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Very Low-grade Metamorphism in Basement Greywacke Terranes of the Northern and Central North Island, New Zealand
Gondwana Research, V 5, No. 4, pp. 857-865. 02002 International Association for Gondwana Research, Japan. ISSN: 1342-937X
Gondwana Research
Very Low-grade Metamorphism in Basement Greywacke Terranes of the Northern and Central North Island, New Zealand S. Woldemichael' and P.M. Black2
' Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan Geology Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand (Manuscript received October 3,2001; accepted April 6,2002)
Abstract Metamorphism in the late Permian to early Cretaceous North Island basement greywackes has been investigated using petrography and clay mineral crystallinity. Several terranes are represented in the North Island greywackes and the study area includes Murihiku, Manaia Hill, Bay of Islands and Omahuta terranes and the Melange Zone. Very lowgrade metamorphic events in the greywackes have produced mineral assemblages of zeolite to pumpellyite-actinolite greywacke facies. Zeolite facies greywackes are characterized by the assemblage Zeo (Lmt, Anl, Hul) +Qtzf Abk Calk Chl? I? I/S* observed in the entire Murihiku terrane and in the eastern part of the Bay of Islands terrane and the Melange Zone. The entire Manaia Hill, most of the Bay of Islands, the eastern area of the Omahuta terranes and the central part of the Melange Zone are a t prehnite-pumpellyite facies with mineral assemblages of Prh+Qtz+Chl+Pmp+Ab++ Ill+ Calf Lmt. Pumpellyite-actinolite facies with the mineral assemblage of Pmp? Act+Qtz+Ab+Chl? Eprt: Ill* Calk Chl occurs in the western part of the Melange Zone and the Omahuta terrane. Illite (IC) and chlorite (ChC) crystallinity values of greywackes are very similar and range from diagenetic zone to anchizone. Metamorphic conditions indicated by the IC and ChC and mineral facies are in excellent agreement and correlate as follows: crystallinity diagenetic-zone with the zeolite mineral facies, crystallinity lower anchizone with prehnite-pumpellyite mineral facies and crystallinity upper anchizone with pumpellyite-actinolite mineral facies. The general increase in the metamorphic grade from east to west, except in Murihiku terrane, is compatiblewith the sequence of accretion expected in a subduction environment.
Key words: Crystallinityzone, prehnite-pumpellyitefacies, pumpellyite-actinolitefacies,very low-grade metamorphism, zeolite facies.
Introduction Late Permian to early Cretaceous greywackes are exposed over a wide area of the North Island of New Zealand, and are inferred to underlie the thick Cenozoic volcanic and sedimentary sequences that now cover much of the remainder (Fig. la). The exposed North Island greywackes consist of several tectonostratigraphic terranes and this study includes the entire North Island exposure of the Murihiku, the Manaia Hill, the Bay of Islands and the Omahuta terranes and the Melange Zone (Fig. la). This paper is concerned mainly with aspects of the metamorphism of the greywackes and gives only a brief summary of the characteristics of the terranes. The northern and Central North Island greywackes have long been recognized as being metamorphosed at very low grade (Brothers, 1956; Coombs, 1960) and several *Kretz (1983)
researchers (e.g., Black, 1989; Black et al., 1993) have described zeolite, prehnite-pumpellyite and pumpellyiteactinolite facies assemblages. Minerals characteristicof very-low grade metamorphism such as zeolite, prehnite and pumpellyite usually develop from fine-grained unstable starting materials such as volcanic glass. Metamorphic belts which lack suitable starting materials also lack indicator minerals and, out of necessity, researchers have used other means (e.g., clay mineral crystallinity) to define metamorphic conditions. Crystallinity studies are also widely applied in geological environments where metamorphic indicator minerals are available and correlation of crystallinityzones and mineral facies usually lead to a better understanding of very lowgrade metamorphism (Kisch, 1987). There are little or no crystallinity data for North Island greywackes and this study was carried out to bridge that gap and facilitate
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Fig. 1. Simplified map of the North Island, New Zealand; (a) the distribution of greywacke outcrops showing the main greywacke terrane boundaries (Sporli, 1978; Black, 1994) TVZ = Taupo Volcanic Zone, (b) greywacke sample localities in the northern half of the North Island. Bold lines show approximate boundaries of the greywackes terranes.
comparison of the metamorphic conditions in North Island greywackeswith those in similar geological environments.
Samples and Methods Samples for this study were collected from 55 localities distributed throughout the study area (Fig. lb). In most cases, several samples were collected from each locality in order to represent variation in lithology,which is mainly
grain-size related such as metasandstone and metasiltstone. More than 300 representative thin sections were examined. Most of the samples were collected specifically for this study but some samples collected by previous workers and housed in the Universityof Auckland Geology Department collections, were also examined. Clay mineral crystallinity determinations were carried out on <2pm grain-size clay mineral-rich fractions that were prepared from 165 representative samples using the Gondwana Research, V. 5, No. 4,2002
VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
method similar to Warr and Rice (1994) and described in Woldemichael(l998). The IC and ChC were determined from the X-ray diffraction pattern of illite 001 (10 A) and chlorite 002 (7 A) reflections, respectively. The diffractograms were run at the Geology Department, Auckland University, using a Philips generator 1130/ goniometer 1050, operated at 40 kV and 20 mA, CuKa radiation, normal focus tube, slits 1'4.2 mm-lo,graphite monochromator PW 1762, and a xenon proportional detector. Each sample was scanned from 2 to 16 "20, at a scan rate of 0.6 "20/min and a step size of 0.01". The error of the crystallinity measurements depends on peak broadening and was determined by making replicate measurements on several samples. In general, the reproducibility of the crystallinity values was about 15% variance from the mean.
Tectonic Setting The New Zealand micro-continent is a fragment of Gondwanaland and its offshore extensions, which separated from the supercontinent in late Cretaceous to early-Tertiarytime and later, during a Cenozoic tectonic event, foundered, elevated and rearranged in blocks to the present position (Sporli, 1978; Bradshaw, 1989). The Australian-Antarctic part of Gondwanaland is thought to have attained continental-type thickness and structure by the end of the Carboniferous (Bradshaw, 1989). During the late Paleozoic to mid-Cretaceous, accretion of terranes took place in a subduction-related arc-forearc-trench system that extended along the eastern margin of the Gondwana plate. The resulting accretionary complex is composed of dominantly terrigenous clastic deposits with minor pelagic and volcanogenic assemblages, characteristic of a typical convergent margin. These sedimentary sequences now make up a series of discrete fault-bounded greywacke terranes, each with a distinct litho-stratigraphy.
Greywacke Terranes The distribution of the greywacke terranes in the study area is shown in figure l a . The characteristics of the Murihiku, Manaia Hill, Bay of Islands and the Omahuta terranes and the Mblange Zone are briefly summarized below.
