Buchitic metagreywacke xenoliths from Mount Ngauruhoe, Taupo Volcanic Zone, New Zealand

Journal of Volcanology and Geothermal Research, 35 (1988) 205-216

205

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

BUCHITIC METAGREYWACKE XENOLITHS FROM MOUNT NGAURUHOE, TAUPO VOLCANIC ZONE, NEW ZEALAND I.J. GRAHAM*, R.H. GRAPES and K. KIFLE Research School of Earth Sciences, Victoria University of Wellington, Private Bag, Wellington, New Zealand (Received August, 1987; revised and accepted May 25, 1988 )

Abstract Graham, I.J., Grapes, R.H. and Kifle, K., 1988. Buchitic metagreywacke xenoliths from Mount Ngauruhoe, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res., 35: 205-216. Buchitic sedimentary xenoliths, a few centimetres to several decimetres diameter, occur in Recent andesite from Mount Ngauruhoe, Tongariro Volcanic Center, Taupo Volcanic Zone, New Zealand. Bulk chemistry and Sr isotope compositions of the xenoliths indicate that they are greywacke and argillite derived from Mesozoic Torlesse terrane basement that partly underlies the Taupo Volcanic Zone. The xenoliths contain up to 80% glass with quartz, apatite and zircon remaining as unmelted phases. Glasses within the xenoliths are peraluminous (A/CNK = 1.0-1.4), have high normative corundum (2 -7 % ), appreciable FeO ( 2 - 4 wt. % ), MgO ( 0.2 - 1.5 wt. % ), Ti02 ( 0.17-0.84 wt. % ), relatively high normative An (1,0-5.3%), and do not represent S-type granitic melts. In the argillite the glass has higher amounts of A1203, FeO, MgO, CaO and K20, and has less Si02 and Na20 than glass in the greywacke. Silica-rich glass (up to 80 wt. % Si02) surrounds partially melted quartz. Variable glass chemistry reflects the heterogeneous (layered) nature of the xenoliths. Cordierite (Mg/(Mg + Fe + Mn) = 0.78-0.58), orthopyroxene (En43_s6), Mg-rich ilmenite, rutile, pleonaste, V-Cr-Ti spinel, and pyrrhotite occur in the glass of the xenoliths. The dominant cordierite, orthopyroxene, spinel assemblage can be accounted for by disequilibrium breakdown reactions under low oxidation conditions ( < QFM) involving phengite and chlorite which are abundant in Torlesse greywacke and argillite cropping out along the eastern side of the Taupo Volcanic Zone. Comparison with glass compositions and phase relations of disequilibrium melting experiments on Torlesse greywacke and argillite indicates a minimum temperature of 775 °C and a maximum pressure of 1.5 kbar for fusion of the xenoliths that underwent a rapid rate of heating at a depth of less than 5 km and a cooling period constrained by the time of quenching when they were erupted.

Introduction Metasedimentary xenoliths occur within andesite lava flows from Mount Ngauruhoe, Tongariro Volcanic Center, Taupo Volcanic Zone, North Island of New Zealand (Speight, 1908; Grange and Williamson, 1930; Battey, 1949; Steiner, 1958). In particular, lava flows of *Present address: Institute of Nuclear Sciences, Private Bag, Lower Hutt, New Zealand.

0377-0273/88/$03.50

18.8.1954 and the glowing avalanche of 16.9.1954 from Ngauruhoe contain many buchitic xenoliths. Because of quenching on eruption these xenoliths provide direct evidence of precursor rocks, processes of pyrometamorphic reconstitution, and melt generation. The petrography of many of the xenoliths has been given by Steiner (1958) who considered that they were derived from quartzo-feldspathic gneisses. In this paper we have re-examined five buchitic xenoliths using XRF and isotopic

© 1988 Elsevier Science Publishers B.V.

206

methods to determine their composition and affinity, together with electron microprobe analyses of glass and minerals in two representative samples. These data are compared with glass compositions and mineral stabilities in experimentally melted greywacke and argillite from the Taupo Volcanic Zone to assess the T-P conditions of buchite formation. Description of the xenoliths In hand specimen the buchite xenoliths (up to 80% glass and up to 50% vesicles) in the Ngauruhoe 1954 flows range from a few centimetres to several decimetres in diameter. They are typified by layering and lensoid structures of alternating light coloured quartz-rich layers and darker, quartz-poor layers (Fig. 1A and B ).

A o

10

I

I

mm

B.

