eclogitic xenoliths from volcanic breccia at kakanui, new zealand

Contr. Mineral. and Petrol. 19, 316--327 (1968)

Eclogitic Xenoliths from Volcanic Breccia at Kakanui, New Zealand

BRIAN MASON U.S. National Museum, Washington, D.C.

Received July 25, 1968

Abstract. Eclogitie xenoliths consisting of tschermakitic augite and pyrope garnet, together with variable amounts of kaersutitic hornblende, are common in a volcanic breccia of Lower Oligocene age at Kakanui, New Zealand. The breccia also contains xenocrysts of these minerals, and xenoliths of peridotite. Modal analyses are given of a number of the eclogitic xenoliths, and chemical analyses of two of them and their component minerals. They are compared with similar xenoliths from Hoggar (Algeria), SMt Lake Crater (Hawaii), and Delegate (Australia), with eelogite xenoliths from kimberlites, and with garnet peridoti~s. These three types of igneous eclogites can be characterized by the nature of their clinopyroxene: tschermakitic in the xenoliths from basaltic rocks, jadcitic in the xenoliths from kimberlites, and chrome diopside in the garnet peridotites. The eclogitic xenoliths in basaltic rocks probably crystallized in the mantle at depths of about 60 kin, but their rarity in contrast to the numerous occurrences of peridotite xenoliths poses some significant problems.

Introduction

The volcanic breccia forms the extremities of Kakanui North and South Heads, on the coast of South Island, New Zealand. The breccia itself is of limited extent at each of these localities and grades laterMly into submarine tufts. This is the Deborah Volcanic Formation, and the strat igraphy and paleontology indicate a Lower Oligoeene age. The breccia has been described by THo~sozr (1907), and in greater petrographic detail by Dzcx~r (1968). The most remarkable feature is the presence of large xenocrysts, often beautifully polished and rounded, of pyrope, kaersutitie hornblende, and tschermaldtie augite (Miso~r 1966); these range up to fist-size, and are usually pieces of single crystals. Other mineral fragments in the breccia include anorthoclase, olivine, chrome diopside, and ilmenite. The breccia also contains numerous peridotitie and eclogitie xenotiths up to several centimeters in diameter. The peridotite xenoliths consist largley of olivine, together with bronzite, chrome diopside, and accessory spinel (pieotite). The eelogitic xenoliths are made up of augite and pyrope, and many contain considerable amounts of kaersutitie hornblende. The peridotitie and eelogitie nodules are quite different in appearance and mineral composition, and transi- tional types have not been found (although compound xenoliths, in which eclogite is in contact with peridotite, are found). Also in the breccia are fragments of fine-grained melanephelinite (DIcKey), presumably representative of the eruptive magma. The peridotite xenoliths are essentially identical with those described from many localities throughout the world; the eclogitic xenoliths are almost unique and form the subject of this paper.

Eclogitic Xenoliths from Volcanic Breccia, New Zealand 317

Rock Descriptions The eclogitic xenoliths have a characteristic appearance in hand specimen. They are coal-black in color, spotted red or reddish-brown with grains of garnet. Hornblende, if present in appreciable amount, shows up by its lustrous cleavage surfaces. The xenoliths are usually spherical to ellipsoidal in form, and average 5- -8 cm in greatest diameter. From a large collection of the xenoliths a limited number was selected for further investigation. Selection was based on the refractive index of the garnet, which is a useful indicator of the overall composition of the rock, and particularly one of the main variables, the Mg/Fe ratio. Modal analyses of the selected xenoliths were made by the point-counting technique, and the results are presented in Table 1.

Table 1. Modal analyses (volume per cent) o/ eelogitie xenoliths ]rom Kakanui

Specimen Pyrope R.I. Augite Horn- Ilmenite No. blende

K55 trace 1.757 69 31 trace K57 18 1.758 70 10 2 K8 6 1.758 94 none none K3 48 1.759 46 6 none K59 3 1.760 66 28 3 K15 25 1.762 68 6 1 K56 43 1.762 38 7 12 K39 31 1.763 47 20 2 K58 6 1.764 82 9 3 K12 22 1.768 45 21 12 K l l 4 1.769 73 20 3 K16 26 1.769 58 8 8 K13 5 1.770 46 30 19 K14 18 1.773 18 62 2

