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Geochemistry of actinolitic hornblendes from tonalitic rocks, Northern Portugal
&oohimicaet Cosmootimioa Acts,1974,vol. 38,pg. 789to 803.Pergamon Press.PrintedIn Northern Ireland
Geochmistry of a~~o~~ic ho~blend~ from tonalitic rocks, Northern Porgy CARLOS A. R. DE ALBUQUERQUE Department of Geology, Saint Mary’s University, Hal&x, Nova Scotia, Canada (Received 16 July 1973; accepted in revised fm
11 December
1973)
Abs~aB&Chemios,l snalyses are given for actinolitic hornblendes of tonalitic rocks from the Hercynian belt of Northern Portugal. The distribution of elements between amphibole and co-existing biotite is studied. The composition of the amphiboles is analysed in the light of experimental data on amp~boles and the physical conditions of crysta~iz&tion inferred from the study of the biotite and rock series. The data on the biotites lead to the def%tion of 5 temperature of 800°C for the crystallization of aetinolitic hornblendes with Mg/(Mg f Fe) ratios of 0.7%0.61 at pressures of about 3 Kb and f0, defined by FMQ. THE INTEGRATED studies of the geochemistry of the rooks and minerals of plutonic talc-alkali complexes have greatly increased our understanding of their magmatic evolution and of the role of intensive variables. However, the problem of the origin of some magmas, in particular magmas of tonalitie and dioritic composition, is largely unresolved. Additional information is therefore required for a more complete knowledge of magmatism in erogenic belts. It became apparent from the investigations of DEER (1935) and NOCKOLDS and M?CTCHELL(1948) that amphiboles constitute excellent petrogenetic indicators as they are very complex in composition. This concept was emphasized by ERNST (1968), and examples of its application can be found in reoent studies (HIETANEEN, 1971; LYORS,
1972).
In this paper we will be concerned with the study of the geochemistry of the amphiboles from tonalitic rocks of the Aregos region of the Hercynian belt of Central and Northern Portugal. The distribution of elements between amphibole and biotite will also be considered. The petrogenesis of the rocks from which these minerals were separated has been studied (ALBUQUERQUE, 1971) as well as the geochemistry of the biotites (ALBUQUERQUE, 1973). The amphiboles -were separated from the crushed rock powders employing & Frantz Electromagnetic Separator. These impure concentrates were then purified in methylene iodide until a concentrate of better than 99.0-99.5 per cent purity was obtained. The impurities consist mostly of small grains of biotite and of composite grains of feldspar and ilmenite. The major element analyses were carried out by wet, classical methods for all elements except sodium and potassium determined by flame photometry. The trace elements were determined by emission spectrography using the technique described in earlier papers (ALBUQUERQUE, 1971, 19733. Struct~al formulae of the analysed amphiboles were calculated by computer programme. The chemical analyses of amphibole presented here fulfil the criteria of analysis evaluation defined by LEAKXG(1968). The only possible exception is that of the actinolitic hornblende from the xenolith (Table 2, analysis 3). This analysis shows a value of the Y group (5.256) close to the upper limit which can be considered acceptable for this group. However, LEAEE (1968, p. 34) notes that this may not necessarily mean analytical error. It may also be noted that the biotite from the same specimen also shows a high value of the Y group (ALBUQTJERQUE, 1973, Table 1, analysis 3). 789
790 Table
CAFCLOS
1. Modal
composition
of
A.
DE
ALBUQUERQUE
monzonites and average granodiorites
Monzonites 10 11 Quartz Potassium feldspar Plagioclase Hornblende Biotite Accessories Colour index
R.
85 26.6 31.2 14.4 17.8 1.5 33.7
11.0 28.7 32.9 5.3 21.2 0.9 27.4
Av.
Granodiorites (4) Range
SiO, TiO, AN Fe@, Fe0 MIlO MgG CaO Na,O KS0 H,O+ HsO-
Fet/(Fe,
+ Mg)
of
ton&es
Av.
(7)
and
Tonalites Range
196-23.0 7.3-13.2 386-41.1 0.2-1.1 25.0-28.7 -
15.8 0.5 43.2 8.0 30.2 2.3
12.3-20.0 0.0-1.7 394-46.6 3.5-12.7 27.0-34.4 -
28.7
26.8-31.4
40.5
35~246.7
2. Chemical
analvses 5
and structural 6
AIV’ TiV’ Fe3+ Fe2+ Mn Mg Ca Na K H Z Y X R2+ R3+
form-
2
3
4
50.36 0.72 5.45 2.71 9.96 0.36 15.03 12.36 0.69 0.44 2.04 0.05
49.80 0.77 6.09 1.57 12.16 0.42 13.96 12.32 0.57 0.39 2.13 0.02
50.14 0.84 5.77 1.09 14.38 0.49 13.31 11.11 0.88 0.39 1.77 0.03
49.69 0.80 5.48 2.18 13.15 0.41 13.06 12.25 0.72 0.48 1.86 0.02
51.64 0.65 4.19 0.77 14.26 0.47 13.75 11.36 0.53 0.41 2.11 0.03 ---
51.06 0.56 4.56 1.00 13.93 0.51 13.63 12.18 0.48 0.35 2.04 0.01
51.30 0.59 4.30 0.94 13.96 0.49 13.52 12.09 0.43 0.37 2.12 0.01
100.17 0.52
100.20 056
100.20 0.60
100~10 0.60
100*17 0.58
100.31 0.58
100.12 0.59
Structural E1v
related
21.4 9.7 40.2 05 26.8 1.4
Table 1
modes
7.258 0.742 0.184 0.078 0.294 1.200 0.044 3.229 1.909 0.193 0.081 1.961 8.000 5.029 2.183 4.473 0.556
7.211 0.789 0.250 0.084 0.171 1.473 0.052 3.013 1.912 0.160 0.072 2.057 8.000 5.043 2.144 4.538 0.505
7.330 0.670 0.324 0.092 0.120 1.758 0.061 2.901 1.740 0.249 0.073 1.726 8.000 5.256 2.062 4.720 0.536
7.277 0.723 0.223 0.088 0.240 1.611 0.051 2.851 1.922 0.204 0.090 1.817 8.000 5.064 2.216 4.513 0.551
7.496 0.504 0.213 0.071 0.084 1.731 0.058 2.975 1.767 0.149 0.076 2.043 8.000 5.132 1.992 4.764 0.368
7.423 0.577 0.204 0.061 0.109 1.694 0.063 2.954 1.897 0.135 0.065 1.978 8.000 5.085 2.097 4.711 0.374
* Includes Rb 0.008. Analysis by C. A. R. de Albuquerque. 1, 2. Green actinolitic hornblende, tonalite (northern outcrop) (A-205, A-112). 3. Brownish green actinolitic hornblende, tonalite (xenolith) (A-143). 4. Green actinolitic hornblende, tonalite (southern outcrop) (A-52). 5. Light green actinolitic hornblende, tonalite (southern outcrop) (A-51-A).
7
formulae
on
7.459 0.541 0.196 0.065 0.103 1.697 0.060 2.930 1.883 0.121 0.069 2.056 8.000 5.051 2.073 4.687 0.364
Geochemistry
of actinolitic hornblendesfrom tom&tic rocks, Northern Portugal
791
OCCURRENCE OF AMPHIBOLE Igneous rocks of the Hercynian belt of Western Europe are representedin Central Northern Portugal by a large batholith composed mainly of c&-alkali granodiorites and granites. These rocks are typically biotite or/and muscovite-bearing. Although rocks containing hornblende occupy small areas, their occurrence has been reported from several localities (SCHERMERHORN,1956; OEN, 1958; MAIJER, 1965) where they outcrop spatially associated with the Younger granites. In the Aregos region, in particular, the most mafic rocks are hornblendebiotite tonalites and related hornblende-bearing biotite granodiorites which have been inter1971). These rocks show aEnities with the high-K preted as hybrid rocks (ALBUQUERQUE, diorites of island arcs (GULSONet al., 1972). Amphiboles from hornblende-biotite monzonites (Albuquerque, unpublished data) are also included in this study as they are similar to those of the tonalitic rocks. The monzonites occur ulae of the amnhiboles and monzonite
10
8
9
61.09 0.68 4.46 1.07 13.05 0.45 13.92 12.47 0.60 O-38 2.08 0.04
48.52 0.86 6.63 2.80 12.41 0.47 12.83 12.54 0.76 0.46 1.95 0.01
51.41 0.62 4.52 1.04 11.80 0.37 14.74 12.70 0.65 0.36 1.93 0.04
100.29 0.56
100.24 0.60
99.91 0.53
biotites
11
12
13
47.84 0.94 6.60 2.33 13.58 0.50 12.14 12.33 0.88 0.64 1.95 0.02
37.05 2.56 15.81 0.85 16.34 0.24 13.00 0.45 0.20 9.35 3.97 0.09
36.72 2.57 15.46 0.91 18.23 0.31 11.65 0.30 0.21 9.65 3.27 0.07
99.78 0.62
99.91 0.63
99.35 0.68
SiO, TiO, A&& Fe& Fe0 MIlO MgC CaO Na,O Es0 H,O+ HsOFeJ(Fet + Mg)
basis of 24 (0, OH, F) 7.410 0.590 0.172 0.074 0.117 1.583 0.055 3.010 1.938 0.169 0.070 2.012 8.000 5.011 2.177 4.648 0.363 6, 7. 8. 9. 10. 11. 12, 13.
7.097 0.903 0.240 0.095 0.308 1.518 0.058 2.797 l-965 0.216 0.086 1.902 8.000 5.116 2.267 4.473 0.643
7.419 0.581 0.192 0.068 0.114 1.432 0.045 3.188 1.974 0.183 0.067 1.867 8.000 5.039 2.224 4.665 0.374
7.078 o-922 0.228 0.105 0.259 1.679 0.063 2.676 1.954 0.252 0.121 1.923 8.000 5.010 2.327 4.418 0.592
5.557 2.443 0.352 0.289 0.096 2.050 0.030 2.907 0.072 0.058 1.789 3.971 8.000 5.724 1.927* 4.987 0.737
5.644 2.356 0.444 0.297 0.105 2.343 0.040 2.669 0.049 0.063 1.892 3.352 8.000 5.898 2.012* 5.052 0.846
$v Alvr Tip1 Few Fe2+ Mn Mg Ca Na K H Z Y X R2+ R3+
Light green actinolitic hornblende, granodiorite, (southern outcrop) (A-51, A-55). Light green actinolitic hornblende, granodiorite, (northern outcrop) (A-79). Green actinolitic hornblende, granodiorite (northern outcrop) (A-202). Brownish green actinolitic hornblende, monzonite (A-370). Green actinolitic hornblende, monzonite (A-268). Biotite, monzonite (A-370, A-268).
