyllitic textures (Table 1).The provisional terrane affinity in the scheme ofAita et al. (2001) and Black (1994) for each sampleis shown in Table 1.

Representative photomicrographs of the 27samples examined are shown in Figure 5. Photo-micrographs (a) to (f) are of fine-grained siltstonesfrom Zones A to D, displaying well the variationsin degree of recrystallization and texture withincreasing grade of metamorphism. Photomicro-graphs (g) and (h) are coarse-grained siltstonesfrom Zones B and D. Brief descriptions are asfollows.1. N-01 from Zone A (Fig. 5a): Recrystallization

is very weak; silt-sized detrital grains ofquartz, feldspar, white mica and biotite arecommonly preserved; very fine-grained whitemica recrystallized in the matrix; this isassigned to slate.

2. N-03 from Zone B (Fig. 5b): Weak concentra-tions of parallel platelets of very fine-grainedrecrystallized white mica following a disturbedsedimentary lamination (center); relict detritalgrains of quartz, feldspar and white mica arestill preserved; this is also slate.

3. N-06 from the low-temperature part of Zone C(Fig. 5c): Recrystallization slightly increased,with pervasive parallel streaks of very fine-grained recrystallized white mica; relict detritalgrains of quartz, feldspar and white mica arestill often preserved; this may be assigned toslate and to phyllite in part.

4. N-12 from the high-temperature part of Zone C(Fig. 5d): Notably more recrystallized than N-06; partially phyllitic texture of very fine-grained recrystallized white mica; rare relictdetrital grains of white mica; this is also slate tophyllite in part.

5. N-22 from the low-temperature part of Zone D(Fig. 5e): This is almost identical to rocks in thehigh-temperature part of Zone C (N-12), and isreferred to as phyllite.

6. N-23 from the high-temperature part of ZoneD (Fig. 5f): Recrystallization is the most pro-nounced in this area, with a distinct preferredorientation of fine-grained recrystallized whitemica; weak segregation lamellae of quartz andalbite beginning to occur; relict detrital grainsof quartz and feldspar still remain, whereasthose of white mica are just recognizable; thisis phyllite.

7. N-02 from Zone B (Fig. 5g): Coarse-grainedsiltstone to fine-grained sandstone, in whichdetrital texture and grains including micas arepredominant; minute fibers of recrystallizedwhite mica scattered in the matrix.

8. N-16 from Zone D (Fig. 5h): Coarse-grainedsiltstone, in which minute fibers of recrystal-lized white mica have preferred a weakly paral-lel orientation; larger grains of detrital whitemica often preserved.

GRAPHITIZATION OF CARBONACEOUS MATERIAL

Carbonaceous material is a minor but commoncomponent in pelitic metamorphic rocks. It is wellknown that the chemical composition and crystalstructure of carbonaceous material change sys-tematically toward fully ordered graphite in peliticmetamorphic rocks with increasing metamorphictemperature, and that the process of graphitiza-tion, which can be tracked by chemical analysis, X-ray diffraction, vitrinite reflectance and Ramanspectroscopy, is useful as a potential relative geo-thermometer (e.g. Landis 1971; Grew 1974; Itaya1981; Wang 1989; Nishimura et al. 2000). Tofurther investigate and clarify the relationshipbetween the mineral zonation described above, thegraphitization of carbonaceous material in the pel-itic and semipelitic metamorphic rocks of theNorthland and Coromandel peninsulas has beenexamined using the X-ray diffraction method ofItaya (1981). The X-ray diffraction operating con-ditions were as follows: scanning speed, 1∞/min;chart speed, 10 mm/min; accelerating voltage,45 kV; current, 25 mA; and range, 1000 cps. Agraphite monochrometer was used.