The Murihiku terrane Late Triassic to early Cretaceous Murihiku terrane greywackes outcrop along the west coast of the North Island, from Port Waikato to Awakino in a relatively simple N-plunging fold, with a subvertical to steeply east-dipping Gondwana Research, V. 5, No. 4,2002
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axial surface (Sporli, 1978). In most exposures of this terrane, stratigraphic units are well preserved with excellent fossil control and can be traced for several kilometres. The Murihiku terrane consists of volcanoclastic siltstones, sandstones, conglomerates and tuffaceous sediments. Along the south shore of Kawhia Harbour there is a continuous sequence of late Triassic to late Jurassic sediments, whereas around Port Waikato late-Jurassic to early-Cretaceous sediments are exposed. The Murihiku terrane is interpreted as a forearc basin, which contains a continuous record of arc-related volcanism from early Triassic to early Cretaceous (Ballance and Campbell, 1993).
The Manaia Hill terrane The late Jurassic to early Cretaceous Manaia Hill terrane (Black, 1994) is exposed in the Central North Island and Cormandel Peninsula. It is separated from the Murihiku terrane to the west by a narrow zone of serpentinite and ultramafics rocks (Sporli, 1978; Black et al., 1993), and to the north the Melange Zone separates the Manaia Hill terrane from the Bay of Islands terrane (Black, 1994). The eastern and southern boundaries of the Manaia Hill terrane are not well defined. However, greywackes encountered in deep geothermal drill-holes within the Taupo Volcanic Zone (TVZ, Fig. l a ) have been described as belonging to the Mania Hill terrane (Kear, 1993; Mortimer, 1995). The Manaia Hill terrane dominantly consists of siltstones, sandstones and conglomerate. Within the Manaia Hill terrane, typical conglomerates consist of rounded to sub-angular, up to pebble size clasts of sedimentary and volcanic fragments in a sandy grade matrix, but conglomerates from the North Coromandel Peninsula contain clastic debris from a wide variety of sources (McFarlane, 1993; Skinner, 1993). The conglomerate beds usually underlie a sandstone bed, either forming massive beds, or grading into the overlying sandstone within a few meters. These conglomeratesandstone sequences are interpreted to be thick debris flow and fan deposits on the evidence of the dominantly argillite pebbles and rare limestone, chert and granite material.
The Bay of Islands terrane The Bay of Islands terrane, exposed in the east coast of Northland from around Bay of Islands south to near Whangarei, contains sediments of late Paleozoic to early Mesozoic age deposited some distance away from the subduction trench and believed to have been accreted to the subduction complex in the early to Middle Jurassic (Black, 1994). The Bay of Islands terrane is locally very
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S. WOLDEMICHAEL AND P.M. BLACK ~
deformed and the stratigraphic sequence is disrupted. The sediments are dominantly quartzofeldspathic sandstone and clastic-poor siliceous siltstone; the former is more common in the western half of the terrane and the latter in the east.
The Omahuta terrane The late Paleozoic to early Mesozoic rocks of the Omahuta terrane (Black, 1994) are exposed in the Omahuta and Puketi forests, Northland. The Omahuta terrane has two distinct stratigraphic units, the Omahuta and Puketi forest units, which are in fault contact (Jennings, 1987). The Omahuta units contain dominantly plutoniclastic and volcaniclastic sandstone and siltstone sequences very similar to sediments accumulated at evolved or dissected magmatic arc-relatedbasins (Dickinson and Suczek, 1979; Dickinson, 1985). The Puketi forest unit consists of a sequence of massive basalts, and pillow lavas interbedded with tuffs, sandstone and siliceous siltstone and cherts. The basalts show chemical compositions ranging from strongly depleted basalts to chemically enriched andesites, thus indicating source heterogeneity (Jennings, 1991). The time of juxtaposition of the Omahuta terrane against the Bay of Islands terranes is unknown.
The Me‘lange Zone The Melange Zone contains tectonic slices of Jurassic oceanic floor material and debris that may be derived from the Bay of Islands and the Manaia Hill terranes and their metamorphic equivalents (Black, 1996).
Metamorphism Our petrographic study has confirmed that greywackes of the northern and Central North Island show regional zoning of zeolite, prehnite-pumpellyite and pumpellyiteactinolite mineral assemblages.
Zeolite facies The key metamorphic minerals of this facies are zeolites and the main mineral assemblage are Zeo (Lmt, Anl, Hul) +Qtz+Abt Calk Chlf I+ I/S. Greywackes containing zeolite facies mineral assemblages occur in two distinct geographical locations. The first location is the entire greywacke exposure of the Murihiku terrane and the second location, in Northland, in an area which includes the eastern exposures of both the Bay of Islands terrane and the Melange Zone (Fig. 2). In most zeolite facies greywackes laumontite is the abundant zeolite; the exception is the Middle to late Jurassic sequence of the Murihiku terrane where heulandite is more common. Laumontite mostly occurs
Fig. 2. Metamorphic map of the northern half of the North Island, showing the distribution of the zeolite, prehnite-pumpellyite and pumpellyite-actinolite facies and their relations to terrane boundaries (as defined in figure 1).
as original pore, vein or fracture fillings, or replacing plagioclase and lithic fragments, and is usually intergrown with chlorite, quartz or clay minerals. Heulandite usually occurs as cavity fillings or replacing volcanic glass and mostly forms as orange-coloured, short, fibrous, stubby crystals and sometime as meshes of interlocking crystals. Analcime is far less abundant than either laumontite or heulandite and usually occurs as a vein or cavity-filling mineral. Quartz, occurs in all greywackes as clastic grains, cement and vein fillings as well as replacing detrital feldspar and lithic fragments, and is usually associated with zeolite, calcite and clay minerals. Albite is also common but mainly replaces detrital plagioclase where it is usually intergrown with laumontite. Chlorite is a typical constituent of the interstitial matrix; it also replaces rock fragments and detrital ferromagnesian grains and occurs as a vein mineral. Vein chlorite is associated with laumontite, calcite and, less commonly, with quartz. Gondwana Research, K 5, No. 4,2002
VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
Calcite occurs mainly as a vein mineral usually intergrown with laumontite, quartz, albite and chlorite but sometimes it also occurs as discrete grains in the matrix, as cement, and replaces some clastic grains, bioclasts and rock fragments. Calcite is occasionally intergrown with heulandite and analcime. Prehnite-Pumpellyitefacies Prehnite-pumpellyite facies greywackes occur throughout the Central and northern North Island in the Manaia Hill terrane, the western part of the Bay of Islands terrane, the eastern part of Omahuta terrane and the Melange Zone (Fig. 2). Greywackes in these areas contain the mineral assemblage Prh+ Pmp+Qtz+Ab+Chl+ I11 k Calk Lmt. Throughout this area, prehnite is ubiquitous and commonly occurs as a well crystallized vein filling mineral usually associated with quartz, calcite, chlorite and rarely with laumontite. Locally, fine-grained prehnite crystals occur, replacing rock matrix or detrital fragments. Pumpellyite commonly fills cavities but is less abundant than prehnite and rarely occurs as a vein mineral. It is usually intergrown either with chlorite and epidote or with prehnite and epidote. Quartz, as in the zeolite facies, is ubiquitous in veins, cavities and as matrix cement. In veins and cavities, it forms fine-grained aggregates of crystals usually associated with calcite and prehnite and rarely with laumontite, illite and chlorite. In most of the veins examined, quartz predated all the other minerals present. It is often found inter-grown with feldspars as a devitrification product of volcanic glass or matrix. Albite occurs mainly as a replacement of plagioclase but is occasionally in the matrix, mostly accompanied by clay minerals or quartz. Calcite mainly occurs as vein filling, but also as cement and replaces clastic grains. As a vein-forming mineral, it occurs as monomineral veins as well as in association with quartz and prehnite and rarely with laumontite. Whenever it is possible to establish a paragenesis, in almost all cases calcite was the last crystallising mineral. Chlorite and illite commonly occur as matrix cement and replace lithic fragments, as well as crystallising in veins and cavities. Illite shows no variation in optical properties whereas chlorite occurs in green and brown varieties. Laumontite has been observed in a few localities in the Coromandel Peninsula and around Auckland. It occurs as a vein and matrix mineral usually associated with prehnite, quartz and calcite, suggesting a transition from the zeolite facies to prehnite-pumpellyite facies. Pumpellyite-Actinolitefacies Greywackes containing the pumpellyite-actinolitefacies mineral assemblage Pmp+Act+Qtz+Ab+Chlk Epf Ill+ Gondwana Research, V. 5, No. 4,2002
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Calk Chl occur in the western Omahuta terrane and the western part of the Melange Zone. The pumpellyiteactinolite facies is characterized by the absence of prehnite, suggesting that actinolite crystallized at the expense of prehnite. Actinolite with green and brown pleochroism occurs chiefly in the rock matrix. Pumpellyite occurs in both veins and the matrix; in veins it is associated with epidote, calcite, quartz and albite and in the matrix it crystallises with epidote, calcite, quartz, chlorite and white mica in cavities. Epidote occurs as spongy masses, radiating aggregates or small clusters of granules in most of these greywackes. Quartz and albite, as in rocks of the zeolite and prehnite-pumpellyite facies, are common minerals. Quartz occurs as vein fillings and replacing feldspar grains and in the matrix, while albite mainly replaces demtal feldspars. Calcite, chlorite and illite are common metamorphic constituents of pumpellyite-actinolite facies greywackes. Calcite occurs in both the matrix and in veins as irregular masses or euhedral crystals. Chlorite and illite/micas occur as fine-grained, irregular patches in the matrix or replacing clastic grains and rock fragments.