The xenoliths show a limited range of composition, as indicated by the range in the refractive index of the garnet, 1.757--1.773. The xenolith garnet is evidently richer in iron than the xenocryst garnet; measurements on twelve xenoerysts gave a refractive index range of 1.741--1.751. The modal analyses show that augite is usually the dominant mineral; pyrope ranges from trace amounts ( < 1% ) to nearly 50 % ; hornblende is usually present, sometimes in considerable amounts; the only accessory mineral is ilmenite, and its amount seldom exceeds 3%. These xenoliths are noteworthy for their simple phase composition; in particular only one pyroxene is present, and they contain no olivine. Microscopic examination of thin sections shows a xenomorphie-granular texture; grain size varies somewhat, but is usually about 2 mm for both augite and pyrope. In a few specimens pyrope is present as phenocrysts up to 5 mm across; these phenoerysts usually contain numerous poiMlitic inclusions of augite and hornblende. Hornblende occurs in a variety of forms : as grains similar in size to those of augite and pyrope, as large xenomorphic crystals poikilitieally enclosing small grains of augite, and as inclusions in augite with uniform crystallographic orientation. Pyrope is colorless to pale pink in thin section; hornblende strongly

318 B. MAso~r

pleochroic, yellow to brown; augite colorless to pale grey, sometimes with greenish or bluish tints, and weakly pleochroic. Pyrope grains sometimes show thin kelyphitic rims of brown, weakly birefringent material, possibly a chlorite; the same material may line fractures in the garnet. In a few specimens the garnet is almost completely altered to this brown material. Exsolution of orthopyroxene from clinopyroxene, or of garnet from clinopyroxene, such as has been recorded in similar xenoliths from other localities [Salt Lake Crater, Oahu, Hawaii by Y o D ~ and TII~V~y (1962); t toggar, Algerian Sahara, by GIRoD (1967)], has not been observed in the Kakanui xenoliths. Some specimens contain small vesicles filled with calcite, zeolites, and chlorite, probably of deuterie origin. Two xenoliths, K 14 and K 15 (Table 1), were selected for comprehensive chemical and mineralogical analyses. Of these, K 15 is an "ave rage" xenolith, insofar as one can select an average composition from those given in Table 1, whereas K 14 is exceptionally rich in hornblende and also has pyrope with the highest refractive index recorded for the Kakanui specimens. The analysis of K 15 is given in Table 2, along with published analyses of rocks of comparable composition, an olivine basalt f rom the island of Skye, a garnet

Table 2. Analyses o/ Kakanui eelogite K15 (analyst H. B. W~Ir and similar rocks 1: K15; 2: olivine basalt (TR6oE~, 1935, no. 379); 3: garnet ariegite (LxcRoix, 1917); 4: eclogite (WmT.I~S, 1932); 5: norm of K 15.

1 2 3 4 5

SiO 2 46.11 46.61 46.00 46.60 Olivine 7.9 Ti02 1.97 1.81 3.50 0 . 4 5 Hypersthene 21.9 Al~O a 12.56 15.14 14.49 11.87 Diopside 22.1 Fe20 a 2.67 3.49 4.02 4.56 Anor~hite 29.5 FeO 9.13 7.71 7.67 7.41 Albite 8.0 MnO 0.17 0.13 - - 0.21 Orthoclase 1.0 MgO 12.83 8.66 9.72 11.88 Ilmenite 3.7 CaO 12.16 10.08 11.34 10.36 magnetite 3.8 Na20 0.94 2.43 1.49 3.78 Apatite 0.4 K.20 0.16 0.67 0.51 0.83 Calcite 0.8 1)~0~ 0.15 0.10 0.07 0.07 II20+ 0.23 2.07 1.21 1.68 I-I20- 0.00 1.10 0.12 0.00 CO s 0.36 trace - - 0.15

99.44 100.08 a 100.14 99.89 b

S.G. 3.41 2.87 - - 3.32

a includes Cr, O a 0.04, V~O a 0.04. b includes NiO 0.04.

ari6gite from the Pyrenees, and an eclogite from the Roberts-Victor Mine, South Africa. Of the four rocks, the eclogites and the ari6gite are verysimilar in mineralogical composition, whereas the olivine basalt is quite different (note the contrast between the density of the Kakanui eelogite, 3.41, and tha t of the olivine basalt, 2.87). T~6Gv~I~ gives the mineralogical composition of the basalt as 51% plagio- clase (An54), 31% augite, 12% olivine, 6% accessory minerals. LAOROIX does not