Results are shown in Table 1. Representativediffractograms from 15 to 35∞ 2q Cu-Ka of carbon-aceous material, along with minor peaks for acid-insoluble minerals (mainly zircon, tourmaline andpyrite), are shown in Figure 6. The carbonaceous

422 Y. Nishimura et al.

Fig. 5 Representative photomicrographs of pelitic and semipelitic metamorphic rocks for which carbonaceous material d002 and white mica K–Ar ageswere measured. All have crossed polars and are the same scale (scale bar, 0.1 mm). (a) N-01: Zone A (slate); (b) N-03: Zone B (slate); (c) N-06: Zone C(slate to phyllite); (d) N-12: Zone C (phyllite); (e) N-22: Zone D (phyllite); (f) N-23: Zone D (phyllite); (g) N-02: Zone B (coarse-grained siltstone to fine-grained sandstone); and (h) N-16: Zone D (coarse-grained siltstone). See text for details.


material gave two peaks: one is a sharp but smallpeak corresponding to (002) of fully orderedgraphite, and the other is a diffuse peak located ata somewhat lower 2q than the (002) peak. Theformer is superimposed on the latter, and is attrib-uted to the presence of relict graphite of detritalorigin (e.g. Itaya 1981; Nishimura et al. 1989;Wang 1989). With increasing grade of metamor-phism, the intensity of the diffuse peak increasesslightly, its width decreases, and its position shiftstoward higher 2q, but it does not approach thesharp peaks recorded in high grades of metamor-phism. This change can be explained by a gradualincrease in the degree of graphitization withincreasing metamorphic temperature.

Each diffuse peak shown in Figure 6, however,is a combination of two peaks: the carbonaceous

material and the slide glass used in the samplepreparation (Tomuro et al. 1996; Itaya et al. 1997).The top three diffractograms (N-01, N-03 and N-10) in Figure 6, which are nearly the same as thediffraction pattern of the slide glass used, shouldbe excluded when determining the apparent inter-planar spacing d002 values of the carbonaceousmaterial. In this paper, d002 values calculated fromdiffractograms which give a D2q∞ (peak width athalf-peak height) value of 10.0∞ or more were notconsidered useful for evaluating metamorphicgrade and therefore were eliminated from discus-sion (Table 1). The available d002 data from North-land therefore range from 3.642 Å in Zone C to3.564 Å in Zone D.

The measured peak widths at half-peak height(D2q∞) ranged from 11.8 to 7.7∞, and formed a clus-ter in each mineral zone with considerable overlap.The clusters shift to the lower degree side fromZone A to Zone D (Fig. 7). These data support thevalidity of the mineral zoning as an indicator ofmetamorphic grade.

K–Ar AGE DETERMINATIONS

SAMPLE PREPARATION AND ANALYTICAL PROCEDURE

Because the siltstones studied have been meta-morphosed under very low-grade conditions, therocks are very fine-grained and it is difficult to

Fig. 6 Representative X-ray diffractograms of the carbonaceous mate-rial from pelitic and semipelitic metamorphic rocks of the Northland areaand slide glass used. The sharp but small peak (gr) represents fullyordered graphite interpreted as of detrital origin. d002 values for thesepatterns are shown in Table 1 and the values from Zones C and D areplotted in Figure 10.

Fig. 7 Relationship between peak widths at half-peak height for car-bonaceous material and mineral zones of the Northland and Coromandelareas. The numeral in the box represents the sample number from theNorthland area, and C/1 is C-01 from the Coromandel area.


concentrate recrystallized white mica to a suffi-ciently high purity for K–Ar age determination.To overcome this problem, we concentrated therecrystallized white mica by both magnetic sepa-ration and by hydraulic elutriation methods, andmeasured and compared the K–Ar ages of both ofthe separates for each sample. The purity of thewhite mica separates was checked by polarizedmicroscope observations as well as by X-raydiffraction.

