Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions

Journal of Volcanology and Geothermal Research, 4 (1978) 315--331

315

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MAGMA MIXING IN MAFIC ALKALINE VOLCANIC ROCKS: THE EVIDENCE FROM RELICT PHENOCRYST PHASES AND OTHER INCLUSIONS

C.K. BROOKS and I. PRINTZLAU

Institut for Petrologi, Oster Voldgade 1 O, DK 1350 Copenhagen (Denmark) Geologisk Museum, Oster Voldgade 5--7, DK 1350 Copenhagen (Denmark) (Received January 9, 1978; revised and accepted July 21, 1978)

ABSTRACT Brooks, C.K. and Printzlau, I., 1978. Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions. J. Volcanol. Geotherm. Res., 4: 315--331. Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles.

INTRODUCTION

Magma mixing, first proposed by Bunsen (1851) to explain the genesis of intermediate rocks in Iceland, is not widely accepted (e.g. Carmichael, et al., 1974). However, it has been proposed to explain some calc-alkaline rocks of the circum-Pacific belt (Anderson, 1976), and tholeiitic rocks from Kilauea, Hawaii (Wright and Fiske, 1971; Anderson and Wright, 1972; Wright, 1973). Skye, Scotland (Wager et al., 1965), East Greenland {Brooks, 1977) and the Mid-Atlantic Ridge (Donaldson and Brown, 1977). The purpose of this paper is to show that alkaline rocks from many localities commonly have mineralogical and rarely textural evidence of magma mixing. We also attempt to show that, in some cases at least, there is evidence that magma mixing took place at mantle depths. Consequently we infer that 'evolved'* phases may form in the mantle, most probably by partial melting *By evolved phases we mean those phases which have a Mg/(Mg+Fe) ratio less than 65 and are unlikely to have been in equilibrium with normal mantle material (Green, 1971)

316

of anomalous mantle material, which has undergone metasomatic enrichment in certain components. The suggestions p u t forward here are tentative and much additional work is needed before it will be possible to say h o w c o m m o n the process of magma mixing is and to what extent the variations shown by alkaline rocks can be ascribed to it. OCCURRENCE OF GREEN PYROXENE IN MAFIC ALKALINE ROCKS

In the course of our work in eastern Greenland and southern Sweden, and by our knowledge of other areas such as western Greenland, the Canary Islands and the Roman province, we became aware that green, pleochroic clinopyroxene megacrysts (no genetic implication intended) commonly occur alongside colourless megacrysts and phenocrysts in many basic, alkaline, volcanic and hypabyssal rocks. A search of the literature confirmed this impression and resulted in the list of reported occurrences shown in Table 1. Such green pyroxenes have been known for a long time -- they were apparently first described by Zirkel (1870) -- b u t have hitherto received no particular attention. Table 1 should be regarded as tentative, some occurrences have probably escaped our attention as descriptions are sometimes meagre. However, it is sufficient to show that they are widespread. Also, due to the scanty information often given, we have not attempted to collect specific information on the nature of the host rocks for the examples given in Table 1. However, these are invariably basic and alkaline, often strongly so, and have been designated as basanite, camptonite, monchiquite, nephelinite, ankaramite, leucitite and analcimite. In many cases, chemical analyses of the host rocks have not been given, and indeed may be unrepresentative of the host rock due to the abundance of included material (megacrysts of various minerals, ultramafic nodules believed to be both of mantle and cumulate origin). Such, for example, was the case with the camptonite described by Brooks and Rucklidge (1973) and the nephelinite described by Wilkinson (1975). We conclude that these green pyroxenes are a c o m m o n feature of strongly nepheline-normative, basic volcanic and hypabyssal rocks and frequently occur in association with other clinopyroxenes, megacrysts of other minerals and ultramafic nodules of mantle origin (e.g., the examples described by Borley et al., 1971; Br~indle et al., 1974; Brooks and Rucklidge, 1973; Griffin and Taylor, 1975 and Wilkinson,1975). Babkine et al. (1968) conducted a search of basic alkaline volcanic rocks for occurrences of green pyroxenes and found them in 107 o u t of 319 samples. They noted that they were generally more abundant and less resorbed in potassic lavas (e.g. from Birunga and Vesuvius) than in the commoner sodic rocks, where they only occurred as small, strongly resorbed cores in phenocrysts.

