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Petrochemistry and feldspar crystallisation in the silicic volcanic rocks, Central North Island, New Zealand
PETROCHEMISTRY AND FELDSPAR CRYSTALLISATION IN THE SILICIC VOLCANIC ROCKS, CENTRAL NORTH ISLAND, NEW ZEALAND A. E W A R T
EWART, A. 1969: Petrochemlstry and feldspar crystallisation in the silicic volcanic rocks, central North Island, New Zealand. Lithos 2, 371-388. Chemical analyses of various andesitic, dacitie, and especially rhyolitic volcanic rocks are presented, including groundmass and phenocrystie feldspar components separated from a number of these rocks. The rhyolites are characterised by the early separation of phenocrystic plagioclase (An2~-46), a second potash-rich feldspar(Or65-6s) occurring only rarely. It is shown that in terms of the A b - O r - A n system, the rhyolitic total rock compositions all project into the plagioclase field, and that only a few of the groundmass compositions have reached the feldspar cotectic surface, thus explaining the low frequency of occurrence of sanidine. Analyses of 5 groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the projection (within the ternary feldspar system) of the curve defining the intersection of the feldspar cotectic and quartz surfaces. These analyses plot close to the position of this curve as delineated by Carmichael (1963). An apparent correlation is also noted between the compositions (in terms of the normative feldspar components) of coexisting plagioclasegroundmass pairs and the nature of their coexisting phenocrystic ferromagnesian assemblage i.e. hypersthene; hypersthene + hornblende; or biotite + hornblende 4- hypersthene.
Introduction Pleistocene and Recent calc-alkali rhyolitic lavas, pumice deposits and especiaUy ignimbrites, occupy an estimated 4,000 cubic miles in the Central North Island, New Zealand. Associated with these rhyolitic extrusives are smaller volumes of andesites, dacites, and calc-alkali basalts. The volcanic rocks described in this paper are taken from two major calc-alkali provinces, namely the Coramandel Peninsula (mainly Pleistocene) and the Taupo Volcanic Zone (Pleistocene to Recent). Accounts of the volcanic geology of these areas are given in Grindley (1960), Healy (1962, 1964), and Healy, Schofield & Thompson (1964). The rhyolitic magmas have generally been interpreted to be the products of widespread crustal fusion, and on the basis of the Sr isotopic data (Ewart & Stipp 1968), it has been suggested that the Triassic-Jurassic eugeosynclinal greywacke-argillite sediments which outcrop along the western margins of these cale-alkali provinces constitute a possible crustal magma source.
372
a. EWART
Table 1. Chemical analyses of total rocks and coexisting groundmasses of volcanic rocks from
R = t o t a l rock. G = g r o u n d m a s s . n . d . = n o t determined Numbers prefixed by letter P belong to the New Zealand Geological Survey. Numbers with no prefix belong to the Department of Geology, University of Queensland Rhyolitic Lavas
SiO2 AI2Oa Fe2Oa FeO MgO CaO Na20 K20 TiOz PzO~ MnO Ho_O(+) HzO(--)
P.29203 R
P.28361 R
P.29115
72.6 14.3 0.9 1.6 0.8 2.4 4.2 2.7 0.34 0.06 0.06 0.51 0.08
76.8 12.9 0.4 0.9 0.16 1.3 4.4 3.8 0.33 0.03 0.03 0.10 0.10
72.9 13.2 0.7 1.10 0.41 1.7 4.1 3.2 0.29 0.06 0.09 1.88 0.34
100.55
101.25
99.97
R
P.29210 R
P.27854 R
P.29200 R
74.1 11.8 0.49 0.70 0.05 0.95 3,95 4.10 0.03 0.13 0.05 | ~ 3.00
73.4 13.4 0.8 1.1 0.3 1.7 4.0 2.8 0.26 0.07 0.03 1.88 0.31
75.0 12.3 1.1 0.5 0.05 1.2 3.9 3.4 0.23 0.03 1.00 0.41
71.2 14.6 1.9 0.7 0.2 1.9 4.3 3.00 0.46 0.06 0.06 0.65 0.65
99.35
100.05
99.12
99.68
G
Rhyolitic Lavas(continued) P.27574 R SiO~ A120a FezOa FeO MgO CaO NazO K~O TiO2 P2Os MnO H20(+) HzO(--)
P.27576 G
R
P.29170 G
R
P.27573 R
G
75.2 13.3 0.70 0.80 0.20 1.55 5.10 2.95 0.24 0.02 0.04 0.14 0.06
76.3 12.8 0.30 0.80 0.05 1.00 4.30 4.00 0.19 0.01 0.07 0.26 0.06
74.6 13.3 0.5 0.9 0.28 1.45 5.05 3.00 0.27 0.03 0.07 0.32 0.04
75.4 12.5 0.45 0.75 0.15 0.95 4.45 3.25 0.17 0.01 0.08 0.86 0.14
77.2 12.2 0.7 0.3 0.10 1.1 4.0 3.6 0.17 0.02 0.02 0.31 0.22
73.0 13.55 0.95 0.80 0.32 1.3 3.80 3.55 0.35 0.10 0.01 1.78 0.42
74.1 12.9 0.45 0.75 0.14 0.9 3.55 4.10 0.15 0.03 0.03 2.10 0.64
100.30
100.14
99.81
99.16
99.94
99.93
99.84
In this paper, the major element chemistry of the rhyolitic rocks is presented, with emphasis on the relationship between chemistry and feldspar crystallisation. This relationship has been approached by analysing not only the rhyolitic total rock and feldspar phenocryst compositions, but also the groundmass compositions, which are, with one exception (No. 22843, Table 1), taken to represent the original residual liquid compositions (see
373
FELDSPAR CRYSTALLISATION the Taupo Volcanic Zone
Rhyolitic Lavas(continued) P.29171 R
P.29194 R
R
P.28360 G
P.27580
73.4 13.9 1.3 0.8 0.38 1.9 4.0 3.0 0.49 0.06 0.02 0.34 0.33
75.9 12.8 1.1 0.7 0.1 1.1 4.5 3.3 0.26 0.06 0.05 1.21 0.11
77.0 12.6 0.2 1.0 0.2 1.3 4.2 3.2 0.27 0.08 0.06 0.21 0.07
76.9 12.6 0.3 0.6 0.1 0.5 4.4 4.0 0.30 0.02 0.08 0.20 0.15
99.92
101.26
100.39
100.15
R
P.27575 G
R
G
74.9 13.0 0.41 1.00 0.32 1.6 4.2 2.8 0.08 0.03 0.06 [ 1.50 |
74.1 12.6 0.35 0.65 0.13 0.9 3.95 3.20 0.16 0.03 0.04 2.28 0.62
74.7 13.4 0.55 1.05 0.32 1.65 4.80 3.00 0.27 0.01 0.06 0.12 0.04
76.5 12.4 0.35 0.80 0.18 1.15 4.45 3.55 0.10 0.04 0.04 0.30 0.02
99.90
99.01
99.97
99.88
Rhyolitic Ignimbrites P.27577
P.27579 Manunui
P.31386 Matahina R
P.31391 Matahina R
R
G
R
G
70.9 14.3 1.8 1.0 0.65 2.35 3.65 3.40 0.35 0.07 0.01 0.44
73.0 14.15 1.80 0.35 1.15 3.15 3.25 0.19 0.04 0.04 2.14 0.26
72.3 12.9 1.05 0.7 0.25 1.20 3.30 4.10 0.15 0.02 0.03 2.66 0.38
72.9 11.65 1.05 0.45 0.8 3.25 4.55 0.25 0.05 0.02 1.80 1.44
76.40 12.55 1.1 0.25 0.09 1.33 4.24 3.38 0.13 0.07 0.02 ~0.73 I
98.92
99.52
99.04
98.21
100.29
P.34906 Paeroa R
G
75.86 12.21 0.55 0.55 0.13 1.45 4.13 3.23 0.15 0.06 0.02 ~ 1.57 I
75.5 13.3 1.03 0.43 0.18 0.95 4.0 4.0 0.21 0.01 0.04 0.46 -
76.5 12.7 0.69 0.16 0.05 0.36 3.50 5.20 0.10 0.01 0.03 0.55 -
99.91
100.11
99.85
also Carmichael 1963). Thus, not only can crystal fractionation trends be studied, but also limits on certain critical boundary surfaces can be established. In a number of the plagioclase concentrates, quartz was not entirely removed, and the feldspar compositions were recalculated assuming no alkalis or calcium in the quartz. In some of these concentrates, quartz could have 25 - -
Lithos
2:4
374
SiO2 Al2Oa Fe203 FeO MgO CaO Na.~O K20 TiO2 P205 1XlnO H20(+) H20(--)
A. EWART Rhyolitic Ignimbrites (cont.)
Rhyolitic Pumice Deposits
22843 T e Weta
P.28362
] Dacitic Eruptives
P.29177
P.25708 Waiteariki Ignimbrite R G
G
R
G
R
G
73.4 11.6 0.63 0.13 0.19 1.24 1.51 5.65 0.09 0.01 0.02 5.25 -
74.7 12.4 0.4 0.8 0.2 1.25 4.3 2.95 0.20 0.10 0.04 2.05 0.45
74.8 12.1 0.2 0.8 <0.1 0.8 4.3 3.4 0.24 0.01 0.07 2.80 0.20
67.4 15.3 3.6 1.1 1.34 3.4 4.6 2.4 0.71 0.20 0.10 0.40 0.35
72.3 14.6 1.6 0.6 0.3 2.1 4.6 2.8 0.49 0.07 0.07 0.50 0.10
65.4 16.8 1.3 2.4 1.7 4.1 4.1 2.3 0.52 0.17 0.10 1.04 0.40
72.4 13.25 0.65 0.70 1.15 3.20 3.75 0.18 0.06 0.04 3.24 0.32
99.72
99.84
99.72
100.90
100.13
100.33
98.94
Analysts: J.A. Ritchle
I Chemistry Division, iilrs. I~I.G. Rundle I N.Z. Dept. Scientific P. Curtis and htdustrial Research A. fforgenson (nos. P. 34906, 22843), Australian 2~IineralDevelopment Laboratories.
been removed by centrifuging, but only by also removing much plagioclase with overlapping specific gravity, thus resulting in a severe bias to the determined bulk composition of the feldspar. In Table 2, refractive indices and partial chemical analyses of the feldspars are presented. The indices were measured as ~. min and g max, in order to obtain an estimation of the compositional range resulting from the complex oscillatory zoning displayed by the plagioclase phenocrysts. In addition, an optium 0~range was obtained, this being the measured range in which about 70% of the plagioelase grains were estimated to fall. It can be seen from Table 2 that there is generally a very good agreement between the bulk compositions determined from the partial analyses, and the An-content estimated from the optimum 0~indices. D i s c u s s i o n o f c h e m i c a l data
Rhyolitic compositions in relation to the experimental quartz-feldspar systems. The total rock and groundmass analyses are presented in Table 1, and the normative Q, Ab, and Or components of the rhyolitic coexisting total rockgroundmass pairs are plotted in Fig. 1. These data are compared to the rhyolitic total rock compositional fields in the inset of Fig. 1. In addition, the various determinations of the minimum melting compositions in the
375
FELDSPAR CRYSTALLISATION
Andesitic Eruptives
Dacitic Eruptives
P.28364
R 67.7 n.d. n.d. n.d. n.d. 2.5 3.9 2.2 n.d. n.d. n.d. n.d. n.d.
P.28363
G 70.0 13.0 0.4 1.7 0.5 2.0 4.2 2.4 0.38 0.06 ~ 4.80 I
P.29166
P.17171
P.29114
R
G
R
G
R
G
R
G
67.4 n.d. n.d. n.d. n.d. 2.4 4.5 2.2 n.d. n.d. n.d. n.d. n.d.
