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Discussion on the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie
Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
DISCUSSION ON THE PAPER “THE ORIGIN OF ULTRAMAFIC BASIC ROCKS” BY P.J. WYLLIE
AND ULTRA-
G.A. CHALLIS New Zealand Geological Survey, Lower Hutt (New Zealand) (Received March 6, 1969)
SUMMARY In recent years, speculation on the origin of “alpinotype” ultramafic rocks has tended to ignore the field relationships of the rocks, and in particular, their very common association with basic volcanic rocks, or greenschists. In New Zealand, the almost invariable association of ultramafic intrusions and basaltic volcanics is so striking that it has dominated the thinking of field geologists on the origin of these rocks, and some have proposed that they represent ultramafic submarine flows contemporaneous with the Permian volcanics. However, pronounced mineralogical layering, and other textural features suggest that gravitational differentiation has played a part in their formation, and high temperature metamorphism at unfaulted contacts excludes cold, solid intrusion. Geophysical data suggest that they have a lopolithic form and a thickness of 5,OOO-10,000 ft. for the Dun Mountain and Red Hill masses. A detailed study of both ultramafic rocks and the surrounding volcanics has shown that there are chemical and mineralogical similarities between the ultramafic and volcanic rocks in any particular area. In the Permian sequence in New Zealand, large thicknesses of andesitic-basaltic volcanics form pronounced belts on both limbs of a syncline, marginal to the main Upper Paleozoic and Mesozoic New Zealand geosyncline, and a number of individual ultramafic bodies are found within both belts. On the eastern limb, olivine-poor tholeiitic basalts with very low K90/Na90 ratios of 0.09, enclose the ultramafic intrusions of Dun Mountain and Red Hill in which the clinopyroxenes show features common to pyroxenes from tholeiitic basalts and their derivatives (low Al,, K90, Na90 and TiO9). The pegmatitic phase in these ultramafic rocks is a pyroxenite, and there is very little sulphide mineralisation associated. On the western limb, the basic volcanics have an average K20/Na20 ratio of 0.40, and the associated ultramafic bodies show more alkaline features with Ti-rich pyroxenes and primary hornblende. The pegmatitic phase in these rocks is a hornblendite, and much of the known sulphide mineralisation and platinum in New Zealand is associated with ultramafics of the western belt. I have previously suggested that the New Zealand ultramafic masses represent the magma chambers of basaltic and andesitic volcanoes in a Permian double volcanic arc, analogous to the present-day arcs of the Circum-Pacific Belt. The depths of the magma chambers, and hence the Tectonophysics, 7 (5-6) (1969) 495-505
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levels now exposed by erosion, are dependent on the composition of the original magma, and the crustal thickness in the area. It is possible that in more deformed areas, the ultramafic portions of the magma chambers could be displaced, either horizontally or vertically, by faulting.
INTRODUCTION
New Zealand is a good area for study of field and laboratory relationships that may help to elucidate some problems of the origin of “alpinotype” ultramafic rocks, as the larger masses, such as Dun Mountain and Red Hill, are well-exposed, and largely undeformed and un~erpentinized. In recent years, speculation on the origin of these rocks has tended to ignore field relationships, in particular the common association with basic volcanic rocks, or their metamorphosed equivalents? the green-and blue-schists. In New Zealand, the larger ultramafic bodies occur in Upper Paleozoic and Mesozoic basic volcanics and tuffaceous sediments in the Southern Island. They are associated with basic volcanics of presumed Permian age on both sides of a marginal syncline on the western edge of the New Zealand geosynciine of Upper Paleozoic-Mesozoic age. In fact, this association is so close that until recently the ultramafic rocks and Permian volcanic rocks were mapped as a single unit, termed the “mineral belt” or “ultrabasic be.lt”, and some field geologists have suggested that the ultramafic rocks represent submarine lava flows contemporaneous with the enclosing volcanics. On the other hand, Dun ~ount~n and other ultramafic masses in the New Zealand “ultrabasic belt” have always been classified as “alpinotype”, and an origin by solid, or semi-solid intrusion along major thrust faults has been proposed by a number of New Zealand and overseas geologists. More recently, some petrologists have suggested that the New Zealand ultramafic bodies represent sub-volcanic, or intra-volcanic accumulates from olivine-rich basaltic magma. Over the past ten years the geological mapping of New Zealand, on a scale of four miles to the inch, has been completed, and much new information is available. In this paper some new data is presented that may help to reconcile field and laboratory observations.
POSITION
AND FORM OF THE ULTRAMAFIC
MASSES
The larger, and better known ultramafic masses, such as Dun Mountain and Red Hill, occur with basic volcanics of Permian age (Fig.1, arcs 1 and 2) on the eastern side of the Nelson and Southland synclines which are marginal troughs west of the main New Zealand geosyncline. During the Upper Paleozoic, Triassic and Jurassic, shallow-water shelf and transitional Fig. 1. Basic volcanic rocks in the Permian and Cretaceous of in solid black. Crosses alongside rocks within the volcanics of that 496
and associated mafic-ultramafic intrusions New Zealand. Areas of volcanics are shown indicate the presence of mafic-ultramafic area. Tectonophysics, ‘7 (5-6) (1969) 495-505
SCALE 50
0
SO
100
IS0 MILES
@ Ulrramafic
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lntrubnf
497
sediments with a large content of volcanic material were deposited in the marginal trough, whiIe deep-water “greywacke type” sediments were deposited in the main geosyncline to the east (Wellman, 1956). Considerable thicknesses of basaltic and andesitic volcanics are found on both sides of the marginal synclines, and Wellman (1956) suggested that the volcanics of the western limb represented a Carboniferous belt of island volcanoes. Examining the petrology and chemistry of volcanics from bvth sides of the marginal synclines, Challis (1968) came to the conclusion that there was a double volcanic arc present in the Upper Paleozoic, and recent mapping (Wood, 1962, 1966; Waterhouse, 1964) has shown that ultramafic rocks are associated with volcanics on both sides of the synclines. Because the ultramafic rocks and basic volcanics on the eastern side of the Southland and Nelson synclines were mapped as a single unit, the ultramafic rocks appeared to form a thick, continuous horizon extending for a total of nearly 150 miles, with the northern Nelson lens displaced by 300 miles transcurrent movement on the Alpine fauIt (Wellman, 1956). In fact, ultramafic rocks form individual, reIatively small lopolithic, or sill-like bodies within the Permian volcanics in most places. The largest of the ultramafic masses is Red Hill, in the Wairau Valley (Fig.1) which is 49 sq. miles in area and, from geophysical data (Malahof, 1961) is approximately 10,000 ft. thick. Ultramafic rocks at Dun Mountai~l cover an area of about 4 sq. miles, and the Red Moulltain massif in