792
Coos
A. R.
DE
~~UQUERQ~
spatially associated with the porphyritio biotite granodiorite of the suite of Younger granites and it may be noted that no ton&k rocks are spatially associated with the porphyritic granodiorite or the monzonites. The tonal&es contain quartz, plagioclase, biotite and hornblende, the ratio biotite/hornblende being 4 (Table 1). The granodiorites are ch~acterized by the presence of potassium feldspar @a. 10 per cent) and the ratio biotitejhornblende is higher, approximately 50. Although the monzonites con&& the same minerals as the granodiorites, the modal proportions are different (Table 1) and some features am of interest: hornblende and potassium feldspar are more abundant in the monzonites while quartz, plagioclase md biotite are distinctly higher in the granodiorites. However, the values of the colour index of both monzonites and gmnodiorites are identioal. Aggregates of grains of amphibole and biotite are seen in various specimens and occasionally biotite replaces the amphibole. Rare relies of clinopyroxene occur in the amphibole in a few specimens of monzonites. The pyroxene appears as small gr&ns in the core of crystals of amphibole. The amphiboles can be divided in two groups based on their pleochroism and other optical properties: the green amphiboles with Z-green or bluish green, Y-brownish green, X-light yellow, and the light green amphiboles with pleoahroism Z-light green, Y-light greenish brown, X-pale yellow, or Z-light greenish brown, Y-pale brown, X-pale yellow. The values of 2Vx are lower in the green amphiboles for comparable values of the Fe/Mg ratios. The range of vJues is 68-72” for these amphiboles md 2Vx of the light green amphiboles is in the range 76-79”. The two groups also show slight differences in chemioal composition. The green hornblendes have slightly lower contents of Si and higher Al, Ti, l?e3+ and Na than the light green, more actinolitic amphiboles. The use of the terms ‘prim&ry’ or ‘secondary’ amphiboles is avoided hero as the aetinolitio hornblende8 are believed to have crystallized in a. reaction rel&tions~p with pyroxene and melt. &WPOSITION
OF THE AICPHIBOLES
Major elements The chemical analyses, of major elements, and the structural formulae of the amphiboles are given in Table 2. The analyses and structural formulae of the two biotites from the monzonites are also given in Table 2. Those of the biotites coexisting with the amphiboles from the tonalites and related grano~orites have been pub~sh~ (~BUQUERQU~, 1973). The chemical composition of the Aregos amphiboles is distinctive as they can be classified as actinolitic hornblendes (LEAKE, 1968) unlike most amphiboles from talc-alkali rocks which are hornblendes. Their most characteristic features are the high contents of Si and low contents of Al (Al,O, in the range 4-19-6-63 per cent). Ti, Fes+, Na and K are also low in the Aregos amphiboles while Fe2+ and Mg show some variation expressed as the Fe/Mg ratio. ~tr~ct~~~l~~rn~la~ From inspection of Figs. 1, 2 and 3, it can be seen that the composition of these amphiboles does not show any marked deviation towards the alkali amphibole group and can be expressed in terms of the calcium amphibole end members tschermakite-ferrotschermapargasite-ferrohastingsite, tremoliteferroactinolite, kite and eden&e-ferroedenite. It can also be observed that these actinolitic hornblendes show little depa~~e from the tremo~t~fe~o~ctinolite end member and the tscherm~kiti~ substitution MgSi + R3+Al appears to play the dominant role conditioning the variations in composition.
Geochemistry of actinolitic hornblendes from ton&tic rocks, Northern Portugal
793
0
0.6
0
0
0.6
Al’”
---
0'2
04
0.6
(No+K) atoms Fig. 1. Relation between AlI’ and (Na + K) atoms. Variations in the composition of calcium amphiboles are well illustrated by HALLIMOND’S (1943) triangular diagram with apexes Ca,(Mg, Fe),Si,O,,(OH),, Ca,(Mg, Fe),Al,Si,Al,O,,(OH), and the hypothetical molecule Na,Ca,(Mg, Fe),Si,Al,O,,(OH), (Fig. 4). Thr‘s re p resentation has the advantage that edenite and pargasite-ferrohastingsite plot on the sides of the triangle. As could be expected, the Aregos actinolitic hornblendes occupy two areas near the tremolite-ferroactinolite apex, the light green amphiboles nearer this apex than The actinolitic hornblendes of the Ben Nevis complex are the green amphiboles. plotted in an area near that defined for the Aregos green amphiboles, while most amphiboles from other complexes such as the Sierra Nevada batholith and Southern California batholith (DODGE et al., 1968; LARSEN and DRAISIN, 1950) have a different composition, being richer in both the tschermakite and pargasite-ferrohastingsite end members. The hornblendes of the oldest (Jurassic) rocks of the Sierra Nevada batholith are, however, more actinolitic than those of rocks of intermediate and younger ages. Whereas actinolitic hornblendes occur in a few complexes, most amphiboles of talc-alkali rocks are hornblendes containing moderate to high amounts of the pargasite-ferrohastingsite molecule and to a smaller extent, the tschermakite molecule.
794
CARLOS
A. R.
DE
ALBUQUERQUE
0
0.6 -
0
0.6 -
R”+atoms Fig. 2. Relation between Al”
and (Fe3+ + Ti t_ AlV’) atoms.
06 -
Al'" O-6 -
[R3% (No t K)] atoms Fig. 3. Relation between AlI’ and (Fe3+ + Ti + Al”)
+ (Na + K) atoms.
Geochemistry
of actinolitic hornblendes from tonalitic rocks, Northern Portugal
795
No,Ca@g, Fe), Si,AI,C&(OH),
Tremlite-ferroxtimlite
Wg, Fe&i
=
ALA1
Tsckrmkite-ferrotsckfmkite
tremolite-ferrotremolite-tschermakite-ferrotscherma~t~ Fig. 4. Diagram ‘Na,Cas(Mg, Fe),Si,Al,O,,(OH),’ of the calcium amphiboles (after HALLIMOND, 1943). eAregos actinolitic hornblendes. x-Ben Nevis amphiboles. +T-Amphiboles of Scottish Caledonian complexes. fJ-Amphiboles of the Southern California batholith. Amphiboles of the Sierra Nevada batholith: V-Feather River, Northern Sierra Nevada; +-rocks of oldest age (Jurassic); A-rocks of intermediate and youngest ages. Fields outlined: solid line-Aregos amphiboles; dashed line-Ben Nevis amphiboles; Sierra Nevada amphiboles: dotted lineJurassic rocks; dash-dot line-rocks of intermediate and youngest ages.
No correlation can be established between amphibole composition and rock type for these calcium amphiboles. Although the data are scanty, this conclusion appears to be valid as the composition of amphibole depends on the physical conditions of crystallization of the magma (GREEN and RINGWOOD, 1968; HOLLOWAYand BTJRNHAM, 1972; HELZ, 1973). Trace elements
The trace element contents and element ratios of the amphiboles are given in Table 3, as well as those of the biotites from the monzonites. The concentrations of most trace elements and the values of the element ratios of the Aregos amphiboles are similar to those of amphiboles from comparable rocks of talc-alkali complexes. The exceptions are Sr and Ba, which are low in the Aregos actinolitic hornblendes. However, the actinolitic hornblendes of the Ben Nevis complex are also low in these elements. DISTRIBUTIONOF ELEMENTSBETWEENAMPI~BOLEAND BIOTITE Major elements
The actinolitic hornblendes have higher contents of Fez+ and MInthan the coexisting biotites. The relatively low contents of Ti and Al found in the Aregos actinolitic hornblendes are related to the low temperature of crystallization of these amphiboles. High temperature favours the entry of Ti and Al in the structure (VERHOOOEN, 1962; GREENand RINGWOOD,1968; HOLLOWAYand BURNHAM,1972). The Fe/Mg ratio in the Aregos biotites relative to the actinolitic hornblendes is particularly high
CARLOS A. R. DE ALBUQUERQUE
796
Table 3. Trace elements and element ratios of the emphiboles and monzonite biotites 1 elements G8 20 Cr 480 V 140 Li 9 Ni 110 CO 34 CU 21 S0 110 Zr 20 Y 60 Sr 16 Pb Be 22 Rb CS Element rrttios Ge x 103/ 0.42 (Al + FeS+) Cr x 102/F@+ 2.6 0.74 V x 10a/FeS+ Ni x lOS/Mg 1.2 Ni x 10a/FeB+ 1.4 Co x 10S/Fe2+ 0.44 Ni/Co 3.2
Tram
2
3
4
6
6
I
8
9
23 430 180 5 75 35 12 110 15 80 11 11 -
27 380 250 9 63 43 18 170 27 180 12 21 -
24 275 180 4 60 38 20 140 26 100 12 17 -
14 380 160 4 63 42 18 125 29 130 6 39 -
19 380 250 2 63 42 12 150 33 130 5 15 -
15
410 240 3 68 41 12 135 30 130 5 14 -
23 335 210 5 85 40 24 170 18 30 6 16 -
30 570 210 6 85 38 19 200 20 90 17 12 -
0.53 3.9 I.65 0.89 0.79 0.37 2.16
0.71 5.0 3.3 0.78 0.56 0.38 1.45
0.54 1.8 1.2 0.76 0.69 0.37 1.6
0.61 7.05 2.95 0.76 0.57 0.38 1.5
0.61 5.4 3.6 0.77 0.58 0.39 1.5
0.51 6.2 3.65 0.83 0.63 0.38 1.65
074 4.5 2.8 1.0 0.84 0.39 2.18
0.56 2.9 1.05 1.1 0.88 0.39 2.25
10 19 335 180 4 135 35 8 100 15 25 16 10 -
0.61 4.6 2.5 1.5 1.45 0.38 3.85
11
29
205 280 9 50 30 11 170 17 90 24 16 0.57 1.25 1.7 0.69 0.47 0.28 1.65
12 67 850 280 410 260 60 22 15 I 22 550 810 85
13 64 136 290 425 8.5 40 22 12 15 6 14 666 770 ii5
0.64 143 4.7 33 2.05 0.47 4.35
Analysis by C. A. R. de Albuquerque. Key w in Table 2.