Fig. 1. A. Contact between xenolith 474 and host andesite lava (dark grey). The white vein fragments consist of quartz, wollastonite and calcic plagioclase. B. Quartz-rich and quartz-poor layeri=lg in xenolith 469. Light coloured fragments have the same composition as those in Fig. A.

The layers are either parallel, contorted and/or discontinuous over a few centimetres, suggestive of viscous flow in a partially molten state. Some of the xenoliths have white veins which are often fragmented and composed of quartz, wollastonite and calcic plagioclase (Fig. 1A and B). Contacts between xenoliths and andesite are sharp. Megascopic and microscopic evidence of interaction, such as a reaction zone is lacking. In some cases (e.g. 474) the contact is irregular and pieces of the xenolith are broken offand lodged in the andesite (Fig. 1A). In others, andesite has intruded along fractures or along the margins of veins. Bulk rock chemistry Analyses of five vitrified xenoliths from Ngauruhoe are given in Table 1. SiO2 contents range from 62 to 75 wt.% and they are all peraluminous with mol A l z O J ( C a O + N a e O + K 2 0 ) ratios between 1.10 and 1.63. Normative corundum varies from 7.2 to 1.4% and the wt.% K 2 0 / ( K 2 0 + Na20) ratio varies between 0.39 to 0.68. Volatile contents (expressed as loss on ignition) are less than 1 wt.%. Despite the high degree of melting, there is an extremely close correspondence for a wide range of major and trace elements between the xenoliths and Mesozoic Torlesse greywacke and argillite basement rocks (e.g. Figs. 2 and 3; Graham, 1985a), which indicates that most of the xenoliths are greywacke sandstones (SIO2>65 wt.% ) while 465 is an argillite (Si02 < 65 wt.% ). Torlesse rocks crop out along the eastern part of the Taupo Volcanic Zone whereas chemically and mineralogically distinct (dominantly volcanogenic) greywacke and argillite of the Waipapa terrane (Fig. 2) occur in the western part of the Taupo Volcanic Zone. The chemical evidence implies that the xenoliths have not been compositionally modified by diffusive interaction with the andesite, possibly because of short residence time in the magma. Sr-isotopic ratios (Table 1; Fig. 4) also support the correlation: three samples (471,469, 474) fall on a low grade

207 TABLE 1 XRF analyses 1, C.I.P.W. norms, and isotope ratios 2 of buchite xenoliths ~ from Ngauruhoe, Tongariro Volcanic Center Sample no: 4

465

471

469

474

461

SiO2 TiO~ Al~O:/ FeO ~ MnO MgO CaO Na20 K20 P20~ L.O.I. ~ Total

61.90 0.82 18.95 5.35 0.05 1.94 1.33 2.52 5.25 0.17 0.87 99.15

70.82 0.55 15.23 3.19 0.05 1.23 1.59 3.32 2.98 0.11 0.34 99.41

71.90 0.50 15.27 3.32 0.03 1.42 0.60 3.19 2.78 0.12 0.24 99.37

73.77 0.47 13.83 3.63 0.04 1.26 0.77 2.61 2.70 0.10 0.15 99.34

74.37 0.43 12.97 2.51 0.06 1.14 2.27 3.27 2.10 0.10 0.24 99.46

Selected trace elements (ppm): Ba Cr Rb Sr V Zr

678 55 181 134 113 212

470 45 130 211 73 241

536 40 124 159 72 222

414 35 115 146 72 171

449 37 77 347 56 187

C.I.P.W. norms: Q C Or Ab An Opx I1 Ap

18.3 7.2 31.6 21.7 5.6 13.6 1.6 0.4

33.1 4.0 17.7 28.4 7.2 8.2 1.1 0.3

37.5 6.3 16.6 27.2 2.2 8.9 1.0 0.3

42.6 5.5 16.1 22.3 3.2 9.2 0.9 0.2

39.6 1.4 12.5 27.9 10.7 6.9 0.8 0.2

Rb/Sr 8~Sr/S6Sr A/CNK 7 K 2 0 / ( K 2 0 + Na20)