give a quant i ta t ive mineralogical composit ion for the ari6gite, bu t states t ha t it contains garnet and cl inopyroxene; the garnet in ari6gites is a magnesium-rich type, containing up to 73% of the pyrope component ( R A v I ~ , 1964). The ari6gites occur as layers and lenses in peridotite bodies, and RAvrE~ considers these rocks to have originated in the upper mantle. The mineralogical composition of the South African eclogite is not given, bu t it presumably is an omphacite- pyrope rock typical of this locality. The remarkable character of the eclogite is demonst ra ted by the contrast between its mode (Table 1) and its norm (Table2). The norm shows almost 40% of feldspar, which is completely absent f rom the rock. P a r t of the normat ive anorthi te is present as the grossularite component of the garnet, the remainder of the normat ive feldspar is in the augite and the small amount of hornblende. The norm is t ha t of an olivine tholeiite. The second specimen analyzed, K 14, is an eclogite unusual ly rich in hornblende, and also contains the mos~ iron-rich garnet ye t found at Kakanui . I t s analysis is given in Table 3, together with those of an olivine nephelhfite f rom the Eifel,

Table 3. Analyses o/Kalcanui hornblende eclogite K14 (analyst H. B. WIIK), and similar rocks 1: K14; 2: olivine nephelinite (TRb(~E~, 1935, no. 617); 3: hornblende garnet ariegite (LAc~oIx, 1917) ; 4: eclogite (WrLLIA~S, 1932) ; 5 : norm o~ K 14.

1 2 3 4 5

SiO 2 41,20 40.01 42.30 42.50 TiQ 3.20 2.18 4.60 0.60 AI~O 3 16.35 14.13 13.71 17.72 Fe~O 3 2,87 4.57 1.93 2.84 FeO 11,40 6.04 7.36 7.52 M~O 0.20 0.19 - - 0.36 MgO 10.93 11.18 12.47 13.03 CaO 9,61 12.07 12.94 9.76 Na~O 2.33 3.27 1.80 2.19 K20 0.86 1.11 1.07 0.58 P~O s 0.14 0.66 0.05 0.05 H20+ 0,38 2.96 1.91 3.07 It20- 0.20 0.52 0.11 0.28 CO S 0.00 0.66 - - 0.04

99.71 a 99.83 b 100.25 100.80 c

S.G. 3.32 - - - - 3.27

Olivine 25.2 Diopside 12.3 Anorthite 31.5 Albite 7.9 hTepheline 6.4 Orthoclase 5.1 Ilmenite 6.1 Magnetite 4.2 Apatite 0.3

a includes VeO~ 0.04. b includes SO~ 0.28. c includes Cr203 0.07, NiO 0.19.

a hornblende ari~gite f rom the Pyrenees, and another eclogite f rom the Roberts- Victor mine. The densi ty of K 14 is 3.32, and tha t of the Roberts-Victor eclogite 3.27; densities are no t available for the other analyzed rocks, bu t for the olivine nephelinite would be about 2.9, and for the ari6gite about 3.3. The modal composit ion of K 14 (Table 1) is 62% hornblende, 18% pyrope, 18% augite, and 2 % ilmenite. The mineralogical composition of the ari~gite is evidently similar to t ha t of the hornblende eclogite. According to T~SG~a, the olivine

21 Contr. Mineral. and Petrol., VoL ]9

320 B. MAso~:

nephelinite consists of 44% nepheline, 40% titanaugite, with minor olivine and plagioelase, and accessory minerals; however, this high content of nepheline is clearly inconsistent with the relatively small amounts of Na20 and K20 in the analysis. ~evertheless, the comparison between the eelogite and the olivine nephelinite, and between the mode and the norm of the eclogite, provide an informative contrast of the crystallization of s~milar bulk compositions under very different physicoehemical conditions. The eclogite K 14 is nepheline-normative, whereas K 15 is hypersthene-normative. They thus represent two different basalt magma types, K 14 representing an alkali basalt, K 15 a tholeiitie basalt. This difference is produced by the high content of kaersutitie hornblende in K 14, this hornblende being extremely nndersaturated.