relative to many talc-alkali rocks. The distribution coefficient
K
_ [Fe/Mg] hornblende D[Fe/Mg] biotite
is presented in Fig. 5, which is a plot of the Fe/Mg ratios of co-existing amphibole and biotite (modified after KRETZ, 1963, and HIETANEN, 1971). Structural considerations and the possible influence of other elements on the values of K, are discussed briefly. BINNS (1962) pointed out that for co-existing orthopyroxene and clinopyroxene the values of K, cannot be determined directly from the Mg/Fe ratios as the influence of Ca should not be ignored. This was expressed in quantitative terms by BLANDER (1972), the correction being necessary as Fe2+ may enter the M(2) sites to a considerable extent. Similarly, Fez+ and Mn normally included in the Y group in the calculation of the structural formulae of amphiboles can also enter the M(4) sites of this mineral (BURNS and STRENS, 1966; BANCROFT et al., 1967; PAPIXE et al., 1969). It has also to be postulated that the amphiboles have ordered structures and this can reasonably be expected in the light of the available evidence for amphiboles of various compositions (BURNS and STRENS, 1966; PAPIKE et al., 1969). In most of the Aregos amphiboles, the values of Ca in the structural formulae lie in the range 1.90-1.97, suggesting that only minor amounts of other cations enter the M(4) sites. As the values of the Y group are generally slightly in excess of 5-Oand probably Mn enters the M(4) site preferentially to Fez+ and Mg (cf. PAPIRE et al.,
0.61 2.1 4.55 I.2 0.60 0.28 2.15
Geochemistry of actinolitic hornblendes from tonalitic rocks, Northern Portugal
0.2
03
c-*
05
Ob
Ot
OB
09
to
797
I6
FSlM@
Biotite
Fig. 5. Distribution coefficients K&Fe/Mg] hornblende/bfotite k fX&-~Eali imeous complexes (modified after KRETZ, 1963, and HIETAXEN, 19’71). Symbols EWin Fig. 4.
1969), it can be assumed that only minor amounts of Fe2+ enter the M(4) sites and therefore no correction need be applied in the calculation of K,. These considerations are also valid for the composition of most amphiboles used in the calculation of Ki, (Fig. 5). One exception is the amp~bole of the tonal&e xenolith (Table 2, analysis 3) with the high value of 525 for the occupancy of the Y group. If the excess occupancy over 5-O is allocated to the M(4) sites (cf. Ross et aE., 1969), and Mn and Fez+ enter preferentially this site over Mg, the value of K, for this amphibole-biotite pair is lowered from 0.82 to O-73, a value still higher than those of the other amphiboles. From the inspection of Fig. 5, it is apparent that no systematic variation of K, can be correlated with the values of the Fe/Mg ratios of the minerals. The K, values plotted in the diagram (Fig. 5) for various amp~bol~biotite pairs from rocks of talc-alkali complexes have been calculated in the same manner from the data of LARSEN and DRAISIN (1950) (Southern California batholith), NOCKOLDSand ~XITCHELL(1948) (Scottish Caledonian rocks), HASLAM (1968) (Ben Nevis complex), and DODGEet al. (1968, 1969) (Sierra Nevada batholith). The values of K, for the Aregos pairs vary in a narrow range 0*56-O-64 with an average of O-60 if two pairs are excluded. The latter show higher values of that coefficient (O&Z and O-72) and are from a tonalite xenolith in the po~h~it,io granite and from a monzonite. Similar values of Kft are found for the same mineral pairs from rocks of the Ben Nevis complex (K, = O-61) and rocks of the oldest age and of the Feather River area of the Sierra Nevada batholith (K, = 0.64). The values of K, of amphibole-biotite pairs of rocks of the Southern California batholith, of rocks of intermediate and youngest ages of the Sierra Nevada pluton and of primary ampbibolebiotite pairs of Scottish Caledonian rocks lie mainly in the range O-75-0.87. The 9
798
CKRLOS
A. R.
DE
~BUQ~~Q~
observed difference in the values of R, for pairs from quartz-diorites and monzotonalites of the Feather River (Sierra Nevada), with an average of 0.64 (HIETANEN, 1971), and those of pairs from the Southern California batholith with an average K, of 0.83, was attributed by HIETANEN(1971) to different operating pressures during crystallization, the latter rocks being formed at lower pressures. However, the amphibole-biotite pairs with low values of K, are found in rocks which crystallized at low pressures such as the Aregos rocks (ALBUQUERQUE, 1971; 1973) and those of the Ben Nevis subvolcanic complex (HASLAM,1968), while the co-existing primary hornblendes and biotites from ‘normal’ diorites (~~~~~LDS and ~~CE~LL, 1948) of the Garabal Hill-Glen Fyne and ~orven-Strontian complexes, two deep-seated intrusions (HASLAM,1968), have K, in the range 0+79-0.87. The petrologic evidenoe and the inspection of Fig, 5 suggest that, in fact, low values of the distribution coefficient [Fe/Mg] hornblendebiotite can be correlated with relatively low temperature during the crystallization of those ferromagnesian minerals. Tmce elements The trace element contents in the hornblend~biotite pairs are plotted in the diagram of Fig. 6. A good correlation is observed for the ~st~bution of the elements N-i and Ba. Gallium is concentrated in biotite although the Ga/(AI + FeS+)ratios are identical in both minerals. Chromium shows preference for the amphibole while vanadium is concentrated in
Fig. 6. Distributionof tra.ceelementsin the Axegos mineral p&s actinolitic ho~blend~bioti~e.
Geochemistry of actinolitic hornblendes from tonalitic rocks, Northern Portugal
799
biotite. As these elements are generally thought to follow Fe3+, this distribution needs some explanation. It appears from the values of the ionic radii that V also replaces Fez+, involving a coupled substitution of the type Fe2+Si + VAl. This would explain the enrichment observed in biotite. Lithium is strongly concentrated in biotite, the contents of the co-existing amphibole being of the order of a few ppm only. According to various authors, Li follows Mg. However, this does not explain the observed distribution and a coupled substitution of the type 2Mg + LiA.lvl has to be postulated. Therefore, the extent of this substitution (and consequently the Li content of the mineral) is limited by structural considerations, in particular, the amount of Al in octahedral co-ordination. Nickel and cobalt are concentrated in biotite. However, the Co/Fez+ ratio is higher in amphibole, while the Ni/Mg ratio is higher in biotite. The same distribution has been observed in other amphibolebiotite pairs from talc-alkali rocks. The Ni/Co ratio is systematically higher in the biotites. Barium, rubidium and caesium, as can be expected from their affinity to potassium, are strongly concentrated in biotite. Yttrium and scandium are concentrated in the amphibole. The distribution of the latter element in amphibole-biotite pairs has been studied extensively (TILLING et al., 1969) for its potential use as a geothermometer. It has been suggested that SC replaces Mg. The erratic correlation observed here tends to contirm the conclusions of NORMAN and HASKIX (1968) and TILLINGet al. (1969) on the lack of affinity of SC with any major element. COMPOSITION OF AMPHIBOLEAND CONDITIONS OF CRYSTALLIZATION Experimental studies of the relations between amphibole composition and physical factors of crystallization have largely been confined to end-members of this mineral group (BOYD, 1959; ERNST, 1966; GILBERT, 1966; HOLLOWAY,1973). However, recent experimental work on the crystallization of systems of andesitic to basaltic compositions have greatly added to the understanding of the conditions of crystallization of amphiboles of various compositions (GREEN and RINGWOOD, 1968; LAMBERTand WYLLIE, 1968; EGGLER, 1972; HOLLOWAYand BURNHAM, 1972; HELZ, 1973). These data appear to confirm the value of this mineral group as an important petrogenetic indicator (EGGLERand BURNRAM,1973; HELZ, 1973; HOLLOWAY,1973). An attempt will be made here at correlating amphibole composition with the physical conditions of crystallization inferred from the petrologic and geochemical study of the rocks (ALBUQUERQUE, 1971) and of their biotites (ALBUQUERQUE, 1973). As discussed above, the Aregos amphiboles can be classified as actinolitic hornblendes in which tremolite-ferroactinolite is the dominant constituent (approximate range 52-71 per cent). Other important end-members are pargasite-ferrohastingsite (range 14-37 per cent) and tschermakite-ferrotschermakite (6-19 per cent). Experimental data show that the upper thermal stability limit of ferroactinolite is lower than that of other end-members of calcium amphiboles for the same values of the pressure andf0, (cf. ERNST, 1968).
800
CARLOS
A. R.
DE
ALBUQUERQUE
The quantitative influence of cations such as Ti and Al is difficult to evaluate. It is known, however, that Ti and Alrv increase with temperature (VERHOOOEN, 1962; ERNST,1968; HELZ, 1973). An increase in the (Ti + Alvl) contents has been observed with increasing pressure (HOLLOWAYand BURNHAM,1972). The evidence for Fe3+ is not conclusive. Fe 3+ contents probably depend mostly onf0, (cf. SEMET, 1973). The importance of f0, is well illustrated in the experimental data of GILBERT (1966) for ferropargasite. The upper stability limit of this mineral is lowered to about 550°C for systems buffered by NNO from 870°C when the system is buffered by MW at a pressure of 2 kb. This influence was confirmed by SEMET(1973) for magnesiohastingsite. Amphibole disappears in systems of andesitic composition at temperatures of the order of 930-950°C under pressures of 5-15 kb while the amphibole breakdown occurs at slightly lower temperatures under lower pressures (EOGLER,1972; GREEN, 1972 ; EGGLERand BURNHAM,1973). In systems of basaltic composition amphibole melts at about 1000°C in the same pressure range (5-15 kb) (GREENand RINGWOOD 1968; LAMBERTand WYLLIE, 1968; HOLLOWAYand BURNHAM,1972; HELZ, 1973). However, the analysed amphiboles in these systems contain relatively high Ti and Al and therefore high melting temperatures are to be expected, PIW~NSKII(1968) observed the disappearance of hornblende at temperatures of 930-950°C at pressures of 2 kb in granodiorites of the Sierra Nevada batholith. These amphiboles are also hornblendes with more than 1-OAIIv atoms per formula unit. Amphibole with the composition Tr,,Ts,, (tremolite-tschermakite solid solution) breaks down at 826°C under a pressure of 2 kb (JASMUNDand SCHAFER, 1972). It is apparent, therefore, that the occurrence of actinolitic hornblendes and possibly other amphiboles in igneous rocks places several limitations on the conditions of crystallization of the host rock. In the particular case of the Aregos amphiboles, the occurrence of the actinolitic hornblendes appears to be a consequence of increased fH, (and fH,O) during crystallization of a partly crystalline, pyroxene-bearing magma. The f0, would remain close to that defined by the FM& buffer as estimated from the biotite compositions. The temperature of SOO”C,deduced in the same study, does not conflict with the temperature of crystallization of actinolite-rich amphiboles of relatively high Mg/(Mg + Fe) ratio (in the range O-72-0.61) predicted from experimental data for systems buffered by FM&. CONCLUSIONS The amphiboles of talc-alkali igneous rocks have generally been considered as hornblendes; however, actinolitic hornblendes are more common than previously believed. The identification of such amphibole in these rocks places several limitations on their physical conditions of crystallization and would therefore permit a more precise definition of f0, and T. The Aregos amphiboles are actinolitic hornblendes characterized by low Ti, Al, Fe3+, Na and K.