1.352 0.71190 1.54 0.68

0.695 0.70858 1.25 0.47

0.778 0.71097 1.63 0.47

0.793 0.71200 1.60 0.51

0.223 0.71058 1.10 0.39

1Analyses were carried out with an automated Siemens SRS-1 X-ray spectrometer at the Analytical Facility, Victoria University of Wellington. 2Sr was analysed isotopically with a VG MM30B mass spectrometer at the Institute of Nuclear Sciences, Lower Hutt. Experimental details are given in Graham (1983a,b). 8VSr/S~Sr ratios are normalised internally against 86Sr/SSSr--0.1194 and precision is given as the standard errors of the mean for the 95% confidence interval. For comparison, five analyses of NBS987 gave a mean SVSr/S6Sr ratio of 0.71023 ± 0.00008. :~The relatively high Sr isotopic composition of quartz-wollastonite-calcic plagioclase veins in some xenoliths (Graham, 1985a) indicated that such veins are not in chemical or isotopic equilibrium with the rest of the xenolith. Therefore care was taken to exclude such veins from material prepared for bulk X R F and isotope analyses. 4Samples housed in the Geology Department, Victoria University of Wellington. All numbers prefixed by 17. ~Total iron given as FeO. ~Loss on ignition. VMolar A12OJ (CaO + NaeO + KeO ).

208

SiO2

~2 90

9O

80

FeO ÷ ~

v 20

MgO

v

v

60

10 ÷

K20

10

AI,

Fig. 2. Bulk compositions of the buchitic xenoliths and fields of glass compositions for xenoliths 474 and 465 (stippled areas ) in terms of weight percent Si02-A1203- (Na20 + KeO ), (A) and (FeOtot~liro, + MgO ), (B). Fields for Torlesse and Waipapa terrane metasediments are labelled T and W, respectively, q = glasses near partially melted quartz. m lO

465 474

The reason for this is not clear but may be due to partial equilibration with host lava Sr, or may simply reflect regional variation in isotopic composition as observed elsewhere (Graham, 1985a).

z

9 .715; .714

AGE=139+-6~

.713 o 0.1

I

I

J

I

Z

Sr

K

Rb

Ba

Th

I

I

J

Nb La Ce

I

I

P Zr

I

I

Ti

Y

Fig. 3. Spider diagram of trace-element concentrations in xenoliths 465 (open triangles) and 474 (open diamonds), normalized to average Torlesse greywacke (after Graham, 1985b). Shaded area outlines the composition range of Torlesse metasediments in the vicinity of the Tongariro Volcanic Center, Taupo Volcanic Zone.

h.

474~

.712

~

465

.711 L_

~461

.710 .709 .708 .7O7 .706 .705 0.0

.s

,.o

1.,

~.o

,.s

~.o

~.,

,.o

87Rb/""Sr metamorphic isochron for Torlesse greywacke and argillite (Graham, 1985b) and sample 461 plots close to it; only the argillite xenolith (465) shows significant departure from the isochron.

Fig. 4. Rb-Sr whole-rock plot of buchitic xenoliths from the 1954 lava flows, Mount Ngauruhoe. Regression line for lowgrade prehnite-pumpellyite - - lower greenschist facies metamorphism of Torlesse greywacke and argillite (Rangipo Suite, Graham, 1985b) is given for comparison.

209

Glass c h e m i s t r y

in glass chemistry is largely the result of original compositional heterogeneity in the xenoliths as indicated by the layering which, in turn, controls mineral distribution as described below.

Representative analyses of glasses in a bucho itic greywacke and argillite xenolith are given in Table 2 and composition fields of all glass analyses are plotted in Figs. 2A and B. In general, glass compositions reflect the bulk compositions of the host rocks. The glasses are peraluminous with molecular Al2Off(CaO+ Na20 + K20) ratios ranging from 1.01 to 1.44 in the argillite (normative corundum of 0.75.5%) and from 1.55-1.87 in the greywacke (normative corundum of 1.3-7.2% ). In the argillite the glass is enriched relative to glass in the greywacke in A12Oa, FeO, MgO, CaO, K20, has comparable TiO2, is less siliceous and has slightly lower Na20. In the immediate vicinity (i.e. closer than 40 microns) of partially melted quartz grains in both xenoliths the glass is silica-rich (Table 2). Apart from this, variation

Mineral chemistry Representative electron microprobe analyses of minerals within the glass of the argillite and greywacke are listed in Table 3. Cordierite. Cordierite typically forms euhedral crystals within the glass of both xenoliths. In the greywacke, cordierite tends to occur within the darker, quartz-poor layers that contain less silica-rich glass. Large crystals of cordierite, such as that illustrated in Fig. 5, appear to be the result of growth coalescence of many smaller, optically continuous crystals, each of

TABLE2 Representative electron microprobe analyses a and C.I.P.W. norms of glasses in buchite argillite (465) and greywacke (474) xenoliths Argillite (465) ~- Range--,