Clay Mineral Crystallinity IC and ChC values presented here are all measured values. Since the machine conditions and the sample preparation procedures were calibrated using InterLaboratory Polished Slate (Kisch, 1990) and CIS (Warr and Rice, 1994) standards, the crystallinity values in this study can easily be converted to either the Kubler or CIS scales by the following equations: IC (Kubler) = IC (this study) X 0.92752+0.01752 IC (CIS) = IC (this study) X 1.4516 - 0.035974 Overall the IC and ChC values of samples show very similar ranges with some minor differences (Fig. 3). The measured IC values range between 0.18 and 0.96 "A26 but the results show that most greywackes have IC values between 0.20 and 0.45 "A26. Similarly, the ChC ranges from 0.20 to 0.53 "A26, but only few samples have ChC values greater than 0.44OA26. The IC and ChC values (Fig. 3) show little difference at values < 0.50 "A26. At < 0.30 "A26 the difference between the IC and ChC values for the same sample is within the limits of measurement error, f 0.03 "A26 and between 0.30 and O.SO"A26 the differences are not significant and show no clear trend. In contrast, at values >0.50 "A26, although there are few data points, all show significant discrepancy between IC and ChC values. Most of the differences at >0.50 "A26 are probably caused, in part, by a systematic analytical error and in part by other factors, such as the presence of detrital phases or interlayered smectite.
S. WOLDEMICHAEL AND P.M. BLACK
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ChC (OA28) Very Low-grade Metamorphism Crystallinity Zones Very-low grade metamorphic crystallinity zones were defined by Kubler in 1967 as diagenetic zone, anchizone and epizone, in order of increasing metamorphic grade. Kiibler's anchizone limits calibrated for analytical conditions of this study are 0.38 "A20 and 0.21 "A20 for the lower and upper boundaries, respectively. The measured IC and ChC values show regional zonation trends within the limits of very low-grade crystallinity zones and indicate metamorphic conditions varying from diagenetic zone to anchizone (Figs. 4a and b).
Diagenetic-zone The IC and ChC values of the diagenetic zone are defined by the upper limit of 0.38 "A20 and most IC and ChC values of samples from the Murihiku terrane and samples from the eastern part of the Bay of Islands terrane are > 0.38 "A20 suggesting diagentic zone condition. The Murihiku terrane is represented by eight localities and the IC values of 15 samples (Fig. 4a) range from 0.30 to 0.64 "A20, with one (from locality 6, Fig. l b ) value at 0.25 "A20. The IC values of 11 of the samples (Fig. 4a) indicate diagenetic zone. Three samples (from localities 4, 5 and 7 respectively, Fig. lb) have lower anchizone IC values ranging from 0.30 and 0.36 "A20 and only one sample has an upper anchizone IC value (0.25 "A20). The ChC values of samples from the Murihiku terrane range
between 0.28 and 0.50 "A28, however, most are 0.38rt 0.02 "A20, and cluster near the diagenetic zone boundary. The Bay of Islands terrane greywackes with diagenetic zone IC and ChC values (Figs. 4a and b respectively) come from localities (46,47,49 and 50, Fig. lb) at or near the coast, which constitutes the easternmost exposure of the Bay of Islands terrane. The diagenetic IC and ChC zone in the eastern Bay of Islands terrane may extend south into the Mdange Zone but unfortunately we do not have data to verify this assumption.
Anchizone The IC and ChC anchizone is defined by its upper 0.21 "A20 and lower 0.38 "A20 limits and most greywackes from the Manaia Hill, Bay of Islands (except its eastern exposures), and all greywackesfrom the Omahuta terranes and the MGlange Zone fall within this range. The IC and ChC values of these samples show further distinction into upper and lower anchi-subzones, this distinction is more obvious in some areas (Figs. 4a and b). The IC and ChC values of samples from the Manaia Hill terrane are mostly anchizone with few exceptions. All samples from localities 9, 10, 11, 12, 15, 17, 18, 19, 21, 22, 25, 28, 30 and 33 (Fig. lb) have anchizone IC values (Fig. 4a). Samples from localities 13 (six of seven samples), 16 (seven of nine samples), 32 (three of four samples) and 34 (two of three samples) have anchizone IC values. All samples from locality 22 and locality 18 Gondwana Research, V. 5, No. 4,2002
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VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
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have upper anchi-subzone IC values of 0.28+ 0.01 "A28. IC values of samples from localities 13,14,16,17 and 34 show a bi-modal form with upper anchi-subzone IC values of 0.29f 0.03 "A28 and lower anchi-subzone values at about 0.35k 0.03 "A28. Samples from localities 20, 23 and 26 show diagenetic zone and lower anchi-subzone IC values (0.35 to 0.44 OA20). The IC values (from 0.26 to 0.32 OA28) of samples from localities 43, 44, 45 and 48 in the Bay of Islands terrane suggest lower anchisubzone metamorphic conditions. In the Mklange Zone, the IC values of 18 samples from localities 35, 36, 37, 38, 39, 40 and 41 (Fig. lb) range from 0.23 to 0.30 "A20 (Fig. 4a), all indicating upper anchi-subzone. These samples also have very similar ChC values between 0.22 and O.3O0A28,with one value of 0.34 "A20. The IC values of the eight samples from localities 51, 52,53,54 and 55 (Fig. lb) in the Omahuta terrane range Gondwana Research, V. 5, No. 4,2002
from 0.21 to 0.28 "A28 (Fig. 4a). Most of these samples also have upper anchi-subzone ChC values ranging from 0.22 to 0.32 "A28, with one value of 0.37"A20.