northwestern @ago is about 20 sq. miles in area. Apart from the above localities, the “ultramafic belt” consists of basic volcanic rocks, and ultramafic rocks are only present as small, discontinuous, highly sheared, and completely serpentinised lenses within the volcanics. In some instances, material mapped as serpentinite is actually chloritised volcanics. The eastern bound&ry of the “ultramafic belt” on the eastern limb of the Southland and Nelson synclines is shown as a major fault, and this led Benson (1926) and Turner (1930) to propose solid intrusion of the ultramafics along major thrust faults. However, in many places, such as at Dun Mountain and in the Mossburn area, the Permian volcanics intervene, and the major fault is at the margin of the volcanic-ultramafic belt as a whole, rather than specifically within the ultramafic rocks. On the western limb of the Nelson and Soutliland synclines, ultramafic rocks are not as plentiful. but are nevertheless associated with the Brook Street Volcanics on,D’Urville Island, where there is a small gabbroserpentinite complex at Otu Bay (Coleman, 1966), and small lenses of gabbroserpentinite occur in the Eglinton Volcanics, near the southern end of the Alpine fault (Fig.1) and in the Takitimu Mountains. In the Longwood Range, and at Bluff, layered gabbro-norite complexes include layers of dunite, pyroxene peridotite, pyroxenite and anorthosite. It should be noted that there is an unusual occurrence of a similar layered gabbro-peridotite mass 011 the eastern side of the Southland syncline, at Cow Saddle at the southern end of the Red Mountain lens (Benson and Holloway, 1940) where it forms part of the so-called “ultramafic belt” and further to the southeast is the Otama gabbro-norite complex. Geophysical surveys (Hathertoii, 1966, lS67) of the Southland and Nelson synclines reveal large positive isostatic gravity anomalies on both sides of the synclines. The largest (+85 mgal) is associated with the BluffLongwood gabbro-peridotite complex on the southwestern margin of the 498
Tectonophysics, 7 (5-6) (1969) 495-506
Southland syncline (Fig. 1). Another positive anomaly of approximately +60 mgal is associated with a gabbro-norite complex near,Otama on the northeastern margin. The anomaly appears too large to be accounted for by the rocks exposed at the surface, and Hatherton (1966) postulates the existence of a basic-ultrabasic mass at depth extending in a northwesterly direction towards Mossburn and the exposed portion of the “ultramafic belt”. In the Nelson syncline, the largest isostatic gravity anomaly is associated with Red Hill (+45 mgal). The anomaly over Dun Mountain is relatively small (+15-+20 mgal). On the western side of the Nelson syncline there is positive anomaly of up to +40 mgal associated with the Brook Street Volcanics and Rotoroa Complex (Hatherton, 1967), but as the area is overlain by thick gravels, the significance of the anomaly is in doubt. A large positive magnetic anomaly is also present on the western side of the syncline, and Hatherton has interpreted this as serpentinised ultramafic rocks at depth.
Ultramafic
rocks
associated
with Cretaceous
volcanics
In the inland Kaikoura Range, centred on Mount Tapuaenuku (9,465 ft.) there is a layered peridotite-pyroxenite-anorthosite-gabbro complex that intrudes and metamorphoses Upper Jurassic sandstones (Fig.1, arc 3). A second, smaller intrusion occurs at the northern end of the range. Block faulting and uplift in the Tertiary, followed by deep dissection, have exposed a section of at least 10,000 ft. Exposures are nearly perfect and the basicultrabasic complex is seen to feed a radial dyke swarm with the dykes as feeders for overlying alkaline olivine basalt flows. There are strong mineralogical and chemical affinities between the plutonic, hypabyssal and volcanic rocks, and xenoliths and xenocrysts from the layered intrusion are found in the dykes and flows (Challis, 1963). Some of the dykes are ultramafic in character, but these never reach the overlying basalts and terminate a few hundred feet above the top of the layered intrusion. No other occurrences of ultramafic rocks with Cretaceous volcanics are known, but uplift and erosion has not exposed such a sequence elsewhere.
CHEMICAL VOLCANIC
AND MINERALOGICAL ROCKS
CHARACTERISTICS
OF THE ULTRAMAFIC
AND
During a study of some ultramafic rocks on the eastern limbs of the Southland and Nelson synclines, Challis (1965a) noted the close association of the ultramafic rocks with olivine-poor spilitic volcanics. Chemically, these volcanics are tholeiitic in character, and it was found that the clinopyroxenes from the associated ultramafic rocks plotted in the same field (for AIIV, Na, Ti, etc.) as clinopyroxenes from tholeiitic basalts, and layered intrusions presumed to be derived by differentiation from tholeiitic basaltic magma. Challis suggested that the ultramafic rocks represented the deeplevel magma chambers of a line of Upper Paleozoic volcanoes, and that the original magma from which the ultramafic rocks differentiated might have been an olivine tholeiite or picrite. Later, Challis and Lauder (1966) concluded that alkaline and talc-alkaline magmas might also be expected to
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yield ultramafic differentiates which could be distinguished by their mineralogy and chemistry. Work on the KzO/NaZO ratios of Permian and Cretaceous basaltic and andesitic volcanics (Challis, 1968) has shown that double volcanic arcs can be defined in both the Permian and the Cretaceous. The eastern arcs (Fig.1, arcs 2 and 41, on the edge of the Upper Paleozoic and Mesozoic geosynclines, are characterised by volcanics with very low KzO/NazO ratios, typical of those found in present-day oceanic tholeiites and basalts of volcanic arcs bordering deep ocean basins (Sugimura, 1961; Kupo, 1966). However, the volcanics of the Permian and Cretaceous western arcs (Fig.1, arcs 1 and 5’) have much higher KzO/NazO ratios, which are similar to ratios from present-day talc-alkaline and alkaline basalts and andesites from the continental side of the Japanese and other volcanic arcs of the CircumPacific Belt. It is inferred that a landmass was adjacent to the western volcanics during the Permian and Cretaceous in New Zealand (Grindley, 1961). The ultramafic rocks associated with volcanics of the western Permian belt and the western Cretaceous belt were examined in more detail, and some of the more significant differences in the mineralogy and chemistry of ultramafic rocks of the eastern and western Permian belts and the western Cretaceous belt are set out in Table I. TABLE
I
Comparison of New Zealand ultramafic rocks and associated volcanicsl
-...-.-~~_
Perminn
east belt
Permian
west belt
Cretaceous
west bell
Ultramafics: Clinopyroxene
diopside-endiopside
diopside-augite
diopside-titanaugite
A1203 in Cpx (%)
l.Oh-3.92,
4.28-6.02,
::.!)1-7.16,
av. (8) 2.43
av. (6) 5.13
a”. (5) 5.1
SiO2 in Cpx (u/o)
49.13~34.41,
Olivine
FoH’J-_!)_I
Fo79-N.5
Orthopyrosenc
%h.5-93
EllbS_si
Accessories
chromitc
ilmenite-magnetite, green spinel: rare chrome spine1
iimenite-magnetite
Pegmatites
ortho- and clinopyroxenites
hornblendites; rare pyroxenites
hornblendites
Rock types
dunite, pyroxeneperidotite, anorthosite, gabbro
dunite, pyroxeneperidotite, anorthosite, norite
pyroxene-peridotite anorthosite, hornble gabbro, syenite
Volcanics:
tholeiitic basnlts and spilites
basalts, high alumina basalt, andesite
alkaline olivine trachyte
av. 51.h!)