Geochemistry of actinolitic hornblendesfrom tonalitic rocks, Northern Portugal
801
The distribution coefficient Fe/Mg between amphibole and biotite is systematically low (0.56-0.64 except for two specimens) and the average value is 0.60. This pairs of the Ben Nevis figure is similar to the average K, of amphibole-biotite complex (0.61) and oldest rocks of the Sierra Nevada batholith (O-64). K, appears to depend on temperature, low K, values being characteristic of crystallization at low temperature. The distribution of trace elements between actinolitic hornblendes and biotites shows similarities with those of the same mineral pairs of talc-alkali igneous rocks such as Scottish Caledonian (NOCKOLDS and MITCHELL, 1948 ; HASLAM, 1968), Sierra Nevada batholith (HIETANEN, 1971) and Southern California batholith (SEN et al., 1959). V, Ni and Co are concentrated in biotite while higher contents of Cr and SC are found in the amphibole, a nearly equal distribution between the two minerals being observed for all these elements with the exception of SC. However, the contents of Ba and Sr of the Aregos actinolitic hornblendes are particularly low. The composition of the actinolitic hornblendes of the Aregos tonalitic rocks was largely conditioned by the physical factors such as f0, buffered by FM&, relatively low temperature (of the order of SOO’C), and low pressure (about 3 kb). An actinolitic amphibole, magnesian in composition (Mg/Mg + Mn + Fe ratio in the range 0*72-O-61), would therefore crystallize under such conditions. The agreement with experimental data is good considering the limited information available. The complementary studies of the petrology of the tonalitic rocks and of the geochemistry of the biotites and amphiboles seem to warrant the conclusion that amphiboles and, in particular, actinolitic hornblendes can define a stage in the evolution of magmas. These considerations serve to emphasize the importance of defining the intensive variables at a determined stage of crystallization of the magma, and the constituent minerals of the rocks appear to be the best source of data on those parameters. If this is coupled to detailed petrologic study of the rocks, the changes in the physical conditions of crystallization of the magmas during ascent in the crust can be traced. Acknowledgements-ProfessorW. A. DEER is thanked for making available the facilities of the Department of Mineralogy and Petrology (University of Cambridge). The author is grateful to Drs. J. R. HOLLOWAYand A. J. PIWINSKIIfor their critical review of the manuscript from which the paper benefited. Thanks are due to the Instituto de Alta Culturs and Comiss&o Coordenadora da Investigap&opara a O.T.A.N. (Lisbon, Portugal) for support of the project. REFERENCES DE ALBUQUERQUE C. A. R. (1971) Petrochemistry of a series of granitic rocks from Northern Portugal. Bull. Geol. Sot. Amer. 82, 2783-2798. DE ALBUQUERQUE C. A. R. (1973) Geochemistry of biotites from granitic rocks, Northern Portugal. Cfeochim. Cosmochim.Acta 37, 1779-1802. BANCROFT G. M., BURNSR. G. and &DOCK A. G. (1967) Determination of cation distribution in the cummingtonite-grunerite series. Amer. Miwal. 52, 1009-1026. BINNSR. (1962) Metamorphic pyroxenes from Broken Hill district, New South Wales. Mined Mag.
83,
320-338.
BLANDERM. (1972) Thermodynamic properties of orthopyroxenes end clinopyroxenes based on the ideal two-site model. Geochim. Cosmochim. Acta 36, 787-799.
802
CARLOS A. R. DE ALBUQUERQUE
BOYD F. R. (1959) Hydrothermal investigations of amphiboles. In Researchesin Geochemistry, (editor P. IX. Abelson), pp. 377-396. Wiley. BURNS Et.G. and STRENS R. G. J. (1966) Infrared study of the hydroxyl bands in clinoamphibole. Science 153, 890-892. DEER W. A. (1936) The Cairnsmore of Carsphairn igneous complex. Qwwt. J. Geol. Xoc. 91, 47-76. DODGE F. C!. W., PAPIU J. J. and MAYS R. E. (1968) Hornblendes from granitic rocks of the Central Sierm Nevada batholith, California. J. Petrol. 9, 378-410. DODGE F. C. W., SMJTEL V. C. and MAYS R. E. (1969) Biotites from granitic rocks of the Central Sierra Nevada batholith, California. J. Petrol. 10, 250-271. EGGLER D. H. (1972) Amphibole stability in HsO-undersaturated talc-alkaline melts. Earth. Planet. S’ci. L&t. 15,28-34. EGGLERD. H. and BURN~AM C. W. (1973) Crystallizationand fractionationtrends in the system andesits-H,O-CO,-0s at pressuresto 10 kb. Bull. Geol. Sot. Amer. 84, 2517-2532. ERNST W. G. (1966) Synthesis and stability relations of ferrotremolite. Amer. J. Soi. $364, 37-65. ERNST W. G. (1968) Amphiboles, 125 pp. Springer-Ye&g, New York. GILBERT M. C. (1966) Synthesis and stability relations of the hornblende ferropargasite. Amer. J. Sci. 234, 698-742. GREIENT. H. (1972) C~stal~zation of e~c-~k~~e andesite under controlled high-pressure hydrous conditions. Co&rib. ~~ne~~l. Petrol. 34, 156166. GREEN T. H. and RIN~WOO~ A. E. (1968) Genesis of the talc-bake igneous rock suite. Cm&rib. MiseraL Petrol. 18,105-162. GULSOXB. L., LOWERINGJ. F., TAYLOR S. R. and WHITE A. J. R. (1972) High-K diorites, their place in the talc-alkaline association and relationship to andesites. Lithos 5, 269-279. HAZUMOND A. F. (1943) On the graphical representation of the calciferous amphiboles. Amer. &fineral. 28, 65-89. HASIXM H. W. (1968) The crystallization of intermediate and acid magmas at Baa Nevis, Scotland. J. Petrol. 9,84-104. HELZ R. T. (19’73) Phase relationsof basalts in their melting range at PHlo = 5 kb as a fmotion of oxygen fugaoity. J. Petrol. 14, 249-302. H~TANXN A. (1971) Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons, Northern Sierra Nevada, California. Oontrib. Mineral. Petrol. 30, 161-176. HOLLOWAY J. R. (1973) The system pargasite-HsO-COs: a model for melting of a hydrous mineral with a mixed-volatile fluid-I. Experimental results to 8 kbar. Beocibim. Cosmochim. Acta 87,651-666. HOLLOWAY J. R. and BURNKAM C. W. (1972) Melting relations of basalt with equilibrium water pressure less than total pressure. J. Petrol. 13, 1-29. JASIVTUND K. and SCR~~FER R. (1972) Experimentelle Bestimmung der P-T-stabilitatsbereiche in der mischkristallreihe-Tremolit-Tschermakit. Contrib. Mined. Petrol. 34, 106-115. KRETZ R. (1963) Distribution of magnesium and iron between orthopyroxene and calcic pyroxene in natural mineral assemblages. J. Geol. 71, 773-785. LAMBERTI. B, and WYXUE P. J. (1968) Stability of hornblende and a model for the low velocity zone. Nature 219, 1240-1241. LARSEN E. S. and DRAISIN W. M. (1950) Composition of the minerals in rocks of the Southern California Batholith. I&. Geol. Cmg., &eut Br~tu~n, Sess. IS, 66-79. LEAKE B. (1968) A catdogue of analyzed ~alc~e~~ and subc~c~erous amp~boles together with their nomenclat~e and associated mine&s. QeoE.SIX. Amp. Epec. Paper 23, 1-210, LYONS P. C. (1972) Significance of riebeckite and ferroh~t~gsite in microperthite granites. Amer. Mineral. 57, 1404-1412. MAIJER C. (1965) Geological investigations in the Amarante region (Northern Portugal) with special referenceto the mineralogy of the cassiterite-bearingalbite pegmatites. Ph.D. thesis, Amsterdam University, 155 pp.