Epizone Samples with epizone IC and ChC values come from locality 29 (Fig. l b ) , in the northern Coromandel Peninsula where the greywackes are in contact with plutonic rocks. Illite in highly altered, clay-rich samples, from a few kilometres south of the contact (locality 27, Fig. l b ) also has epizonal IC and ChC values. The enhanced crystallinity at these localities is believed to reflect local contact metamorphism.
Discussion and Conclusions Mineral assemblages in the North Island greywackes show regional consistency and zonation. Zeolite
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S. WOLDEMICHAEL AND PM. BLACK
Table 1. Correlation between metamorphic conditions indicated by mineral facies and crystallinity zones. Mineral facies
IC and ChC zones
Area/locality
Zeolite
Diagenetic zone
The Murihiku terrane, eastern area of the Bay of Islands terrane and eastern part of the Melange Zone*
Prehnite-pumpellyite
Lower/upper anchi-subzone Lower anchi-subzone Upper anchi-subzone
Manaia Hill terrane Bay of Islands terrane Melange Zone, Omahuta terrane
Pumpellyite-actinolite
Upper anchi-subzone
Western part of the Omahuta terrane and the Melange Zone
* Crystallinity data are not available. IC= illite crystallinity, ChC= chlorite crystallinity
metamorphism in the Murihiku terrane suggests that the Murihiku sediments were buried only to a shallow depth, which is in agreement with the interpretation that the Murihiku terrane contains sediment deposited in a relatively shallow forearc basin. The Manaia Hill and the Bay of Islands terranes are metamorphosed up to prehnitepumpellyite facies, and the Omahuta terrane and the Melange Zone to pumpellyite-actinolite facies. Across these terranes from east to west there is a general increase in metamorphic grade. The Manaia Hill terrane greywackes are mainly prehnite-pumpellyite facies. However, in some of its eastern exposures around Auckland and Coromandel, Iaumontite is found with prehnite-pumpellyite assemblages. The Bay of Islands terrane and the Melange Zone prehnite-pumpellyite greywacke facies occur just to the west of the zeolite facies greywackes. In the Melange Zone and the Omhuta terrane, pumpellyite-actinolite greywacke facies occur further to the west of the pumpellite-prehnite greywackes facies. The metamorphic conditions indicated by the IC and ChC values are very similar to those indicated by the mineral facies and a general correlation of metamorphic IC and ChC zones with metamorphic mineral facies is shown in table 1. The general pattern of correlation is: the crystallinity diagenetic zone with the zeolite mineral facies; the crystallinity lower anchi-subzone with the prehnite-pumpellyite mineral facies; the crystallinity upper anchi-subzone with the pumpellyite-actinolite mineral facies. Overall, the pattern of very low-grade metamorphism revealed by this study is compatible with the model developed for accretionary complexes, where subduction driven fast accumulation/accretion of sediments and burial is accompanied by rapid uplifting and erosion (exhumation). The metamorphism in the Murihiku terrane, in the west, is in contrast to the other terranes. The pattern of metamorphism in the other terranes is one of increasing grade away from the east coast of the North Island towards the west. This evidence supports the suggestion that the Murihiku terrane represents the forearc basin of subduction system and evolved independently.
The emplacement of the Bay of Islands terrane against the Manaia Hill terrane is believed to have occurred prior to, and contemporaneous with, the metamorphism (Kano, 1995), suggesting that the two terranes have been metamorphosed as a single unit. The increase in metamorphic grade from east to west within these terranes, most likely represents increasing depth of burial. The time of emplacement of the Omahuta terrane against Bay of Island terrane is not known. If the juxtaposition of the Omahuta terrane with Bay of Island terrane was postmetamorphism, then the Omahuta terrane could represent a separate metamorphic event.
References Ballance, PE and Campbell, J.D. (1993) The Murihiku arc-related basin of New Zealand (Triassic-Jurassic). In: Balance, P.E (Ed.), South Pacific sedimentary basins. Sedimentary basins of the world, No. 2, pp. 21-33. Black, P.M. (1989) Regional metamorphism in basement Waipapa Group, Northland, New Zealand. R. SOC.of New Zealand, Bull., v. 26, pp. 15-22. Black, P.M. (1994) The “Waipapa Terrane”, North Island ,New Zealand: subdivision and correlation. Geoscience reports of Shizuoka University, v. 20, pp. 55-62. Black, P.M. (1996) Omahuta, Bay of Islands and Manaia Hill terranes: Waipapa composite terrane, North Island, New Zealand. Geol. SOC.New Zealand, Misc. Publ., 82A. Black, P.M., Clark, A.S.B. and Hawke, A.A. (1993) Diagenesis and very low-grade metamorphism of volcaniclastic sandstones from contrasting geodynamic environments, North Island, New Zealand: the Murihiku and the Waipapa terranes. J. Metam. Geol., v. 11, pp. 429-435. Bradshaw, J.D. (1989) Cretaceous geotectonic patterns in the New Zealand region. Tectonics, v. 8, pp. 803-820. Brothers, R.N. (1956) The structure and petrography of greywackes near Auckland, New Zealand. Trans. R. SOC.New Zealand, v. 83, pp. 465-482. Coombs, D.S. (1960) Lower grade mineral facies in New Zealand. Report of the 21st International Geological Congress part 13, pp. 339-357. Dickinson, W.R. (1985) Interpreting provenance relations from detrital modes of sandstones. In: Zuffa, G.G. (Ed.), Provenance of Arenites, pp. 333-361. Gondwana Research, K 5, No. 4,2002
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Dickinson, W.R. and Suczek, C.A. (1979) Plate tectonics and sandstone compositions. Assoc. Amer. Petrol. Geol. Bull., V. 63, pp. 2164-2182. Jennings, W.L. (1987) The stratigraphy, structure, geochemistry and metamorphism of the Waipapa basement in the Omahuta and Puketi forest district, Northland, New Zealand. M.Sc. thesis, University of Auckland, p. 146. Jennings, W.L. (1991) The geochemistry of the Waipapa terrane metabasalts. Ph.D. thesis, University of Auckland, p. 208. Kano, K. (1995) Early deformational fabrics of melange in the Mesozoic Waipapa terrane, North Island, New Zealand. The Island Arc, v. 4, pp. 69-87. Kear, D. (1993) Reflections on major North Island basement rock assemblages and megafaults. J. R. SOC.New Zealand, V. 83, pp. 29-41. Kisch, H.J. (1987) Correlation between indicators of very lowgrade metamorphism. In: Frey, M. (Ed.), Low temperature metamorphism, Blackie, Glasgow, pp. 227-300. Kisch, H.J. (1990) Calibration of the anchizone: a critical comparison of illite ‘crystallinity’ scales used for definition. J. Metam. Geol., v. 8, pp. 31-46. Kretz, R. (1983) Symbols for rock-forming minerals. Amer., Mineralogist, v. 68, pp. 277-279.