TABLE

1

S u m m a r y of occurrences of reverse-zoned sodian salitesin basic alkaline volcanic rocks Africa

Algeria, Hoggar (Atokar massif) South Africa (Okonjeje complex)

Girod (1971) Simpson (1954)

America

U.S.A. (western Colorado)

Best and Brimhall (1974)

Antarctica

McMurdo volcanic group

Weiblen et al. (1974) K. Kyle (1976, and written communication, 1974)

Atlantic Ocean

Gran Canaria (Roque Nublo Formation) Gran Canaria ('basaltic formation IV') Tenerife (various occurrences) La Palma (Tenegufa volcano) Trindade Island, Brazil Jan Mayen Island and surroundings

Frisch and Schmincke (1969, 1971) C.K. Brooks and K. Hansen (unpublished) Borley et al. (1971) Scott (1976) Br£ndle et al. (1974) Almeida (1961) N. Hald (personal communication)

Australia

Nandewar volcanics, New South Wales

Wilkinson (1975)

Europe

Sweden (Mesozoic volcanics, Sk,Jlne) Sweden (kimberlitic dikes, Kalix) Norway (Fen complex) Norway (Oslo province volcanics) Scotland (Orkney Islands) Germany (Eifel district) France Italy (Colli Albani) Italy (Campanlan district)

I. Printzlau (in preparation) I. Printzlau (unpublished) Bragger (1921) Griffin and Taylor (1975) T.V. Segeistad (written communication, 1976) Flett (1935) Zirkel (1870) Huckenholz (1973) Babkine et al. (1968) Velde and Frain de la Gaulayrie (1974) Thompson (1977) Ghiara and Lirer (1976-1977)

Greenland

Mosozoic dikes (West Greenland) Tertiary lavas and dikes (West Greenland) Tertiary dikes (East Greenland) Tertiary nephelinites (northern East Greenland)

Walton and Arnold (1970) K. Hansen (personal communication) J. Gutzon Larsen (1977, and personal communication) Brooks and Rucklidge (1973) Brooks and Platt (1975, and unpublished) C.IL Brooks, A.K. Pedersen and D.C. Rex (in preparation)

Nicholls and Carmichael (1969), Smith and Carmichael (1968) and Anderson (1974) found reverse-zoned clinopyroxenes in calc-alkalinevolcanic rocks and Anderson and Wright (1971) in tholeiiticlavas from Hawaii, but these are not considered here as we restrictour discussion to alkaline rocks. However, these cases probably also reveal m a g m a mixing.

318

NATURE OF THE GREEN PYROXENES The green p y r o x e n e s are variable in t e x t u r e b u t have similar c h e m i c a l f e a t u r e s and are invariably reverse-zoned. A t y p i c a l e x a m p l e is s h o w n in Fig.1. T h e y o c c u r as cores to p h e n o c r y s t s a n d as m e g a c r y s t s r i m m e d b y m a t e r i a l

Fig.1. Resorbed green pyroxene core mantled by more Mg-rich lilac-coloured titansalite in a lamprophyre dike from Wiedemanns Fjord, East Greenland. A thin zone of eolourless, Ti-poor, Mg-rich pyroxene is visible between the core (towards which it is sharply-bounded) and the rim and this probably corresponds to a normal phenocryst composition. For analyses of core and rim see Brooks and Rucklidge (1973, table 1, p. 202). The crystal is ca. 1 cm across. A fragment of ortbopyroxene from a mantle lherzolite is visible top centre. similar to t h e g r o u n d m a s s p y r o x e n e , which is generally a lilac titansalite. S o m e t i m e s a colourless m a n t l e is d e v e l o p e d b e t w e e n t h e green c o r e and t h e p u r p l e r i m which a p p e a r s to be identical to the a c c o m p a n y i n g p h e n o c r y s t s . M o r p h o l o g i c a l l y the green cores vary f r o m angular, t h r o u g h r o u n d e d , t o euhedral, suggesting t h a t a v a r i e t y o f histories m a y be r e c o r d e d . In o u r experience, t h e green p a r t is invariably s h a r p l y b o u n d e d against t h e m a n t l i n g material. In s o m e cases t h e green core is r e p r e s e n t e d b y a sponge-like mass and this, in c o m b i n a t i o n with t h e i r r o u n d e d f o r m , is i n t e r p r e t e d as t h e result o f r e s o r p t i o n b y the enclosing material, with w h i c h t h e y are o u t o f equilibrium. T h e p l e o c h r o i s m is variable in its i n t e n s i t y o f green a n d is grass-green to yellowish-green. Analyses s h o w t h a t this variation in i n t e n s i t y o f c o l o u r i n g reflects t h e a m o u n t o f a c m i t e present. A n u m b e r o f minerals are r e c o r d e d as inclusions in t h e green p y r o x e n e s . These include a p a t i t e (East and West G r e e n l a n d , b o t h T e r t i a r y a n d Mesozoic o c c u r r e n c e s ; Colli Albani, Italy; Hoggar, central Sahara} a n d less c o m m o n l y