Geochemistry of actinolitic hornblendesfrom ton&tic rocks, Northern Portugal
803
NOCKOLDSS. R. and MITCHELL R. L. (1948) The geochemistry of some Caledonian plutonic rocks; a study in the relationship between the major and trace elements of igneous rocks and their minerals. Trane. Roy. Sot. Edinb. 61,635-575. NORMAYN J. C. and HASKINL. A. (1968) The geochemistry of SC: a comparisonto the rare-earths and Fe. Beochim. Cosmochim. Acta 32,93-108. OEN ING SOEN (1958) The geology, petrology and ore deposits of the Viseu region, northern Portugal. Ph.D. thesis, Amsterdam University, 195 pp. PAPIKE J. J., Ross M. and CLARKJ. R. (1969) Crystal-chemical characterization of clinoamphiboles based on five new structure refinements. Mineral Sot. Anaer. Spec. Paper 2, 11’7-136. PIWINSKIIA. J. (1968) Experimental studies of igneous rock series, Central Sierra Nevada batholith, California. J. Beol. 76,548-570. Ross M., PAPIKEJ. J. and SHAWK. W. (1969) Exsolution textures in amphiboles as indicators of subsohdus thermal histories. Mineral. Sot. Amer. Spec. Paper 2, 275-299. SCHERMERHORN L. J. C. (1956) Igneous, metamorphic and ore geology of the Castro DaireSHo Pedro do Sul-S&tBoregion (northern Portugal). Ph.D. thesis, Amsterdam University, 617 pp. SEMETM. P. (1973) A crystal-chemical study of synthetic magnesiohastingsite. Amer. Mineral. 58, 480494. SEN N., NOCKOLDSS. R. and ALLEN R. (1959) Trace elements in minerals from rocks of the Southern California batholith. Beochim. Cosmochim. Acta 16,58-78. TILLINGR. I., GREENLANDL. P. and GOTTBXIED D. (1969) Distribution of scandium between co-existing biotite and hornblendein igneous rocks. Bull. Qeol. Sot. Amer. 80, 651-668. VERHOOGEN J. (1962) Distribution of titanium between silicates and oxides in igneous rocks. Amer. J. Sci. 260, 211-220.
Geochmistry of a~~o~~ic ho~blend~ from tonalitic rocks, Northern Porgy CARLOS A. R. DE ALBUQUERQUE Department of Geology, Saint Mary’s University, Hal&x, Nova Scotia, Canada (Received 16 July 1973; accepted in revised fm
11 December
1973)
Abs~aB&Chemios,l snalyses are given for actinolitic hornblendes of tonalitic rocks from the Hercynian belt of Northern Portugal. The distribution of elements between amphibole and co-existing biotite is studied. The composition of the amphiboles is analysed in the light of experimental data on amp~boles and the physical conditions of crysta~iz&tion inferred from the study of the biotite and rock series. The data on the biotites lead to the def%tion of 5 temperature of 800°C for the crystallization of aetinolitic hornblendes with Mg/(Mg f Fe) ratios of 0.7%0.61 at pressures of about 3 Kb and f0, defined by FMQ. THE INTEGRATED studies of the geochemistry of the rooks and minerals of plutonic talc-alkali complexes have greatly increased our understanding of their magmatic evolution and of the role of intensive variables. However, the problem of the origin of some magmas, in particular magmas of tonalitie and dioritic composition, is largely unresolved. Additional information is therefore required for a more complete knowledge of magmatism in erogenic belts. It became apparent from the investigations of DEER (1935) and NOCKOLDS and M?CTCHELL(1948) that amphiboles constitute excellent petrogenetic indicators as they are very complex in composition. This concept was emphasized by ERNST (1968), and examples of its application can be found in reoent studies (HIETANEEN, 1971; LYORS,
1972).
In this paper we will be concerned with the study of the geochemistry of the amphiboles from tonalitic rocks of the Aregos region of the Hercynian belt of Central and Northern Portugal. The distribution of elements between amphibole and biotite will also be considered. The petrogenesis of the rocks from which these minerals were separated has been studied (ALBUQUERQUE, 1971) as well as the geochemistry of the biotites (ALBUQUERQUE, 1973). The amphiboles -were separated from the crushed rock powders employing & Frantz Electromagnetic Separator. These impure concentrates were then purified in methylene iodide until a concentrate of better than 99.0-99.5 per cent purity was obtained. The impurities consist mostly of small grains of biotite and of composite grains of feldspar and ilmenite. The major element analyses were carried out by wet, classical methods for all elements except sodium and potassium determined by flame photometry. The trace elements were determined by emission spectrography using the technique described in earlier papers (ALBUQUERQUE, 1971, 19733. Struct~al formulae of the analysed amphiboles were calculated by computer programme. The chemical analyses of amphibole presented here fulfil the criteria of analysis evaluation defined by LEAKXG(1968). The only possible exception is that of the actinolitic hornblende from the xenolith (Table 2, analysis 3). This analysis shows a value of the Y group (5.256) close to the upper limit which can be considered acceptable for this group. However, LEAEE (1968, p. 34) notes that this may not necessarily mean analytical error. It may also be noted that the biotite from the same specimen also shows a high value of the Y group (ALBUQTJERQUE, 1973, Table 1, analysis 3). 789
790 Table
CAFCLOS
1. Modal
composition
of
A.
DE
ALBUQUERQUE
monzonites and average granodiorites
Monzonites 10 11 Quartz Potassium feldspar Plagioclase Hornblende Biotite Accessories Colour index
R.
85 26.6 31.2 14.4 17.8 1.5 33.7
11.0 28.7 32.9 5.3 21.2 0.9 27.4
Av.
Granodiorites (4) Range
SiO, TiO, AN Fe@, Fe0 MIlO MgG CaO Na,O KS0 H,O+ HsO-
Fet/(Fe,
+ Mg)
of
ton&es
Av.
(7)
and
Tonalites Range
196-23.0 7.3-13.2 386-41.1 0.2-1.1 25.0-28.7 -
15.8 0.5 43.2 8.0 30.2 2.3
12.3-20.0 0.0-1.7 394-46.6 3.5-12.7 27.0-34.4 -
28.7
26.8-31.4
40.5
35~246.7
2. Chemical
analvses 5
and structural 6
AIV’ TiV’ Fe3+ Fe2+ Mn Mg Ca Na K H Z Y X R2+ R3+
form-
2
3
4
50.36 0.72 5.45 2.71 9.96 0.36 15.03 12.36 0.69 0.44 2.04 0.05
49.80 0.77 6.09 1.57 12.16 0.42 13.96 12.32 0.57 0.39 2.13 0.02
50.14 0.84 5.77 1.09 14.38 0.49 13.31 11.11 0.88 0.39 1.77 0.03
49.69 0.80 5.48 2.18 13.15 0.41 13.06 12.25 0.72 0.48 1.86 0.02
51.64 0.65 4.19 0.77 14.26 0.47 13.75 11.36 0.53 0.41 2.11 0.03 ---
51.06 0.56 4.56 1.00 13.93 0.51 13.63 12.18 0.48 0.35 2.04 0.01
51.30 0.59 4.30 0.94 13.96 0.49 13.52 12.09 0.43 0.37 2.12 0.01
100.17 0.52
100.20 056
100.20 0.60
100~10 0.60
100*17 0.58
100.31 0.58
100.12 0.59
Structural E1v
related
21.4 9.7 40.2 05 26.8 1.4
Table 1
modes
7.258 0.742 0.184 0.078 0.294 1.200 0.044 3.229 1.909 0.193 0.081 1.961 8.000 5.029 2.183 4.473 0.556
7.211 0.789 0.250 0.084 0.171 1.473 0.052 3.013 1.912 0.160 0.072 2.057 8.000 5.043 2.144 4.538 0.505
7.330 0.670 0.324 0.092 0.120 1.758 0.061 2.901 1.740 0.249 0.073 1.726 8.000 5.256 2.062 4.720 0.536
7.277 0.723 0.223 0.088 0.240 1.611 0.051 2.851 1.922 0.204 0.090 1.817 8.000 5.064 2.216 4.513 0.551
7.496 0.504 0.213 0.071 0.084 1.731 0.058 2.975 1.767 0.149 0.076 2.043 8.000 5.132 1.992 4.764 0.368
7.423 0.577 0.204 0.061 0.109 1.694 0.063 2.954 1.897 0.135 0.065 1.978 8.000 5.085 2.097 4.711 0.374
* Includes Rb 0.008. Analysis by C. A. R. de Albuquerque. 1, 2. Green actinolitic hornblende, tonalite (northern outcrop) (A-205, A-112). 3. Brownish green actinolitic hornblende, tonalite (xenolith) (A-143). 4. Green actinolitic hornblende, tonalite (southern outcrop) (A-52). 5. Light green actinolitic hornblende, tonalite (southern outcrop) (A-51-A).
7
formulae
on
7.459 0.541 0.196 0.065 0.103 1.697 0.060 2.930 1.883 0.121 0.069 2.056 8.000 5.051 2.073 4.687 0.364
Geochemistry
of actinolitic hornblendesfrom tom&tic rocks, Northern Portugal
791
OCCURRENCE OF AMPHIBOLE Igneous rocks of the Hercynian belt of Western Europe are representedin Central Northern Portugal by a large batholith composed mainly of c&-alkali granodiorites and granites. These rocks are typically biotite or/and muscovite-bearing. Although rocks containing hornblende occupy small areas, their occurrence has been reported from several localities (SCHERMERHORN,1956; OEN, 1958; MAIJER, 1965) where they outcrop spatially associated with the Younger granites. In the Aregos region, in particular, the most mafic rocks are hornblendebiotite tonalites and related hornblende-bearing biotite granodiorites which have been inter1971). These rocks show aEnities with the high-K preted as hybrid rocks (ALBUQUERQUE, diorites of island arcs (GULSONet al., 1972). Amphiboles from hornblende-biotite monzonites (Albuquerque, unpublished data) are also included in this study as they are similar to those of the tonalitic rocks. The monzonites occur ulae of the amnhiboles and monzonite
10
8
9
61.09 0.68 4.46 1.07 13.05 0.45 13.92 12.47 0.60 O-38 2.08 0.04
48.52 0.86 6.63 2.80 12.41 0.47 12.83 12.54 0.76 0.46 1.95 0.01
51.41 0.62 4.52 1.04 11.80 0.37 14.74 12.70 0.65 0.36 1.93 0.04
100.29 0.56
100.24 0.60
99.91 0.53
biotites
11
12
13
47.84 0.94 6.60 2.33 13.58 0.50 12.14 12.33 0.88 0.64 1.95 0.02
37.05 2.56 15.81 0.85 16.34 0.24 13.00 0.45 0.20 9.35 3.97 0.09
36.72 2.57 15.46 0.91 18.23 0.31 11.65 0.30 0.21 9.65 3.27 0.07
99.78 0.62
99.91 0.63
99.35 0.68
SiO, TiO, A&& Fe& Fe0 MIlO MgC CaO Na,O Es0 H,O+ HsOFeJ(Fet + Mg)
basis of 24 (0, OH, F) 7.410 0.590 0.172 0.074 0.117 1.583 0.055 3.010 1.938 0.169 0.070 2.012 8.000 5.011 2.177 4.648 0.363 6, 7. 8. 9. 10. 11. 12, 13.