Mineral Descriptions Partial analyses were made with the microprobe of the minerals in some of the Kakanui eclogites, utilizing the previously analyzed xenocryst minerals (MAsoN, 1966) as standards. Because of the close similarity in composition between the standards and the corresponding minerals in the eclogites the only correction applied to the microprobe data was tha t for drift. The results are presented in Table 4. The eclogites chosen for microprobe analysis of the constituent minerals cover the range of composition indicated by range of refractive index of the pyrope (Table 1), and include the chemically analyzed specimens K 14 and K 15. In Table 4 the analyses are arranged in order of increasing refractive index of the

Table 4. Compositions o/ augite, pyrope, and hornblende in xenoerysts (~Aso~, 1966) and in eclogitio xenoliths

Xenocrysts K57 K 15 K56 K 14

Augite TiO~ 0.7 1.2 1.5 1.5 1.5 A1.20 ~ 7.9 7.9 8.3 8.3 8.4 FeO* 6.8 8.1 9.2 9.7 10.8 MgO 16.7 13.6 13.1 12.0 10.5 CaO 15.8 16.5 16.4 17.2 16.4 Na~O 1.3 3.1 2.8 3.0 3.0

Pyrope TiO 2 0.5 0.4 0.4 0.4 0.5 AI~O 3 23.5 22.4 24.8 23.5 20.2 FeO* 10.8 18.0 18.2 21.2 22.8 MgO 18.8 15.4 13.8 13.2 10.3 CaO 5.1 4.8 4.9 5.7 6.8

Hornblende TiO~ 4.4 4.5 5.0 5.0 5.1 AlcOa 13.9 13.4 14.0 13.7 13.7 FeO* 11.2 10.7 12.0 12.9 13.9 MgO 13.0 13.5 13.0 11.6 10.2 CaO 10.3 9.4 9.4 9.7 9.7 Na~O 3.0 3.8 4.1 3.6 3.6 K~O 2.1 1.5 1.2 1.2 1.4

FeO* = all Fe as FeO.


pyrope (the xenocryst pyrope has n -~ 1.741), and, as is to be expected, this is the order of increasing FeO and decreasing MgO. This relationship holds for all three coexisting minerals in the four xenollths analyzed, and is evidence for equilibrium crystallization. The other components show a remarkable uniformity in each of the three minerals.

The augite in the xenoliths resembles the xenocryst augite in having a low CaO content and a high Al~O~ content; it differs in having somewhat higher TiO~ content and notably higher Na20 content; and as pointed out previously it has a somewhat higher Fe/Mg ratio. Since the microprobe does not discriminate between Fe ~ and Fe 3, it is not possible to calculate the xenolith augites into pyroxene components according to the procedure of W~T~ (1964), because the amount of ferric iron to be calculated as acmite is not known. However, the amount of ferric iron in the xenolith augite is not likely to differ greatly from that in the analyzed xenocryst. This analysis calculated to 50% diopside (CaMgSi206), 12% enstatite (MgSi03), 18% Mg-tsehermakite (MgAI~SiOG) , 11% hedenbergite (CaFeSi~O~), 9% acmite (NaFeSi~Q), and no jadeite (NaA1Si~OG). The xenolith augites, with more than twice as much Na~O, probably have an appreciable jadeite component, and hence are omphacitic in nature; this is also indicated by their bluish and greenish tints in thin section. Contents of Na~O around 3 % are recorded for many omphacites in the literature. The low clcinm and high aluminum content of the eelogite augites indicates that they contain an appreciable content of the Mg-tschermakite component, comparable to that in the xenocryst. The low calcium content distinguishes these augites from the fassaites, which are also aluminous, but in which the aluminum calculates as the Ca-tsehermakite component, CaA12SiO 6.

The presence of a considerable content of the Mg-tschermakite component can also be interpreted as a considerable amount of pyrope in solid solution in the augite. Following the method of calculation used by O'HA~A and YODER (1967), the xenoeryst augite contains 45 % garnet in solid solution in the pyroxene, which they ~ound is about the maximum in the clinopyroxene-garnet system at about 30 kflobars and 1570~ The compositions they studied, however, had consider- ably higher Mg/Fe ratios than the Kakanui specimens. The texture of the Kakanui eclogites indicates that the augite and the pyrope crystallized independently, and that the pyrope is not an exsolution product. Hewever, the large analyzed xenoeryst contains small pyrope inclusions which may well represent anexsolution product; this pyrope has essentially identical composition with the pyrope xenocryst previously described (MAson, 1966).

In the garnet in the Kakanui eclogites the pyrope component is always dominant, except perhaps in K 14; exact calculation is not possible in absence of Fe s determinations. There is some evidence of an increase in CaO with increase in the Fe/Mg ratio, but the trend is not prominent.