-1x.94-49.95,
av. 49.4:<
(scarce)
3.41
4.40
jl.bX
49.50
K2O/Na20
0.09 (av. of 14)
0.40 (av. of 14)
in Cpx (%)
;,v. 4h.!
Fo(;5-76
SiO2 in Cpx (YJ)
Al203
‘1S.32~30.02,
very
rare
basa
0.53 (av. of 16)
1AnaIyses: Challis 1963, 1965a, 1965b, 1968. Additional mineral analyses from unpublishe micro-analysis by the author records of Chemistry Division, D.S.I.R., and electron-probe
500
Tectonophysies,
7 (3-6)
(1969) 495-505
53
0’
02 rc
51
,
cl ,
.
Tholeiitic % SiC$
field
-
,
r, rrr 49-
,
,
/
.
,
,
.4
A3
15
’
,
Alkaline
field
47 -
I 1
, 2
I ‘i.
3 A1203
I 4
I
I
5
6
Fig.2. Plot of Si02/A1203 for clinopyroxenes from basic volcanic rocks and associated ultramafic intrusions in the New Zealand Permian and Cretaceous. 1 = average of 8 clinopyroxenes from ultramafics of the eastern Permian belt (Challis, 1965a); 2 = clinopyroxene from volcanics of the eastern Permian belt, Red Hill (Analyst: G.A. Challis); 3 = clinopyroxene from the western Permian Brook Street Volcanics, D’Urville Island (unpublished Dominion Laboratory analysis, AA 121/2); 4 = average of 6 clinopyroxenes from ultramafics of the western Permian belt (Analyst: G.A. Challis); 5 = clinopyroxene from an ultramafic of the western Cretaceous belt (Challis, 1963). 2 and 4: analyses by electron-probe microanalyser, using natural and synthetic clinopyroxene standards.
From Table I, it is obvious that there are important differences in the mineralogy and chemistry of the ultramafic rocks from the eastern and western Permian belts. The olivines and orthopyroxenes of the fluff-Lon~oodTakitimu complex are generally more iron-rich, and the clinopyroxenes have lower SiO2 and higher Al.203 compared with clinopyroxenes from ultramafic rocks of the eastern belt (Challis, 1965a,b). The clinopyroxenes from the western Cretaceous belt show an even stronger trend to low SiO2 and high A1203. Clinopyroxenes from the associated volcanics show similar trends for two clinopyroxenes separated from the Te Anau volcanics (eastern belt) and the Brook Street volcanics (western belt). On a plot of SiO2/A1203 (Fig.2) it is clearly seen that the clinopyroxenes from both the volcanics and ultramafics of the eastern Permian belt fall in the tholeiitic field, as defined by Le Bas (1962), whereas the clinopyroxenes from volcanics and ultramafics of the western Permian and Cretaceous belts fall well within the alkaline field. Le Bas (1962, p.267) has already suggested that the aluminium content ‘of a clinopyroxene might be used to determine the magmatic parentage of a Tectonophysics, 7 (5-6) (1969) 495-505
501
rock. This is particularly so in the case of examining possible genetic associations of ultramafic and basic volcanic rocks, as clinopyroxene is usually the last mineral to crystallise in the former and one of the first to crystallise in the volcanic rocks. Other points of interest in the mineralogy of the ultramafic rocks are the scarcity, or complete absence, of chromite in the western belts, and its replacement by magnetite-ilmenite and green spinel, and the difference in the pegmatites. In the western ultramafics hornble~~dites are more usual, whereas only pyroxenites occur in the ultramafics of the eastern Permian belt.
Where the ultramafic masses are layered, the rock types found also differ. In the eastern Permian belt layering is present in a few of the bodies, notably at Red Hill and Cow Saddle (southern end of Red Mountain). The layering is between dunite, pyroxene peridotite, anorthosite (Ango-97) and normal gabbro. In the Bluff-Longwood complex of the western belt, the layering is between dunite, pyroxene-peridotite, anorthosite (Arq+&, troctolite and norite, or orthopyroxene gabbros. In the inland Kaikoura intrusions, the layering is pyroxene-peridotite (no orthopyroxene), anorthosite (An,j8_Gd, hornblende gabbro, and, at the top of the intrusion, syenite.
Ultramafic intrusions have produced contact metamorphism in volcanics of both Permian belts. At Bluff, Service (1937) describes hightemperature contact metamorphism of basic volcanic rocks to pyroxene, and pyroxene-hornblende hornfelses. At Red Hill, Challis (1965b) has described a high-temperature contact aureole which also reaches pyroxene hornfels grade immediately adjacent to the contact with the ultramafic rocks. It is probable that other high-temperature contacts will be found in the Permian volcanics, particularly at unfaulted contacts where there has been little metasomatism during serpentinization. It should be noted that the contact effects at Bluff and Red Hill cannot be ascribed to regional metamorphism as both belts of volcanics lie within the greenschist and prehnite-pumpellyite facies (Landis and Coombs, 1967). Furthermore, these authors note that prehnite-pumpellyite assemblages are always present between lawsonite-albite rocks and the ultramafics indicating a higher temperature/pressure ratio. Absence of hightemperature contact effects at the contacts of the smaller, completely serpentinized lenses may be due to calcium metasomatism, which can obscure earlier-formed minerals and textures, or to tectonic displacement of the lenses. Some lenses may actually be slivers torn off larger masses, and displaced horizontally or vertically.
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495-305
AGE OF THE ULTRAMAFrC
ROCKS
Although the volcanics of the Southland and Nelson synclines can be dated as about Middle Permian on interbedded fossiliferous tuffs, the ultramafic rocks have only been dated in the Bluff intrusion. Two samples from the Bluff ultramafic-mafic complex gave similar dates of about 240 m.y. (Aronson, 1965, Devereux et al., 1968). Gabbros from the Longwood complex gave dates of between 235 and 120 million years, but it is very likely that the younger dates have been influenced by later granitic intrusions along the margins of the Longwood Range. It is significant that the oldest date (235 million years) was obtained on a rock occurring furthest from any of the later granites. No attempts to date the ultramafic rocks of the eastern belt have yet been successful. However, in view of their occurrence in volcanics of approximately the same age as those of the western belt, and the chemical and mineralogical affinities between the eastern volcanics and ultramafic rocks demonstrated previously, it is reasonable to suggest a Permian age for the ultramafics also. The dating of the ultramafic-mafic intrusion in the inland Kaikoura Range is simpler. The intrusion can be seen to feed a dyke swarm, which in turn feeds a thick sequence of basalt flows, interbedded at the top of the sequence with fossiliferous tuffs, and underlain by fossiliferous sandstone. The age of the basalts is Cenomanian-Turonian.