Magnetic separation method

Approximately 2 kg of each of the rocks used forthe graphitization study were crushed in a jawcrusher to less than approximately 200 mesh insize, and because of the very fine grains of therecrystallized white mica (cf. Fig. 5), were sievedto provide a mesh fraction size range of 280–330mesh. These sieved fractions were then washed toremove adhering powder residues, and the heavymaterials were separated out by decanting. An iso-dynamic magnetic separator was used to remove

most of the impurity minerals such as chlorite,stilpnomelane, albite and quartz. The white micaconcentrates were then treated with dilute HClsolution (approximately 4 N) to dissolve theremaining chlorite and Fe contamination, and thenthey were washed repeatedly. Magnetic separationwas then repeated on the acid-treated fractions.With this separation method, it was difficult toexclude completely the carbonaceous material thatis commonly contained in white mica in all of themineral zones. It was also impossible to completelyremove quartz and feldspar from the white micasowing to their very fine grain size. For these rea-sons, the K contents are rather lower than for purewhite mica separates (Table 3). However, quartz,albite and carbonaceous material do not signifi-cantly affect white mica K–Ar ages (Itaya &Takasugi 1988). Some detrital white mica andorthoclase occur in the samples examined, and wecannot rule out the possibility that some might bepresent in the very fine-grained white mica con-centrates, and that they might make a minor con-tribution to the measured K contents.

Table 3 K–Ar ages of recrystallized white micas separated by magnetic separation and hydraulic elutriation methods forpelitic and semipelitic metamorphic rocks, Northland (N) and Coromandel (C) areas, New Zealand

Sampleno.

Magnetic separation (# 280–330 = 45–53 mm) Hydraulic elutriation (2 mm under) MineralzoneK (wt%) Rad. 40Ar

(10-8 ccSTP/g)K–Ar age

(Ma)Non-rad.40Ar (%)

K (wt%) Rad. 40Ar(10-8 ccSTP/g)

K–Ar age(Ma)

Non-rad.40Ar (%)