The hornblendes in the Kakanui eelogites are all very similar in composition, apart from a moderate increase in Fe/Mg ratio from K 57 to K 14. In a recent paper (Mason, 1968) I have discussed these hornblendes from a number of occurrences, and shown that they can be classed as titaniferous pargasites, or kaersu- tires, although the ti tanium content is somewhat lower than the type kaersutite

21"

322 B. MAso~:

f rom Kaersut , Greenland. G n ~ N and RINowooD (1968) have shown experiment- ally t ha t hornblende of this composition crystallizes f rom high-alumina quartz tholeiite melts under hydrous conditions at 10 kilobars and 900~176 Since the hornblende is the only potassium-bearing phase in the association pyrope- augite-hornblende, the amount of this mineral in any specimen m a y reflect the potassium content of the original material.

I lmenite is a common accessory in the Kakanu i eclogites, a l though it is absent f rom some of them. Microprobe analyses show a moderate MgO content, 4 .2- -5 .7%.

Discussion

A search of the li terature has revealed a number of analogous occurrences to t ha t at Kakanui , i.e. eclogitic xenoliths and xenocrysts in basaltic tufts and breecias. These are enumerated in Table 5. The list, while probably not complete, is

Table 5. Comparable occurrences o/xenoliths xand enocrysts

Xenoliths Xenoerysts

eclogite peridotite pyrope augRe kaersutite

Kakanui • • • • • Salt Lake Crater • • - - • • ttoggar, Sahara • - - - - - - • Delegate, N.S.W. • x - - - - • Elie Ness, Scotland - - • • • • 1Navajo Reservation x • • - - • San Carlos, Ariz. -- • - - • • ]~glazines, France - - • - - X - - Taka-Sima, Japan -- • -- • • Solomon Is. - - • • - - - -

remarkably short, and Kakanai appears to be unique in its variety of xenoliths

and xenocrysts, although each type is known from one or more other localities.

Similar eclogite xenoliths have been described from Salt Lake Crater, Oahu, Hawaii, by a number of authors (WHITE, 1966); f rom Hoggar, Algerian Sahara (GraoD, 1967); and f rom Delegate, N . S . W . (Lov~I~CG and W H I ~ , 1964). These all contain augite and pyrope similar in composition to these minerals in the Kakanu i eelogites, a l though the amount of pyrope is usually less. The eelogite xenoliths described f rom volcanic pipes in the I~avajo Reservat ion by O'ttA~A and M ~ c Y (1966) are distinctly different, in tha t the pyroxenes are much more jadeitie, containing up to 9% Na20. Peridoti te xenoliths occur together with the eclogite xenoliths, except in the Hoggar occurrence.

Of the xenocrysts, pyrope is the mineral most diagnostic of an eclogitic environment, but is rare, being recorded from the Navajo Reservation, f rom Elie l~ess (Bz~T~SrLI~, 1927), and f rom Malaita in the Solomon Islands ( A L L ~ and D~A~s, 1965). Augite xenocrysts of the type found at Kakanui , i.e. low in CaO and high in A12Q , are more common, having been recorded at Salt Lake Crater, Etie Ness,


San Carlos (Livs~N, 1927), t~glazines (Bnovss~ and B~no~R, 1965), and Taka- sima (Kv~o, 1964). These augite xenocrysts usually have a characteristic appearance - - coal-black, lustrous, with a conchoidal fracture, and practically no trace of the usual pyroxene cleavage. Kaersutite xenocrysts are much more common, and are found in many occurrences where none of the other xenocrysts or xenoliths occur (MAsoN, 1968).

An illuminating procedure for shOwing similarities and differences between some of these occurrences is to plot mole percentages of MgO, CaO, and FeO for coexisting clinopyroxene and garnet. COL~,MA~ etal. (1965) showed that this procedure illustrates chemical differences between the three groups of eclogites which they distinguished on the basis of geologically similar occurrence. Their three groups are:

Group A. Inclusions in kimberlites, basalts, or layers in ultrabasic rocks. Group ]3. Bands or lenses within migmatite gneissie terrains. Group C. Bands or lenses within the metamorphic rocks of the alpine-type

orogenic zones.

The Kakanui eclogitcs clearly belong in Group A, along with the eclogite xenoliths in kimberlites, the garnet ari6gites of the Pyrenees, and the garnet peridotites described by O'HA~i and M]~RcY (1963). Fig. 1 is an at tempt to summarize the available information, and to compare and contrast the various eclogites included in Group A. Since for many of the mineral pairs the analytical data were obtained by the microprobe, FeO represents total iron reported as FeO.