Greywacke (474) ac

~ Range-~

bc

SiQ Ti02 A1203 FeO b MnO MgO CaO Na20 K20 Total

65.33-68.95 0.84- 0.43 16.54-15.50 3.75- 3.19 n.d. n.d. 1.07- 1.05 0.98- 1.05 3.11- 3.21 6.50- 5.79 98.12-99.17

77.17 0.21 10.64 1.55 n.d. 0.30 0.35 1.87 5.78 97.87

73.72-74.80 0.17- 0.58 15.32-13.58 2.04- 2.05 n.d. n.d. 1.53- 0.27 0.20- 0.28 2.98- 3.76 2.73- 2.89 98.69-98.21

80.03 0.25 10.06 1.20 n.d. 0.19 0.14 3.06 3.27 98.20

Q C Or Ab An Opx Ilm

16.5 2.7 39.2 26.8 5.0 8.3 1.5

42.7 0.7 34.9 16.2 1.8 3.3 0.4

42.2 -40.4 7.2 - 3.8 16.4 -17.4 25.6 -32.4 1 . 0 - 1.4 7.4 - 3.5 0.2 - 1.1

49.2 1.2 19.7 26.4 0.7 2.3 0.5

-22.1 - 2.1 -34.5 -27.4 - 5.3 - 7.8 - 0.8

aGlass analyses were made using a beam diameter of 20 #m, 0.8 × 10 8 A, using a J E O L Superprobe 733 at the Analytical Facility, Victoria University of Wellington. bAll iron as FeO; n.d. = below detection limit. Ca, b, Glass near partially fused quartz grains in argillite and greywacke respectively.

210 TABLE3 R e p r e s e n t a t i v e electron m i c r o p r o b e a n a l y s e s ~ a n d formulae of h i g h - t e m p e r a t u r e m i n e r a l s in b u c h i t e argillite (465) a n d greywacke (474) xenoliths

A rgillite (465) Cordierite

Orthopyroxene

core SiO2 TiO~ A120:~ Cr20:~ V~O:~ Fe20:~ FeO MnO MgO CaO Na20 K~O Total

48.33 n.d. 33.33 -. . 9.20 0.23 7.80 n.d. n.d. 0.12 99.01

rim

core

50.67 n.d. 32.23 -. . 6.73 + 0.17 10.02 n.d. n.d. 0.26 100.08

4.966 4.037 . .

V :~+

.

Fe 2 + Mn Mg Ti ~+ Ca Na K

0.79l 0.020 1.194 --. 0.016 11.023

52.97 n.d. 1.53 n.d.

0.15 0.74 61.49 0.22

21.19 + 0.36 22.27 0.36 n.d. n.d. 98.68

0.43** 25.08 n.d. 11.36 n.d. n.d. n.d. 99.47

n.d. 54.37 0.37 0.57 n.d. 0.67** 38.26 0.35 5.77 n.d. n.d. n.d. 100.36

.

. 32.10 + 0.67 13.68 0.56 n.d. n.d. 100.00

18(0)

Si A1 Fe :~+ Cr :~+

Ilmenite

rim

50.31 0.31 2.37 -.

.

.

Pleonaste

6(0)

5.089 3.816 . .

1.964 0.109 . .

.

.

.

. . 3.972

which has euhedral terminations when in contact with the glass. M g / ( M g + Fe + Mn) ratios (XM~) of cordierite vary from 0.73 to 0.58 (Table 2 ). This variation is due to zoning. The bulk of individual crystals have XMg=0.59 and are surrounded by a thin rim (up to 10 microns wide) of XM~ = 0.72-0.69. The distortion index (A) of cordierite in the argillite is 0.28 indicating a "low" structural state (Schreyer and Schairer, 1969).

Orthopyroxene. Hypersthene

.

1.048 0.022 0.796 0.009 0.023 .

0.033 11.019

1.983 0.068

. .

0.565 0.014 1.500 --.

32(0)

occurs as euhe-

.

0.664 0.011 1.243 -0.014 . . 3.983

6(0)

0.032 15.588 0.070 0.037

-0.021 0.024 0.022

4.511 -3.641 0.120 --

1.539 0.014 0.414 1.966 --

23.998

4.000

.

. .

dral, often lath-like, crystals with compositions that range from Mg49.8-42.6Fe4,.7 .~.1Cao.5_1.2. Like cordierite, the crystals have a thin Mg-rich rim of Mg64.7-~7.sFe34.6 42.~Ca0.7_o.o. AleO:~ varies from 1.00 to 1.53 wt.%. Hypersthene is invariably found within more siliceous glass areas that contain numerous grains of partially melted quartz.