N-01 3.95 ± 0.08 3029 ± 30 187.4 ± 4.0 2.9 3.99 ± 0.08 2932 ± 30 180.2 ± 3.8 3.0 AN-02 3.39 ± 0.07 2275 ± 23 165.4 ± 3.5 2.8 3.71 ± 0.07 2302 ± 23 153.1 ± 3.3 3.2 BN-03 3.94 ± 0.08 2177 ± 22 137.1 ± 3.0 4.7 3.56 ± 0.07 2004 ± 20 139.7 ± 3.0 4.8 BN-04 3.72 ± 0.07 2125 ± 21 141.3 ± 3.0 2.1 3.68 ± 0.07 1910 ± 19 129.1 ± 2.8 3.8 CN-05 5.81 ± 0.12 3723 ± 37 158.1 ± 3.4 1.1 4.93 ± 0.10 2753 ± 28 138.5 ± 3.0 2.4 CN-06 5.02 ± 0.10 3619 ± 36 176.8 ± 3.8 1.9 4.68 ± 0.09 3112 ± 31 163.7 ± 3.5 3.0 CN-07 7.04 ± 0.14 3815 ± 38 134.5 ± 2.9 1.2 5.38 ± 0.11 2894 ± 29 133.6 ± 2.9 2.7 CN-08 3.80 ± 0.08 2115 ± 22 138.1 ± 3.0 5.0 2.97 ± 0.06 1701 ± 17 142.0 ± 3.1 4.5 CN-09 4.97 ± 0.10 3022 ± 30 150.4 ± 3.2 1.6 5.76 ± 0.12 3240 ± 33 139.5 ± 3.0 2.6 CN-10 4.11 ± 0.08 2523 ± 25 151.8 ± 3.2 1.7 4.68 ± 0.09 2840 ± 29 149.9 ± 3.2 2.9 CN-11 4.94 ± 0.10 3186 ± 31 158.9 ± 3.4 1.6 5.18 ± 0.10 3189 ± 33 152.0 ± 3.3 2.6 CN-12 4.88 ± 0.10 2923 ± 29 148.3 ± 3.2 1.8 5.28 ± 0.11 2867 ± 29 134.7 ± 2.9 2.6 CN-13 4.53 ± 0.09 2872 ± 29 156.6 ± 3.4 0.7 4.62 ± 0.09 2791 ± 28 149.3 ± 3.2 2.2 CN-14 4.66 ± 0.09 3375 ± 35 177.8 ± 3.8 1.3 5.18 ± 0.10 3505 ± 35 166.6 ± 3.6 2.1 CN-15 3.87 ± 0.08 2334 ± 23 149.0 ± 3.2 1.2 5.85 ± 0.12 3323 ± 34 140.7 ± 3.0 2.2 CN-16 5.09 ± 0.10 3788 ± 38 182.3 ± 3.9 1.6 4.94 ± 0.10 3472 ± 36 172.6 ± 3.7 2.8 DN-17 4.93 ± 0.10 3206 ± 33 160.3 ± 3.4 2.0 5.23 ± 0.11 3193 ± 33 151.0 ± 3.3 3.0 DN-18 5.07 ± 0.10 3254 ± 33 158.3 ± 3.4 1.3 6.28 ± 0.13 3708 ± 38 146.0 ± 3.1 2.4 DN-19 5.50 ± 0.11 4041 ± 41 180.0 ± 3.8 0.7 6.41 ± 0.13 4302 ± 44 165.2 ± 3.6 3.0 DN-20 3.77 ± 0.08 2136 ± 22 140.4 ± 3.0 1.0 6.62 ± 0.13 3571 ± 36 134.0 ± 2.9 1.8 DN-21 3.45 ± 0.07 1885 ± 19 135.6 ± 2.9 1.3 6.56 ± 0.13 3426 ± 34 129.8 ± 2.8 2.3 DN-22 2.76 ± 0.06 1628 ± 16 145.9 ± 3.1 1.4 5.23 ± 0.11 2891 ± 30 137.1 ± 3.0 3.3 DN-23 3.46 ± 0.07 2039 ± 20 145.9 ± 3.1 1.3 5.82 ± 0.12 3195 ± 32 136.2 ± 2.9 2.3 DN-24 3.10 ± 0.06 1891 ± 20 150.8 ± 3.3 1.3 5.48 ± 0.11 3072 ± 31 139.0 ± 3.0 2.3 DN-25 3.55 ± 0.07 2057 ± 20 143.3 ± 3.1 1.7 6.32 ± 0.13 3476 ± 34 136.4 ± 2.9 2.1 DC-01 4.16 ± 0.08 2469 ± 25 147.0 ± 3.2 6.5 4.41 ± 0.09 2540 ± 25 142.6 ± 3.1 2.6 BC-02 4.19 ± 0.08 2142 ± 21 127.2 ± 2.7 1.7 4.27 ± 0.09 2190 ± 21 127.6 ± 2.7 1.8 B


Hydraulic elutriation method

The less than approximately 200 mesh sizecrushed samples were dispersed in water, and theless than 2 mm size fractions were separated byapplying Stokes’ law, and then they were treatedwith dilute HCl solution (approximately 4 N) todissolve chlorite and Fe contamination. The acid-treated fractions were then separated by centri-fuging into white mica separates with high purityand quartz- and feldspar-rich separates. The whitemica separates were washed repeatedly and theirpurity was checked by X-ray diffraction.

Analytical procedure

The white mica separates, concentrated by meansof magnetic separation and hydraulic elutriation,were analyzed for K and Ar at Okayama Univer-sity of Science, and calculations of ages and errorswere determined using the methods described byNagao et al. (1984) and Itaya et al. (1991). Multipleruns of some chemical standards (JG-1, JB-1 andbiotite of JG-1) indicated that the accuracy andreproducibility of this method were within 2%.Decay constants for 40Ar and 40Ca and the 40K con-tent in potassium used in the age calculation arefrom Steiger and Jäger (1977): 0.581 ¥ 10-10/y,4.962 ¥ 10-10/y and 1.167 ¥ 10-4, respectively.