68.5 13.4 0.8 1.6 0.8 1.9 4.8 2.5 0.56 .0.10 0.09 5.00 0.20
61.2 16.9 2.4 4.0 4.46 6.9 2.9 1.7 0.17 0.02 0.02 0.31 0.22
71.9 12.5 1.3 1.5 0.88 3.32 3.6 2.3 0.39 0.14 0.02 n.d. n.d.
59.2 17.2 3.1 3.3 5.26 6.5 3.3 0.9 0.52 0.09 0.05 n.d. n.d.
68.8 16.7 0.9 1.3 1.56 4.0 4.3 1.5 0.16 0.07 0.02 n.d. n.d.
55.9 16.9 2.1 6.3 5.2 8.4 2.6 1.0 0.76 0.10 0.15 0.06
57.2 17.1 1.9 6.9 3.3 7.6 3.3 1.5 0.91 0.19 0.09 -
100.25
101.20
97.73
99.42
99.31
99.47
99.99
99.44
Q
Q
\ 221643
D
Rhyolitic Iovas
Ab
~
v %
Ab/
~
~
v • •
-
•
-
x
--
~
,
Y
-
Y
,
Or
\Or
TOlOl Rock. G r o u n d m o s s e s o f s c , n i d i n e - f r e e rhyolitic Iovos a n d i ~ n i m ~ i t e s . Groundmasses
©f s t m i d i n e - 1 0 e o r i n g
rhyolitic ignimbrltes. Pumice residuQI glosses ( o i l 5onidine-free). Triassic - Jurassic
® -- eu~eosynclinal (3reywocke-orgillite s e d i m e n t s .
Fig. 1. Normative quartz, orthoclase, and albite components (weight %) of the rhyolitic coexisting total rock-groundmass pairs from Table 1, and the eugeosynclinal sediments from Fig. 6. plotted in the quartz-feldspar system. Each coexisting pair are joined by tielines. The inset shows the compositional fields (based on all available analyses which show no evidence of post-eruptive leaching) of the Taupo rhyolitic lavas and ignimbrites. T h e hollow circles represent the minimum melting compositions, at 2000 bars pressure, for obsidian containing 4, 6, 8, and 15% normative anorthlte respectively (v. Platen 1965). T h e cross marked C indicates the appropriate minimum in the Q - A b - O r - H 2 0 system (Turtle & Bowen 1958), while the line C - D represents Barth's (1966) extrapolation of the minimum melting composition with increasing anorthite molecule.
376
A. E~VART
Table 2. Partial chemical analyses, refractive indices, and calculated compositions of feld spar phenocrysts from volcanic rocks of the T a u p o Volcanic Zone Pg=plagioclase phenocryst S = s a n i d i n e phenocryst [Note: Specimens arranged in order of increasing An-content within each subdivision] Sample No.
CaO
NazO
KzO
1. Rhyolitic Eruptives P.34906Pg (<:200 mesh) P.34906Pg ( > 200 mesh) P.349065 P.29115Pg (light fraction)
3.10 1.96 0.22 4.28
4.90 2.70 3.90 6.23
0.49 0.25 11.10 0.53
P.29115Pg (hea~3, fraction)
5.54
6.88
0.47
22843Pg 228435 P.28360Pg
3.00 0.34 5.06
4.00 3.25 5.54
0.37 10.30 0.30
P.27579Pg
7.04
7.20
0.57
P.28362Pg P.27573Pg
6.20 7.38
6.20 7.23
0.30 0.41
P.27580Pg
6.45
6.33
0.32
P.27854Pg P.27853Pg
7.50 7.51
7.20 7.13
0.50 0.30
P.27576Pg
7.68
7.07
0.28
P.27574Pg
8.12
6.83
0.25
P.27575Pg
8.45
6.68
0.27
P.28366Pg
8.41
6.52
0.29
P.27832Pg
9.18
6.22
0.27
P.27577Pg II. Dacitic Eruptives P.25708Pg
9.35
6.10
0.35
9.00
6.10
0.32
P.28363Pg
9.88
5.75
0.27
P.29177Pg
10.85
5.25
0.35
III. Andesitic Eruptives P.17171Pg
13.50
3.50
0.20
P.29114Pg
15.00
2.90
0.14
P.29166Pg
16.31
2.15
0.11
IV. Basalt P.29205Pg (microphenocrysts)
14.40
3.30
0.23
y max. and cc min.
Range of An content O p t i m u m from cc indices+ range
n.d. n.d. n.d. 1.5511.538 1.5551.540 n.d. n.d. 1.5591.541 1.5551.544 n.d. 1.5601.544 1.5571.543 n.d. 1.5601.546 1.5631.544 1.5611.546 1.5631.546 1.5641.547 1.5601.547 n.d.
An30An20 An3z-An23 Ana4An2s An37-An~o An4~An30 Anao-Anz9 An4GAn3a An51An3o AnasAn34 An51An3a Ansz-AnzG An46An36 -
n.d. n.d. n.d. 1.543
1.5611.547 1.5641.546 1.5681.553
An4sAn36 Ans3An3a Ans0Ana7
1.5511.552 1.5531.554 1.5571.559
1.5721.554 1.5761.560 1.5821.551
AnsTAn49 AnTaAnG1 AnssAnsz
1.5621.563 1.5631.565 1.5671.570
1.5781.561
AnTsAns,,
1.5661.567
1.5431.544 n.d. n.d. 1.5461.548 1.546 n.d. 1.5461.548 1.5451.546 n.d. 1.547 1.5471.549 1.548 1.549 1.550 1.550 n.d.
FELDSPAR CRYSTALLISATION
377
Refractive indices 4-0.002; *Quartz not separated from plagioclase; + U s i n g curves of Chayes (1952). Analysts: J.A. Ritchie, Mrs. M.G. Rundle, W. Kitt (Chemistry Division N.Z.D.S.I.R.), & A. Ewart: A. Jorgensen (Nos. 34906, 22843), Australian Mineral Development Laboratories. Optimum An content from indices+
An+Ab+ Or
Recalculated to 100 % An Ab Or
An
Ab
Or
15.4 9.7 1.1 21.2
41.5 22.8 33.0 52.7
2.9 1.5 65.6 3.1
59.8* 34.0* 99.7 77.0 #
25.8 28.6 1.1 27.6
69.4 67.1 33.1 68.4
4.8 4.4 65.8 4.1
27.5
58.2
2.8
88.5*
31.1
65.8
3.1
14.9 1.7 25.1
33.9 27.5 46.9
2.2 60.8 1.8
51.0" 90.0* 73.8*
29.2 1.9 34.0
66.5 30.6 . 63.6
4.3 67.6 2.4
34.9
60.9
3.4
99.2
35.2
61.4
3.4
30.8 36.6
52.4 61.1
1.8 2.5
85.0* 100.2
36.2 36.5
61.7 61.0
2.1 2.5
32.0
53.5
1.9
87.4*
36.6
61.2
2.2
AnsG
37.2 37.2
60.9 60.3
3.0 1.8
101.1 99.3
36.8 37.5
60.3 60.7
2.9 1.8
Ans~
38.1
59.8
1.7
99.6
38.3
60.1
1.7
An3s
40.3
57.8
1.5
99.6
40.5
58.1
1.5
An-to
41.9
56.5
1.6
100.0
41.9
56.5
1.6
An42
41.7
55.2
1.7
98.6
42.3
55.9
1.8
An42
45.5
52.6
1.6
99.7
45.7
52.7
1.6
-
46.4
51.6
2.1
100.1
46.4
51.6
2.1
An44An4~ An4~An~9 AnnaAnss
44.6
51.6
1.9
98.1
45.5
52.6
1.9
49.0
48.7
1.6
99.3
49.4
49.0
1.6
53.9
44.4
2.1
100.4
53.7
44.3
2.1
AnG:3An66 AnGGAn~o An~5Ans2
67.0
29.6
1.1
97.7
68.5
30.3
1.1
74.4
24.5
0.8
99.7
74.6
24.6
0.8
80.9
18.2
0.7
99.8
81.1
18.2
0.7
71.4
27.9
1.3
100.6
71.0
27.7
1.3
-
-
An2s An2sAnso AnaaAnss Ans~ -
Aria4Anas AnszAns4 -
An4o
AreaAnTs
378
A. E'~VART
quartz-feldspar system, at 2000 bars water pressure, for differing anorthite contents are plotted (Tuttle & Bowen 1958, v. Platen 1965, Barth 1966). Before attempting to interpret the analytical data in terms of the quartzfeldspar and ternary feldspar systems the following factors must be noted and considered: (a) All the rhyolitic extrusives contain phenocrystie plagioelase, but a second, potash-rich phenocrystic feldspar is very rare (Table 3). Furthermore, statistical studies of the variations of modal plagioclase and quartz in many of the rhyolites indicate that with few exceptions, plagioclase has begun to crystallise before quartz (Ewart 1968). It is, therefore, clearly evident that the total rock compositions of the Taupo rhyolitic magmas must lie within the primary plagioclase field. (b) As previously noted, the rhyolitic magmas are interpreted to represent the products of crustal fusion. If this hypothesis is correct, the bulk chemistry of the rhyolitic magmas was determined at relatively high pressures, whereas the subsequent crystallisation paths (represented, for example, by the groundmass compositions) were determined at low pressures. Thus, the compositional trends defined by coexisting total rock-groundmass pairs may not necessarily coincide with the trends shown by the whole rock compositional fields. (c) T h e possibility is always present that modification of the rhyolitic chemistry by post-eruptive processes has occurred. T h e most likely are: (i)Alkalileaching (especially selective sodium loss) and oxidation of iron during hydration of natural volcanic glasses (Lipman 1965, Noble 1967). (ii) Changes due to devitrification of natural glasses (e.g. Noble, Smith & Peck 1967), especially oxidation of iron. This results in higher normative quartz, although the maximum effect is estimated to be less than 1% normative quartz for the compositions in question. (iii) Secondary deposition, especially silicification, which is most likely to effect the ignimbritic deposits. T h e effects of the first process are believed to be relatively minor in the rocks studied (see discussion in Ewart 1966, 1967b) with the notable exception of groundmass No. 22843 (Table 1) which seems to have undergone preferential sodium lOSS. (d) In applying the normative data to the experimental quartz-feldspar systems, it is assumed that the magmas were saturated with respect to water prior to their eruption. This assumption is, however, in some doubt (e.g. Carrniehael 1967). Burnham (1967) has concluded that the majority ofanatectic felsic magmas intruded to shallow crustal levels were undersaturated with water for the prevailing load pressure at their presumed source. Nevertheless, he clearly shows that these same magmas could well become saturated during subsequent intrusion to shallow depths. Thus, if the volcanic magmas in question were erupted from shallow magma chambers, as seems likely from field data (Healy 1962, 1964), saturation with respect to water was very possibly attained. No independent evidence is available on this point.
379
FELDSPAR CRYSTALLISATION
Table 3. l]Iodal data for the Rhyolitic volcanic rocks Average of the Rhyolitic lavas I. Modal Per Cent Groundmass Quartz Plagioclase Sanidine Hypersthene Hornblende Biotite Magnetite + ilmenite No. of Samples
Average of the Rhyolitic Ignimbrites
87.6 2.4 8.7 0.4 0.3 0.3 0.3 246
82.5 3.3 12.6 0.1 0.8 0.2 0.1 0.4 123
II. Percentage of specimens in which phenocrysts occur Quartz 80.9 Sanidine Hypersthene 90.2 Hornblende 68.7 Biotite 28.9
94.3 6.5 96.7 68.3 28.5
Plagioclase and magnetite/ilmenite occur in all samples examined.