As pointed out by Y o D ~ and TILLEY (1962, p. 485), i~ eclogites of varying bulk composition crystallized under similar pressure-temperature conditions, the clinopyroxene-garnet tie lines should not cross one another but should assume a continuous sweep across the diagram. Fig. 1 shows that the eclogite xenoliths from basaltic rocks - - Kakantfi, I-Ioggar, Salt Lake Crater, and Delegate (nos. 1--9) - - do show such a continuous sweep of tie lines (the slight crossing of no. 4 (SMt Lake Crater) and no. 5 (Kakanui) is probably within the experimental error of the microprobe analyses). In nos. 2 and 4 (Salt Lake Crater) and no. 3 (Hoggar) the augite is clearly more calcic than in the Kakanui specimens; this can be equated with a higher content of the pyrope component in solid solution in the Kakanni augites, which possibly indicates a somewhat higher temperature of equilibration for them. The garnet compositions are notably uniform in calcium content, CaO ranging from 4.7--7.1%, with a trend for CaO content to increase with increasing iron content. The calcium content corresponds to 13--20% of the grossular-andradite component in the garnet.

O'I{~a~i and Mv,~cy (1963) have analyzed a number of garnet-peridotites and their coexisting minerals, and the two extremes in Mg/Fe ratio are plotted as A and B in Fig. 1. Pair A is their sample A. 3, a garnet-lherzolite xenolith from a kimberHte pipe in South Africa; it has a lower Fe/Mg ratio than any of the eclogites previously discussed, but the tie line is compatible with those already plotted. Pair ]3 is their sample N. 23, a garnet-websterite from Tafjord, Norway, and this tie line is incompatible with those of the eclogite xenoliths, cutting

324 B. MAsoN:

across then at a considerable angle. Actually, this is hardly surprising, since the clinopyroxene in the garnet peridotites is quite different from that in the eclogite xenoliths; it is a chrome diopside low in A1203. The garnet in the garnet peridotites is similar to that in the eclogite xenoliths, except for an appreciable chromium content (2.32 % Cr203 in A. 3). In order to compare the Kakanui eclogites with those occurring as xenoliths in kimberlites, three of the latter (C, D, E, Fig. 1) were chosen to cover as wide a range of Fe/Mg ratios as possible. These three were also selected as having rather

9

: ~" 0 ~s

/ f l

CaO Mg0

Fig. 1. CFM plot (mole per cent) of coexisting garnet-c]inopyroxene pairs. Nos. 1--9: eelogite xenolRhs from basaltic rocks; 1 Kakanui (augite xenocryst with garnet inclusions); 2 Salt Lake Crater (WmT]~, 1966); 3 ttoggar (GIROD, 1967); 4 Salt Lake Crater (W~TE, 1966); 5 Kakanui (K 57); 6 Kakanui (K 15); 7 Kakanui (K 56); 8 Delegate (LovERING and W~ITE, 1964); 9 Kakanui (K 14). Nos A - - E : other types; A garnet peridotite xenolith, South Africa (O'ttA~_ and MERCY, 1963); B garnet peridotite, Norway (O'ttAR~ and ~RCu 1963); C eclogite xenolith from kimberlite, u (SoBoLEV and KUZNETSOVA, 1965); D eclogite xenolith from kimberlite, Basutoland (E 4, NlxoN et al., 1963); E eclogite xenolith from kimberlite, Tanzania (37079, O'H~A and Yo])~R, 1967)

low sodium contents in the clinopyroxene, comparable with that in the Kakanui eclogites (C, 2.80% lkTa20; D, 4.62% I~a20; E, 2.22% Na20 ). For eclogites (D and E) of comparable composition to the I<akanui specimens, the tie lines show quite a distinct trend. This reflects the different nature of the pyroxene; in the eelogites from kimberlites most of the clinopyroxene aluminum is in six-fold coordination, whereas in the Kakanui eclogites the clinopyroxene aluminum is about equally divided between six-fold and four-fold coordination, i.e. the former are jadeitic, the latter tschermakitie. The garnet in eclogites D and E is also notably more calcic than the garnet in the Kakanui eclogites. Although the eclogite xenoliths from basaltic rocks and those from kimberlite pipes can readily be distinguished by their mineral compositions (and even by their macroscopic appearance), it is possible to match them fairly closely in bulk chemical composition. Theoretically it is possible for the xenoliths from basaltic rocks to evolve towards those from kimberlites by exsolution of garnet from the clinopyroxene. Since the garnet contains little or no sodium, exsolution of this phase will increase the sodium content and hence the jadeite component in the clinopyroxene. If the exsolved garnet increases in calcium content, then