DEPTH OF DIFFERENTIATION
Many geologists (e.g., Thayer, 1960) recognise that ~‘~pinot~e” ultramafic rocks are the product of a differentiation process, and most argument centres around the question of whether the differentiation takes place in the mantle, or in the crust. Challis and Lauder (1966) attempted to show that there is a gradual transition from large lopolithic complexes (Bushveld, etc.) through trough-like layered complexes (Great Dyke), and central volcanic complexes (Rhum) to layered “alpinotype” ultramafid complexes. They considered that most ultramafic rocks were originally part of volcanic pipes or magma chambers, and that the present surface features could be explained by depth of intrusion, level of erosion and subsequent tectonic and metamorphic events. O’Hara (1965) considered that picritic magma, generated in the mantle, would not reach the surface without some settIing of olivine. It is therefore possible that the nearest approach to unmodified magma might be found in the early stages of volcanism on mid-ocean ridges and oceanic volcanoes. The lowest portions of mid-ocean ridges and volcanoes might consist of highly picritic material which was able to reach the surface because of the absence of continental crust, and the relatively shallow depth of derivation proposed for magmas of oceanic and marginal oceanic volcanoes (e.g., 60 km at Hawaii; Eaton, 1962). As the volcanic pile grows, however, the picritic magma is less able to reach the surface and is held in relatively shallowlevel intra-volcanic magma chambers prior to eruption (Eaton, 1962), where there is opportunity for settling of olivine, possibly accompanied by pyroxene. In areas of thicker sialic crust, on the continental side of island arcs, or on Tectonophysics, 7 (5-6) (1969) 495-505
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the continents themselves, unmodified “primitive” magma would be less likely to reach the surface, even in the initial stages of volcanism, and a series of magma chambers, where differentiation can take place, would be formed within the crustal rocks. These magma chambers, exposed by uplift and erosion, may contain more alkaline rocks, as in the inland Kaikoura example. Because “alpinotype” ultramafic rocks are, by definition, associated with a geosyncline, or, in the case of New Zealand with the margin of a geosyncline, initial volcanism was probably of oceanic type, with sialic crust thin or absent. Here, differentiation could be expected to be intra-volcanic, or shallow sub-volcanic. This intra-volcanic relationship of ultramafics to basic volcanics is observed in the eastern Permian belt (Fig. 1, arc 2)> particul.arly at the northern end of the Southland syncline (Red Mountain lens) and in the Nelson syncline. At the southeastern end of the Southland syncline (Otama complex), only gabbro-norite is exposed, although the geophysical anomaly suggests ultramafic material at depth,,and it is probable that the southerly plunge of the syncline results in deeper levels of erosion at the northern end. In the western Permian belt, relatively little ultramafic material is exposed, but thick sequences of volcanics are present, particularly in the Takitimu Mountains. The small amount of ultramafic material exposed at Bluff and in the Longwood Range hardly accounts for the large positive gravity anomaly already mentioned, and it is probable that the main mass of ultramafic rocks lies, at relatively shallow depth, below the Bluff-Longwood-Takitimu complex. This position would either be deep within the volcanic pile, or shallow sub-volcanic. In the case of the western Cretaceous belt, the upper part of an ultramafic-mafic complex is exposed mainly within Upper Jurassic sediments, with up to 10,000 ft. of sediments between the intrusion and the base of the volcanic pile. In the inland Kaikoura region, the Cretaceous basaltic volcanism is mainly sub-aerial and the underlying sediments contain coal measures in contrast to the pillow lavas and shelf sediments of the Permian belts. It is probable that the sialic crust in the inland Kaikoura region was rather thicker than on the margin of the New Zealand geosyncline, and that in the Cretaceous, differentiation took place within the underlying sediments. Numerous small dykes of feldspar peridotite and melagabbro terminate in the sediments only a few hundred feet above the top of the intrusion, which is additional evidence that the more ultramafic types of magma were unable to reach the surface under these conditions. Finally, a number of authors, notably Kuno (1959, 1966) and Sugimura (1961) have related volcanism in the Japanese arc to thrust zones, inferred from earthquake foci, which dip beneath the island arc towards the continent. Lf “alpinotype” ultramafic rocks represent the intra-volcanic or sub-volcanic differentiates of volcanoes in an island arc environment, then the association of volcanism, ultramafics and major thrust faults becomes clear. The present author finds it difficult to believe that this close association of basic volcanic rocks with “alpinotype” ultramafic rocks is purely accidental, and considers that it is essential to examine these relationships in as many areas as possible.
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REFERENCES Aronson, J., 1965. Reconnaissance rubidium-strontium geochronology of New Zealand plutonic and metamorphic rocks. New Zealand J. Geol. Geophys., 8: 401-423. Benson, N.N., 1926. The tectonic conditions accompanying the intrusion of basic and ultrabasic igneous rocks. Mem. Natl. Acad. Sci., Washington, 19(l): l-90. Benson, N.N. and Holloway, J., 1940. Notes on the geography and rocks of the ranges between the Pyke and Matukituki Rivers, northwest Otago. Trans. Roy. Sot. New Zealand, 70: l-24. Challis, G.A., 1963. Layered xenoliths in a dyke, Awatere Valley, New Zealand. Geol. Mag., 100: 11-16. Challis, G.A., 1965a. The origin of New Zealand ultramafic intrusions. J. Petrol., 6.: 322-364. Challis, G.A., 1965b. High-temperature contact metamorphism at the Red Hills ultramafic intrusion, Wairau Vallev, New Zealand. J. Petrol., 6: 395-419. Challis, G.A., 1968. The K2O/Na20 ratios of ancient volcanic arcs in New Zealand. New Zealand Geol. Geophys., 11: 200-211. Challis, G.A. and Lauder, W.R., 1966. The genetic position of “Alpine” type ultramafic rocks. Bull. Volcanol., 29: 283-306. Coleman, R.G., 1966. New Zealand serpentinites and associated metasomatic rocks. Bull. New Zealand Geol. Surv., 76: l-102. dates on Devereux, I., McDougall, I. and Watters, W.A., 1968. Potassium-argon intrusive rocks from the Foveaux Strait area. New Zealand J. Geol. Geophys., in press. Eaton, J.P., 1962. Crustal structure and volcanism in Hawaii. In: G.A. MacDonald and H. Kuno (Editors), The crust of the Pacific Basin - Am. Geophys. Union, Mem., 6: 13-29. Grindley, G.W., 1961. Mesozoic orogenies in New Zealand. Proc. Congr. Pacific Sci. Assoc., 9th, 12: 