(e) T h e occurrence of modal biotite, which is not included in the norm calculation. Reference to Table 3, however, indicates that this is not likely to give rise to serious errors in the Taupo rhyolites, in view of the absence of biotite in over 70% of the rocks examined, and its very low modal abundance in those rocks in which it does occur. Modal biotite is absent from all the analysed groundmass and residual glass samples. A qualitative comparison of the total rock and groundmass compositions, in terms of the quartz-feldspar systems, can now be made. First, it is noted that the rhyolitic total rock compositional fields (inset of Fig. 1) exhibit a marked concentration in the area of the minimum melting compositions containing up to 6% anorthite. T h e normative anorthite content of the Taupo rhyolites ranges from 3 to 12.5%, averaging 7.4% (Ewart 1966), there being no consistent difference between the various eruptive types. In spite of coincidence of the compositional fields with the minima, these fields also extend appreciably towards the Q side of the minima; this may be indicative that crystallisation of plagioclase occurred at pressures considerably less than 2000 bars in many of the rhyolites. When compared to their coexisting total rock compositions, the groundmasses are all displaced towards the Q-Or sideline. T h e normative anorthite of the groundmasses range from 1.7-5.8%. Of particular significance are the five groundmass compositions indicated as sanidine-bearing (Fig. 1), as these coexist with phenocrystic plagioclase, quartz, and sanidine and should theoretically define the projected position of the minima appropriateuto their chemistry and crystallisation conditions prior to eruption. T h e latter factor will depend in part on the history of upward movement of the magmas, which will continuously modify the conditions and saturation requirements
380
a. ~WaRT
Q
Q
/,
\
Ab
v
v
.....
~
v
Groundmoss
compositions of obsidian with increasing normative onorlhite from 4to 15 *~* [v •P l a t e n 1965] " c o m p o s i t i o n s - C o r m i c h a e l (1963).
• -
Groundmnss
compositions of Toupotwo-feldspar rhyolites.
•
Sonidine eompositions-Carmichoel
Or
0 - Minimum melting •
-
x -
$ a n i d i n e compositions of T a u p o
(1963).
two-feldspar rhyolites.
Fig. 2. The normative quartz, orthoclase, and albite components (weight per cent) of the
rhyolitic groundmass compositions that coexist with phenoct3,stic quartz, plagloclase, and sanidine, based on data from Carmichael (1963) and this paper. The groundmass compositions are joined by tie-lines to the coexisting sanidine compositions. The hollow circles represent the minimum melting compositions, at 2000 bars pressure, for obsidian containing 4, 6, 8, and 15% anorthite respectively (v. Platen 1965). The cross at C indicates the minimum in the Ab-Or-Q-HzO system (Tuttle & Bowen 1958). The line C-D is Barth's (1966) extrapolation of minimum melting composition with increasing anorthite molecule. of the magmas. It is, therefore, significant that these five groundmass compositions plot close together; their normative anorthite contents range from 1.7 to 5.0%, and thus they project slightly closer to the Q - O r sideline than indicated by v. Platen's (1965) data. In Fig. 2, the five Taupo sanidine-bearing groundmasses are compared to those groundmass compositions analysed by Carmichael (1963) that coexist with phenocrystic quartz, plagioclase, and sanidine. It is tempting to ascribe the systematically lower Ab contents of the Taupo groundmasses to the generally higher normative anorthite contents of Taupo rhyolitic compositions, compared to those rhyolites studied by Carmichael (1963). Finally, attention is drawn to the rhyolitic pumice residual glasses which show a tendency (Fig. 1) to project closer to the Ab corner when compared to the rhyolitic lava and ignimbritic groundmass compositions. This can possibly be interpreted as indicative that crystallisation of the magmas giving rise to the pumice deposits occurred under higher water pressures compared to the magmas giving rise to lavas and ignimbrites, as would be anticipated from their modes of eruption. This is based on the shift of the minima in t h e A b - O r - Q - H z O system towards the Ab corner with increasing pressure (Tuttle & Bowen 1958, Luth, J a h n s & Tuttle 1964).
FELDSPAR CRYSTALLISATION
381
Chemistry in relation to feldspar erystallisation It has already been noted that the volcanic rocks of these calc-alkali provinces, from rhyolitic to basaltic, are characterised by phenocrystic plagioclase. Phenocrystie sanidine has a low frequency of occurrence and has so far been positively identified in only four rhyolitic ignimbritic units (Table 3; Martin 1961); analytical data are available from these units (Table 1, and Ewart 1965). In Fig. 3, the total rock-groundmass pairs from Table 1, plus previously published residual glass and groundmass compositions (Ewart 1963, 1965) are plotted in the system Ab-Or-An. From Fig. 3, the following points emerge: (a) It is clear that the total rock compositions all lie within the plagioclase field, which is broadly separated from the orthoclase field in the Ab-Or-An system by Kleeman's (1965) low temperature trough, also shown on Fig. 3. (b) The groundmasses all show a fractionation trend towards this low temperature trough. In fact, the total rock-groundmass trends show a marked similarity with the overall total rock variational trends shown in the inset in Fig. 3. These trends lie between the Thingmuli and San Juan trends delineated by Carmichael (1963). An
)
/,:.:/
Or No 22843
e-Total Rock.
A-Groundmass. No phenocrystic sanidine. =-Groundrnass. Sanidine-beorincj. Coexisting pairs are joined by tie-lines.
Fig. 3. Normative feldspar components (weight per cent) of the coexisting total rock and groundmass compositions from Table 1 and Ewart (1965), plotted in the ternary A b - O r - A n diagram. The low temperature trough is after Kleeman (1965). The short-dash curved line indicates the position of the projection (from the SiOz apex) of the intersection of the feldspar cotectic surface with the silica surface, delineated by Carmichael (1963). The 'differentiation' and 'assimilation' trends are based on the data for the Thingmuli and San Juan Provinces respectively (Carmichael 1963). The inset is a plot of all the published analysed (total rock) volcanic rocks from the Taupo Volcanic Zone (Steiner 1958, 1963; Ewart 1965, this paper).
382
A. EXVART
(c) In Fig. 3, the trace of the intersection of the feldspar cotectic surface with the quartz surface is plotted after Carmichael (1963). The groundmasses coexisting with phenocrystic sanidine are distinguished (which as previously noted, should delineate the cotectic surface), and it is evident that, with one exception, there is a marked consistency in the positions of the sanidinebearing and sanidine-free groundmass compositions with respect to the position of this quartz-plagioclase-sanidine cotectic curve. This evidently indicates that the position of this curve estimated by Carmichael (1963), based on a study of rhyolitic groundmass compositions, is applicable to the Taupo rhyolitic eruptives. The one exception (analysis 22843, Table 1) can almost certainly be attributed to preferential sodium leaching.
Feldspar Compositions In Fig. 4 the bulk compositions of the feldspar phenocrysts (determined from partial chemical analyses, see Table 2) are plotted in the ternary Ab-Or-An system. Previous studies of the plagioclases (Ewart 1963, 1965) have drawn attention to the extremely well developed and complex normal oscillatory zoning shown by practically all plagioclase phenocrysts examined, with the result that appreciable variation of plagioclase composition occurs
An FELDSPAR PHENOCRYSTS. ~ O- Rhyolitic Iovos. " ~ e-Rhyolltic ignimbrites.
~
~ ~
+-Rhyolitic pumice deposits.
~ A - Dacitlcu -Andesitic.
A°Bosaltic. x - Feldspars separated from 9ranodiorite xenoliths occurring in the rhyolitic eruptives (Ewarte, Cole 1967).
a-Feldspar phenocrysts from Mayor Island panteHerites
{Ewort,Toylor aCapp 1968} Tie-llnes connect phases separated frcm the same rock.
Ab
Fig. 4. Ternary diagram Ab-Or-An showing a plot of the bulk compositions (weight per cent) of the feldspar phenoerysts (from Table 2; Ewart 1965; and Ewart & Cole 1967). The phenocrysts are subdivided according to rock t3"pefrom which they were separated. Coexisting feldspars are joined by tie-lines.
FELDSPAR CRYSTALLISATION
i
/ ++++\/ An "
,,~,,,,h,°,
\
+0:+.o. An
/~l
°°'Y
~ t~
An
An ~
°
o'T0101[¢ckC0m~llil[0~l.
":"-°;:2:;?:g2:1~::'27'
It Ab
383
~
............
",. "..... +
~
~
Or
Fig. $.(a)-(c). Composite terna D" A b - O r - A n diagrams showing the normative feldspar compositions (weight per cent) of coexisting phenocr3"stic plagioclase-groundmass pairs, separated from the rhyolitic and dacitic eruptives. The pairs have been subdivided according to the coexisting phenocrystic ferromagnesian assemblage. In (d), the groundmass compositions are joined by tie-lines to their respective total rock compositions. The dotted fields include all the analysed groundmass compositions, split according to the coexisting ferromagnesian assemblages. Data from Tables 1 and 2, and Ewart (1963, 1965).
in any one rock; this can be judged from Table 2, where the variation in individual samples ranges from Ant0 to An25, as indicated by the refractive index determinations. Nevertheless, the bulk compositions do show up a number of consistent features. The most notable feature is the generally relatively calcic nature of the plagioclase compositions. For example, the bulk composition of the rhyolitic plagioclases ranges from An25 to Ana6, while the three analysed andesitic plagioclases range from An68 to Ansi. Little evidence of systematic compositional variations of the plagioclases are apparent between the different rhyolitic extrusive types. The occurrence of such calcic plagioclases in rhyolitic magmas raises the obvious question of whether the plagioclase phenocrysts are in fact xenocrystic. Such an origin is, however, discounted on the basis of previously published data. For example, it has been shown that close, and statistically significant correlations exist between modal quartz and plagioclase, and between modal abundance and average diameter (Ewart 1968), while systematic trace element variations were found to exist through the complete plagioclase compositional range (Ewart & Taylor 1969). Furthermore, certain systematic variations between plagioclase and groundmass compositions will be shown to exist in the next section of this paper, again pointing against a xenocrystic origin for the phenocrysts. T h e data may not, however, necessarily rule out the possibility of small, xenocrystic, and incompletely resorbed calcic plagioclase fragments acting as nuclei for subsequent growth, forming the basic cores to the observed phenocrysts. (cf. Piwinskii 1968).
384
a. EWaRT
The compositions of the rhyolitic sanidine phenocrysts (Fig. 5) fall within the narrow compositional range Or65_68, which as seen from Fig. 2 is very close to the compositions of most of the sanidines from the two-feldspar rhyolites analysed by Carmichael (1963). The Taupo sanidines occur normally as euhedral to subhedral, frequently unzoned or poorly zoned, optically homogeneous phenocrysts. The absence or simplicity of the zoning pattern, compared to the coexisting plagioclases, is considered to be a reflection of the relatively short crystallisation history of thesanidines compared to the plagioclase phenocrysts. In Fig. 4, the feldspar crystallisation trends have been extended by including two coexisting plagioclase-sanidine pairs from granodiorite xenoliths occurring within certain rhyolitic pumice breccia deposits (Ewart & Cole 1967). These granodiorites have been interpreted as comagmatic with the acid eruptives, representing magma which has completely crystallised before reaching the surface. If the geothermometry data of Barth (1962, 1968) are applied to the compositions of the three coexisting plagioclase-sanidine pairs, lemperatures in the range 680-730°C are indicated. This temperature range could possibly be consistent with the fact that the groundmasses coexisting with these twofeldspar ( + quartz) assemblages lie at the appropriate minimum melting composition in the Ab-Or-Q-An-(HzO) system, if water saturation of the magma was attained prior to eruption.