319

magnetite and sphene, as at Hoggar and in West Greenland. In one example from Ubekendt Ejland, West Greenland, the green pyroxenes occur with apatite and magnetite (J.G. Larsen, unpublished) in a texture similar to that found where crystal settling is generally believed to have operated. Although variable, the chemical composition always shows certain characteristics as displayed by the examples in Table 2. They are always reversezoned, the cores being richer in Fe, Na and Mn than other pyroxenes in the rock which is quite unlike the situation normally encountered in basaltic phenocrysts. They generally have much less Ti than the accompanying groundmass pyroxenes and are usually, but not invariably poorer in Cr than the colourless megacrysts. Most reported analyses are by electron microprobe so that the oxidation state of iron is uncertain. However, where wet chemical analyses are available (e.g. Girod, 1971; Ghiara and Lirer, 1 9 7 6 - 1 9 7 7 ) , large amounts of Fe 3÷ are present, a feature also shown by stoichiometric calculation of the microprobe analyses. In some cases the acmite content of the green pyroxenes reaches 30% (e.g., sample 7 of Griffin and Taylor, 1975), a composition corresponding to that encountered in the late soda syenites at Shonkin Sag (Nash and Wilkinson, 1970). Similarly, as noted below, the green pyroxenes and accompanying reverse-zoned kaersutites from Tenerife could be matched by those in an anorthoclase syenite (Borley et al., 1971), and Girod (1971) noted that green 'sodian ferrisalite' cores had a similar acmite content to the pyroxenes of the comagmatic mugearites, differing only in their higher content of the Ca-Tschermak's component. In some varieties (e.g., Table 2, columns 3 and 4) Si is high and all the A1 is in octahedral coordination, b u t in other cases (e.g., Table 2, columns 1 and 2) considerable tetrahedral Al is present. These chemical characteristics lead to an enrichment of ferrosilite, acmite and Ca ferri-Tschermak's c o m p o n e n t relative to the groundmass pyroxenes when calculated according to standard procedures (Kushiro, 1962; Papike et al., 1974). Ca-Tschermak's component, CaA12SiO6, reaches high values, i.e. more than 10%, in a number of cases. The reverse zonation in these pyroxenes confirms the textural conclusion that they are not in equilibrium with the groundmass. Similar pyroxenes occur in some ultramafic nodules. Thus, Lloyd and Bailey (1975) reported green rims, with compositions similar to those discussed here (Table 2, column 4), on pyroxenes in ultramafic nodules from Uganda and Eifel, which they lucidly argued had arisen by metasomatic introduction of alkalies, Ti, Fe, Al, Mn, H20 and CO2 into normal mantle lherzolite. Similar nodules have been described from West Greenland (Walton and Arnold, 1978) and have been recognized by one of us (I.P.) in the basanites of southern Sweden. In the latter case, the nodules contain veins of brown phonolitic glass (composition reported in Table 3, column 4, and compared with a tinguaite from Trindade, a green pyroxene-bearing locality). Because the glass intimately veins the green pyroxenes and because the pyroxenes are ragged and apparently undergoing breakdown, the glass is inferred

99.45

Sum

99.8

47.9 6.2 2.0 -4.8 7.2 -9.4 20.4 1.9

99.75

53.68 2.87 0.22 0.39 3.89 7.87 0.38 10.25 16.85 3.55

0.162 0.160

0.009

0.568 0.905 0.085

Mn

Mg Ca Na

0.529 0.825 0.142

--

1.808 0.276 0.057 O. 0.136 0.227

0.569 0.672 0.256

0.012

1.999 0.126 0.006 0.011 0.109 0.239

51.8 0.65 0.16 -9.92 5.27 0.51 9.2 19.1 3.58

0.518 0.774 0.262

0.016

0.282 0.167

--

1.959 0.029 0.005

100.19

F o r n o t e a n d e x p l a n a t i o n o f c o l u m n s , see p. 321.