RESULTS

The results of the K–Ar age determinations for themagnetic and elutriation separates are presentedin Table 3. K contents measured in the two sepa-rates are plotted in Figure 8. The magnetic sepa-rates from N-20 to N-25, which have lower Kcontents by approximately 2.5 wt% than those ofthe elutriation separates from the same sample,were checked for purity only by microscopic obser-vation, while the other magnetic separates werechecked by both microscopic observation and X-ray diffraction, and as a consequence the double-checked magnetic separates had obviouslyimproved purity in spite of their white micas hav-ing lower recrystallization values.

The elutriation separates tend to have higher Kcontents than the magnetic separates from thesame zone, and the K contents decrease from 6.5to 3.5 wt% from Zone D to Zone A (Fig. 8). Thisimplies that the white micas in Zones A and B (andpartially also in Zone C) are rich in illitecomponent.

The K–Ar ages obtained from the magnetic andelutriation separates are plotted for each mineralzone in Figure 9 along with the lithologies and tex-tures of the host rocks. The ages range from 187to 127 Ma; that is, from the late Early Jurassic toearly Early Cretaceous. It is likely that there areno marked differences between the ages of thewhite micas in the four mineral zones, but thereare small differences in age between the two sep-arates, and these are discussed below.

DISCUSSION

METAMORPHIC FACIES AND HIGHEST METAMORPHIC TEMPERATURE

As described in the previous section, the subduc-tion-related metamorphism in the Northland andCoromandel areas can be divided into four zonesbased on mineral parageneses, and the highestgrade attained is pumpellyite-actinolite facies

Fig. 8 K contents of magnetic separates compared with elutriationseparates from the Northland and Coromandel areas.


(Fig. 4). The relationship between mineral zonesor facies and apparent interplanar spacing (d002

values) of carbonaceous material is variablebetween different metamorphic terranes, asshown in Figure 10. Nishimura et al. (2000) mea-sured the d002 values of carbonaceous material insediments of the Chrystalls Beach–Brighton sec-

tion (Wakatipu Belt, South Island) that showed arange from 3.619 Å for a diffuse peak at the lowestmetamorphic grade (Pmp-Chl zone) to 3.385 Å fora sharp peak in the highest metamorphic grade (Btzone), and correlated the values with the sequenceof mineral zones (Fig. 10). The range of d002 valuesfor all zones of the Chrystalls Beach–Brighton

Fig. 9 Relationship between K–Ar ages formagnetic and elutriation separates and mineralzones of the Northland and Coromandel areasshowing the lithologies and textures of the hostrocks (cf. Table 1).

Fig. 10 Relationship between d002 values for carbonaceous material and mineral zones of the Northland area, compared with those of the Wakatipu Belt(Nishimura et al. 2000) in New Zealand and the Sanbagawa (Itaya 1981) and Suo (Nishimura et al. 1989) belts in Japan.


section falls within the range of those for theSuo Belt in Japan (Nishimura et al. 1989),although in the latter terrane, the values are bothlower and higher for the lowest and highestgrades, respectively. In Northland, where no rocksof the epidote-actinolite and biotite zones occur,the range of d002 for Zone D (Pmp-Act zone) isconsiderably different from values for the samezone from the Chrystalls Beach–Brighton sectionand the Suo Belt (Fig. 10). This comparison isimportant and indicates that even in the highestgrade part of Northland the rocks have only expe-rienced conditions equivalent to the lowermostgrade of the pumpellyite-actinolite facies.