Feldspar crystallisation ht relation to ferromagnesian crystallisation In Fig. 5, the rhyolitic and dacitie coexisting plagioclase-groundmass compositions are plotted, and subdivided according to the eoe~xisting phenoerystic ferromagnesian assemblages, these being: Hypersthene (only); hornblende+hypersthene; and biotite+hornblende +hypersthene (Table 3; Ewart 1967a). The occurrence of these assemblages was shown (in the rhyolite lavas) to be statistically correlated with the modal plagioclase/ quartz ratios and total phenocryst contents of the lavas in which the assemblages occur. For example, the biotite-bearing assemblage occurs most commonly in the lavas containing the highest total phenocryst contents and lowest plagioelase/quartz ratios, while the hypersthene (only) assemblage typically occurs in lavas with little or no quartz and low total phenocryst contents. It was suggested (Ewart 1967a) that progressively decreasing liquidus temperature, concurrent with progressive crystallisation, was one of the main factors in controlling the erystallisation of the ferromagnesian assemblages, the sequence hypersthene-+hornblende-+biotite representing progressively decreasing temperature. From Fig. 5(a to e), it is evident that the slopes ofthetie-lines connecting coexisting plagioclase-groundmass pairs show a systematic difference of slope when considered in terms of the coexisting phenocrystic ferromagnesian assemblages, the tie-lines of the pairs coexisting with hypersthene (only) being consistently steepest. This change of slope of the tie-lines is due both to
FELDSPAR CRYSTALLISATION
385
the more sodic nature of the plagioclases coexisting with the biotite assemblage, and to the variations of the groundmass compositions coexisting with the different ferromagnesian assemblages. From Figs. 3 and 5d, it can be seen that in detail, the groundmass compositions are to a large extent independent of the total rock compositions, being the result of variations in the degree of feldspar crystallisation from given starting (total rock) compositions. T h e most Or enriched 'liquids' in Fig. 5d will lie closest to the low temperature trough, and thus should have been at lower liquidus temperatures immediately prior to their eruption than the liquids whose compositions lie further from the low temperature trough. The chemical data, therefore, appear to be consistent with the previous conclusion concerning the role of temperature in controlling the ferromagnesian assemblages.
Conclusions (1) Qualitatively, the Taupo rhyolitic total rock compositions show a concentration around the minimum melting compositions in the A b - 0 r - Q - A n H20 system for up to 6% anorthite, based on v. Platen's (1965) data for 2000 bars water pressure. T h e normative anorthite content of the rhyolitic eruptives ranges from 3.0-12.5%, averaging 7.4%. It may thus be considered possibly anomalous that the rhyolitic compositional fields do not show some indication of a pronounced 'tail' towards the Q-Or sideline as required by v. Platen's data, assuming that the hypothesis of their origin by partial melting of a more basic parent material is correct. An
•
A/~//
Compositionol field of
I
TRIASSIC-JURASSIC ~,~
7/5 '/~ ~/
eugeosynclinal greywlickes /
/~"
An
"o
/
3/
~/
-"~"/" .+'' r~'" +"
,+,t/,
I,~
.-Argillile ~ x-,..,reywackes
/" t~ Compositional fi 'l \ +.+~j+../"doci,ic eruplives. i#
\~\ '
"~
A
Fig. 6. T h e normative feldspar components (weight per cent) of Triassic-Jurassic eugeosynclinal greywacke and argillite from the North Island, N.Z., plotted in the ternary A b - O r - A n system. T h e analyses were performed at the Chemistry Division N.Z.D.S.I.R., and at the Australian Mineral Development Laboratories. T h e y are compared to the general compositional fields of the Taupo rhyolitic and dacitic eruptives. Copies of the analyses are available on request addressed to the author. Other data as in Fig. 3.
386
A. EWART
(2) T h e rhyolitic magmas were characterised by the early separation of plagioclase. A second, potash-rich feldspar does occur, but only rarely. A plot of the rhyolitic (and also the more basic) total rock compositions within the Ab-Or-An system shows that their compositions clearly project into the plagioclase field. The coexisting groundmass compositions all show enrichment of the normative Or component i.e. trending towards the feldspar cotectic surface. The groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the location of the projected position, within the ternary feldspar system, of the curve defining the intersection of the feldspar cotectic surface with the quartz surface. Analyses of 5 such groundmasses are presented, and these plot close to the position of this curve as defined by Carmichael (1963). It is shown that only a few of the groundmass compositions have reached this curve, and thus the rarity of sanidine and the early separation of plagioclase are adequately explained. (3) T h e fact that the Taupo rhyolites are (or potentially are) two-feldspar bearing appears to be at variance with Carmichael's (1963) conclusion that ' . . . . . the precipitation of sanidine depends solely upon the potassium-sodium ratio of the initial liquid '. This arises from the fact that the KzO/Na20 iatios of the Taupo rhyolites are such that most project into the one feldspar field delineated by Carmichael in the AbOr-Q-An tetrahedron (Ewart 1965, Ewart & Cole 1967). T h e explanation presumably lies in the normative anorthite contents of the Taupo rhyolites. (4) It is shown that an apparent correlation exists between the coexisting plagioclase-groundmass compositions and the type of coexisting phem)crystic ferromagnesian assemblage, viz. hypersthene; hornblende+hypersthene; or biotite+hornblendeztzhypersthene. This apparent correlation is interpreted to be consistent with a previousconclusion indicating the importance of temperature in controlling the erystallisation of the ferromagnesian minerals in the Taupo rhyolites. (5) It has been tentatively suggested (Ewart & Stipp 1968) that the TriassicJurassic eugeosynclinal sediments of the central North Island could provide a possible parent source (of overall acid intermediate composition) for the rhyolitic magmas. The few available analyses of these sediments are plotted in Figs. 1 and 6. From these data, it appears that in a general way these sediment compositions are consistent with the above hypothesis. The overall excess of Naz0 over K20, characteristic of these greywacke-type sediments, could well account for the observed Naz0/Kz0 ratios in the rhyolitic eruptives. ACICNOWLEDGEMENTS. T h e author wishes to thank Professor J.F.G. Wilkinson, University of New England, and Dr. M.J. Abbott, University of Queensland, for many helpful comments on the manuscript. March 1969.
Dept. Geology and 3lhleralogy, University of Queensland, St. Lucia, Brisbane, Queensland4067, ,4ustralia.
FELDSPAR CRYSTALLISATION
387
REFERENCES
BARTIt, T.F.x,V. 1962: T h e feldspar geologic thermometers. Norsk Geol. Tidsskrift 42, 330-9. Bartzlt, T.F.W. 1966: Aspects of the crystallization of quartzo-feldspathic plutonic rocks. Tschermaks J~lineral. und Petrog. l~litteihtngen 11, 209-22. BARTH, T.F.W. 1968: Additional data for the two-feldspar geothermometer. Lithos 1, 305-6. Bur~'~HA.~I, C. WAYNE 1967: Hydrothermal fluids at the magmatic stage. In BARNES, H.L. (Editor) Geochemistry of Hydrothermal Ore Deposits. Holt, Rinehart and Winston, Inc., pp. 34-76. CAR.XIICHAEL, I.S.E. 1963: T h e crystallization of feldspar in volcanic acid liquids. Q. flour. Geol. Soc. London 119, 95-131. CAR~MICItAEL, I.S.E. 1967: T h e iron-titanium oxides in salic volcanic rocks and their associated ferromagnesian silicates. Contr. itlineral, and Petrol. 14, 36-64. CHAYES, F. 1952 : Relations between ~:omposition and indices of refraction in natural plagioclase..,liner, flour. ScL, Bozcen vol., pp. 85-105. EWART, A. 1963: Petrology and petrogenesis of the Quaternary pumice ash in the T a u p o area, New Zealand. flour. Petrology 4, 392-431. EWART A. 1965: Mineralogy and petrogenesis of the Whakamaru Ignimbrite in the Maraetai area of the Taupo Volcanic Zone, New Zealand, N.Z. _7l. Geol. Geophys. 8, 611-77. EWART A. 1966: Review of mineralogy and chemistry of the acidic volcanic rocks of T a u p o Volcanic Zone, New Zealand. Bull. Irolcanol. 29, 147-72. EWART A. 1967a: T h e petrography of the central N o r t h Island rhyolitic lavas. Part 1. N.Z. Jl. Geol. Geophys. 10, 182-97. EWART A. 1967b: Discussion. Water pressures during differentiation and crystallisation of some ash-flow magmas from Southern Nevada. Amer. flour. Sci. 265, 898-904. EW,XRT A. 1968: T h e petrography of the central N o r t h Island rhyolitic lavas. Part 2. 1V.Z. ffl. Geol. Geophys. 11,478-545. EWART A. & COLE, J.W. 1967: Textural and mineralogical significance of the granitic xenoliths from the central volcanic region, North Island, New Zealand. N.Z..7l. GeoL Geophys. 10, 31-54. EWART, A. & STIPP, J.J. 1968: Petrogenesis of the volcanic rocks of the central N o r t h Island, New Zealand, as indicated by a study of SrST/Sr s6 ratios, and Sr, Rb, K, U, and T h abundances. Geochim. et Cosmochim. .4eta, 32, 699-735. EWaRT, A., TAVLOa, S.R. & CAPP, A.C. 1968: Geochemistry of the pantellerites of Mayor Island, New Zealand. Contr. J~lineral. and Petrol. 17, 116--40. EWART, A. & TAYLOa, S.R. 1969: Trace and minor element geochemistry of the rhyolitic volcanic rocks, central North Island, New Zealand. Phenocryst data. Contr. z~Ihteral. and Petrol. 22, 127-46. GmNDLEY, G.W. 1960: Sheet 8 - Taupo : Geological l~fap of New Zealand 1:250,000. Dep. Sci. Industr. Res., Wellington. HEALV, J. 1962: Structure and volcanism in the T a u p o Volcanic Zone, New Zealand. ,4mer. Geophys. Un. Geophys. l~Ionogr. 6, 151-7. HEALY, J. 1964: Volcanic mechanisms in the Taupo Volcanic Zone, New Zealand. N.Z..71. Geol. Geophys. 7, 6-23. HEALY, J., SCHOFIELO, J.C. & THOMI'SON, B.N. 1964: Sheet 5 - Rotorua : Geological map of New Zealand, 1:250,000. Dep. Sci. Industr. Res., Wellington. KLEE.XIAN, A.W. 1965: T h e origin of granitic magmas, flour. Geol. Soc. Australia 12, 35-52. LIPM.~'% P.W. 1965: Chemical comparison of glassy and crystalline volcanic rocks. U.S. Geol. Surv. Bull. 1201-D, 24 pp. LUTH, W.C., JAHXS, R.H. & TUTTLE, O.F. 1964: T h e granite system at pressures of 4 to 10 kilobars. Journ. Geophys. Research 69, 759-73. MARXIN, R.C. 1961 : Stratigraphy and structural outline of the T a u p o Volcanic Zone. N . Z . Jl. Geol. Geophys. 4, 449-78. NOBLE, D.C. 1967: Sodium, potassium, and ferrous iron contents of some secondarily hydrated natural silicic glasses. Am. l)lhteral. 52, 280-6. NOBLE, D.C., S.XtlTII, V.C. & PECK, L.C. 1967: Loss of halogens from crystallized and glassy silicic volcanic rocks. Geochim. et Cosmochim. Acta 31, 215-23. PlWINSKH, A.J. 1968: Studies of batholithic feldspars: Sierra Nevada, California. Contr. 3lineral. and Petrol. 17, 204-23.