the garnet-pyroxene tie line will rotate towards the tie line direction for the eclogites from kimberlites. This is of course consistent with a higher temperature of crystallization for the eclogite xenoliths from basaltic rocks than for those from kimberlites. Such a relationship has been postulated by G ~ E ~ (1966), in his discussion of the origin of the eelogite from Salt Lake Crater. He correlates the chemistry and petrography of this rock with experimental studies of melting and subsolidus relations in basaltic compositions at high pressure. He concludes that this eclogite originated as an accumulate of liquidus or near-liqnidus subcalcic clinopyroxene derived from alkali olivine basalt or basanite at a pressure of approximately 13--18 kilobars. The original accumulate cooled from temperatures of 1350 ~ 1400~ to about 1000~ at constant pressure, resulting in exsolution of garnet from the original highly aluminous pyroxene. G ~ ' s work is equally applicable to the Kakanui eclogites, although there is no textural evidence in the Kakanni eclogites that the garnet formed by exsoln- tion from the pyroxene. Either crystal growth has completely destroyed any exsolution textures, or more likely the garnet and pyroxene crystallized simulta- neously in equilibrium, and rapid cooling has inhibited exsolution of garnet from the pyroxene. The Kakanni eclogites have a notably higher Fe/Mg ratio than those from Salt Lake Crater, and this higher ratio may have resulted in a lower liqnidus temperature for the Kakanni material. One pertinent feature of the Kakanui material is the relationship between the eclogite xenoliths and the xenocrysts of augite, pyrope, and kaersutite that are also present in this volcanic breccia. The range of refractive index in the garnet shows that the xenocryst pyrope has a higher Mg/Fe ratio then does the xenolith pyrope. The most magnesium-rich pyrope among the xenocrysts is that previously analyzed (MAson, 1966), and its composition would place it at 1 in Fig. 1 ; the most magnesium-rich pyrope in the xenoliths is that indicated as 5 on Fig. 1. Compositions between 1 and 5 are represented by unanalyzed xenocrysts, judging from their range in refractive index. I t appears that, under the conditions of crystallization of the parent material, pyrope, augite, and kaersutite formed individual large xenocrysts until the Fe/Fe~-Mg ratio reached a certain value (approximately 0.4 for the pyrope), but at higher Fe/Fe ~-Mg ratios these minerals crystallized together to give the eclogite xenoliths. The origin of the Kakanui eclogites, and those from other basaltic rocks - - Salt Lake Crater, Hoggar, Delegate - - poses some interesting questions. The experimental data of G R ~ x (1966), O'HAtCA and Y o n ~ (1967), and others indicate that this mineral association can crystallize from a variety of basaltic compositions under temperatures and pressures consistent with those in the upper mantle at depths of about 60 kin. However, the geological environment at each of the above localities differs markedly. The Kakanui breccia pipe penetrates a com- paratively thin sequence of Cretaceous and Lower Tertiary sediments overlying schists extending to an unknown but probably great depth. Salt Lake Crater is on a typical oceanic basalt volcano. The Delegate pipe penetrates the folded Paleozoie rocks of the Australian Alps. The Hoggar occurrence, in Tertiary and Recent voleanics, is on a Precambrian shield area. The mineralogy of the eelogite xenoliths, especially the large amount of potential pyrope in solid solution in

326 B.M_~so~:

the clinopyroxene, indicates extremely rapid transfer f rom their place of origin, rapid enough to prevent internal reaction (exsolution) or external reaction (with the magma).

A pert inent question is why eelogitie xenoliths of this kind are so rare, in contrast to the numerous occurrences of peridotite xenoliths, also believed b y m a n y to be derived from the upper mantle. Two possible hypotheses come to mind: (1) material of this kind requires rare and special conditions of formation, its occurrence in the mant le is therefore extremely sporadic, and hence it is seldom brought up in volcanoes; (2) material of this kind is present at greater depths in the mantle than peridotltle material, and only rare volcanic pipes drawing material f rom greater depths will contain it. An objection to the first hypothesis is the experimental evidence showing tha t the characteristic minerals of these eclogites crystallize readily f rom basaltic melts under upper mant le conditions. Of course, the rar i ty of these eclogites m a y be conditioned by the difficulty in moving them rapidly f rom the mantle to the surface; this material m a y be common within the mantle, bu t be usually remelted and incorporated in intrusive and extrusive magmas.

References

ALLE~, J. B., and T. DEANS: Ultrabasic ernptives with alnoitic-kimberlitic affinities from Malaita, Solomon Islands. Mineral. Mag. 34, 16--34 (1965).