An upper temperature limit for the pumpellyite-actinolite facies with co-existing pumpellyite andchlorite is estimated to be approximately 360∞C at4.5 kbar, or at somewhat lower temperatures withincreasing Fe content of reactants and at lowerfluid pressures (Liou et al. 1987). Frey et al. (1991),using thermodynamic calculations and mineralparageneses for low-grade basalts, determined therelative pressure–temperature (P–T) stabilityfields for prehnite-pumpellyite facies to rangefrom approximately 175 to 280∞C at 0.5–4.5 kbarand for pumpellyite-actinolite facies to range fromapproximately 220 to 360∞C at 1.5–9.5 kbar. AsZone D in Northland corresponds to the lower-most grade of the pumpellyite-actinolite facies, thehighest metamorphic temperature experienced bythe basement rocks is interpreted to be approxi-mately 300∞C, which is lower than the estimate of350∞C given previously by Jennings (1989).

EFFECT OF DETRITAL WHITE MICA ON K–Ar AGE DETERMINATIONS

K content and K–Ar age differences betweenwhite mica concentrates prepared by the magnetic

and elutriation methods, and thus representingdifferent grain-size fractions, are plotted inFigure 11.

The six magnetic separates from N-20 to N-25that had been checked for purity only by micro-scopic observation show considerably lower valuesof K content (-3.11 to -2.47 wt%) when comparedwith the K content determined for the finer grainedelutriation separate of the same sample (Figs 8,11).There is, however, very little difference in the K–Ar ages determined for the two different separatetypes. When these six samples are excluded, theother K content differences range from -1.98 to+1.66 wt%, with a computed average differenceof -0.13 wt%, which converges nearly on zero(Fig. 11). However, the K–Ar age differences rangefrom -3.9 to +19.6 m.y., with an average differenceof +8.1 m.y. (Fig. 11). This indicates that the K–Arages for the magnetic separates are older byapproximately 8.1 m.y. than those for the elutria-tion separates, and the difference is significantlygreater than their error ranges (cf. Table 3).Because detrital white micas are commonly pre-served in rocks of the Northland and Coromandelareas, particularly in the coarse-grained samples,it is reasonable to assume that the coarser grainedmagnetic separates include more detrital micasand therefore give older K–Ar ages than the finergrained elutriation separates. Although the K–Arages for elutriation separates would be alsoaffected to some degree by the presence of detritalwhite mica, the effect is likely to be small.

The closure temperature for the muscovite K–Ar system is approximately 350∞C (Jäger 1979),and the highest temperature experienced by therocks in Northland is approximately 300∞C. Thus,the K–Ar ages for the elutriation separates fromall zones indicate that the time of peak metamor-phism was in the range 180–130 Ma; that is, earli-

Fig. 11 K contents and K–Ar age differencesbetween magnetic and elutriation separates fromthe Northland and Coromandel areas showing thelithologies and textures of the host rocks (cf.Table 1). �, coarse-grained; �, detrital texture;▲, detrital/phyllitic texture; �, phyllitic texture.


est Middle Jurassic to early Early Cretaceous(Fig. 9).

SUBDIVISION OF NORTHLAND AND METAMORPHIC AGES OF THE UNITS

To understand accretionary complexes, it isimportant to reconstruct the stratigraphy of theoceanic plate as it existed before being tectoni-cally sliced and mixed by the subduction process,and to establish reasonable ages of the metamor-phism related to the subduction. The oceanic platestratigraphy may comprise in ascending order (i)mid-oceanic ridge basalts (MORB); (ii) limestone,bedded chert and siliceous mudstone of pelagic or

hemipelagic origin; and (iii) trench fill, turbiditicsandstone and mudstone, similar to rocks recov-ered from drilling through modern trench floors(Isozaki et al. 1990).

Several stratigraphic sequences (Fig. 3) thathave been demonstrated for the north and southof Northland by Aita and Spörli (1992) and revisedmore recently by Aita et al. (2001) suggest thatthere are two types of oceanic plate in the North-land and Auckland areas. This implies that the‘Waipapa’ Terrane might be divisible into at leasttwo units, but because there are as yet only a fewareas in which the stratigraphy has been verifiedby fossil data, the boundary between the two unitshas not yet been defined.