388
A. EWART
Ptcx'rr_r,r, H. yon 1965: Kristallisation granitischer Schmelzen. Beitr. ~Iineral. Petrogr. 11, 334-81. STrINER, A. 1958: Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths. N.Z..7l. Geol. Geophys. 2, 325-63. STERNER,A. 1963: Crystallization behaviour and origin of the acidic ignimbrite and rhyolite magma in the North Island of New Zealand. Bull. Volcanol. 25, 217-41. TUa"rLE, O.F. & Bo'~-,r, N.L. 1958: Origin of granite in light of experimental studies. Geol. Soc. Amer. 2~lem. 7d, 153 pp. Revised manuscript accepted June 1969
Printed October 1969
EWART, A. 1969: Petrochemlstry and feldspar crystallisation in the silicic volcanic rocks, central North Island, New Zealand. Lithos 2, 371-388. Chemical analyses of various andesitic, dacitie, and especially rhyolitic volcanic rocks are presented, including groundmass and phenocrystie feldspar components separated from a number of these rocks. The rhyolites are characterised by the early separation of phenocrystic plagioclase (An2~-46), a second potash-rich feldspar(Or65-6s) occurring only rarely. It is shown that in terms of the A b - O r - A n system, the rhyolitic total rock compositions all project into the plagioclase field, and that only a few of the groundmass compositions have reached the feldspar cotectic surface, thus explaining the low frequency of occurrence of sanidine. Analyses of 5 groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the projection (within the ternary feldspar system) of the curve defining the intersection of the feldspar cotectic and quartz surfaces. These analyses plot close to the position of this curve as delineated by Carmichael (1963). An apparent correlation is also noted between the compositions (in terms of the normative feldspar components) of coexisting plagioclasegroundmass pairs and the nature of their coexisting phenocrystic ferromagnesian assemblage i.e. hypersthene; hypersthene + hornblende; or biotite + hornblende 4- hypersthene.
Introduction Pleistocene and Recent calc-alkali rhyolitic lavas, pumice deposits and especiaUy ignimbrites, occupy an estimated 4,000 cubic miles in the Central North Island, New Zealand. Associated with these rhyolitic extrusives are smaller volumes of andesites, dacites, and calc-alkali basalts. The volcanic rocks described in this paper are taken from two major calc-alkali provinces, namely the Coramandel Peninsula (mainly Pleistocene) and the Taupo Volcanic Zone (Pleistocene to Recent). Accounts of the volcanic geology of these areas are given in Grindley (1960), Healy (1962, 1964), and Healy, Schofield & Thompson (1964). The rhyolitic magmas have generally been interpreted to be the products of widespread crustal fusion, and on the basis of the Sr isotopic data (Ewart & Stipp 1968), it has been suggested that the Triassic-Jurassic eugeosynclinal greywacke-argillite sediments which outcrop along the western margins of these cale-alkali provinces constitute a possible crustal magma source.
372
a. EWART
Table 1. Chemical analyses of total rocks and coexisting groundmasses of volcanic rocks from
R = t o t a l rock. G = g r o u n d m a s s . n . d . = n o t determined Numbers prefixed by letter P belong to the New Zealand Geological Survey. Numbers with no prefix belong to the Department of Geology, University of Queensland Rhyolitic Lavas
SiO2 AI2Oa Fe2Oa FeO MgO CaO Na20 K20 TiOz PzO~ MnO Ho_O(+) HzO(--)
P.29203 R
P.28361 R
P.29115
72.6 14.3 0.9 1.6 0.8 2.4 4.2 2.7 0.34 0.06 0.06 0.51 0.08
76.8 12.9 0.4 0.9 0.16 1.3 4.4 3.8 0.33 0.03 0.03 0.10 0.10
72.9 13.2 0.7 1.10 0.41 1.7 4.1 3.2 0.29 0.06 0.09 1.88 0.34
100.55
101.25
99.97
R
P.29210 R
P.27854 R
P.29200 R
74.1 11.8 0.49 0.70 0.05 0.95 3,95 4.10 0.03 0.13 0.05 | ~ 3.00
73.4 13.4 0.8 1.1 0.3 1.7 4.0 2.8 0.26 0.07 0.03 1.88 0.31
75.0 12.3 1.1 0.5 0.05 1.2 3.9 3.4 0.23 0.03 1.00 0.41
71.2 14.6 1.9 0.7 0.2 1.9 4.3 3.00 0.46 0.06 0.06 0.65 0.65
99.35
100.05
99.12
99.68
G
Rhyolitic Lavas(continued) P.27574 R SiO~ A120a FezOa FeO MgO CaO NazO K~O TiO2 P2Os MnO H20(+) HzO(--)
P.27576 G
R
P.29170 G
R
P.27573 R
G
75.2 13.3 0.70 0.80 0.20 1.55 5.10 2.95 0.24 0.02 0.04 0.14 0.06
76.3 12.8 0.30 0.80 0.05 1.00 4.30 4.00 0.19 0.01 0.07 0.26 0.06
74.6 13.3 0.5 0.9 0.28 1.45 5.05 3.00 0.27 0.03 0.07 0.32 0.04
75.4 12.5 0.45 0.75 0.15 0.95 4.45 3.25 0.17 0.01 0.08 0.86 0.14
77.2 12.2 0.7 0.3 0.10 1.1 4.0 3.6 0.17 0.02 0.02 0.31 0.22
73.0 13.55 0.95 0.80 0.32 1.3 3.80 3.55 0.35 0.10 0.01 1.78 0.42
74.1 12.9 0.45 0.75 0.14 0.9 3.55 4.10 0.15 0.03 0.03 2.10 0.64
100.30
100.14
99.81
99.16
99.94
99.93
99.84
In this paper, the major element chemistry of the rhyolitic rocks is presented, with emphasis on the relationship between chemistry and feldspar crystallisation. This relationship has been approached by analysing not only the rhyolitic total rock and feldspar phenocryst compositions, but also the groundmass compositions, which are, with one exception (No. 22843, Table 1), taken to represent the original residual liquid compositions (see
373
FELDSPAR CRYSTALLISATION the Taupo Volcanic Zone
Rhyolitic Lavas(continued) P.29171 R
P.29194 R
R
P.28360 G
P.27580
73.4 13.9 1.3 0.8 0.38 1.9 4.0 3.0 0.49 0.06 0.02 0.34 0.33
75.9 12.8 1.1 0.7 0.1 1.1 4.5 3.3 0.26 0.06 0.05 1.21 0.11
77.0 12.6 0.2 1.0 0.2 1.3 4.2 3.2 0.27 0.08 0.06 0.21 0.07
76.9 12.6 0.3 0.6 0.1 0.5 4.4 4.0 0.30 0.02 0.08 0.20 0.15
99.92
101.26
100.39
100.15
R
P.27575 G
R
G
74.9 13.0 0.41 1.00 0.32 1.6 4.2 2.8 0.08 0.03 0.06 [ 1.50 |
74.1 12.6 0.35 0.65 0.13 0.9 3.95 3.20 0.16 0.03 0.04 2.28 0.62
74.7 13.4 0.55 1.05 0.32 1.65 4.80 3.00 0.27 0.01 0.06 0.12 0.04
76.5 12.4 0.35 0.80 0.18 1.15 4.45 3.55 0.10 0.04 0.04 0.30 0.02
99.90
99.01
99.97
99.88
Rhyolitic Ignimbrites P.27577
P.27579 Manunui
P.31386 Matahina R
P.31391 Matahina R
R
G
R
G
70.9 14.3 1.8 1.0 0.65 2.35 3.65 3.40 0.35 0.07 0.01 0.44
73.0 14.15 1.80 0.35 1.15 3.15 3.25 0.19 0.04 0.04 2.14 0.26
72.3 12.9 1.05 0.7 0.25 1.20 3.30 4.10 0.15 0.02 0.03 2.66 0.38
72.9 11.65 1.05 0.45 0.8 3.25 4.55 0.25 0.05 0.02 1.80 1.44
76.40 12.55 1.1 0.25 0.09 1.33 4.24 3.38 0.13 0.07 0.02 ~0.73 I
98.92
99.52
99.04
98.21
100.29
P.34906 Paeroa R
G
75.86 12.21 0.55 0.55 0.13 1.45 4.13 3.23 0.15 0.06 0.02 ~ 1.57 I
75.5 13.3 1.03 0.43 0.18 0.95 4.0 4.0 0.21 0.01 0.04 0.46 -
76.5 12.7 0.69 0.16 0.05 0.36 3.50 5.20 0.10 0.01 0.03 0.55 -
99.91
100.11
99.85
also Carmichael 1963). Thus, not only can crystal fractionation trends be studied, but also limits on certain critical boundary surfaces can be established. In a number of the plagioclase concentrates, quartz was not entirely removed, and the feldspar compositions were recalculated assuming no alkalis or calcium in the quartz. In some of these concentrates, quartz could have 25 - -
Lithos
2:4
374
SiO2 Al2Oa Fe203 FeO MgO CaO Na.~O K20 TiO2 P205 1XlnO H20(+) H20(--)
A. EWART Rhyolitic Ignimbrites (cont.)
Rhyolitic Pumice Deposits
22843 T e Weta
P.28362
] Dacitic Eruptives
P.29177
P.25708 Waiteariki Ignimbrite R G
G
R
G
R
G
73.4 11.6 0.63 0.13 0.19 1.24 1.51 5.65 0.09 0.01 0.02 5.25 -
74.7 12.4 0.4 0.8 0.2 1.25 4.3 2.95 0.20 0.10 0.04 2.05 0.45
74.8 12.1 0.2 0.8 <0.1 0.8 4.3 3.4 0.24 0.01 0.07 2.80 0.20
67.4 15.3 3.6 1.1 1.34 3.4 4.6 2.4 0.71 0.20 0.10 0.40 0.35
72.3 14.6 1.6 0.6 0.3 2.1 4.6 2.8 0.49 0.07 0.07 0.50 0.10
65.4 16.8 1.3 2.4 1.7 4.1 4.1 2.3 0.52 0.17 0.10 1.04 0.40
72.4 13.25 0.65 0.70 1.15 3.20 3.75 0.18 0.06 0.04 3.24 0.32
99.72
99.84
99.72
100.90
100.13
100.33
98.94
Analysts: J.A. Ritchle
I Chemistry Division, iilrs. I~I.G. Rundle I N.Z. Dept. Scientific P. Curtis and htdustrial Research A. fforgenson (nos. P. 34906, 22843), Australian 2~IineralDevelopment Laboratories.
been removed by centrifuging, but only by also removing much plagioclase with overlapping specific gravity, thus resulting in a severe bias to the determined bulk composition of the feldspar. In Table 2, refractive indices and partial chemical analyses of the feldspars are presented. The indices were measured as ~. min and g max, in order to obtain an estimation of the compositional range resulting from the complex oscillatory zoning displayed by the plagioclase phenocrysts. In addition, an optium 0~range was obtained, this being the measured range in which about 70% of the plagioelase grains were estimated to fall. It can be seen from Table 2 that there is generally a very good agreement between the bulk compositions determined from the partial analyses, and the An-content estimated from the optimum 0~indices. D i s c u s s i o n o f c h e m i c a l data
Rhyolitic compositions in relation to the experimental quartz-feldspar systems. The total rock and groundmass analyses are presented in Table 1, and the normative Q, Ab, and Or components of the rhyolitic coexisting total rockgroundmass pairs are plotted in Fig. 1. These data are compared to the rhyolitic total rock compositional fields in the inset of Fig. 1. In addition, the various determinations of the minimum melting compositions in the
375
FELDSPAR CRYSTALLISATION
Andesitic Eruptives
Dacitic Eruptives
P.28364
R 67.7 n.d. n.d. n.d. n.d. 2.5 3.9 2.2 n.d. n.d. n.d. n.d. n.d.
P.28363
G 70.0 13.0 0.4 1.7 0.5 2.0 4.2 2.4 0.38 0.06 ~ 4.80 I
P.29166
P.17171
P.29114
R
G
R
G
R
G
R
G
67.4 n.d. n.d. n.d. n.d. 2.4 4.5 2.2 n.d. n.d. n.d. n.d. n.d.