0.000

1.744 0.298 0.069

Si A1 Ti Or Fe 3* Fe 2*

Cation proportions on basis o f 6 oxygens

45.93 5.67 2.40 0.00 5.67 5.05 0.29 10.04 22.25 1.15

SiO 2 AI~O~ T/O 2 Cr203 Fe~O3* FeO MnO MgO CaO Na=O

4

0.826 0.848 0.046

0.004

1.845 0.227 0.030 0.019 0.051 0.104

99.67

50.14 5.23 1.09 0.65 1.83 3.38 0.12 15.07 21.51 0.65

0.795 0.925 0.060

--

0.190 0.014

--

1.812 0.162 0.041

99.93

14.4 23.3 0.83

6.8 0.5

48.9 3.7 1.5

6

5

3

1

2

Colourless p y r o x e n e s

G r e e n salites

Analyses o f s o m e g r e e n c l i n o p y r o x e n e s a n d associated c l i n o p y r o x e n e s

TABLE 2

1.790 0.275 0.052 0.002 0.091 0.111 0.003 0.815 0.808 0.053

99.24

48.28 6.29 1.87 0.06 3.27 3.57 0.08 14.75 20.33 0.74

7

99.47

3.25 1.63 0.11 15.5 23.6 0.86

-

1.909 0.081 0.026 -0.090 0.050 0.003 0.853 0.934 0.062

-

51.7 1.87 0.95

8

1.719 0.340 0.095 0.000 0.068 0.136 0.004 0.680 0.922 0.036

98.85

45.59 7.65 3.36 0.00 2.39 4.30 0.13 12.11 22.83 0.49

9

-

1.806 0.175 0.056 -0.150 0.071 -0.759 0.936 0.048

100.17

5.4 2.3 -13.7 23.5 0.67

-

48.6 4.0 2.0

10

Lilac g r o u n d m a s s

1.745 0.358 0.108 0.000 0.006 0.277 0.006 0.611 0.820 0.069

99.38

45.83 8.87 3.75 0.00 0.21 8.70 0.19 10.78 20.12 0.93

11

bO

321

to be derived from partial melting of the host nodule. There is no indication of where this partial melting t o o k place but it shows the potential of this nodule (of apparent mantle material) to give an evolved partial melt. We conclude that the green pyroxene megacrysts are invariably richer in Na, Fe and Mn than the accompanying groundmass pyroxenes and were resorbed to differing extents. Tetrahedral A1 and Cr contents also seem to be highly variable. The high Fe, Na and Mn contents suggest crystallization from a more evolved liquid than their present host, a conclusion supported by the presence of other evolved minerals in these rocks (see below), while the variability in textures reflect differing lengths of time in the present host. High Cr and octahedral A1 (which is not accompanied by high Ti) are possibly, but not necessarily, reflections of a mantle environment, and it is possible that variability in these components represents variable depths of origin. This last point is supported by the presence of similar pyroxenes in probable mantle fragments and these examples are particularly Cr- and AlVI-rich. NATURE OF ACCOMPANYING PHASES

Other phases which may be present in the green pyroxene-bearing rocks include the lilac groundmass titansalites and colourless clinopyroxene megacrysts and phenocrysts already referred to, together with a variety of evolved minerals which possibly crystallized as part of the same assemblage as the green pyroxenes. DioPside megacrysts and phenocrysts are both colourless in thin section but may be readily distinguished from each other. The megacrysts are larger (around 1 cm as against a few millimeters), they are rounded and subhedral, whereas the phenocrysts are euhedral, and they have higher Mg and Cr contents. Previous workers have shown that the megacrysts originate from two distinct sources: disaggregated mantle lherzolites and high-pressure cumulates.