7.097 0.903 0.240 0.095 0.308 1.518 0.058 2.797 l-965 0.216 0.086 1.902 8.000 5.116 2.267 4.473 0.643
7.419 0.581 0.192 0.068 0.114 1.432 0.045 3.188 1.974 0.183 0.067 1.867 8.000 5.039 2.224 4.665 0.374
7.078 o-922 0.228 0.105 0.259 1.679 0.063 2.676 1.954 0.252 0.121 1.923 8.000 5.010 2.327 4.418 0.592
5.557 2.443 0.352 0.289 0.096 2.050 0.030 2.907 0.072 0.058 1.789 3.971 8.000 5.724 1.927* 4.987 0.737
5.644 2.356 0.444 0.297 0.105 2.343 0.040 2.669 0.049 0.063 1.892 3.352 8.000 5.898 2.012* 5.052 0.846
$v Alvr Tip1 Few Fe2+ Mn Mg Ca Na K H Z Y X R2+ R3+
Light green actinolitic hornblende, granodiorite, (southern outcrop) (A-51, A-55). Light green actinolitic hornblende, granodiorite, (northern outcrop) (A-79). Green actinolitic hornblende, granodiorite (northern outcrop) (A-202). Brownish green actinolitic hornblende, monzonite (A-370). Green actinolitic hornblende, monzonite (A-268). Biotite, monzonite (A-370, A-268).
792
Coos
A. R.
DE
~~UQUERQ~
spatially associated with the porphyritio biotite granodiorite of the suite of Younger granites and it may be noted that no ton&k rocks are spatially associated with the porphyritic granodiorite or the monzonites. The tonal&es contain quartz, plagioclase, biotite and hornblende, the ratio biotite/hornblende being 4 (Table 1). The granodiorites are ch~acterized by the presence of potassium feldspar @a. 10 per cent) and the ratio biotitejhornblende is higher, approximately 50. Although the monzonites con&& the same minerals as the granodiorites, the modal proportions are different (Table 1) and some features am of interest: hornblende and potassium feldspar are more abundant in the monzonites while quartz, plagioclase md biotite are distinctly higher in the granodiorites. However, the values of the colour index of both monzonites and gmnodiorites are identioal. Aggregates of grains of amphibole and biotite are seen in various specimens and occasionally biotite replaces the amphibole. Rare relies of clinopyroxene occur in the amphibole in a few specimens of monzonites. The pyroxene appears as small gr&ns in the core of crystals of amphibole. The amphiboles can be divided in two groups based on their pleochroism and other optical properties: the green amphiboles with Z-green or bluish green, Y-brownish green, X-light yellow, and the light green amphiboles with pleoahroism Z-light green, Y-light greenish brown, X-pale yellow, or Z-light greenish brown, Y-pale brown, X-pale yellow. The values of 2Vx are lower in the green amphiboles for comparable values of the Fe/Mg ratios. The range of vJues is 68-72” for these amphiboles md 2Vx of the light green amphiboles is in the range 76-79”. The two groups also show slight differences in chemioal composition. The green hornblendes have slightly lower contents of Si and higher Al, Ti, l?e3+ and Na than the light green, more actinolitic amphiboles. The use of the terms ‘prim&ry’ or ‘secondary’ amphiboles is avoided hero as the aetinolitio hornblende8 are believed to have crystallized in a. reaction rel&tions~p with pyroxene and melt. &WPOSITION
OF THE AICPHIBOLES
Major elements The chemical analyses, of major elements, and the structural formulae of the amphiboles are given in Table 2. The analyses and structural formulae of the two biotites from the monzonites are also given in Table 2. Those of the biotites coexisting with the amphiboles from the tonalites and related grano~orites have been pub~sh~ (~BUQUERQU~, 1973). The chemical composition of the Aregos amphiboles is distinctive as they can be classified as actinolitic hornblendes (LEAKE, 1968) unlike most amphiboles from talc-alkali rocks which are hornblendes. Their most characteristic features are the high contents of Si and low contents of Al (Al,O, in the range 4-19-6-63 per cent). Ti, Fes+, Na and K are also low in the Aregos amphiboles while Fe2+ and Mg show some variation expressed as the Fe/Mg ratio. ~tr~ct~~~l~~rn~la~ From inspection of Figs. 1, 2 and 3, it can be seen that the composition of these amphiboles does not show any marked deviation towards the alkali amphibole group and can be expressed in terms of the calcium amphibole end members tschermakite-ferrotschermapargasite-ferrohastingsite, tremoliteferroactinolite, kite and eden&e-ferroedenite. It can also be observed that these actinolitic hornblendes show little depa~~e from the tremo~t~fe~o~ctinolite end member and the tscherm~kiti~ substitution MgSi + R3+Al appears to play the dominant role conditioning the variations in composition.
Geochemistry of actinolitic hornblendes from ton&tic rocks, Northern Portugal
793
0
0.6
0
0
0.6
Al’”
---
0'2
04
0.6
(No+K) atoms Fig. 1. Relation between AlI’ and (Na + K) atoms. Variations in the composition of calcium amphiboles are well illustrated by HALLIMOND’S (1943) triangular diagram with apexes Ca,(Mg, Fe),Si,O,,(OH),, Ca,(Mg, Fe),Al,Si,Al,O,,(OH), and the hypothetical molecule Na,Ca,(Mg, Fe),Si,Al,O,,(OH), (Fig. 4). Thr‘s re p resentation has the advantage that edenite and pargasite-ferrohastingsite plot on the sides of the triangle. As could be expected, the Aregos actinolitic hornblendes occupy two areas near the tremolite-ferroactinolite apex, the light green amphiboles nearer this apex than The actinolitic hornblendes of the Ben Nevis complex are the green amphiboles. plotted in an area near that defined for the Aregos green amphiboles, while most amphiboles from other complexes such as the Sierra Nevada batholith and Southern California batholith (DODGE et al., 1968; LARSEN and DRAISIN, 1950) have a different composition, being richer in both the tschermakite and pargasite-ferrohastingsite end members. The hornblendes of the oldest (Jurassic) rocks of the Sierra Nevada batholith are, however, more actinolitic than those of rocks of intermediate and younger ages. Whereas actinolitic hornblendes occur in a few complexes, most amphiboles of talc-alkali rocks are hornblendes containing moderate to high amounts of the pargasite-ferrohastingsite molecule and to a smaller extent, the tschermakite molecule.
794
CARLOS
A. R.
DE
ALBUQUERQUE
0
0.6 -
0
0.6 -
R”+atoms Fig. 2. Relation between Al”
and (Fe3+ + Ti t_ AlV’) atoms.
06 -
Al'" O-6 -
[R3% (No t K)] atoms Fig. 3. Relation between AlI’ and (Fe3+ + Ti + Al”)
+ (Na + K) atoms.
Geochemistry
of actinolitic hornblendes from tonalitic rocks, Northern Portugal
795
No,Ca@g, Fe), Si,AI,C&(OH),
Tremlite-ferroxtimlite
Wg, Fe&i
=
ALA1
Tsckrmkite-ferrotsckfmkite
tremolite-ferrotremolite-tschermakite-ferrotscherma~t~ Fig. 4. Diagram ‘Na,Cas(Mg, Fe),Si,Al,O,,(OH),’ of the calcium amphiboles (after HALLIMOND, 1943). eAregos actinolitic hornblendes. x-Ben Nevis amphiboles. +T-Amphiboles of Scottish Caledonian complexes. fJ-Amphiboles of the Southern California batholith. Amphiboles of the Sierra Nevada batholith: V-Feather River, Northern Sierra Nevada; +-rocks of oldest age (Jurassic); A-rocks of intermediate and youngest ages. Fields outlined: solid line-Aregos amphiboles; dashed line-Ben Nevis amphiboles; Sierra Nevada amphiboles: dotted lineJurassic rocks; dash-dot line-rocks of intermediate and youngest ages.
No correlation can be established between amphibole composition and rock type for these calcium amphiboles. Although the data are scanty, this conclusion appears to be valid as the composition of amphibole depends on the physical conditions of crystallization of the magma (GREEN and RINGWOOD, 1968; HOLLOWAYand BTJRNHAM, 1972; HELZ, 1973). Trace elements
The trace element contents and element ratios of the amphiboles are given in Table 3, as well as those of the biotites from the monzonites. The concentrations of most trace elements and the values of the element ratios of the Aregos amphiboles are similar to those of amphiboles from comparable rocks of talc-alkali complexes. The exceptions are Sr and Ba, which are low in the Aregos actinolitic hornblendes. However, the actinolitic hornblendes of the Ben Nevis complex are also low in these elements. DISTRIBUTIONOF ELEMENTSBETWEENAMPI~BOLEAND BIOTITE Major elements
The actinolitic hornblendes have higher contents of Fez+ and MInthan the coexisting biotites. The relatively low contents of Ti and Al found in the Aregos actinolitic hornblendes are related to the low temperature of crystallization of these amphiboles. High temperature favours the entry of Ti and Al in the structure (VERHOOOEN, 1962; GREENand RINGWOOD,1968; HOLLOWAYand BURNHAM,1972). The Fe/Mg ratio in the Aregos biotites relative to the actinolitic hornblendes is particularly high
CARLOS A. R. DE ALBUQUERQUE
796
Table 3. Trace elements and element ratios of the emphiboles and monzonite biotites 1 elements G8 20 Cr 480 V 140 Li 9 Ni 110 CO 34 CU 21 S0 110 Zr 20 Y 60 Sr 16 Pb Be 22 Rb CS Element rrttios Ge x 103/ 0.42 (Al + FeS+) Cr x 102/F@+ 2.6 0.74 V x 10a/FeS+ Ni x lOS/Mg 1.2 Ni x 10a/FeB+ 1.4 Co x 10S/Fe2+ 0.44 Ni/Co 3.2
Tram
2
3
4
6
6
I
8
9
23 430 180 5 75 35 12 110 15 80 11 11 -
27 380 250 9 63 43 18 170 27 180 12 21 -
24 275 180 4 60 38 20 140 26 100 12 17 -
14 380 160 4 63 42 18 125 29 130 6 39 -
19 380 250 2 63 42 12 150 33 130 5 15 -
15
410 240 3 68 41 12 135 30 130 5 14 -
23 335 210 5 85 40 24 170 18 30 6 16 -
30 570 210 6 85 38 19 200 20 90 17 12 -
0.53 3.9 I.65 0.89 0.79 0.37 2.16
0.71 5.0 3.3 0.78 0.56 0.38 1.45
0.54 1.8 1.2 0.76 0.69 0.37 1.6
0.61 7.05 2.95 0.76 0.57 0.38 1.5
0.61 5.4 3.6 0.77 0.58 0.39 1.5
0.51 6.2 3.65 0.83 0.63 0.38 1.65
074 4.5 2.8 1.0 0.84 0.39 2.18
0.56 2.9 1.05 1.1 0.88 0.39 2.25
10 19 335 180 4 135 35 8 100 15 25 16 10 -
0.61 4.6 2.5 1.5 1.45 0.38 3.85
11
29
205 280 9 50 30 11 170 17 90 24 16 0.57 1.25 1.7 0.69 0.47 0.28 1.65
12 67 850 280 410 260 60 22 15 I 22 550 810 85
13 64 136 290 425 8.5 40 22 12 15 6 14 666 770 ii5
0.64 143 4.7 33 2.05 0.47 4.35
Analysis by C. A. R. de Albuquerque. Key w in Table 2.