68.5 13.4 0.8 1.6 0.8 1.9 4.8 2.5 0.56 .0.10 0.09 5.00 0.20
61.2 16.9 2.4 4.0 4.46 6.9 2.9 1.7 0.17 0.02 0.02 0.31 0.22
71.9 12.5 1.3 1.5 0.88 3.32 3.6 2.3 0.39 0.14 0.02 n.d. n.d.
59.2 17.2 3.1 3.3 5.26 6.5 3.3 0.9 0.52 0.09 0.05 n.d. n.d.
68.8 16.7 0.9 1.3 1.56 4.0 4.3 1.5 0.16 0.07 0.02 n.d. n.d.
55.9 16.9 2.1 6.3 5.2 8.4 2.6 1.0 0.76 0.10 0.15 0.06
57.2 17.1 1.9 6.9 3.3 7.6 3.3 1.5 0.91 0.19 0.09 -
100.25
101.20
97.73
99.42
99.31
99.47
99.99
99.44
Q
Q
\ 221643
D
Rhyolitic Iovas
Ab
~
v %
Ab/
~
~
v • •
-
•
-
x
--
~
,
Y
-
Y
,
Or
\Or
TOlOl Rock. G r o u n d m o s s e s o f s c , n i d i n e - f r e e rhyolitic Iovos a n d i ~ n i m ~ i t e s . Groundmasses
©f s t m i d i n e - 1 0 e o r i n g
rhyolitic ignimbrltes. Pumice residuQI glosses ( o i l 5onidine-free). Triassic - Jurassic
® -- eu~eosynclinal (3reywocke-orgillite s e d i m e n t s .
Fig. 1. Normative quartz, orthoclase, and albite components (weight %) of the rhyolitic coexisting total rock-groundmass pairs from Table 1, and the eugeosynclinal sediments from Fig. 6. plotted in the quartz-feldspar system. Each coexisting pair are joined by tielines. The inset shows the compositional fields (based on all available analyses which show no evidence of post-eruptive leaching) of the Taupo rhyolitic lavas and ignimbrites. T h e hollow circles represent the minimum melting compositions, at 2000 bars pressure, for obsidian containing 4, 6, 8, and 15% normative anorthlte respectively (v. Platen 1965). T h e cross marked C indicates the appropriate minimum in the Q - A b - O r - H 2 0 system (Turtle & Bowen 1958), while the line C - D represents Barth's (1966) extrapolation of the minimum melting composition with increasing anorthite molecule.
376
A. E~VART
Table 2. Partial chemical analyses, refractive indices, and calculated compositions of feld spar phenocrysts from volcanic rocks of the T a u p o Volcanic Zone Pg=plagioclase phenocryst S = s a n i d i n e phenocryst [Note: Specimens arranged in order of increasing An-content within each subdivision] Sample No.
CaO
NazO
KzO
1. Rhyolitic Eruptives P.34906Pg (<:200 mesh) P.34906Pg ( > 200 mesh) P.349065 P.29115Pg (light fraction)
3.10 1.96 0.22 4.28
4.90 2.70 3.90 6.23
0.49 0.25 11.10 0.53
P.29115Pg (hea~3, fraction)
5.54
6.88
0.47
22843Pg 228435 P.28360Pg
3.00 0.34 5.06
4.00 3.25 5.54
0.37 10.30 0.30
P.27579Pg
7.04
7.20
0.57
P.28362Pg P.27573Pg
6.20 7.38
6.20 7.23
0.30 0.41
P.27580Pg
6.45
6.33
0.32
P.27854Pg P.27853Pg
7.50 7.51
7.20 7.13
0.50 0.30
P.27576Pg
7.68
7.07
0.28
P.27574Pg
8.12
6.83
0.25
P.27575Pg
8.45
6.68
0.27
P.28366Pg
8.41
6.52
0.29
P.27832Pg
9.18
6.22
0.27
P.27577Pg II. Dacitic Eruptives P.25708Pg
9.35
6.10
0.35
9.00
6.10
0.32
P.28363Pg
9.88
5.75
0.27
P.29177Pg
10.85
5.25
0.35
III. Andesitic Eruptives P.17171Pg
13.50
3.50
0.20
P.29114Pg
15.00
2.90
0.14
P.29166Pg
16.31
2.15
0.11
IV. Basalt P.29205Pg (microphenocrysts)
14.40
3.30
0.23
y max. and cc min.
Range of An content O p t i m u m from cc indices+ range
n.d. n.d. n.d. 1.5511.538 1.5551.540 n.d. n.d. 1.5591.541 1.5551.544 n.d. 1.5601.544 1.5571.543 n.d. 1.5601.546 1.5631.544 1.5611.546 1.5631.546 1.5641.547 1.5601.547 n.d.
An30An20 An3z-An23 Ana4An2s An37-An~o An4~An30 Anao-Anz9 An4GAn3a An51An3o AnasAn34 An51An3a Ansz-AnzG An46An36 -
n.d. n.d. n.d. 1.543
1.5611.547 1.5641.546 1.5681.553
An4sAn36 Ans3An3a Ans0Ana7
1.5511.552 1.5531.554 1.5571.559
1.5721.554 1.5761.560 1.5821.551
AnsTAn49 AnTaAnG1 AnssAnsz
1.5621.563 1.5631.565 1.5671.570
1.5781.561
AnTsAns,,
1.5661.567
1.5431.544 n.d. n.d. 1.5461.548 1.546 n.d. 1.5461.548 1.5451.546 n.d. 1.547 1.5471.549 1.548 1.549 1.550 1.550 n.d.
FELDSPAR CRYSTALLISATION
377
Refractive indices 4-0.002; *Quartz not separated from plagioclase; + U s i n g curves of Chayes (1952). Analysts: J.A. Ritchie, Mrs. M.G. Rundle, W. Kitt (Chemistry Division N.Z.D.S.I.R.), & A. Ewart: A. Jorgensen (Nos. 34906, 22843), Australian Mineral Development Laboratories. Optimum An content from indices+
An+Ab+ Or
Recalculated to 100 % An Ab Or
An
Ab
Or
15.4 9.7 1.1 21.2
41.5 22.8 33.0 52.7
2.9 1.5 65.6 3.1
59.8* 34.0* 99.7 77.0 #
25.8 28.6 1.1 27.6
69.4 67.1 33.1 68.4
4.8 4.4 65.8 4.1
27.5
58.2
2.8
88.5*
31.1
65.8
3.1
14.9 1.7 25.1
33.9 27.5 46.9
2.2 60.8 1.8
51.0" 90.0* 73.8*
29.2 1.9 34.0
66.5 30.6 . 63.6
4.3 67.6 2.4
34.9
60.9
3.4
99.2
35.2
61.4
3.4
30.8 36.6
52.4 61.1
1.8 2.5
85.0* 100.2
36.2 36.5
61.7 61.0
2.1 2.5
32.0
53.5
1.9
87.4*
36.6
61.2
2.2
AnsG
37.2 37.2
60.9 60.3
3.0 1.8
101.1 99.3
36.8 37.5
60.3 60.7
2.9 1.8
Ans~
38.1
59.8
1.7
99.6
38.3
60.1
1.7
An3s
40.3
57.8
1.5
99.6
40.5
58.1
1.5
An-to
41.9
56.5
1.6
100.0
41.9
56.5
1.6
An42
41.7
55.2
1.7
98.6
42.3
55.9
1.8
An42
45.5
52.6
1.6
99.7
45.7
52.7
1.6
-
46.4
51.6
2.1
100.1
46.4
51.6
2.1
An44An4~ An4~An~9 AnnaAnss
44.6
51.6
1.9
98.1
45.5
52.6
1.9
49.0
48.7
1.6
99.3
49.4
49.0
1.6
53.9
44.4
2.1
100.4
53.7
44.3
2.1
AnG:3An66 AnGGAn~o An~5Ans2
67.0
29.6
1.1
97.7
68.5
30.3
1.1
74.4
24.5
0.8
99.7
74.6
24.6
0.8
80.9
18.2
0.7
99.8
81.1
18.2
0.7
71.4
27.9
1.3
100.6
71.0
27.7
1.3
-
-
An2s An2sAnso AnaaAnss Ans~ -
Aria4Anas AnszAns4 -
An4o
AreaAnTs
378
A. E'~VART
quartz-feldspar system, at 2000 bars water pressure, for differing anorthite contents are plotted (Tuttle & Bowen 1958, v. Platen 1965, Barth 1966). Before attempting to interpret the analytical data in terms of the quartzfeldspar and ternary feldspar systems the following factors must be noted and considered: (a) All the rhyolitic extrusives contain phenocrystie plagioelase, but a second, potash-rich phenocrystic feldspar is very rare (Table 3). Furthermore, statistical studies of the variations of modal plagioclase and quartz in many of the rhyolites indicate that with few exceptions, plagioclase has begun to crystallise before quartz (Ewart 1968). It is, therefore, clearly evident that the total rock compositions of the Taupo rhyolitic magmas must lie within the primary plagioclase field. (b) As previously noted, the rhyolitic magmas are interpreted to represent the products of crustal fusion. If this hypothesis is correct, the bulk chemistry of the rhyolitic magmas was determined at relatively high pressures, whereas the subsequent crystallisation paths (represented, for example, by the groundmass compositions) were determined at low pressures. Thus, the compositional trends defined by coexisting total rock-groundmass pairs may not necessarily coincide with the trends shown by the whole rock compositional fields. (c) T h e possibility is always present that modification of the rhyolitic chemistry by post-eruptive processes has occurred. T h e most likely are: (i)Alkalileaching (especially selective sodium loss) and oxidation of iron during hydration of natural volcanic glasses (Lipman 1965, Noble 1967). (ii) Changes due to devitrification of natural glasses (e.g. Noble, Smith & Peck 1967), especially oxidation of iron. This results in higher normative quartz, although the maximum effect is estimated to be less than 1% normative quartz for the compositions in question. (iii) Secondary deposition, especially silicification, which is most likely to effect the ignimbritic deposits. T h e effects of the first process are believed to be relatively minor in the rocks studied (see discussion in Ewart 1966, 1967b) with the notable exception of groundmass No. 22843 (Table 1) which seems to have undergone preferential sodium lOSS. (d) In applying the normative data to the experimental quartz-feldspar systems, it is assumed that the magmas were saturated with respect to water prior to their eruption. This assumption is, however, in some doubt (e.g. Carrniehael 1967). Burnham (1967) has concluded that the majority ofanatectic felsic magmas intruded to shallow crustal levels were undersaturated with water for the prevailing load pressure at their presumed source. Nevertheless, he clearly shows that these same magmas could well become saturated during subsequent intrusion to shallow depths. Thus, if the volcanic magmas in question were erupted from shallow magma chambers, as seems likely from field data (Healy 1962, 1964), saturation with respect to water was very possibly attained. No independent evidence is available on this point.
379
FELDSPAR CRYSTALLISATION
Table 3. l]Iodal data for the Rhyolitic volcanic rocks Average of the Rhyolitic lavas I. Modal Per Cent Groundmass Quartz Plagioclase Sanidine Hypersthene Hornblende Biotite Magnetite + ilmenite No. of Samples
Average of the Rhyolitic Ignimbrites
87.6 2.4 8.7 0.4 0.3 0.3 0.3 246
82.5 3.3 12.6 0.1 0.8 0.2 0.1 0.4 123
II. Percentage of specimens in which phenocrysts occur Quartz 80.9 Sanidine Hypersthene 90.2 Hornblende 68.7 Biotite 28.9
94.3 6.5 96.7 68.3 28.5
Plagioclase and magnetite/ilmenite occur in all samples examined.