Fig. 12 Summary of metamorphic history forMesozoic accretionary complexes in the Northlandand Coromandel areas, compared with age datafrom previous studies. OPS, oceanic plate strati-graphy. A-B, Adams et al. (1985); A-G, Adams andGraham (1996); Aita, Aita and Spörli (1992) andAita et al. (2001); A-J, Adams et al. (1999); A-R,Adams and Robinson (1993) and Adams and Gra-ham (1997); G-M, Graham and Mortimer (1992)and Adams and Graham (1997); N-C, Nishimuraet al. (2000). See text for details.


In this paper, we tentatively assign sample N-16from the Omahuta–Puketi area to the north unit,because chert from there contains poorly pre-served radiolarians of Permian age (R. Hori, pers.comm., 2002), and classify the rest of the samplesinto either the north or south units on the basis ofthe lithologic features after Black (1994) and asshown in Table 1.

The K–Ar age distributions obtained fromelutriation separates are plotted in Figure 12,together with the simplified oceanic plate stratig-raphies after Aita et al. (2001). It is notable thatthe oldest date was obtained from the lowest graderock (N-01), which has been altered only to zeolitefacies. The age of peak metamorphism in the northunit is inferred to be 180–130 Ma; that is, earliestMiddle Jurassic to early Early Cretaceous,whereas that of the south unit is 150–130 Ma; thatis, late Late Jurassic to early Early Cretaceous,except for two ages of approximately 170 Ma (N-14 and N-19) in the up-faulted Waipu block (Isaacet al. 1994), which may represent a block of thenorth unit (Figs 1,12). These estimations of meta-morphic ages are consistent with the fossil ages ofthe terrigenous clastics and siliceous mudstonefrom the top of the oceanic plate according to thestratigraphic sections of Aita et al. (2001), asshown in Figures 3 and 12.

CORRELATION WITH OTHER AREAS ON THE BASIS OF RADIOMETRIC AGES

A number of radiometric ages have been reportedfrom the Caples, Torlesse and other terranes ofuncertain origin in New Zealand. Age data forrocks regarded as nearly the same metamorphicgrade (lower Pmp-Act facies or TZ IIA and below)as those of Northland have been collated from pre-viously published reports (Adams et al. 1985; Gra-ham & Mortimer 1992; Adams & Robinson 1993;Adams & Graham 1996, 1997; Adams et al. 1999;Nishimura et al. 2000), and are plotted inFigure 12. Most data, except for those of Nishi-mura et al. (2000), are K–Ar total-rock ages, andtherefore may provide much broader age rangesthan those of the present study, because the occur-rence of detrital mica and potassium feldspar inthe dated rocks would lead to older apparent ages,and the presence of any K-bearing clay mineralsproduced by low-temperature alteration wouldlead to reduced apparent ages.

Judging from the results shown in Figure 12,the age cluster for the north unit (including the up-faulted Waipu block) and/or ‘Caples Terrane’ and

‘Older Torlesse Subterrane’ after Black (1994)most likely correlates with the Caples (uncertain)Terrane in the Chrystalls Beach–Taieri Mouth sec-tion (Adams & Robinson 1993; Adams & Graham1997; Nishimura et al. 2000). However, the agecluster of the south unit (excluding the up-faultedWaipu block) and/or ‘Younger Torlesse Subter-rane’ of Black (1994) is older than rocks of theYounger Torlesse Subterrane in the Wellingtonarea (Adams & Graham 1996), but is comparablewith the Caples and Waipapa terranes of the Nel-son/Marlborough area (Adams et al. 1999).

ACKNOWLEDGEMENTS

We are grateful to Professor D. S. Coombs and DrY. Aita for discussion and critical comments, to DrT. Okada for assistance with K–Ar analyses, and toDr H. Muraoka for providing some literature. Thiswork was supported by grants from the Ministryof Education, Science, Culture and Sport of Japanto Y. N., who would also like to thank many mem-bers of staff in the Geology Department of theUniversity of Auckland, New Zealand.

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