71-75. Hatherton, T., 1966. A geophysical study of the Southland Syncline. New Zealand, Dept. Sci. Ind. Res. Bull., 168: l-49. Hatherton, T., 1967. A geophysical study of Nelson-Cook Strait Region, New Zealand. New Zealand J. Geol. Geophys., 10: 1330-1347. provinces of Japan and surrounding Kuno, H., 1959. Origin of Cenozoic petrographic areas. Bull. Volcanol., 20: 37-76. Kuno, H., 1966. Lateral variation of basalt magma types across continental margins and island arcs. Bull. Volcanol., 29: 195-222. Landis, C.A. and Coombs, D.S., 1967. Metamorphic belts and orogenesis in southern New Zealand. Tectonophysics, 4: 501-518. LeBas, M.J., 1962. The role of aluminium in igneous clinopyroxenes with relation to their parentage. Am. J. Sci., 260: 267-288. Malahof, A., 1961. A Gravity Survey of the Red Hills Area, Wairau Valley. Thesis, Victoria University, Wellington, 87 pp. O’Hara, M.J., 1965. Primary magmas and the origin of basalts. Scot. J. Geol., 1: 19-40. Service, H., 1937. An intrusion of norite and its accompanying contact metamorphism at Bluff, New Zealand. Trans. Roy. Sot. New Zealand, 67: 185-217. Sugimura, A., 1961. Regional variation of the K2O/Na20 ratios of volcanic rocks in Japan and environs. J. Geol. Sot. Japan, 67: 292-300. Thayer, T.P., 1960. Some critical differences between Alpinetype and stratiform peridotite-gabbro complexes. Intern. Geol. Congr., 21st, Copenhagen, Rept. Session, Norden, 13: 247-259. Turner, F.J.; 1930. The metamorphic and ultrabasic rocks of the Lower Cascade Valley, South Westland. Trans. New Zealand Inst., 61: 170-201. Waterhouse, J.B., 1964. Permian stratigraphy and faunas of New Zealand. Bull. New Zealand Geol. Surv., 72: l-101. Wellman, H.W., 1956. Structural outline of New Zealand. New Zealand Dept. Sci. Ind. Res. Bull., 121: l-36. Wood, B.L., 1962. Sheet 22 Wakatipu, Geological Map of New Zealand, 1:250,000, 1st Ed., Dept. Sci. Ind. Res., Wellington, map sheet. Wood, B.L., 1966. Sheet 24 Invercargill, tieological Map of New Zealand, 1:250,000, 1st Ed., Dept. Sci. Ind. Res., Wellington, map sheet.
DISCUSSION ON THE PAPER “THE ORIGIN OF ULTRAMAFIC BASIC ROCKS” BY P.J. WYLLIE
AND ULTRA-
G.A. CHALLIS New Zealand Geological Survey, Lower Hutt (New Zealand) (Received March 6, 1969)
SUMMARY In recent years, speculation on the origin of “alpinotype” ultramafic rocks has tended to ignore the field relationships of the rocks, and in particular, their very common association with basic volcanic rocks, or greenschists. In New Zealand, the almost invariable association of ultramafic intrusions and basaltic volcanics is so striking that it has dominated the thinking of field geologists on the origin of these rocks, and some have proposed that they represent ultramafic submarine flows contemporaneous with the Permian volcanics. However, pronounced mineralogical layering, and other textural features suggest that gravitational differentiation has played a part in their formation, and high temperature metamorphism at unfaulted contacts excludes cold, solid intrusion. Geophysical data suggest that they have a lopolithic form and a thickness of 5,OOO-10,000 ft. for the Dun Mountain and Red Hill masses. A detailed study of both ultramafic rocks and the surrounding volcanics has shown that there are chemical and mineralogical similarities between the ultramafic and volcanic rocks in any particular area. In the Permian sequence in New Zealand, large thicknesses of andesitic-basaltic volcanics form pronounced belts on both limbs of a syncline, marginal to the main Upper Paleozoic and Mesozoic New Zealand geosyncline, and a number of individual ultramafic bodies are found within both belts. On the eastern limb, olivine-poor tholeiitic basalts with very low K90/Na90 ratios of 0.09, enclose the ultramafic intrusions of Dun Mountain and Red Hill in which the clinopyroxenes show features common to pyroxenes from tholeiitic basalts and their derivatives (low Al,, K90, Na90 and TiO9). The pegmatitic phase in these ultramafic rocks is a pyroxenite, and there is very little sulphide mineralisation associated. On the western limb, the basic volcanics have an average K20/Na20 ratio of 0.40, and the associated ultramafic bodies show more alkaline features with Ti-rich pyroxenes and primary hornblende. The pegmatitic phase in these rocks is a hornblendite, and much of the known sulphide mineralisation and platinum in New Zealand is associated with ultramafics of the western belt. I have previously suggested that the New Zealand ultramafic masses represent the magma chambers of basaltic and andesitic volcanoes in a Permian double volcanic arc, analogous to the present-day arcs of the Circum-Pacific Belt. The depths of the magma chambers, and hence the Tectonophysics, 7 (5-6) (1969) 495-505
495
levels now exposed by erosion, are dependent on the composition of the original magma, and the crustal thickness in the area. It is possible that in more deformed areas, the ultramafic portions of the magma chambers could be displaced, either horizontally or vertically, by faulting.
INTRODUCTION
New Zealand is a good area for study of field and laboratory relationships that may help to elucidate some problems of the origin of “alpinotype” ultramafic rocks, as the larger masses, such as Dun Mountain and Red Hill, are well-exposed, and largely undeformed and un~erpentinized. In recent years, speculation on the origin of these rocks has tended to ignore field relationships, in particular the common association with basic volcanic rocks, or their metamorphosed equivalents? the green-and blue-schists. In New Zealand, the larger ultramafic bodies occur in Upper Paleozoic and Mesozoic basic volcanics and tuffaceous sediments in the Southern Island. They are associated with basic volcanics of presumed Permian age on both sides of a marginal syncline on the western edge of the New Zealand geosynciine of Upper Paleozoic-Mesozoic age. In fact, this association is so close that until recently the ultramafic rocks and Permian volcanic rocks were mapped as a single unit, termed the “mineral belt” or “ultrabasic be.lt”, and some field geologists have suggested that the ultramafic rocks represent submarine lava flows contemporaneous with the enclosing volcanics. On the other hand, Dun ~ount~n and other ultramafic masses in the New Zealand “ultrabasic belt” have always been classified as “alpinotype”, and an origin by solid, or semi-solid intrusion along major thrust faults has been proposed by a number of New Zealand and overseas geologists. More recently, some petrologists have suggested that the New Zealand ultramafic bodies represent sub-volcanic, or intra-volcanic accumulates from olivine-rich basaltic magma. Over the past ten years the geological mapping of New Zealand, on a scale of four miles to the inch, has been completed, and much new information is available. In this paper some new data is presented that may help to reconcile field and laboratory observations.