(e) T h e occurrence of modal biotite, which is not included in the norm calculation. Reference to Table 3, however, indicates that this is not likely to give rise to serious errors in the Taupo rhyolites, in view of the absence of biotite in over 70% of the rocks examined, and its very low modal abundance in those rocks in which it does occur. Modal biotite is absent from all the analysed groundmass and residual glass samples. A qualitative comparison of the total rock and groundmass compositions, in terms of the quartz-feldspar systems, can now be made. First, it is noted that the rhyolitic total rock compositional fields (inset of Fig. 1) exhibit a marked concentration in the area of the minimum melting compositions containing up to 6% anorthite. T h e normative anorthite content of the Taupo rhyolites ranges from 3 to 12.5%, averaging 7.4% (Ewart 1966), there being no consistent difference between the various eruptive types. In spite of coincidence of the compositional fields with the minima, these fields also extend appreciably towards the Q side of the minima; this may be indicative that crystallisation of plagioclase occurred at pressures considerably less than 2000 bars in many of the rhyolites. When compared to their coexisting total rock compositions, the groundmasses are all displaced towards the Q-Or sideline. T h e normative anorthite of the groundmasses range from 1.7-5.8%. Of particular significance are the five groundmass compositions indicated as sanidine-bearing (Fig. 1), as these coexist with phenocrystic plagioclase, quartz, and sanidine and should theoretically define the projected position of the minima appropriateuto their chemistry and crystallisation conditions prior to eruption. T h e latter factor will depend in part on the history of upward movement of the magmas, which will continuously modify the conditions and saturation requirements
380
a. ~WaRT
Q
Q
/,
\
Ab
v
v
.....
~
v
Groundmoss
compositions of obsidian with increasing normative onorlhite from 4to 15 *~* [v •P l a t e n 1965] " c o m p o s i t i o n s - C o r m i c h a e l (1963).
• -
Groundmnss
compositions of Toupotwo-feldspar rhyolites.
•
Sonidine eompositions-Carmichoel
Or
0 - Minimum melting •
-
x -
$ a n i d i n e compositions of T a u p o
(1963).
two-feldspar rhyolites.
Fig. 2. The normative quartz, orthoclase, and albite components (weight per cent) of the
rhyolitic groundmass compositions that coexist with phenoct3,stic quartz, plagloclase, and sanidine, based on data from Carmichael (1963) and this paper. The groundmass compositions are joined by tie-lines to the coexisting sanidine compositions. The hollow circles represent the minimum melting compositions, at 2000 bars pressure, for obsidian containing 4, 6, 8, and 15% anorthite respectively (v. Platen 1965). The cross at C indicates the minimum in the Ab-Or-Q-HzO system (Tuttle & Bowen 1958). The line C-D is Barth's (1966) extrapolation of minimum melting composition with increasing anorthite molecule. of the magmas. It is, therefore, significant that these five groundmass compositions plot close together; their normative anorthite contents range from 1.7 to 5.0%, and thus they project slightly closer to the Q - O r sideline than indicated by v. Platen's (1965) data. In Fig. 2, the five Taupo sanidine-bearing groundmasses are compared to those groundmass compositions analysed by Carmichael (1963) that coexist with phenocrystic quartz, plagioclase, and sanidine. It is tempting to ascribe the systematically lower Ab contents of the Taupo groundmasses to the generally higher normative anorthite contents of Taupo rhyolitic compositions, compared to those rhyolites studied by Carmichael (1963). Finally, attention is drawn to the rhyolitic pumice residual glasses which show a tendency (Fig. 1) to project closer to the Ab corner when compared to the rhyolitic lava and ignimbritic groundmass compositions. This can possibly be interpreted as indicative that crystallisation of the magmas giving rise to the pumice deposits occurred under higher water pressures compared to the magmas giving rise to lavas and ignimbrites, as would be anticipated from their modes of eruption. This is based on the shift of the minima in t h e A b - O r - Q - H z O system towards the Ab corner with increasing pressure (Tuttle & Bowen 1958, Luth, J a h n s & Tuttle 1964).
FELDSPAR CRYSTALLISATION
381
Chemistry in relation to feldspar erystallisation It has already been noted that the volcanic rocks of these calc-alkali provinces, from rhyolitic to basaltic, are characterised by phenocrystic plagioclase. Phenocrystie sanidine has a low frequency of occurrence and has so far been positively identified in only four rhyolitic ignimbritic units (Table 3; Martin 1961); analytical data are available from these units (Table 1, and Ewart 1965). In Fig. 3, the total rock-groundmass pairs from Table 1, plus previously published residual glass and groundmass compositions (Ewart 1963, 1965) are plotted in the system Ab-Or-An. From Fig. 3, the following points emerge: (a) It is clear that the total rock compositions all lie within the plagioclase field, which is broadly separated from the orthoclase field in the Ab-Or-An system by Kleeman's (1965) low temperature trough, also shown on Fig. 3. (b) The groundmasses all show a fractionation trend towards this low temperature trough. In fact, the total rock-groundmass trends show a marked similarity with the overall total rock variational trends shown in the inset in Fig. 3. These trends lie between the Thingmuli and San Juan trends delineated by Carmichael (1963). An
)
/,:.:/
Or No 22843
e-Total Rock.
A-Groundmass. No phenocrystic sanidine. =-Groundrnass. Sanidine-beorincj. Coexisting pairs are joined by tie-lines.
Fig. 3. Normative feldspar components (weight per cent) of the coexisting total rock and groundmass compositions from Table 1 and Ewart (1965), plotted in the ternary A b - O r - A n diagram. The low temperature trough is after Kleeman (1965). The short-dash curved line indicates the position of the projection (from the SiOz apex) of the intersection of the feldspar cotectic surface with the silica surface, delineated by Carmichael (1963). The 'differentiation' and 'assimilation' trends are based on the data for the Thingmuli and San Juan Provinces respectively (Carmichael 1963). The inset is a plot of all the published analysed (total rock) volcanic rocks from the Taupo Volcanic Zone (Steiner 1958, 1963; Ewart 1965, this paper).
382
A. EXVART
(c) In Fig. 3, the trace of the intersection of the feldspar cotectic surface with the quartz surface is plotted after Carmichael (1963). The groundmasses coexisting with phenocrystic sanidine are distinguished (which as previously noted, should delineate the cotectic surface), and it is evident that, with one exception, there is a marked consistency in the positions of the sanidinebearing and sanidine-free groundmass compositions with respect to the position of this quartz-plagioclase-sanidine cotectic curve. This evidently indicates that the position of this curve estimated by Carmichael (1963), based on a study of rhyolitic groundmass compositions, is applicable to the Taupo rhyolitic eruptives. The one exception (analysis 22843, Table 1) can almost certainly be attributed to preferential sodium leaching.
Feldspar Compositions In Fig. 4 the bulk compositions of the feldspar phenocrysts (determined from partial chemical analyses, see Table 2) are plotted in the ternary Ab-Or-An system. Previous studies of the plagioclases (Ewart 1963, 1965) have drawn attention to the extremely well developed and complex normal oscillatory zoning shown by practically all plagioclase phenocrysts examined, with the result that appreciable variation of plagioclase composition occurs
An FELDSPAR PHENOCRYSTS. ~ O- Rhyolitic Iovos. " ~ e-Rhyolltic ignimbrites.
~
~ ~
+-Rhyolitic pumice deposits.
~ A - Dacitlcu -Andesitic.
A°Bosaltic. x - Feldspars separated from 9ranodiorite xenoliths occurring in the rhyolitic eruptives (Ewarte, Cole 1967).
a-Feldspar phenocrysts from Mayor Island panteHerites
{Ewort,Toylor aCapp 1968} Tie-llnes connect phases separated frcm the same rock.
Ab
Fig. 4. Ternary diagram Ab-Or-An showing a plot of the bulk compositions (weight per cent) of the feldspar phenoerysts (from Table 2; Ewart 1965; and Ewart & Cole 1967). The phenocrysts are subdivided according to rock t3"pefrom which they were separated. Coexisting feldspars are joined by tie-lines.
FELDSPAR CRYSTALLISATION
i
/ ++++\/ An "
,,~,,,,h,°,
\
+0:+.o. An
/~l
°°'Y
~ t~
An
An ~
°
o'T0101[¢ckC0m~llil[0~l.
":"-°;:2:;?:g2:1~::'27'
It Ab
383
~
............
",. "..... +
~
~
Or
Fig. $.(a)-(c). Composite terna D" A b - O r - A n diagrams showing the normative feldspar compositions (weight per cent) of coexisting phenocr3"stic plagioclase-groundmass pairs, separated from the rhyolitic and dacitic eruptives. The pairs have been subdivided according to the coexisting phenocrystic ferromagnesian assemblage. In (d), the groundmass compositions are joined by tie-lines to their respective total rock compositions. The dotted fields include all the analysed groundmass compositions, split according to the coexisting ferromagnesian assemblages. Data from Tables 1 and 2, and Ewart (1963, 1965).
in any one rock; this can be judged from Table 2, where the variation in individual samples ranges from Ant0 to An25, as indicated by the refractive index determinations. Nevertheless, the bulk compositions do show up a number of consistent features. The most notable feature is the generally relatively calcic nature of the plagioclase compositions. For example, the bulk composition of the rhyolitic plagioclases ranges from An25 to Ana6, while the three analysed andesitic plagioclases range from An68 to Ansi. Little evidence of systematic compositional variations of the plagioclases are apparent between the different rhyolitic extrusive types. The occurrence of such calcic plagioclases in rhyolitic magmas raises the obvious question of whether the plagioclase phenocrysts are in fact xenocrystic. Such an origin is, however, discounted on the basis of previously published data. For example, it has been shown that close, and statistically significant correlations exist between modal quartz and plagioclase, and between modal abundance and average diameter (Ewart 1968), while systematic trace element variations were found to exist through the complete plagioclase compositional range (Ewart & Taylor 1969). Furthermore, certain systematic variations between plagioclase and groundmass compositions will be shown to exist in the next section of this paper, again pointing against a xenocrystic origin for the phenocrysts. T h e data may not, however, necessarily rule out the possibility of small, xenocrystic, and incompletely resorbed calcic plagioclase fragments acting as nuclei for subsequent growth, forming the basic cores to the observed phenocrysts. (cf. Piwinskii 1968).
384
a. EWaRT
The compositions of the rhyolitic sanidine phenocrysts (Fig. 5) fall within the narrow compositional range Or65_68, which as seen from Fig. 2 is very close to the compositions of most of the sanidines from the two-feldspar rhyolites analysed by Carmichael (1963). The Taupo sanidines occur normally as euhedral to subhedral, frequently unzoned or poorly zoned, optically homogeneous phenocrysts. The absence or simplicity of the zoning pattern, compared to the coexisting plagioclases, is considered to be a reflection of the relatively short crystallisation history of thesanidines compared to the plagioclase phenocrysts. In Fig. 4, the feldspar crystallisation trends have been extended by including two coexisting plagioclase-sanidine pairs from granodiorite xenoliths occurring within certain rhyolitic pumice breccia deposits (Ewart & Cole 1967). These granodiorites have been interpreted as comagmatic with the acid eruptives, representing magma which has completely crystallised before reaching the surface. If the geothermometry data of Barth (1962, 1968) are applied to the compositions of the three coexisting plagioclase-sanidine pairs, lemperatures in the range 680-730°C are indicated. This temperature range could possibly be consistent with the fact that the groundmasses coexisting with these twofeldspar ( + quartz) assemblages lie at the appropriate minimum melting composition in the Ab-Or-Q-An-(HzO) system, if water saturation of the magma was attained prior to eruption.