Spinels. Pleonaste euhedra (up to 8 microns diameter) are usually associated with cordierite in silica-poor glass of the argillite. Composi-

211 T A B L E 3 (continued)

Greywacke(474) Cordierite

SiO2 TiO~ AI~O:~ Cr~O:~ V~O:~ Fe~O:~ FeO* MnO MgO CaO Na20 K20 Total

Orthopyroxene

core

rim

48.93 n.d. 33.39 n.d. n.d. . 9.40 n.d. 7.50 n.d. 0.06 n.d. 99.28

50.00 n.d. 32.84 n.d. n.d. . 7.27 n.d. 9.20 0.09 n.d. n.d. 99.40

.

core

5.004 4.025 . . . 0.804 -1.143 --0.012

52.35 0.18 1.23 n.d. --

51.50 0.17 1.91 n.d. --

29.76 0.58 16.73 0.24 n.d. n.d. 101.07

25.57 0.26 19.68 n.d. n.d. n.d. 99.09

.

*K

. . .

.

V

6(0) 5.057 3.916 . . . 0.615 -1.387 -0.010 .

.

Rutile

Pyrrhotite

0.67 6.26 25.44 18.37 7.08 0.08** 36.93 n.d. 3.35 n.d. n.d. n.d. 98.18

n.d. 52.22 0.41 0.88 -3.17 36.15 0.38 5.85 n.d. n.d. n.d. 99.06

n.d. 96.85 0.22 n.d. 1.30 0.55 + + -n.d. n.d. n.d. n.d. n.d. 98.92

61.35 38.15 0.14 0.08 99.75

32(0)

1.992 0.055

1.961 0.086

0.947 0.019 0.949 0.005 0.010

0.814 0.008 1.117 0.005 . .

.

. .

10.984

.

3.976

3.991

6(0)

0.174 7.917 0.021 3.832 1.489 8.099 -1.308 1.229 .

. . .

.

10.989

Ilmenite

rim

18(O) Si A1 Fe :~+ Cr :~+ V :~+ Fe z+ Mn Mg Ti 4 + Ca Na

Spinel

. .

Fe S Ni Cu

2(0)

-0.024 0.116 0.034 -1.473 0.016 0.425 1.913

-0.003 0.006 -0.012 ---0.979

4.000

0.999

. .

.

24.069

.

n.d. = not detected; - - = not analysed. * = All iron as FeO; + + = All iron as F%O~; **Fe :~+ / F e 2÷ ratio calculated using the method of Stormer ( 1983 ). 1Analyses made with a J E O L Superprobe 733 at the Analytical Facility, Victoria University of Wellington; operating conditions and data reduction are given in W a t a n a b e et al. (1981).

tions

(mol

F%TiO4

%)

vary

1.3; F e C r 2 0 4

FezSiO4

0.2-0.3;

0.2-0.6; MgA1204 44.9-49.0

ble 3). Rare grains of an

from

1.9-2.2; FeA1204 47.0-51.4; F%O4 0.5-

unusual

spinel a r e

(~ 4 microns

vanadium-titanium-chromium

found in the greywacke

the composition

(Ta-

in diameter)

wacke. Analyses in Table 3 indicate significant amounts of MgO (2.6-5.9 wt.% ), equivalent to a maximum of 12 mol % MgTiO3 (geikelite) component so that it could be described as picroilmenite.

(Table 3 ) with

( m o l % ): F e z S i O 4 1.7; F e z T i O 4

2 6 . 4 ; F e A 1 2 0 4 2 9 . 4 ; F e V z O 4 7.9; F e C r z Q

20.4;

Fe:~O4 0.2; M g A 1 2 0 4 14.0.

Ilmenite. Homogeneous grains of ilmenite occur with the glass of the argillite and grey-

Rutile. Rutile is associated with ilmenite in the greywacke glass as separate, homogeneous grains. It contains small amounts of F%O3 (0.30-0.51 wt.% ) and V205 (1.30 wt.% ). Pyrrhotite. Pyrrhotite is ubiquitous in both the

212

Fig. 5. Backscattered electron image of a cluster of cordierite crystals (pale grey) surrounded by glass (dark grey). Small, bright inclusions within the cordierite are spinel, ilmenite and pyrrhotite. Black areas are holes ( = vesicles). Note the euhedral outlines of cordierite in contact with the glass. Bar scale = 10 microns. Xenolith 474.

argillite and greywacke and forms bleb-like homogeneous grains. Minor elements present are Ni and Cu (Table 3).