POSITION
AND FORM OF THE ULTRAMAFIC
MASSES
The larger, and better known ultramafic masses, such as Dun Mountain and Red Hill, occur with basic volcanics of Permian age (Fig.1, arcs 1 and 2) on the eastern side of the Nelson and Southland synclines which are marginal troughs west of the main New Zealand geosyncline. During the Upper Paleozoic, Triassic and Jurassic, shallow-water shelf and transitional Fig. 1. Basic volcanic rocks in the Permian and Cretaceous of in solid black. Crosses alongside rocks within the volcanics of that 496
and associated mafic-ultramafic intrusions New Zealand. Areas of volcanics are shown indicate the presence of mafic-ultramafic area. Tectonophysics, ‘7 (5-6) (1969) 495-505
SCALE 50
0
SO
100
IS0 MILES
@ Ulrramafic
Tectonophysics,
7 (.5-6) (1969) 495-505
lntrubnf
497
sediments with a large content of volcanic material were deposited in the marginal trough, whiIe deep-water “greywacke type” sediments were deposited in the main geosyncline to the east (Wellman, 1956). Considerable thicknesses of basaltic and andesitic volcanics are found on both sides of the marginal synclines, and Wellman (1956) suggested that the volcanics of the western limb represented a Carboniferous belt of island volcanoes. Examining the petrology and chemistry of volcanics from bvth sides of the marginal synclines, Challis (1968) came to the conclusion that there was a double volcanic arc present in the Upper Paleozoic, and recent mapping (Wood, 1962, 1966; Waterhouse, 1964) has shown that ultramafic rocks are associated with volcanics on both sides of the synclines. Because the ultramafic rocks and basic volcanics on the eastern side of the Southland and Nelson synclines were mapped as a single unit, the ultramafic rocks appeared to form a thick, continuous horizon extending for a total of nearly 150 miles, with the northern Nelson lens displaced by 300 miles transcurrent movement on the Alpine fauIt (Wellman, 1956). In fact, ultramafic rocks form individual, reIatively small lopolithic, or sill-like bodies within the Permian volcanics in most places. The largest of the ultramafic masses is Red Hill, in the Wairau Valley (Fig.1) which is 49 sq. miles in area and, from geophysical data (Malahof, 1961) is approximately 10,000 ft. thick. Ultramafic rocks at Dun Mountai~l cover an area of about 4 sq. miles, and the Red Moulltain massif in northwestern @ago is about 20 sq. miles in area. Apart from the above localities, the “ultramafic belt” consists of basic volcanic rocks, and ultramafic rocks are only present as small, discontinuous, highly sheared, and completely serpentinised lenses within the volcanics. In some instances, material mapped as serpentinite is actually chloritised volcanics. The eastern bound&ry of the “ultramafic belt” on the eastern limb of the Southland and Nelson synclines is shown as a major fault, and this led Benson (1926) and Turner (1930) to propose solid intrusion of the ultramafics along major thrust faults. However, in many places, such as at Dun Mountain and in the Mossburn area, the Permian volcanics intervene, and the major fault is at the margin of the volcanic-ultramafic belt as a whole, rather than specifically within the ultramafic rocks. On the western limb of the Nelson and Soutliland synclines, ultramafic rocks are not as plentiful. but are nevertheless associated with the Brook Street Volcanics on,D’Urville Island, where there is a small gabbroserpentinite complex at Otu Bay (Coleman, 1966), and small lenses of gabbroserpentinite occur in the Eglinton Volcanics, near the southern end of the Alpine fault (Fig.1) and in the Takitimu Mountains. In the Longwood Range, and at Bluff, layered gabbro-norite complexes include layers of dunite, pyroxene peridotite, pyroxenite and anorthosite. It should be noted that there is an unusual occurrence of a similar layered gabbro-peridotite mass 011 the eastern side of the Southland syncline, at Cow Saddle at the southern end of the Red Mountain lens (Benson and Holloway, 1940) where it forms part of the so-called “ultramafic belt” and further to the southeast is the Otama gabbro-norite complex. Geophysical surveys (Hathertoii, 1966, lS67) of the Southland and Nelson synclines reveal large positive isostatic gravity anomalies on both sides of the synclines. The largest (+85 mgal) is associated with the BluffLongwood gabbro-peridotite complex on the southwestern margin of the 498
Tectonophysics, 7 (5-6) (1969) 495-506
Southland syncline (Fig. 1). Another positive anomaly of approximately +60 mgal is associated with a gabbro-norite complex near,Otama on the northeastern margin. The anomaly appears too large to be accounted for by the rocks exposed at the surface, and Hatherton (1966) postulates the existence of a basic-ultrabasic mass at depth extending in a northwesterly direction towards Mossburn and the exposed portion of the “ultramafic belt”. In the Nelson syncline, the largest isostatic gravity anomaly is associated with Red Hill (+45 mgal). The anomaly over Dun Mountain is relatively small (+15-+20 mgal). On the western side of the Nelson syncline there is positive anomaly of up to +40 mgal associated with the Brook Street Volcanics and Rotoroa Complex (Hatherton, 1967), but as the area is overlain by thick gravels, the significance of the anomaly is in doubt. A large positive magnetic anomaly is also present on the western side of the syncline, and Hatherton has interpreted this as serpentinised ultramafic rocks at depth.
Ultramafic
rocks
associated
with Cretaceous
volcanics
In the inland Kaikoura Range, centred on Mount Tapuaenuku (9,465 ft.) there is a layered peridotite-pyroxenite-anorthosite-gabbro complex that intrudes and metamorphoses Upper Jurassic sandstones (Fig.1, arc 3). A second, smaller intrusion occurs at the northern end of the range. Block faulting and uplift in the Tertiary, followed by deep dissection, have exposed a section of at least 10,000 ft. Exposures are nearly perfect and the basicultrabasic complex is seen to feed a radial dyke swarm with the dykes as feeders for overlying alkaline olivine basalt flows. There are strong mineralogical and chemical affinities between the plutonic, hypabyssal and volcanic rocks, and xenoliths and xenocrysts from the layered intrusion are found in the dykes and flows (Challis, 1963). Some of the dykes are ultramafic in character, but these never reach the overlying basalts and terminate a few hundred feet above the top of the layered intrusion. No other occurrences of ultramafic rocks with Cretaceous volcanics are known, but uplift and erosion has not exposed such a sequence elsewhere.