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Kubler, B. (1967) La Cristallinite de l’illite et les zones tout A fait superieurs du metamorphisme. In: Etages tectoniques, Colloque de Neuchstel (Suisse), pp. 105-122. McFarlane, A.J. (1993) Geology of the Kirita Hill area, Coromandel Peninsula, New Zealand. M.Sc. thesis, University of Auckland, p. 93. Mortimer, N. (1995) Origin of the Torlesse terrane and coeval rocks, North Island New Zealand. Int. Geol. Rev., v. 36, pp. 891-910. Skinner, D.N. (1993) Geology of the Coromandel Harbour area, Scale 1:50,000: Lower Hutt, New Zealand, Inst. Geol. and Nuclear Sci. geological map 4, 1sheet and 44p. Sporli, K.B. (1978) Mesozoic tectonic, North Island, New Zealand. Geol. SOC.Amer. Bull., v. 89, pp. 415-425. Warr, L.N. a n d Rice, A.H.N. (1994) Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data. J. Metam. Geol., v. 12, pp. 141-152. Woldemichael, S. (1998) Low grade metamorphism and hydrothermal alteration in the basement greywacke terranes of the northern and Central North Island, New Zealand: reconnaissance study. Ph.D. thesis, University of Auckland, p. 290.
Gondwana Research
Very Low-grade Metamorphism in Basement Greywacke Terranes of the Northern and Central North Island, New Zealand S. Woldemichael' and P.M. Black2
' Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan Geology Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand (Manuscript received October 3,2001; accepted April 6,2002)
Abstract Metamorphism in the late Permian to early Cretaceous North Island basement greywackes has been investigated using petrography and clay mineral crystallinity. Several terranes are represented in the North Island greywackes and the study area includes Murihiku, Manaia Hill, Bay of Islands and Omahuta terranes and the Melange Zone. Very lowgrade metamorphic events in the greywackes have produced mineral assemblages of zeolite to pumpellyite-actinolite greywacke facies. Zeolite facies greywackes are characterized by the assemblage Zeo (Lmt, Anl, Hul) +Qtzf Abk Calk Chl? I? I/S* observed in the entire Murihiku terrane and in the eastern part of the Bay of Islands terrane and the Melange Zone. The entire Manaia Hill, most of the Bay of Islands, the eastern area of the Omahuta terranes and the central part of the Melange Zone are a t prehnite-pumpellyite facies with mineral assemblages of Prh+Qtz+Chl+Pmp+Ab++ Ill+ Calf Lmt. Pumpellyite-actinolite facies with the mineral assemblage of Pmp? Act+Qtz+Ab+Chl? Eprt: Ill* Calk Chl occurs in the western part of the Melange Zone and the Omahuta terrane. Illite (IC) and chlorite (ChC) crystallinity values of greywackes are very similar and range from diagenetic zone to anchizone. Metamorphic conditions indicated by the IC and ChC and mineral facies are in excellent agreement and correlate as follows: crystallinity diagenetic-zone with the zeolite mineral facies, crystallinity lower anchizone with prehnite-pumpellyite mineral facies and crystallinity upper anchizone with pumpellyite-actinolite mineral facies. The general increase in the metamorphic grade from east to west, except in Murihiku terrane, is compatiblewith the sequence of accretion expected in a subduction environment.
Key words: Crystallinityzone, prehnite-pumpellyitefacies, pumpellyite-actinolitefacies,very low-grade metamorphism, zeolite facies.
Introduction Late Permian to early Cretaceous greywackes are exposed over a wide area of the North Island of New Zealand, and are inferred to underlie the thick Cenozoic volcanic and sedimentary sequences that now cover much of the remainder (Fig. la). The exposed North Island greywackes consist of several tectonostratigraphic terranes and this study includes the entire North Island exposure of the Murihiku, the Manaia Hill, the Bay of Islands and the Omahuta terranes and the Melange Zone (Fig. la). This paper is concerned mainly with aspects of the metamorphism of the greywackes and gives only a brief summary of the characteristics of the terranes. The northern and Central North Island greywackes have long been recognized as being metamorphosed at very low grade (Brothers, 1956; Coombs, 1960) and several *Kretz (1983)
researchers (e.g., Black, 1989; Black et al., 1993) have described zeolite, prehnite-pumpellyite and pumpellyiteactinolite facies assemblages. Minerals characteristicof very-low grade metamorphism such as zeolite, prehnite and pumpellyite usually develop from fine-grained unstable starting materials such as volcanic glass. Metamorphic belts which lack suitable starting materials also lack indicator minerals and, out of necessity, researchers have used other means (e.g., clay mineral crystallinity) to define metamorphic conditions. Crystallinity studies are also widely applied in geological environments where metamorphic indicator minerals are available and correlation of crystallinityzones and mineral facies usually lead to a better understanding of very lowgrade metamorphism (Kisch, 1987). There are little or no crystallinity data for North Island greywackes and this study was carried out to bridge that gap and facilitate
858
S. WOLDEMICHAEL AND PM. BLACK
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Fig. 1. Simplified map of the North Island, New Zealand; (a) the distribution of greywacke outcrops showing the main greywacke terrane boundaries (Sporli, 1978; Black, 1994) TVZ = Taupo Volcanic Zone, (b) greywacke sample localities in the northern half of the North Island. Bold lines show approximate boundaries of the greywackes terranes.
comparison of the metamorphic conditions in North Island greywackeswith those in similar geological environments.
Samples and Methods Samples for this study were collected from 55 localities distributed throughout the study area (Fig. lb). In most cases, several samples were collected from each locality in order to represent variation in lithology,which is mainly
grain-size related such as metasandstone and metasiltstone. More than 300 representative thin sections were examined. Most of the samples were collected specifically for this study but some samples collected by previous workers and housed in the Universityof Auckland Geology Department collections, were also examined. Clay mineral crystallinity determinations were carried out on <2pm grain-size clay mineral-rich fractions that were prepared from 165 representative samples using the Gondwana Research, V. 5, No. 4,2002
VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
method similar to Warr and Rice (1994) and described in Woldemichael(l998). The IC and ChC were determined from the X-ray diffraction pattern of illite 001 (10 A) and chlorite 002 (7 A) reflections, respectively. The diffractograms were run at the Geology Department, Auckland University, using a Philips generator 1130/ goniometer 1050, operated at 40 kV and 20 mA, CuKa radiation, normal focus tube, slits 1'4.2 mm-lo,graphite monochromator PW 1762, and a xenon proportional detector. Each sample was scanned from 2 to 16 "20, at a scan rate of 0.6 "20/min and a step size of 0.01". The error of the crystallinity measurements depends on peak broadening and was determined by making replicate measurements on several samples. In general, the reproducibility of the crystallinity values was about 15% variance from the mean.
Tectonic Setting The New Zealand micro-continent is a fragment of Gondwanaland and its offshore extensions, which separated from the supercontinent in late Cretaceous to early-Tertiarytime and later, during a Cenozoic tectonic event, foundered, elevated and rearranged in blocks to the present position (Sporli, 1978; Bradshaw, 1989). The Australian-Antarctic part of Gondwanaland is thought to have attained continental-type thickness and structure by the end of the Carboniferous (Bradshaw, 1989). During the late Paleozoic to mid-Cretaceous, accretion of terranes took place in a subduction-related arc-forearc-trench system that extended along the eastern margin of the Gondwana plate. The resulting accretionary complex is composed of dominantly terrigenous clastic deposits with minor pelagic and volcanogenic assemblages, characteristic of a typical convergent margin. These sedimentary sequences now make up a series of discrete fault-bounded greywacke terranes, each with a distinct litho-stratigraphy.