BALSILLIE, D.: Contemporaneous volcanic activity in East Fife. Geol. Mag. 64, 481--494 (1927).

B~ovssE, R., et E. BEGGER: Grands cristanx d'augite dans une br~che volcanique intrusive. BulL soc. fran~, min6ral, et crist. 88, 525--526 (1965).

COLE~IAN, R. G., D. E. LEE, L. B. BEATTY, and W. W. BRAddOCK: Eclogites and eclogites: Their differences and similarities. Bull. Geol. Soc. Am. 76, 483--508 (1965).

DICKEY, J. S. : Eclogitic and other inclusions in the mineral breccia member of the Deborah volcanic formation at Kakanui, New Zealand. Am Mineral. 53, 1304--1319 (1968).

GmoD, M. : Donn6es p6trographiques sur des pyrox~nolites s grenat en enclaves dans des basaltes du Hoggar (Sahara central). Bull. soe. frang, mineral, et crist. 90, 202--213 (1967).

G~EmV, D. It. : The origin of the "eclogites" from Salt Lake Crater, Hawaii. Earth Planetary Sei. Letters 1, 414 420 (1966).

GREE~, T. I-L, and A. E. RINOWOOD: Crystallization of basalt and andesite under high pressure hydrous conditions. Earth Planetary Sci. Letters 3, 481---489 (1968).

Kv~o, H.: Ahminian augite and bronzite in alkali olivine basalt from Taka-sima, l~orth Kyusyu, Japan. Advancing frontiers in geology and geophysics, 205--220. ttyderabad: Indian Geophysical Union 1964.

LAe~OIX, A. : Les p6ridotites des Pyr6n6es et les autres roches intrnsives non feldspathiques qui les accompagnent. Compt. rend. 165, 381--387 (1917).

LAUSElV, C. : The occurrence of olivine bombs near Globe, Arizona. Am. J. Sci. 14, 293--306 (1927).

LOVE~n~G, J. F., and A. J. R. WroTE: The significance of primary scapolite in granulitie inclusions from deep-seated pipes. J. Petrology 5, 195--218 (1964).

M~so~, B. : Pyrope, augite, and hornblende from Kakanui, New Zealand. New Zealand J. Geol. Geophys. 9, 474--480 (1966).

- - Kaersutite from San Carlos, Arizona, with comments on the paragenesis of this mineral. Mineral. Mag. 86, 997--1002 (1968).

NIXO~, P. H., O. vo~ KNom~I~c, and J. M. ROOKE: Kimberlites and associated inclusions of Basutoland: A mineralogical and geochemical study. Am. Mineral. 48, 1090--1132 (1963).


O'HARA, M. J., and E . L . P . MERCY: Petrology and loetrogenesis of some garnetiferous peridotites. Trans. Roy. Soc. Edinburgh 55, 251--314 (1963).

- - - - Eclogite, peridotite and loyrope from the Navajo country, Arizona and New l~Iexico. Am. Mineral. 51, 336--352 (1966).

- - , and H. S. YODEl: Formation and fractionation of basic magmas at high pressures. Scot. J . Geol. 3, 67--117 (1967).

R~vIEa, J. : Ari@gites et 6clogites. Bull. soe. fran 7. mineral, et crist. 87, 212--215 (1964). So~OnEV, N.V. , and I. K. KVZ~TSOVA: Recent data contributing r the mineralogy of

eclogites from kimberlite pipes of Yakutia. Dokl. Akad. Nauk S. S. S. R. 163, 137--145 (1965).

T~o~soz~, J. A. : Inclusions in some volcanic rocks. Geol. Mag. 4, 490--500 (1907). TR6GER, W. E. : Spezielle Petrographie der Erulotivgesteine. Berlin: Deutsche l~Iineralogische

Gesellschaft 1935. W~ITE, A. J. 1%. : Clinopyroxenes from eelogites and basic granulites. Am. Mineral. 49, 883--

888 (1964). W~IT~., 1%. W.: Ultramafie inclusions in basaltic rocks from Hawaii. Contr. Mineral. and

Petrol. 12, 245--314 (1966). WILL:AMS, A. F. : The genesis of the diamond. London: Benn 1932. YoDel , H. S., and C. E. TILLEu Origin of basaltic magmas: An experimental study of

natural and synthetic rock systems. J . Petrology 3, 342--532 (1962).

Dr. BRIA~ M~SO~ U.S. National l~useum Washington, D.C. (USA)

eclogitic xenoliths from volcanic breccia at kakanui, new zealand

Documents