Feldspar crystallisation ht relation to ferromagnesian crystallisation In Fig. 5, the rhyolitic and dacitie coexisting plagioclase-groundmass compositions are plotted, and subdivided according to the eoe~xisting phenoerystic ferromagnesian assemblages, these being: Hypersthene (only); hornblende+hypersthene; and biotite+hornblende +hypersthene (Table 3; Ewart 1967a). The occurrence of these assemblages was shown (in the rhyolite lavas) to be statistically correlated with the modal plagioclase/ quartz ratios and total phenocryst contents of the lavas in which the assemblages occur. For example, the biotite-bearing assemblage occurs most commonly in the lavas containing the highest total phenocryst contents and lowest plagioelase/quartz ratios, while the hypersthene (only) assemblage typically occurs in lavas with little or no quartz and low total phenocryst contents. It was suggested (Ewart 1967a) that progressively decreasing liquidus temperature, concurrent with progressive crystallisation, was one of the main factors in controlling the erystallisation of the ferromagnesian assemblages, the sequence hypersthene-+hornblende-+biotite representing progressively decreasing temperature. From Fig. 5(a to e), it is evident that the slopes ofthetie-lines connecting coexisting plagioclase-groundmass pairs show a systematic difference of slope when considered in terms of the coexisting phenocrystic ferromagnesian assemblages, the tie-lines of the pairs coexisting with hypersthene (only) being consistently steepest. This change of slope of the tie-lines is due both to
FELDSPAR CRYSTALLISATION
385
the more sodic nature of the plagioclases coexisting with the biotite assemblage, and to the variations of the groundmass compositions coexisting with the different ferromagnesian assemblages. From Figs. 3 and 5d, it can be seen that in detail, the groundmass compositions are to a large extent independent of the total rock compositions, being the result of variations in the degree of feldspar crystallisation from given starting (total rock) compositions. T h e most Or enriched 'liquids' in Fig. 5d will lie closest to the low temperature trough, and thus should have been at lower liquidus temperatures immediately prior to their eruption than the liquids whose compositions lie further from the low temperature trough. The chemical data, therefore, appear to be consistent with the previous conclusion concerning the role of temperature in controlling the ferromagnesian assemblages.
Conclusions (1) Qualitatively, the Taupo rhyolitic total rock compositions show a concentration around the minimum melting compositions in the A b - 0 r - Q - A n H20 system for up to 6% anorthite, based on v. Platen's (1965) data for 2000 bars water pressure. T h e normative anorthite content of the rhyolitic eruptives ranges from 3.0-12.5%, averaging 7.4%. It may thus be considered possibly anomalous that the rhyolitic compositional fields do not show some indication of a pronounced 'tail' towards the Q-Or sideline as required by v. Platen's data, assuming that the hypothesis of their origin by partial melting of a more basic parent material is correct. An
•
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eugeosynclinal greywlickes /
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~/
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Fig. 6. T h e normative feldspar components (weight per cent) of Triassic-Jurassic eugeosynclinal greywacke and argillite from the North Island, N.Z., plotted in the ternary A b - O r - A n system. T h e analyses were performed at the Chemistry Division N.Z.D.S.I.R., and at the Australian Mineral Development Laboratories. T h e y are compared to the general compositional fields of the Taupo rhyolitic and dacitic eruptives. Copies of the analyses are available on request addressed to the author. Other data as in Fig. 3.
386
A. EWART
(2) T h e rhyolitic magmas were characterised by the early separation of plagioclase. A second, potash-rich feldspar does occur, but only rarely. A plot of the rhyolitic (and also the more basic) total rock compositions within the Ab-Or-An system shows that their compositions clearly project into the plagioclase field. The coexisting groundmass compositions all show enrichment of the normative Or component i.e. trending towards the feldspar cotectic surface. The groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the location of the projected position, within the ternary feldspar system, of the curve defining the intersection of the feldspar cotectic surface with the quartz surface. Analyses of 5 such groundmasses are presented, and these plot close to the position of this curve as defined by Carmichael (1963). It is shown that only a few of the groundmass compositions have reached this curve, and thus the rarity of sanidine and the early separation of plagioclase are adequately explained. (3) T h e fact that the Taupo rhyolites are (or potentially are) two-feldspar bearing appears to be at variance with Carmichael's (1963) conclusion that ' . . . . . the precipitation of sanidine depends solely upon the potassium-sodium ratio of the initial liquid '. This arises from the fact that the KzO/Na20 iatios of the Taupo rhyolites are such that most project into the one feldspar field delineated by Carmichael in the AbOr-Q-An tetrahedron (Ewart 1965, Ewart & Cole 1967). T h e explanation presumably lies in the normative anorthite contents of the Taupo rhyolites. (4) It is shown that an apparent correlation exists between the coexisting plagioclase-groundmass compositions and the type of coexisting phem)crystic ferromagnesian assemblage, viz. hypersthene; hornblende+hypersthene; or biotite+hornblendeztzhypersthene. This apparent correlation is interpreted to be consistent with a previousconclusion indicating the importance of temperature in controlling the erystallisation of the ferromagnesian minerals in the Taupo rhyolites. (5) It has been tentatively suggested (Ewart & Stipp 1968) that the TriassicJurassic eugeosynclinal sediments of the central North Island could provide a possible parent source (of overall acid intermediate composition) for the rhyolitic magmas. The few available analyses of these sediments are plotted in Figs. 1 and 6. From these data, it appears that in a general way these sediment compositions are consistent with the above hypothesis. The overall excess of Naz0 over K20, characteristic of these greywacke-type sediments, could well account for the observed Naz0/Kz0 ratios in the rhyolitic eruptives. ACICNOWLEDGEMENTS. T h e author wishes to thank Professor J.F.G. Wilkinson, University of New England, and Dr. M.J. Abbott, University of Queensland, for many helpful comments on the manuscript. March 1969.
Dept. Geology and 3lhleralogy, University of Queensland, St. Lucia, Brisbane, Queensland4067, ,4ustralia.
FELDSPAR CRYSTALLISATION
387
REFERENCES
BARTIt, T.F.x,V. 1962: T h e feldspar geologic thermometers. Norsk Geol. Tidsskrift 42, 330-9. Bartzlt, T.F.W. 1966: Aspects of the crystallization of quartzo-feldspathic plutonic rocks. Tschermaks J~lineral. und Petrog. l~litteihtngen 11, 209-22. BARTH, T.F.W. 1968: Additional data for the two-feldspar geothermometer. Lithos 1, 305-6. Bur~'~HA.~I, C. WAYNE 1967: Hydrothermal fluids at the magmatic stage. In BARNES, H.L. (Editor) Geochemistry of Hydrothermal Ore Deposits. Holt, Rinehart and Winston, Inc., pp. 34-76. CAR.XIICHAEL, I.S.E. 1963: T h e crystallization of feldspar in volcanic acid liquids. Q. flour. Geol. Soc. London 119, 95-131. CAR~MICItAEL, I.S.E. 1967: T h e iron-titanium oxides in salic volcanic rocks and their associated ferromagnesian silicates. Contr. itlineral, and Petrol. 14, 36-64. CHAYES, F. 1952 : Relations between ~:omposition and indices of refraction in natural plagioclase..,liner, flour. ScL, Bozcen vol., pp. 85-105. EWART, A. 1963: Petrology and petrogenesis of the Quaternary pumice ash in the T a u p o area, New Zealand. flour. Petrology 4, 392-431. EWART A. 1965: Mineralogy and petrogenesis of the Whakamaru Ignimbrite in the Maraetai area of the Taupo Volcanic Zone, New Zealand, N.Z. _7l. Geol. Geophys. 8, 611-77. EWART A. 1966: Review of mineralogy and chemistry of the acidic volcanic rocks of T a u p o Volcanic Zone, New Zealand. Bull. Irolcanol. 29, 147-72. EWART A. 1967a: T h e petrography of the central N o r t h Island rhyolitic lavas. Part 1. N.Z. Jl. Geol. Geophys. 10, 182-97. EWART A. 1967b: Discussion. Water pressures during differentiation and crystallisation of some ash-flow magmas from Southern Nevada. Amer. flour. Sci. 265, 898-904. EW,XRT A. 1968: T h e petrography of the central N o r t h Island rhyolitic lavas. Part 2. 1V.Z. ffl. Geol. Geophys. 11,478-545. EWART A. & COLE, J.W. 1967: Textural and mineralogical significance of the granitic xenoliths from the central volcanic region, North Island, New Zealand. N.Z..7l. GeoL Geophys. 10, 31-54. EWART, A. & STIPP, J.J. 1968: Petrogenesis of the volcanic rocks of the central N o r t h Island, New Zealand, as indicated by a study of SrST/Sr s6 ratios, and Sr, Rb, K, U, and T h abundances. Geochim. et Cosmochim. .4eta, 32, 699-735. EWaRT, A., TAVLOa, S.R. & CAPP, A.C. 1968: Geochemistry of the pantellerites of Mayor Island, New Zealand. Contr. J~lineral. and Petrol. 17, 116--40. EWART, A. & TAYLOa, S.R. 1969: Trace and minor element geochemistry of the rhyolitic volcanic rocks, central North Island, New Zealand. Phenocryst data. Contr. z~Ihteral. and Petrol. 22, 127-46. GmNDLEY, G.W. 1960: Sheet 8 - Taupo : Geological l~fap of New Zealand 1:250,000. Dep. Sci. Industr. Res., Wellington. HEALV, J. 1962: Structure and volcanism in the T a u p o Volcanic Zone, New Zealand. ,4mer. Geophys. Un. Geophys. l~Ionogr. 6, 151-7. HEALY, J. 1964: Volcanic mechanisms in the Taupo Volcanic Zone, New Zealand. N.Z..71. Geol. Geophys. 7, 6-23. HEALY, J., SCHOFIELO, J.C. & THOMI'SON, B.N. 1964: Sheet 5 - Rotorua : Geological map of New Zealand, 1:250,000. Dep. Sci. Industr. Res., Wellington. KLEE.XIAN, A.W. 1965: T h e origin of granitic magmas, flour. Geol. Soc. Australia 12, 35-52. LIPM.~'% P.W. 1965: Chemical comparison of glassy and crystalline volcanic rocks. U.S. Geol. Surv. Bull. 1201-D, 24 pp. LUTH, W.C., JAHXS, R.H. & TUTTLE, O.F. 1964: T h e granite system at pressures of 4 to 10 kilobars. Journ. Geophys. Research 69, 759-73. MARXIN, R.C. 1961 : Stratigraphy and structural outline of the T a u p o Volcanic Zone. N . Z . Jl. Geol. Geophys. 4, 449-78. NOBLE, D.C. 1967: Sodium, potassium, and ferrous iron contents of some secondarily hydrated natural silicic glasses. Am. l)lhteral. 52, 280-6. NOBLE, D.C., S.XtlTII, V.C. & PECK, L.C. 1967: Loss of halogens from crystallized and glassy silicic volcanic rocks. Geochim. et Cosmochim. Acta 31, 215-23. PlWINSKH, A.J. 1968: Studies of batholithic feldspars: Sierra Nevada, California. Contr. 3lineral. and Petrol. 17, 204-23.
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Ptcx'rr_r,r, H. yon 1965: Kristallisation granitischer Schmelzen. Beitr. ~Iineral. Petrogr. 11, 334-81. STrINER, A. 1958: Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths. N.Z..7l. Geol. Geophys. 2, 325-63. STERNER,A. 1963: Crystallization behaviour and origin of the acidic ignimbrite and rhyolite magma in the North Island of New Zealand. Bull. Volcanol. 25, 217-41. TUa"rLE, O.F. & Bo'~-,r, N.L. 1958: Origin of granite in light of experimental studies. Geol. Soc. Amer. 2~lem. 7d, 153 pp. Revised manuscript accepted June 1969
Printed October 1969