relative to many talc-alkali rocks. The distribution coefficient
K
_ [Fe/Mg] hornblende D[Fe/Mg] biotite
is presented in Fig. 5, which is a plot of the Fe/Mg ratios of co-existing amphibole and biotite (modified after KRETZ, 1963, and HIETANEN, 1971). Structural considerations and the possible influence of other elements on the values of K, are discussed briefly. BINNS (1962) pointed out that for co-existing orthopyroxene and clinopyroxene the values of K, cannot be determined directly from the Mg/Fe ratios as the influence of Ca should not be ignored. This was expressed in quantitative terms by BLANDER (1972), the correction being necessary as Fe2+ may enter the M(2) sites to a considerable extent. Similarly, Fez+ and Mn normally included in the Y group in the calculation of the structural formulae of amphiboles can also enter the M(4) sites of this mineral (BURNS and STRENS, 1966; BANCROFT et al., 1967; PAPIXE et al., 1969). It has also to be postulated that the amphiboles have ordered structures and this can reasonably be expected in the light of the available evidence for amphiboles of various compositions (BURNS and STRENS, 1966; PAPIKE et al., 1969). In most of the Aregos amphiboles, the values of Ca in the structural formulae lie in the range 1.90-1.97, suggesting that only minor amounts of other cations enter the M(4) sites. As the values of the Y group are generally slightly in excess of 5-Oand probably Mn enters the M(4) site preferentially to Fez+ and Mg (cf. PAPIRE et al.,
0.61 2.1 4.55 I.2 0.60 0.28 2.15
Geochemistry of actinolitic hornblendes from tonalitic rocks, Northern Portugal
0.2
03
c-*
05
Ob
Ot
OB
09
to
797
I6
FSlM@
Biotite
Fig. 5. Distribution coefficients K&Fe/Mg] hornblende/bfotite k fX&-~Eali imeous complexes (modified after KRETZ, 1963, and HIETAXEN, 19’71). Symbols EWin Fig. 4.
1969), it can be assumed that only minor amounts of Fe2+ enter the M(4) sites and therefore no correction need be applied in the calculation of K,. These considerations are also valid for the composition of most amphiboles used in the calculation of Ki, (Fig. 5). One exception is the amp~bole of the tonal&e xenolith (Table 2, analysis 3) with the high value of 525 for the occupancy of the Y group. If the excess occupancy over 5-O is allocated to the M(4) sites (cf. Ross et aE., 1969), and Mn and Fez+ enter preferentially this site over Mg, the value of K, for this amphibole-biotite pair is lowered from 0.82 to O-73, a value still higher than those of the other amphiboles. From the inspection of Fig. 5, it is apparent that no systematic variation of K, can be correlated with the values of the Fe/Mg ratios of the minerals. The K, values plotted in the diagram (Fig. 5) for various amp~bol~biotite pairs from rocks of talc-alkali complexes have been calculated in the same manner from the data of LARSEN and DRAISIN (1950) (Southern California batholith), NOCKOLDSand ~XITCHELL(1948) (Scottish Caledonian rocks), HASLAM (1968) (Ben Nevis complex), and DODGEet al. (1968, 1969) (Sierra Nevada batholith). The values of K, for the Aregos pairs vary in a narrow range 0*56-O-64 with an average of O-60 if two pairs are excluded. The latter show higher values of that coefficient (O&Z and O-72) and are from a tonalite xenolith in the po~h~it,io granite and from a monzonite. Similar values of Kft are found for the same mineral pairs from rocks of the Ben Nevis complex (K, = O-61) and rocks of the oldest age and of the Feather River area of the Sierra Nevada batholith (K, = 0.64). The values of K, of amphibole-biotite pairs of rocks of the Southern California batholith, of rocks of intermediate and youngest ages of the Sierra Nevada pluton and of primary ampbibolebiotite pairs of Scottish Caledonian rocks lie mainly in the range O-75-0.87. The 9
798
CKRLOS
A. R.
DE
~BUQ~~Q~
observed difference in the values of R, for pairs from quartz-diorites and monzotonalites of the Feather River (Sierra Nevada), with an average of 0.64 (HIETANEN, 1971), and those of pairs from the Southern California batholith with an average K, of 0.83, was attributed by HIETANEN(1971) to different operating pressures during crystallization, the latter rocks being formed at lower pressures. However, the amphibole-biotite pairs with low values of K, are found in rocks which crystallized at low pressures such as the Aregos rocks (ALBUQUERQUE, 1971; 1973) and those of the Ben Nevis subvolcanic complex (HASLAM,1968), while the co-existing primary hornblendes and biotites from ‘normal’ diorites (~~~~~LDS and ~~CE~LL, 1948) of the Garabal Hill-Glen Fyne and ~orven-Strontian complexes, two deep-seated intrusions (HASLAM,1968), have K, in the range 0+79-0.87. The petrologic evidenoe and the inspection of Fig, 5 suggest that, in fact, low values of the distribution coefficient [Fe/Mg] hornblendebiotite can be correlated with relatively low temperature during the crystallization of those ferromagnesian minerals. Tmce elements The trace element contents in the hornblend~biotite pairs are plotted in the diagram of Fig. 6. A good correlation is observed for the ~st~bution of the elements N-i and Ba. Gallium is concentrated in biotite although the Ga/(AI + FeS+)ratios are identical in both minerals. Chromium shows preference for the amphibole while vanadium is concentrated in
Fig. 6. Distributionof tra.ceelementsin the Axegos mineral p&s actinolitic ho~blend~bioti~e.
Geochemistry of actinolitic hornblendes from tonalitic rocks, Northern Portugal
799
biotite. As these elements are generally thought to follow Fe3+, this distribution needs some explanation. It appears from the values of the ionic radii that V also replaces Fez+, involving a coupled substitution of the type Fe2+Si + VAl. This would explain the enrichment observed in biotite. Lithium is strongly concentrated in biotite, the contents of the co-existing amphibole being of the order of a few ppm only. According to various authors, Li follows Mg. However, this does not explain the observed distribution and a coupled substitution of the type 2Mg + LiA.lvl has to be postulated. Therefore, the extent of this substitution (and consequently the Li content of the mineral) is limited by structural considerations, in particular, the amount of Al in octahedral co-ordination. Nickel and cobalt are concentrated in biotite. However, the Co/Fez+ ratio is higher in amphibole, while the Ni/Mg ratio is higher in biotite. The same distribution has been observed in other amphibolebiotite pairs from talc-alkali rocks. The Ni/Co ratio is systematically higher in the biotites. Barium, rubidium and caesium, as can be expected from their affinity to potassium, are strongly concentrated in biotite. Yttrium and scandium are concentrated in the amphibole. The distribution of the latter element in amphibole-biotite pairs has been studied extensively (TILLING et al., 1969) for its potential use as a geothermometer. It has been suggested that SC replaces Mg. The erratic correlation observed here tends to contirm the conclusions of NORMAN and HASKIX (1968) and TILLINGet al. (1969) on the lack of affinity of SC with any major element. COMPOSITION OF AMPHIBOLEAND CONDITIONS OF CRYSTALLIZATION Experimental studies of the relations between amphibole composition and physical factors of crystallization have largely been confined to end-members of this mineral group (BOYD, 1959; ERNST, 1966; GILBERT, 1966; HOLLOWAY,1973). However, recent experimental work on the crystallization of systems of andesitic to basaltic compositions have greatly added to the understanding of the conditions of crystallization of amphiboles of various compositions (GREEN and RINGWOOD, 1968; LAMBERTand WYLLIE, 1968; EGGLER, 1972; HOLLOWAYand BURNHAM, 1972; HELZ, 1973). These data appear to confirm the value of this mineral group as an important petrogenetic indicator (EGGLERand BURNRAM,1973; HELZ, 1973; HOLLOWAY,1973). An attempt will be made here at correlating amphibole composition with the physical conditions of crystallization inferred from the petrologic and geochemical study of the rocks (ALBUQUERQUE, 1971) and of their biotites (ALBUQUERQUE, 1973). As discussed above, the Aregos amphiboles can be classified as actinolitic hornblendes in which tremolite-ferroactinolite is the dominant constituent (approximate range 52-71 per cent). Other important end-members are pargasite-ferrohastingsite (range 14-37 per cent) and tschermakite-ferrotschermakite (6-19 per cent). Experimental data show that the upper thermal stability limit of ferroactinolite is lower than that of other end-members of calcium amphiboles for the same values of the pressure andf0, (cf. ERNST, 1968).