CHEMICAL VOLCANIC
AND MINERALOGICAL ROCKS
CHARACTERISTICS
OF THE ULTRAMAFIC
AND
During a study of some ultramafic rocks on the eastern limbs of the Southland and Nelson synclines, Challis (1965a) noted the close association of the ultramafic rocks with olivine-poor spilitic volcanics. Chemically, these volcanics are tholeiitic in character, and it was found that the clinopyroxenes from the associated ultramafic rocks plotted in the same field (for AIIV, Na, Ti, etc.) as clinopyroxenes from tholeiitic basalts, and layered intrusions presumed to be derived by differentiation from tholeiitic basaltic magma. Challis suggested that the ultramafic rocks represented the deeplevel magma chambers of a line of Upper Paleozoic volcanoes, and that the original magma from which the ultramafic rocks differentiated might have been an olivine tholeiite or picrite. Later, Challis and Lauder (1966) concluded that alkaline and talc-alkaline magmas might also be expected to
Tectonophysics,
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499
yield ultramafic differentiates which could be distinguished by their mineralogy and chemistry. Work on the KzO/NaZO ratios of Permian and Cretaceous basaltic and andesitic volcanics (Challis, 1968) has shown that double volcanic arcs can be defined in both the Permian and the Cretaceous. The eastern arcs (Fig.1, arcs 2 and 41, on the edge of the Upper Paleozoic and Mesozoic geosynclines, are characterised by volcanics with very low KzO/NazO ratios, typical of those found in present-day oceanic tholeiites and basalts of volcanic arcs bordering deep ocean basins (Sugimura, 1961; Kupo, 1966). However, the volcanics of the Permian and Cretaceous western arcs (Fig.1, arcs 1 and 5’) have much higher KzO/NazO ratios, which are similar to ratios from present-day talc-alkaline and alkaline basalts and andesites from the continental side of the Japanese and other volcanic arcs of the CircumPacific Belt. It is inferred that a landmass was adjacent to the western volcanics during the Permian and Cretaceous in New Zealand (Grindley, 1961). The ultramafic rocks associated with volcanics of the western Permian belt and the western Cretaceous belt were examined in more detail, and some of the more significant differences in the mineralogy and chemistry of ultramafic rocks of the eastern and western Permian belts and the western Cretaceous belt are set out in Table I. TABLE
I
Comparison of New Zealand ultramafic rocks and associated volcanicsl
-...-.-~~_
Perminn
east belt
Permian
west belt
Cretaceous
west bell
Ultramafics: Clinopyroxene
diopside-endiopside
diopside-augite
diopside-titanaugite
A1203 in Cpx (%)
l.Oh-3.92,
4.28-6.02,
::.!)1-7.16,
av. (8) 2.43
av. (6) 5.13
a”. (5) 5.1
SiO2 in Cpx (u/o)
49.13~34.41,
Olivine
FoH’J-_!)_I
Fo79-N.5
Orthopyrosenc
%h.5-93
EllbS_si
Accessories
chromitc
ilmenite-magnetite, green spinel: rare chrome spine1
iimenite-magnetite
Pegmatites
ortho- and clinopyroxenites
hornblendites; rare pyroxenites
hornblendites
Rock types
dunite, pyroxeneperidotite, anorthosite, gabbro
dunite, pyroxeneperidotite, anorthosite, norite
pyroxene-peridotite anorthosite, hornble gabbro, syenite
Volcanics:
tholeiitic basnlts and spilites
basalts, high alumina basalt, andesite
alkaline olivine trachyte
av. 51.h!)
-1x.94-49.95,
av. 49.4:<
(scarce)
3.41
4.40
jl.bX
49.50
K2O/Na20
0.09 (av. of 14)
0.40 (av. of 14)
in Cpx (%)
;,v. 4h.!
Fo(;5-76
SiO2 in Cpx (YJ)
Al203
‘1S.32~30.02,
very
rare
basa
0.53 (av. of 16)
1AnaIyses: Challis 1963, 1965a, 1965b, 1968. Additional mineral analyses from unpublishe micro-analysis by the author records of Chemistry Division, D.S.I.R., and electron-probe
500
Tectonophysies,
7 (3-6)
(1969) 495-505
53
0’
02 rc
51
,
cl ,
.
Tholeiitic % SiC$
field
-
,
r, rrr 49-
,
,
/
.
,
,
.4
A3
15
’
,
Alkaline
field
47 -
I 1
, 2
I ‘i.
3 A1203
I 4
I
I
5
6
Fig.2. Plot of Si02/A1203 for clinopyroxenes from basic volcanic rocks and associated ultramafic intrusions in the New Zealand Permian and Cretaceous. 1 = average of 8 clinopyroxenes from ultramafics of the eastern Permian belt (Challis, 1965a); 2 = clinopyroxene from volcanics of the eastern Permian belt, Red Hill (Analyst: G.A. Challis); 3 = clinopyroxene from the western Permian Brook Street Volcanics, D’Urville Island (unpublished Dominion Laboratory analysis, AA 121/2); 4 = average of 6 clinopyroxenes from ultramafics of the western Permian belt (Analyst: G.A. Challis); 5 = clinopyroxene from an ultramafic of the western Cretaceous belt (Challis, 1963). 2 and 4: analyses by electron-probe microanalyser, using natural and synthetic clinopyroxene standards.
From Table I, it is obvious that there are important differences in the mineralogy and chemistry of the ultramafic rocks from the eastern and western Permian belts. The olivines and orthopyroxenes of the fluff-Lon~oodTakitimu complex are generally more iron-rich, and the clinopyroxenes have lower SiO2 and higher Al.203 compared with clinopyroxenes from ultramafic rocks of the eastern belt (Challis, 1965a,b). The clinopyroxenes from the western Cretaceous belt show an even stronger trend to low SiO2 and high A1203. Clinopyroxenes from the associated volcanics show similar trends for two clinopyroxenes separated from the Te Anau volcanics (eastern belt) and the Brook Street volcanics (western belt). On a plot of SiO2/A1203 (Fig.2) it is clearly seen that the clinopyroxenes from both the volcanics and ultramafics of the eastern Permian belt fall in the tholeiitic field, as defined by Le Bas (1962), whereas the clinopyroxenes from volcanics and ultramafics of the western Permian and Cretaceous belts fall well within the alkaline field. Le Bas (1962, p.267) has already suggested that the aluminium content ‘of a clinopyroxene might be used to determine the magmatic parentage of a Tectonophysics, 7 (5-6) (1969) 495-505
501
rock. This is particularly so in the case of examining possible genetic associations of ultramafic and basic volcanic rocks, as clinopyroxene is usually the last mineral to crystallise in the former and one of the first to crystallise in the volcanic rocks. Other points of interest in the mineralogy of the ultramafic rocks are the scarcity, or complete absence, of chromite in the western belts, and its replacement by magnetite-ilmenite and green spinel, and the difference in the pegmatites. In the western ultramafics hornble~~dites are more usual, whereas only pyroxenites occur in the ultramafics of the eastern Permian belt.
Where the ultramafic masses are layered, the rock types found also differ. In the eastern Permian belt layering is present in a few of the bodies, notably at Red Hill and Cow Saddle (southern end of Red Mountain). The layering is between dunite, pyroxene peridotite, anorthosite (Ango-97) and normal gabbro. In the Bluff-Longwood complex of the western belt, the layering is between dunite, pyroxene-peridotite, anorthosite (Arq+&, troctolite and norite, or orthopyroxene gabbros. In the inland Kaikoura intrusions, the layering is pyroxene-peridotite (no orthopyroxene), anorthosite (An,j8_Gd, hornblende gabbro, and, at the top of the intrusion, syenite.