Greywacke Terranes The distribution of the greywacke terranes in the study area is shown in figure l a . The characteristics of the Murihiku, Manaia Hill, Bay of Islands and the Omahuta terranes and the Mblange Zone are briefly summarized below.
The Murihiku terrane Late Triassic to early Cretaceous Murihiku terrane greywackes outcrop along the west coast of the North Island, from Port Waikato to Awakino in a relatively simple N-plunging fold, with a subvertical to steeply east-dipping Gondwana Research, V. 5, No. 4,2002
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axial surface (Sporli, 1978). In most exposures of this terrane, stratigraphic units are well preserved with excellent fossil control and can be traced for several kilometres. The Murihiku terrane consists of volcanoclastic siltstones, sandstones, conglomerates and tuffaceous sediments. Along the south shore of Kawhia Harbour there is a continuous sequence of late Triassic to late Jurassic sediments, whereas around Port Waikato late-Jurassic to early-Cretaceous sediments are exposed. The Murihiku terrane is interpreted as a forearc basin, which contains a continuous record of arc-related volcanism from early Triassic to early Cretaceous (Ballance and Campbell, 1993).
The Manaia Hill terrane The late Jurassic to early Cretaceous Manaia Hill terrane (Black, 1994) is exposed in the Central North Island and Cormandel Peninsula. It is separated from the Murihiku terrane to the west by a narrow zone of serpentinite and ultramafics rocks (Sporli, 1978; Black et al., 1993), and to the north the Melange Zone separates the Manaia Hill terrane from the Bay of Islands terrane (Black, 1994). The eastern and southern boundaries of the Manaia Hill terrane are not well defined. However, greywackes encountered in deep geothermal drill-holes within the Taupo Volcanic Zone (TVZ, Fig. l a ) have been described as belonging to the Mania Hill terrane (Kear, 1993; Mortimer, 1995). The Manaia Hill terrane dominantly consists of siltstones, sandstones and conglomerate. Within the Manaia Hill terrane, typical conglomerates consist of rounded to sub-angular, up to pebble size clasts of sedimentary and volcanic fragments in a sandy grade matrix, but conglomerates from the North Coromandel Peninsula contain clastic debris from a wide variety of sources (McFarlane, 1993; Skinner, 1993). The conglomerate beds usually underlie a sandstone bed, either forming massive beds, or grading into the overlying sandstone within a few meters. These conglomeratesandstone sequences are interpreted to be thick debris flow and fan deposits on the evidence of the dominantly argillite pebbles and rare limestone, chert and granite material.
The Bay of Islands terrane The Bay of Islands terrane, exposed in the east coast of Northland from around Bay of Islands south to near Whangarei, contains sediments of late Paleozoic to early Mesozoic age deposited some distance away from the subduction trench and believed to have been accreted to the subduction complex in the early to Middle Jurassic (Black, 1994). The Bay of Islands terrane is locally very
860
S. WOLDEMICHAEL AND P.M. BLACK ~
deformed and the stratigraphic sequence is disrupted. The sediments are dominantly quartzofeldspathic sandstone and clastic-poor siliceous siltstone; the former is more common in the western half of the terrane and the latter in the east.
The Omahuta terrane The late Paleozoic to early Mesozoic rocks of the Omahuta terrane (Black, 1994) are exposed in the Omahuta and Puketi forests, Northland. The Omahuta terrane has two distinct stratigraphic units, the Omahuta and Puketi forest units, which are in fault contact (Jennings, 1987). The Omahuta units contain dominantly plutoniclastic and volcaniclastic sandstone and siltstone sequences very similar to sediments accumulated at evolved or dissected magmatic arc-relatedbasins (Dickinson and Suczek, 1979; Dickinson, 1985). The Puketi forest unit consists of a sequence of massive basalts, and pillow lavas interbedded with tuffs, sandstone and siliceous siltstone and cherts. The basalts show chemical compositions ranging from strongly depleted basalts to chemically enriched andesites, thus indicating source heterogeneity (Jennings, 1991). The time of juxtaposition of the Omahuta terrane against the Bay of Islands terranes is unknown.
The Me‘lange Zone The Melange Zone contains tectonic slices of Jurassic oceanic floor material and debris that may be derived from the Bay of Islands and the Manaia Hill terranes and their metamorphic equivalents (Black, 1996).
Metamorphism Our petrographic study has confirmed that greywackes of the northern and Central North Island show regional zoning of zeolite, prehnite-pumpellyite and pumpellyiteactinolite mineral assemblages.
Zeolite facies The key metamorphic minerals of this facies are zeolites and the main mineral assemblage are Zeo (Lmt, Anl, Hul) +Qtz+Abt Calk Chlf I+ I/S. Greywackes containing zeolite facies mineral assemblages occur in two distinct geographical locations. The first location is the entire greywacke exposure of the Murihiku terrane and the second location, in Northland, in an area which includes the eastern exposures of both the Bay of Islands terrane and the Melange Zone (Fig. 2). In most zeolite facies greywackes laumontite is the abundant zeolite; the exception is the Middle to late Jurassic sequence of the Murihiku terrane where heulandite is more common. Laumontite mostly occurs
Fig. 2. Metamorphic map of the northern half of the North Island, showing the distribution of the zeolite, prehnite-pumpellyite and pumpellyite-actinolite facies and their relations to terrane boundaries (as defined in figure 1).