Relic phases. Subrounded to embayed quartz, as single grains or mosaics, together with rare grains of apatite and zircon are the only primary phases remaining in the xenoliths. Discussion

Disequilibrium melting and metastable crystallization The large amount and variable composition of glass, and the fine grain size of newly formed high-temperature minerals in the buchitic xenoliths indicate that they have undergone a rapid

rate of heating and a cooling period that was probably constrained to the time of quenching on eruption. As such, disequilibrium melting has occurred and the simultaneous nucleation and growth of crystals in the melt has obeyed the Ostwald Step Rule resulting in the kinetically most favourable metastable phases. These phases, however, will presumably mimic those that would have become stable given a longer residence time of the xenoliths in the magma at high temperatures. Glasses in the xenoliths contain appreciable amounts of refractory components such as Mg, Ti, Ca and Fe, have high normative An (Table 2) and therefore do not represent S-type granitic melts (c.f. White and Chappell, 1977). Torlesse rocks exposed in the eastern part of the Taupo Volcanic Zone contain abundant phengitic mica and chlorite, particularly the argillites, so that with fusion the resulting melt is likely to be peraluminous, have high normative corundum and water content, and also contain moderate amounts of FeO and MgO (e.g. Grapes, 1986; Rubie and Brearley, 1987). An advanced stage of melting of the xenoliths is indicated by the high proportion of glass (up to 80 modal percent) and the fact that quartz, apatite and zircon remain as the only relic phases; quartz due to its initial abundance, apatite because of its high melting point ( 1650 ° C, e.g. Bentor et al., 1981) and zircon because of its stability in peraluminous liquids (Watson, 1979) and high melting temperature (1676 ° C; Butterman and Foster, 1967). The mineral assemblage in the glass can be accounted for by the metastable breakdown reactions involving phengite and chlorite (in the presence of quartz and feldspars) which are abundant in Torlesse greywacke and argillite that crop out along the eastern margin of the Taupo Volcanic Zone. These reactions may be characterized as: phengite~ hercynitic spinel+biotite/cordierite + mullite/corundum + sanidine/ melt + vapour

213

vapour release at the time of quenching producing the vesicular nature of the xenoliths. The absence of magnetite, the presence of pyrrhotite, the low calculated F%O3 and Fe304 components of ilmenite and spinel respectively, and the dominant FeTiO3- MgTi03 solid solution of ilmenite (Table 3), indicates that fusion of the xenoliths took place under reducing conditions ( < Q F M ) that were probably controlled by original graphite that is common in Torlesse metasediments. The Mg-rich rims of hypersthene and cordierite in the buchitic xenoliths reflect continued growth (development of euhedral crystal outlines) with increasing temperature in the melt.

(e.g. Smith, 1969; Grapes, 1986; Brearley, 1986; Rubie and Brearley, 1987; Kifle, unpubl, data), chlorite ~ cordierite + hercynitic spinel + magnetite + Al-orthopyroxene +biotite (Worden et al., 1987; Kifle, unpubl, data). Because of rapid heating of the xenoliths initial dehydration (non-melt producing) reaction boundaries of these hydrous phases could have been overstepped by as much as 100200 °C (e.g. McOnie et al., 1975; Brearley, 1986; Rubie and Brearley, 1987). H20 evolved during the breakdown of phengite and chlorite would have become concentrated in the melt so that a sudden pressure drop on eruption resulted in a

SiO2

i k

SiO2

474

•

}2s7so

\

~ (},:.

~

k

-~8o

"°G \

~

•

5

;465 "-

;A

,o

FeO+MgO .,

v 03

v

15

10

Na20+K20

,,

,6s

5

AI203

Fig. 6. Weight percent Si02-A1203- (Na20 + K20 - (FeO + MgO ) plot of average glass (filled circles ) in xenoliths 474 and 465 (open circles), (dashed tie lines connect glass and rock), and change of melt composition (glasses), (arrowed lines), with increasing temperature (25 ° C intervals) in 1 kbar experimentally melted Torlesse greywacke (G) and argillite (A), Taupo Volcanic Zone, (Kifle, unpublished data). M--S-type granite minimum melt (White and Chappell, 1977 ).