Ultramafic intrusions have produced contact metamorphism in volcanics of both Permian belts. At Bluff, Service (1937) describes hightemperature contact metamorphism of basic volcanic rocks to pyroxene, and pyroxene-hornblende hornfelses. At Red Hill, Challis (1965b) has described a high-temperature contact aureole which also reaches pyroxene hornfels grade immediately adjacent to the contact with the ultramafic rocks. It is probable that other high-temperature contacts will be found in the Permian volcanics, particularly at unfaulted contacts where there has been little metasomatism during serpentinization. It should be noted that the contact effects at Bluff and Red Hill cannot be ascribed to regional metamorphism as both belts of volcanics lie within the greenschist and prehnite-pumpellyite facies (Landis and Coombs, 1967). Furthermore, these authors note that prehnite-pumpellyite assemblages are always present between lawsonite-albite rocks and the ultramafics indicating a higher temperature/pressure ratio. Absence of hightemperature contact effects at the contacts of the smaller, completely serpentinized lenses may be due to calcium metasomatism, which can obscure earlier-formed minerals and textures, or to tectonic displacement of the lenses. Some lenses may actually be slivers torn off larger masses, and displaced horizontally or vertically.
502
Tectonophysics,
7 (S-6)
(i969)
495-305
AGE OF THE ULTRAMAFrC
ROCKS
Although the volcanics of the Southland and Nelson synclines can be dated as about Middle Permian on interbedded fossiliferous tuffs, the ultramafic rocks have only been dated in the Bluff intrusion. Two samples from the Bluff ultramafic-mafic complex gave similar dates of about 240 m.y. (Aronson, 1965, Devereux et al., 1968). Gabbros from the Longwood complex gave dates of between 235 and 120 million years, but it is very likely that the younger dates have been influenced by later granitic intrusions along the margins of the Longwood Range. It is significant that the oldest date (235 million years) was obtained on a rock occurring furthest from any of the later granites. No attempts to date the ultramafic rocks of the eastern belt have yet been successful. However, in view of their occurrence in volcanics of approximately the same age as those of the western belt, and the chemical and mineralogical affinities between the eastern volcanics and ultramafic rocks demonstrated previously, it is reasonable to suggest a Permian age for the ultramafics also. The dating of the ultramafic-mafic intrusion in the inland Kaikoura Range is simpler. The intrusion can be seen to feed a dyke swarm, which in turn feeds a thick sequence of basalt flows, interbedded at the top of the sequence with fossiliferous tuffs, and underlain by fossiliferous sandstone. The age of the basalts is Cenomanian-Turonian.
DEPTH OF DIFFERENTIATION
Many geologists (e.g., Thayer, 1960) recognise that ~‘~pinot~e” ultramafic rocks are the product of a differentiation process, and most argument centres around the question of whether the differentiation takes place in the mantle, or in the crust. Challis and Lauder (1966) attempted to show that there is a gradual transition from large lopolithic complexes (Bushveld, etc.) through trough-like layered complexes (Great Dyke), and central volcanic complexes (Rhum) to layered “alpinotype” ultramafid complexes. They considered that most ultramafic rocks were originally part of volcanic pipes or magma chambers, and that the present surface features could be explained by depth of intrusion, level of erosion and subsequent tectonic and metamorphic events. O’Hara (1965) considered that picritic magma, generated in the mantle, would not reach the surface without some settIing of olivine. It is therefore possible that the nearest approach to unmodified magma might be found in the early stages of volcanism on mid-ocean ridges and oceanic volcanoes. The lowest portions of mid-ocean ridges and volcanoes might consist of highly picritic material which was able to reach the surface because of the absence of continental crust, and the relatively shallow depth of derivation proposed for magmas of oceanic and marginal oceanic volcanoes (e.g., 60 km at Hawaii; Eaton, 1962). As the volcanic pile grows, however, the picritic magma is less able to reach the surface and is held in relatively shallowlevel intra-volcanic magma chambers prior to eruption (Eaton, 1962), where there is opportunity for settling of olivine, possibly accompanied by pyroxene. In areas of thicker sialic crust, on the continental side of island arcs, or on Tectonophysics, 7 (5-6) (1969) 495-505
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the continents themselves, unmodified “primitive” magma would be less likely to reach the surface, even in the initial stages of volcanism, and a series of magma chambers, where differentiation can take place, would be formed within the crustal rocks. These magma chambers, exposed by uplift and erosion, may contain more alkaline rocks, as in the inland Kaikoura example. Because “alpinotype” ultramafic rocks are, by definition, associated with a geosyncline, or, in the case of New Zealand with the margin of a geosyncline, initial volcanism was probably of oceanic type, with sialic crust thin or absent. Here, differentiation could be expected to be intra-volcanic, or shallow sub-volcanic. This intra-volcanic relationship of ultramafics to basic volcanics is observed in the eastern Permian belt (Fig. 1, arc 2)> particul.arly at the northern end of the Southland syncline (Red Mountain lens) and in the Nelson syncline. At the southeastern end of the Southland syncline (Otama complex), only gabbro-norite is exposed, although the geophysical anomaly suggests ultramafic material at depth,,and it is probable that the southerly plunge of the syncline results in deeper levels of erosion at the northern end. In the western Permian belt, relatively little ultramafic material is exposed, but thick sequences of volcanics are present, particularly in the Takitimu Mountains. The small amount of ultramafic material exposed at Bluff and in the Longwood Range hardly accounts for the large positive gravity anomaly already mentioned, and it is probable that the main mass of ultramafic rocks lies, at relatively shallow depth, below the Bluff-Longwood-Takitimu complex. This position would either be deep within the volcanic pile, or shallow sub-volcanic. In the case of the western Cretaceous belt, the upper part of an ultramafic-mafic complex is exposed mainly within Upper Jurassic sediments, with up to 10,000 ft. of sediments between the intrusion and the base of the volcanic pile. In the inland Kaikoura region, the Cretaceous basaltic volcanism is mainly sub-aerial and the underlying sediments contain coal measures in contrast to the pillow lavas and shelf sediments of the Permian belts. It is probable that the sialic crust in the inland Kaikoura region was rather thicker than on the margin of the New Zealand geosyncline, and that in the Cretaceous, differentiation took place within the underlying sediments. Numerous small dykes of feldspar peridotite and melagabbro terminate in the sediments only a few hundred feet above the top of the intrusion, which is additional evidence that the more ultramafic types of magma were unable to reach the surface under these conditions. Finally, a number of authors, notably Kuno (1959, 1966) and Sugimura (1961) have related volcanism in the Japanese arc to thrust zones, inferred from earthquake foci, which dip beneath the island arc towards the continent. Lf “alpinotype” ultramafic rocks represent the intra-volcanic or sub-volcanic differentiates of volcanoes in an island arc environment, then the association of volcanism, ultramafics and major thrust faults becomes clear. The present author finds it difficult to believe that this close association of basic volcanic rocks with “alpinotype” ultramafic rocks is purely accidental, and considers that it is essential to examine these relationships in as many areas as possible.
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