as original pore, vein or fracture fillings, or replacing plagioclase and lithic fragments, and is usually intergrown with chlorite, quartz or clay minerals. Heulandite usually occurs as cavity fillings or replacing volcanic glass and mostly forms as orange-coloured, short, fibrous, stubby crystals and sometime as meshes of interlocking crystals. Analcime is far less abundant than either laumontite or heulandite and usually occurs as a vein or cavity-filling mineral. Quartz, occurs in all greywackes as clastic grains, cement and vein fillings as well as replacing detrital feldspar and lithic fragments, and is usually associated with zeolite, calcite and clay minerals. Albite is also common but mainly replaces detrital plagioclase where it is usually intergrown with laumontite. Chlorite is a typical constituent of the interstitial matrix; it also replaces rock fragments and detrital ferromagnesian grains and occurs as a vein mineral. Vein chlorite is associated with laumontite, calcite and, less commonly, with quartz. Gondwana Research, K 5, No. 4,2002
VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
Calcite occurs mainly as a vein mineral usually intergrown with laumontite, quartz, albite and chlorite but sometimes it also occurs as discrete grains in the matrix, as cement, and replaces some clastic grains, bioclasts and rock fragments. Calcite is occasionally intergrown with heulandite and analcime. Prehnite-Pumpellyitefacies Prehnite-pumpellyite facies greywackes occur throughout the Central and northern North Island in the Manaia Hill terrane, the western part of the Bay of Islands terrane, the eastern part of Omahuta terrane and the Melange Zone (Fig. 2). Greywackes in these areas contain the mineral assemblage Prh+ Pmp+Qtz+Ab+Chl+ I11 k Calk Lmt. Throughout this area, prehnite is ubiquitous and commonly occurs as a well crystallized vein filling mineral usually associated with quartz, calcite, chlorite and rarely with laumontite. Locally, fine-grained prehnite crystals occur, replacing rock matrix or detrital fragments. Pumpellyite commonly fills cavities but is less abundant than prehnite and rarely occurs as a vein mineral. It is usually intergrown either with chlorite and epidote or with prehnite and epidote. Quartz, as in the zeolite facies, is ubiquitous in veins, cavities and as matrix cement. In veins and cavities, it forms fine-grained aggregates of crystals usually associated with calcite and prehnite and rarely with laumontite, illite and chlorite. In most of the veins examined, quartz predated all the other minerals present. It is often found inter-grown with feldspars as a devitrification product of volcanic glass or matrix. Albite occurs mainly as a replacement of plagioclase but is occasionally in the matrix, mostly accompanied by clay minerals or quartz. Calcite mainly occurs as vein filling, but also as cement and replaces clastic grains. As a vein-forming mineral, it occurs as monomineral veins as well as in association with quartz and prehnite and rarely with laumontite. Whenever it is possible to establish a paragenesis, in almost all cases calcite was the last crystallising mineral. Chlorite and illite commonly occur as matrix cement and replace lithic fragments, as well as crystallising in veins and cavities. Illite shows no variation in optical properties whereas chlorite occurs in green and brown varieties. Laumontite has been observed in a few localities in the Coromandel Peninsula and around Auckland. It occurs as a vein and matrix mineral usually associated with prehnite, quartz and calcite, suggesting a transition from the zeolite facies to prehnite-pumpellyite facies. Pumpellyite-Actinolitefacies Greywackes containing the pumpellyite-actinolitefacies mineral assemblage Pmp+Act+Qtz+Ab+Chlk Epf Ill+ Gondwana Research, V. 5, No. 4,2002
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Calk Chl occur in the western Omahuta terrane and the western part of the Melange Zone. The pumpellyiteactinolite facies is characterized by the absence of prehnite, suggesting that actinolite crystallized at the expense of prehnite. Actinolite with green and brown pleochroism occurs chiefly in the rock matrix. Pumpellyite occurs in both veins and the matrix; in veins it is associated with epidote, calcite, quartz and albite and in the matrix it crystallises with epidote, calcite, quartz, chlorite and white mica in cavities. Epidote occurs as spongy masses, radiating aggregates or small clusters of granules in most of these greywackes. Quartz and albite, as in rocks of the zeolite and prehnite-pumpellyite facies, are common minerals. Quartz occurs as vein fillings and replacing feldspar grains and in the matrix, while albite mainly replaces demtal feldspars. Calcite, chlorite and illite are common metamorphic constituents of pumpellyite-actinolite facies greywackes. Calcite occurs in both the matrix and in veins as irregular masses or euhedral crystals. Chlorite and illite/micas occur as fine-grained, irregular patches in the matrix or replacing clastic grains and rock fragments.
Clay Mineral Crystallinity IC and ChC values presented here are all measured values. Since the machine conditions and the sample preparation procedures were calibrated using InterLaboratory Polished Slate (Kisch, 1990) and CIS (Warr and Rice, 1994) standards, the crystallinity values in this study can easily be converted to either the Kubler or CIS scales by the following equations: IC (Kubler) = IC (this study) X 0.92752+0.01752 IC (CIS) = IC (this study) X 1.4516 - 0.035974 Overall the IC and ChC values of samples show very similar ranges with some minor differences (Fig. 3). The measured IC values range between 0.18 and 0.96 "A26 but the results show that most greywackes have IC values between 0.20 and 0.45 "A26. Similarly, the ChC ranges from 0.20 to 0.53 "A26, but only few samples have ChC values greater than 0.44OA26. The IC and ChC values (Fig. 3) show little difference at values < 0.50 "A26. At < 0.30 "A26 the difference between the IC and ChC values for the same sample is within the limits of measurement error, f 0.03 "A26 and between 0.30 and O.SO"A26 the differences are not significant and show no clear trend. In contrast, at values >0.50 "A26, although there are few data points, all show significant discrepancy between IC and ChC values. Most of the differences at >0.50 "A26 are probably caused, in part, by a systematic analytical error and in part by other factors, such as the presence of detrital phases or interlayered smectite.
S. WOLDEMICHAEL AND P.M. BLACK
862
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ChC (OA28) Very Low-grade Metamorphism Crystallinity Zones Very-low grade metamorphic crystallinity zones were defined by Kubler in 1967 as diagenetic zone, anchizone and epizone, in order of increasing metamorphic grade. Kiibler's anchizone limits calibrated for analytical conditions of this study are 0.38 "A20 and 0.21 "A20 for the lower and upper boundaries, respectively. The measured IC and ChC values show regional zonation trends within the limits of very low-grade crystallinity zones and indicate metamorphic conditions varying from diagenetic zone to anchizone (Figs. 4a and b).
Diagenetic-zone The IC and ChC values of the diagenetic zone are defined by the upper limit of 0.38 "A20 and most IC and ChC values of samples from the Murihiku terrane and samples from the eastern part of the Bay of Islands terrane are > 0.38 "A20 suggesting diagentic zone condition. The Murihiku terrane is represented by eight localities and the IC values of 15 samples (Fig. 4a) range from 0.30 to 0.64 "A20, with one (from locality 6, Fig. l b ) value at 0.25 "A20. The IC values of 11 of the samples (Fig. 4a) indicate diagenetic zone. Three samples (from localities 4, 5 and 7 respectively, Fig. lb) have lower anchizone IC values ranging from 0.30 and 0.36 "A20 and only one sample has an upper anchizone IC value (0.25 "A20). The ChC values of samples from the Murihiku terrane range
between 0.28 and 0.50 "A28, however, most are 0.38rt 0.02 "A20, and cluster near the diagenetic zone boundary. The Bay of Islands terrane greywackes with diagenetic zone IC and ChC values (Figs. 4a and b respectively) come from localities (46,47,49 and 50, Fig. lb) at or near the coast, which constitutes the easternmost exposure of the Bay of Islands terrane. The diagenetic IC and ChC zone in the eastern Bay of Islands terrane may extend south into the Mdange Zone but unfortunately we do not have data to verify this assumption.