214 W

I

~2

I

I

(

I

600

I

i

'

"C\,, /

\ \

a.

l

'

]

,

,

700

I

800

T("C)

Fig. 7. Phase relations of disequilibrium melting of Torlesse greywacke, Taupo Volcanic Zone up to 3 kbar water pressure and 800°C (Kifle, unpublished data). Experimental conditions are given in the Appendix. Ksp--K-feldspar; qz = quartz; PI = albite (becomesmore An-rich by reaction with the melt); Mt = AI-Ti magnetite; Sp---hercynitic spinel; Bt=biotite (BtF=Fe/Mg-l.64-1.34; BtM=Fe/ Mg-0.81-0.59); Opx=hypersthene (A1203in Opx= ~5 wt.%; OpxF= Fe/Mg- 1.19- 1.05; OpXM=Fe/ Mg-0.85-0.76); Cd=cordierite (Cdr=Fe/Mg-0.88; CdM= Fe/Mg- 0.35 ). Starting greywackematerial is dorainated by phengitic mica, chlorite, albite, quartz, and Kfeldspar. Accessories are (in decreasing order of abundance) pumpellyite, stilpnomelane, epidote, sphene, zircon, apatite, graphite, ilmenite. T-P

depth conditions of melting

Changes in glass compositions with increasing temperature at I kbar PH20 and preliminary phase relations determined from disequilibrium melting experiments on Torlesse greywacke and argillite from the Taupo Volcanic Zone are plotted in Figs. 6 and 7. At temperatures just above the solidus at lkbar, thin films of melt occur along the edges of quartz, albite and K-feldspar grains. The glass films are too narrow for quantitative electron microprobe analysis but presumably have a composition close to that of a minimum S-type granite melt (Fig. 6). With increasing temperature, up to

725°C the plots in Fig. 6 imply that the glass compositions trend away from the bulk rock composition, i.e. they are enriched in silica. However, over the temperature range 725800°C, A1203, FeO, MgO, (and CaO) increase systematically in the glasses at the expense of S i Q with alkali content remaining almost constant, and compositions trend towards their respective bulk rock compositions as expected. Although compositions of the buchitic greywacke and argillite xenoliths are different from those used in the experiments the glasses from the xenoliths plot well down the melt trends of the experimental glasses and reflect the advanced stage (high temperatures) reached during fusion, i.e. at temperatures above 800°C. The phase relations of melted greywacke given in Fig. 7 indicate that for the assemblage, orthopyroxene-cordierite-spinel-magnetite in the absence of biotite, a minimum temperature of 775 °C at pressure below 1.5 kbar are required. The low pressure of pyrometamorphism implies that the depth of fusion of the Torlesse xenoliths beneath M o u n t Ngauruhoe was probably less than 5 km. Recently, Graham and Hackett (1987) have shown that the buchitic xenolith-bearing 1954 andesite erupted from Ngauruhoe has a relatively high STSr/S6Sr ratio (0.70551) and a low silica content of 56 wt.%. According to petrogenetic modelling these authors concluded that this andesite could have been derived by 30% fractional crystallization of a basalt together with 6% assimilation of melt (average glass composition) of the buchitic xenoliths described here. The depth constraint inferred for the fusion of the xenoliths implies that interaction of greywacke wall rock with basalt, or as blocks engulfed in basalt, must have occurred in the upper part of a shallow crustal magma reservoir or conduit beneath Mount Ngauruhoe.

Acknowledgments We would like to thank Dr. Bill Hackett (Idaho State University) for his early interest

215

in this work and Dr. John Gamble (Research School of Earth Sciences, Victoria University), Professor Jim Cole (Geology Department, University of Canterbury) and Dr. Chris Adams (Institute of Nuclear Science, D.S.I.R. ) for discussion and helpful comments on an initial draft of the manuscript.

Appendix Experiments were carried out by K.K. on powdered samples of Torlesse greywacke and argillite from the Taupo Volcanic Zone using externally heated, cold seal pressure vessels and water as the pressure medium. The experiments were unbuffered but oxidation conditions in the Rene 41 pressure vessels used, is considered to be that of the NNO buffer. The powdered samples ( < 15 micron grain size) were loaded into silver capsules together with about 5% HzO. Run times varied from 2-6 weeks. Temperatures were controlled throughout the experiments by Type K chrome-alumel thermocouples accurate to + 5 ° C. Water pressures were continually monitored by Burdenberg gauges and varied by no more than 0.05 kbar of the desired run pressure during the experiment. Quenching at the end of each experiment took place under pressure to below 150 ° C in less than 10 minutes. After preliminary XRD and optical identification, analyses of glass and fine-grained crystalline phases were made using a JXA-733 electron probe microanalyser with analysing points positioned using back scattered electron imaging. Higher oxidation conditions of the experiments in comparison with those inferred for the buchitic xenoliths resulted in the formation of AI-Ti magnetite rather than ilmenite. Relic phases such as apatite, quartz, zircon, were easily distinguished by their rounded outlines and relatively large grain size. Newly formed cordierite, orthopyroxene, biotite, spinels are fine grained (usually 10 microns diameter or less), have euhedral shapes, especially when in contact with glass, and are unzoned. The mineral-

in and mineral-out boundaries shown in Fig. 6 were not reversed and must be considered in the light of disequilibrium melting and metastable crystallization.