800
CARLOS
A. R.
DE
ALBUQUERQUE
The quantitative influence of cations such as Ti and Al is difficult to evaluate. It is known, however, that Ti and Alrv increase with temperature (VERHOOOEN, 1962; ERNST,1968; HELZ, 1973). An increase in the (Ti + Alvl) contents has been observed with increasing pressure (HOLLOWAYand BURNHAM,1972). The evidence for Fe3+ is not conclusive. Fe 3+ contents probably depend mostly onf0, (cf. SEMET, 1973). The importance of f0, is well illustrated in the experimental data of GILBERT (1966) for ferropargasite. The upper stability limit of this mineral is lowered to about 550°C for systems buffered by NNO from 870°C when the system is buffered by MW at a pressure of 2 kb. This influence was confirmed by SEMET(1973) for magnesiohastingsite. Amphibole disappears in systems of andesitic composition at temperatures of the order of 930-950°C under pressures of 5-15 kb while the amphibole breakdown occurs at slightly lower temperatures under lower pressures (EOGLER,1972; GREEN, 1972 ; EGGLERand BURNHAM,1973). In systems of basaltic composition amphibole melts at about 1000°C in the same pressure range (5-15 kb) (GREENand RINGWOOD 1968; LAMBERTand WYLLIE, 1968; HOLLOWAYand BURNHAM,1972; HELZ, 1973). However, the analysed amphiboles in these systems contain relatively high Ti and Al and therefore high melting temperatures are to be expected, PIW~NSKII(1968) observed the disappearance of hornblende at temperatures of 930-950°C at pressures of 2 kb in granodiorites of the Sierra Nevada batholith. These amphiboles are also hornblendes with more than 1-OAIIv atoms per formula unit. Amphibole with the composition Tr,,Ts,, (tremolite-tschermakite solid solution) breaks down at 826°C under a pressure of 2 kb (JASMUNDand SCHAFER, 1972). It is apparent, therefore, that the occurrence of actinolitic hornblendes and possibly other amphiboles in igneous rocks places several limitations on the conditions of crystallization of the host rock. In the particular case of the Aregos amphiboles, the occurrence of the actinolitic hornblendes appears to be a consequence of increased fH, (and fH,O) during crystallization of a partly crystalline, pyroxene-bearing magma. The f0, would remain close to that defined by the FM& buffer as estimated from the biotite compositions. The temperature of SOO”C,deduced in the same study, does not conflict with the temperature of crystallization of actinolite-rich amphiboles of relatively high Mg/(Mg + Fe) ratio (in the range O-72-0.61) predicted from experimental data for systems buffered by FM&. CONCLUSIONS The amphiboles of talc-alkali igneous rocks have generally been considered as hornblendes; however, actinolitic hornblendes are more common than previously believed. The identification of such amphibole in these rocks places several limitations on their physical conditions of crystallization and would therefore permit a more precise definition of f0, and T. The Aregos amphiboles are actinolitic hornblendes characterized by low Ti, Al, Fe3+, Na and K.
Geochemistry of actinolitic hornblendesfrom tonalitic rocks, Northern Portugal
801
The distribution coefficient Fe/Mg between amphibole and biotite is systematically low (0.56-0.64 except for two specimens) and the average value is 0.60. This pairs of the Ben Nevis figure is similar to the average K, of amphibole-biotite complex (0.61) and oldest rocks of the Sierra Nevada batholith (O-64). K, appears to depend on temperature, low K, values being characteristic of crystallization at low temperature. The distribution of trace elements between actinolitic hornblendes and biotites shows similarities with those of the same mineral pairs of talc-alkali igneous rocks such as Scottish Caledonian (NOCKOLDS and MITCHELL, 1948 ; HASLAM, 1968), Sierra Nevada batholith (HIETANEN, 1971) and Southern California batholith (SEN et al., 1959). V, Ni and Co are concentrated in biotite while higher contents of Cr and SC are found in the amphibole, a nearly equal distribution between the two minerals being observed for all these elements with the exception of SC. However, the contents of Ba and Sr of the Aregos actinolitic hornblendes are particularly low. The composition of the actinolitic hornblendes of the Aregos tonalitic rocks was largely conditioned by the physical factors such as f0, buffered by FM&, relatively low temperature (of the order of SOO’C), and low pressure (about 3 kb). An actinolitic amphibole, magnesian in composition (Mg/Mg + Mn + Fe ratio in the range 0*72-O-61), would therefore crystallize under such conditions. The agreement with experimental data is good considering the limited information available. The complementary studies of the petrology of the tonalitic rocks and of the geochemistry of the biotites and amphiboles seem to warrant the conclusion that amphiboles and, in particular, actinolitic hornblendes can define a stage in the evolution of magmas. These considerations serve to emphasize the importance of defining the intensive variables at a determined stage of crystallization of the magma, and the constituent minerals of the rocks appear to be the best source of data on those parameters. If this is coupled to detailed petrologic study of the rocks, the changes in the physical conditions of crystallization of the magmas during ascent in the crust can be traced. Acknowledgements-ProfessorW. A. DEER is thanked for making available the facilities of the Department of Mineralogy and Petrology (University of Cambridge). The author is grateful to Drs. J. R. HOLLOWAYand A. J. PIWINSKIIfor their critical review of the manuscript from which the paper benefited. Thanks are due to the Instituto de Alta Culturs and Comiss&o Coordenadora da Investigap&opara a O.T.A.N. (Lisbon, Portugal) for support of the project. REFERENCES DE ALBUQUERQUE C. A. R. (1971) Petrochemistry of a series of granitic rocks from Northern Portugal. Bull. Geol. Sot. Amer. 82, 2783-2798. DE ALBUQUERQUE C. A. R. (1973) Geochemistry of biotites from granitic rocks, Northern Portugal. Cfeochim. Cosmochim.Acta 37, 1779-1802. BANCROFT G. M., BURNSR. G. and &DOCK A. G. (1967) Determination of cation distribution in the cummingtonite-grunerite series. Amer. Miwal. 52, 1009-1026. BINNSR. (1962) Metamorphic pyroxenes from Broken Hill district, New South Wales. Mined Mag.
83,
320-338.
BLANDERM. (1972) Thermodynamic properties of orthopyroxenes end clinopyroxenes based on the ideal two-site model. Geochim. Cosmochim. Acta 36, 787-799.
802
CARLOS A. R. DE ALBUQUERQUE
BOYD F. R. (1959) Hydrothermal investigations of amphiboles. In Researchesin Geochemistry, (editor P. IX. Abelson), pp. 377-396. Wiley. BURNS Et.G. and STRENS R. G. J. (1966) Infrared study of the hydroxyl bands in clinoamphibole. Science 153, 890-892. DEER W. A. (1936) The Cairnsmore of Carsphairn igneous complex. Qwwt. J. Geol. Xoc. 91, 47-76. DODGE F. C!. W., PAPIU J. J. and MAYS R. E. (1968) Hornblendes from granitic rocks of the Central Sierm Nevada batholith, California. J. Petrol. 9, 378-410. DODGE F. C. W., SMJTEL V. C. and MAYS R. E. (1969) Biotites from granitic rocks of the Central Sierra Nevada batholith, California. J. Petrol. 10, 250-271. EGGLER D. H. (1972) Amphibole stability in HsO-undersaturated talc-alkaline melts. Earth. Planet. S’ci. L&t. 15,28-34. EGGLERD. H. and BURN~AM C. W. (1973) Crystallizationand fractionationtrends in the system andesits-H,O-CO,-0s at pressuresto 10 kb. Bull. Geol. Sot. Amer. 84, 2517-2532. ERNST W. G. (1966) Synthesis and stability relations of ferrotremolite. Amer. J. Soi. $364, 37-65. ERNST W. G. (1968) Amphiboles, 125 pp. Springer-Ye&g, New York. GILBERT M. C. (1966) Synthesis and stability relations of the hornblende ferropargasite. Amer. J. Sci. 234, 698-742. GREIENT. H. (1972) C~stal~zation of e~c-~k~~e andesite under controlled high-pressure hydrous conditions. Co&rib. ~~ne~~l. Petrol. 34, 156166. GREEN T. H. and RIN~WOO~ A. E. (1968) Genesis of the talc-bake igneous rock suite. Cm&rib. MiseraL Petrol. 18,105-162. GULSOXB. L., LOWERINGJ. F., TAYLOR S. R. and WHITE A. J. R. (1972) High-K diorites, their place in the talc-alkaline association and relationship to andesites. Lithos 5, 269-279. HAZUMOND A. F. (1943) On the graphical representation of the calciferous amphiboles. Amer. &fineral. 28, 65-89. HASIXM H. W. (1968) The crystallization of intermediate and acid magmas at Baa Nevis, Scotland. J. Petrol. 9,84-104. HELZ R. T. (19’73) Phase relationsof basalts in their melting range at PHlo = 5 kb as a fmotion of oxygen fugaoity. J. Petrol. 14, 249-302. H~TANXN A. (1971) Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons, Northern Sierra Nevada, California. Oontrib. Mineral. Petrol. 30, 161-176. HOLLOWAY J. R. (1973) The system pargasite-HsO-COs: a model for melting of a hydrous mineral with a mixed-volatile fluid-I. Experimental results to 8 kbar. Beocibim. Cosmochim. Acta 87,651-666. HOLLOWAY J. R. and BURNKAM C. W. (1972) Melting relations of basalt with equilibrium water pressure less than total pressure. J. Petrol. 13, 1-29. JASIVTUND K. and SCR~~FER R. (1972) Experimentelle Bestimmung der P-T-stabilitatsbereiche in der mischkristallreihe-Tremolit-Tschermakit. Contrib. Mined. Petrol. 34, 106-115. KRETZ R. (1963) Distribution of magnesium and iron between orthopyroxene and calcic pyroxene in natural mineral assemblages. J. Geol. 71, 773-785. LAMBERTI. B, and WYXUE P. J. (1968) Stability of hornblende and a model for the low velocity zone. Nature 219, 1240-1241. LARSEN E. S. and DRAISIN W. M. (1950) Composition of the minerals in rocks of the Southern California Batholith. I&. Geol. Cmg., &eut Br~tu~n, Sess. IS, 66-79. LEAKE B. (1968) A catdogue of analyzed ~alc~e~~ and subc~c~erous amp~boles together with their nomenclat~e and associated mine&s. QeoE.SIX. Amp. Epec. Paper 23, 1-210, LYONS P. C. (1972) Significance of riebeckite and ferroh~t~gsite in microperthite granites. Amer. Mineral. 57, 1404-1412. MAIJER C. (1965) Geological investigations in the Amarante region (Northern Portugal) with special referenceto the mineralogy of the cassiterite-bearingalbite pegmatites. Ph.D. thesis, Amsterdam University, 155 pp.
Geochemistry of actinolitic hornblendesfrom ton&tic rocks, Northern Portugal
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