Anchizone The IC and ChC anchizone is defined by its upper 0.21 "A20 and lower 0.38 "A20 limits and most greywackes from the Manaia Hill, Bay of Islands (except its eastern exposures), and all greywackesfrom the Omahuta terranes and the MGlange Zone fall within this range. The IC and ChC values of these samples show further distinction into upper and lower anchi-subzones, this distinction is more obvious in some areas (Figs. 4a and b). The IC and ChC values of samples from the Manaia Hill terrane are mostly anchizone with few exceptions. All samples from localities 9, 10, 11, 12, 15, 17, 18, 19, 21, 22, 25, 28, 30 and 33 (Fig. lb) have anchizone IC values (Fig. 4a). Samples from localities 13 (six of seven samples), 16 (seven of nine samples), 32 (three of four samples) and 34 (two of three samples) have anchizone IC values. All samples from locality 22 and locality 18 Gondwana Research, V. 5, No. 4,2002
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VERY LOW-GRADE METAMORPHISM IN NORTH ISLAND, NEW ZEALAND
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Locality number Fig. 4. The regional patterns of IC (a) and ChC (b) in greywacke rocks of the North Island, New Zealand. Samples are arranged with locality number increasing from left to right. The solid lines show the crystallinity zone boundaries and the broken lines show the terrane boundaries.
have upper anchi-subzone IC values of 0.28+ 0.01 "A28. IC values of samples from localities 13,14,16,17 and 34 show a bi-modal form with upper anchi-subzone IC values of 0.29f 0.03 "A28 and lower anchi-subzone values at about 0.35k 0.03 "A28. Samples from localities 20, 23 and 26 show diagenetic zone and lower anchi-subzone IC values (0.35 to 0.44 OA20). The IC values (from 0.26 to 0.32 OA28) of samples from localities 43, 44, 45 and 48 in the Bay of Islands terrane suggest lower anchisubzone metamorphic conditions. In the Mklange Zone, the IC values of 18 samples from localities 35, 36, 37, 38, 39, 40 and 41 (Fig. lb) range from 0.23 to 0.30 "A20 (Fig. 4a), all indicating upper anchi-subzone. These samples also have very similar ChC values between 0.22 and O.3O0A28,with one value of 0.34 "A20. The IC values of the eight samples from localities 51, 52,53,54 and 55 (Fig. lb) in the Omahuta terrane range Gondwana Research, V. 5, No. 4,2002
from 0.21 to 0.28 "A28 (Fig. 4a). Most of these samples also have upper anchi-subzone ChC values ranging from 0.22 to 0.32 "A28, with one value of 0.37"A20.
Epizone Samples with epizone IC and ChC values come from locality 29 (Fig. l b ) , in the northern Coromandel Peninsula where the greywackes are in contact with plutonic rocks. Illite in highly altered, clay-rich samples, from a few kilometres south of the contact (locality 27, Fig. l b ) also has epizonal IC and ChC values. The enhanced crystallinity at these localities is believed to reflect local contact metamorphism.
Discussion and Conclusions Mineral assemblages in the North Island greywackes show regional consistency and zonation. Zeolite
864
S. WOLDEMICHAEL AND PM. BLACK
Table 1. Correlation between metamorphic conditions indicated by mineral facies and crystallinity zones. Mineral facies
IC and ChC zones
Area/locality
Zeolite
Diagenetic zone
The Murihiku terrane, eastern area of the Bay of Islands terrane and eastern part of the Melange Zone*
Prehnite-pumpellyite
Lower/upper anchi-subzone Lower anchi-subzone Upper anchi-subzone
Manaia Hill terrane Bay of Islands terrane Melange Zone, Omahuta terrane
Pumpellyite-actinolite
Upper anchi-subzone
Western part of the Omahuta terrane and the Melange Zone
* Crystallinity data are not available. IC= illite crystallinity, ChC= chlorite crystallinity
metamorphism in the Murihiku terrane suggests that the Murihiku sediments were buried only to a shallow depth, which is in agreement with the interpretation that the Murihiku terrane contains sediment deposited in a relatively shallow forearc basin. The Manaia Hill and the Bay of Islands terranes are metamorphosed up to prehnitepumpellyite facies, and the Omahuta terrane and the Melange Zone to pumpellyite-actinolite facies. Across these terranes from east to west there is a general increase in metamorphic grade. The Manaia Hill terrane greywackes are mainly prehnite-pumpellyite facies. However, in some of its eastern exposures around Auckland and Coromandel, Iaumontite is found with prehnite-pumpellyite assemblages. The Bay of Islands terrane and the Melange Zone prehnite-pumpellyite greywacke facies occur just to the west of the zeolite facies greywackes. In the Melange Zone and the Omhuta terrane, pumpellyite-actinolite greywacke facies occur further to the west of the pumpellite-prehnite greywackes facies. The metamorphic conditions indicated by the IC and ChC values are very similar to those indicated by the mineral facies and a general correlation of metamorphic IC and ChC zones with metamorphic mineral facies is shown in table 1. The general pattern of correlation is: the crystallinity diagenetic zone with the zeolite mineral facies; the crystallinity lower anchi-subzone with the prehnite-pumpellyite mineral facies; the crystallinity upper anchi-subzone with the pumpellyite-actinolite mineral facies. Overall, the pattern of very low-grade metamorphism revealed by this study is compatible with the model developed for accretionary complexes, where subduction driven fast accumulation/accretion of sediments and burial is accompanied by rapid uplifting and erosion (exhumation). The metamorphism in the Murihiku terrane, in the west, is in contrast to the other terranes. The pattern of metamorphism in the other terranes is one of increasing grade away from the east coast of the North Island towards the west. This evidence supports the suggestion that the Murihiku terrane represents the forearc basin of subduction system and evolved independently.
The emplacement of the Bay of Islands terrane against the Manaia Hill terrane is believed to have occurred prior to, and contemporaneous with, the metamorphism (Kano, 1995), suggesting that the two terranes have been metamorphosed as a single unit. The increase in metamorphic grade from east to west within these terranes, most likely represents increasing depth of burial. The time of emplacement of the Omahuta terrane against Bay of Island terrane is not known. If the juxtaposition of the Omahuta terrane with Bay of Island terrane was postmetamorphism, then the Omahuta terrane could represent a separate metamorphic event.
References Ballance, PE and Campbell, J.D. (1993) The Murihiku arc-related basin of New Zealand (Triassic-Jurassic). In: Balance, P.E (Ed.), South Pacific sedimentary basins. Sedimentary basins of the world, No. 2, pp. 21-33. Black, P.M. (1989) Regional metamorphism in basement Waipapa Group, Northland, New Zealand. R. SOC.of New Zealand, Bull., v. 26, pp. 15-22. Black, P.M. (1994) The “Waipapa Terrane”, North Island ,New Zealand: subdivision and correlation. Geoscience reports of Shizuoka University, v. 20, pp. 55-62. Black, P.M. (1996) Omahuta, Bay of Islands and Manaia Hill terranes: Waipapa composite terrane, North Island, New Zealand. Geol. SOC.New Zealand, Misc. Publ., 82A. Black, P.M., Clark, A.S.B. and Hawke, A.A. (1993) Diagenesis and very low-grade metamorphism of volcaniclastic sandstones from contrasting geodynamic environments, North Island, New Zealand: the Murihiku and the Waipapa terranes. J. Metam. Geol., v. 11, pp. 429-435. Bradshaw, J.D. (1989) Cretaceous geotectonic patterns in the New Zealand region. Tectonics, v. 8, pp. 803-820. Brothers, R.N. (1956) The structure and petrography of greywackes near Auckland, New Zealand. Trans. R. SOC.New Zealand, v. 83, pp. 465-482. Coombs, D.S. (1960) Lower grade mineral facies in New Zealand. Report of the 21st International Geological Congress part 13, pp. 339-357. Dickinson, W.R. (1985) Interpreting provenance relations from detrital modes of sandstones. In: Zuffa, G.G. (Ed.), Provenance of Arenites, pp. 333-361. Gondwana Research, K 5, No. 4,2002
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