References Battey, M.H., 1949. The recent eruption of Ngauruhoe. Rec. Auckland N.Z. Inst., 3 (6): 387-395. Bentor, Y.K., Kastner, M., Perlman, I and Yellin, Y., 1981. Combustion metamorphism of bituminous sediments and the formation of melt granitic and sedimentary composition. Geochim. Cosmochim. Acta, 45: 22292255. Brearley, A.J., 1986. An electron optic study of muscovite breakdown in pelitic xenoliths during pyromethamorphism. Min. Mag., 50: 385-397. Butterman, W.C. and Foster, W.R., 1967. Zircon stability and the Zr02-SiO2 phase diagram. Am. Mineral., 52: 880885. Graham, I.J., 1983a. The Rb-Sr system. Manual 1: Chemical procedures. N.Z. Dept. Sci. Indust. Res., Pub. INSM-76. Graham, I.J., 1983b. The Rb-Sr system. Manual 2: Mass Spectrometry. N.Z. Dept. Sci. Indust. Res., Pub. INSM-76. Graham, I.J., 1985a. Petrochemical and Sr-isotope studies of lavas and xenoliths from Tongariro Volcanic Center Implications for crustal contamination of calc-alkaline magmas. Ph.D. Thesis, Victoria University of Wellington, 344 pp. Graham, I.J., 1985b. Rb-Sr geochronology and geochemistry of Torlesse metasediments from the central North Island, New Zealand. Chem. Geol. (Isot. Geosci. Sect.) 52: 317-331. Graham, I.J. and Hackett, W.R., 1987. Petrology of calcalkaline lavas from Ruapehu Volcano and related vents, Taupo Volcanic Zone, New Zealand. J. Petrol.,28: 531568. Grange, L.I. and Williamson, J.H., 1930. Tongariro Subdivision N.Z. Geol. Surv., 24th Ann. Rep., 1929-1930: 10-13. Grapes, R.H., 1986. Melting and thermal reconstitution of pelitic xenoliths, Wehr Volcano, East Eifel, Germany. J. Petrol., 27: 343-396. McOnie, A.W., Fawcett, J.J. and James, R.S., 1975. The stability of intermediate chlorites of the clinochloredaphnite solid solution series at 2 kbar water pressure. Am. Mineral., 60: 1047-1062. Rubie, D.C. and Brearley, A.J., 1987. Metastable melting during the breakdown of muscovite + quartz at 1 kbar. Soc. Fr. Mineral Crystallogr. Bull. Mineral., 110: 533549. Schreyer, W. and Schairer, J.F., 1969. Compositions and structural state of anhydrous Mg-cordierite: a re-inves-

216 tigation of the central part of the system MgO-A1203Si02. J. Petrol., 2: 324-406. Smith, D.G.W., 1969. Pyrometamorphism of phyllite by a dolerite plug. J. Petrol., 10: 20-55. Speight, R., 1908. Geology of the Tongariro National Park. Dept. Rep. Parliament Paper C-11: 7-13. Steiner, A., 1958. Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths. N.Z.J. Geol. Geophys., 1: 325-363. Stormer, J.C. Jr., 1983. The effect of recalculation on estimates of temperature and oxygen fugacity from analyses of multicomponent iron-titanium oxides. Am. Mineral., 68: 586-594.

Watanabe, T., Grapes, R.H. and Palmer, K., 1981. Quantitative analyses of rock-forming minerals by JXA-733 electron microprobe X-ray microanalyser. JEOL News, 19E: 15-19. Watson, E.B., 1979. Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry. Contrib. Mineral. Petrol., 70: 407-419. White, A.J.R. and Chappeli, B.W., 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43: 7-22. Worden, R.H., Champness, P.E. and Droop, G.T.R., 1987. Transmission electron microscopy of the pyrometamorphic breakdown of phengite and chlorite. Min. Mag., 51: 107-121.