*Calculated using the program of Papike et al. (1974). 1 = Green core of microphenocryst in basanite from Gran Canaria (sample CKB71--318, analyst: K. Hansen, unpublished). 2 = Green pyroxene core in damtjernite, Fen district, Norway (Griffin and Taylor, 1975, table 2, analysis 5) 3 = Light green core in pyroxene phenocryst from basanite, GSbnehall, Ska°ne, southern Sweden (sample 14816, I. Printzlau, in preparation). 4 = Green edge of clinopyroxene in carbonated biotite clinopyroxenite nodule from Uganda (Lloyd and Bailey, 1975, table 1, sample $23.210). 5 = Colourless clinopyroxene microphenocryst accompanying 1. 6 = Colourless clinopyroxene phenocryst in damtjernite accompanying 2. 7 = Colourless core of phenocryst accompanying 3. 8 = Colourless centre of crystal with margin reported in 4. 9, 10, 11 = Groundmass clinopyroxene compositions co-existing with 1, 2 and 3.

322

TABLE3 Analyses a n d n o r m s o f t h e h o s t r o c k s to t h e p y r o x e n e s in T a b l e 2 a n d p h o n o l i t e glass with c o m p a r i s o n 1

2

3

4

5

SiO 2 A1,O 3 Fe203 FeO MgO CaO Na20 K20 MnO TiO 2 P~O 5 H~O + H~O CO:

41.96 10.80 3.21 8.75 15.10 11.51 2.43 1.40 0.18 3.50 0.93 0.08 ---

29.41 8.15 8.48 6.21 7.32 17.62 2.42 2.06 0.28 4.23 1.55 2.48 0.14 9.49

43.18 14.00 3.31 7.19 10.02 11.00 4.06 1.08 0.19 2.40 0.73 2.58 . . . . . -

51.98 21.70 1.59 3.20 2.09 2.60 8.91 4.90 0.04 1.53 0.42

51.00 19.41 2.97 1.58 1.44 3.40 8.00 5.26 0.15 0.50 0.22 1.82 2.30 1.98

Sum

99.85

99.84

99.74

100.0

100.03

C I P W w e i g h t n o r m (volatile-free a n d w i t h F e ~ O J F e O = 0.15 in c o l u m n s 2 a n d 5) or 8.29 -6.38 28.96 33.22 ab 1.60 -8.76 18.63 18.23 an 14.46 6.08 16.79 4.77 1.62 Ic ne di o]

-10.30 29.14 22.73

10.96 12.74 2.89 26.42

-13.87 26.80 13.65

in mt il ap

-4.66 6.66 2.16

24.51 3.05 9.23 4.12

.

. . 4.80 4.56 1.73

30.75 8.32 2.36

.

.

. . 2.31 2.91 0.99

--29.31 12.50 2.72

0.85 1.01 0.54

1 = S c o r i a c e o u s b o m b in lapilli tufts, Pico de B a n d a m a , G r a n Canaria ( u n p u b l i s h e d analysis o f C K B 7 1 - 3 1 8 b y Geological Survey o f G r e e n l a n d ) . T r a c e e l e m e n t s as follows: (in p p m ) : Rb, 26; Sr, 6 1 0 ; Ba, 4 7 0 ; Zr, 2 5 0 ; Ni, 3 2 0 ; Cr, 670. 2 = D a m t j e r n i t e of Br~nan t y p e ( G r i f f i n a n d Taylor, 1 9 7 5 , t a b l e 1, analysis 5). Original analysis includes 0.26% F. 3 = Basanite f r o m H~sthall, Sk~ne, s o u t h e r n S w e d e n (I. Printzlau, in p r e p a r a t i o n ) . Trace e l e m e n t s (in p p m ) : Rb, 63; Sr, 1 0 8 8 ; Ba, 1 5 8 5 ; Zr, 2 3 8 ; Ni, 3 2 6 ; Cr, 205. 4 = B r o w n p h o n o l i t i c glass in m e t a s o m a t i z e d lherzolite n o d u l e , Sk~ne, s o u t h e r n S w e d e n (I. Printzlau, in p r e p a r a t i o n ) . Analysis r e c a l c u l a t e d to 1 0 0 % f r o m original 9 2 . 3 4 % (i.e. ca. 8% H20, etc.). T h e F e 2 0 3 / F e O r a t i o is a s s u m e d t o be 0.5 as f o u n d in t h e acc o m p a n y i n g basanites, for t h e p u r p o s e o f n o r m calculation. 5 = T i n g u a i t e f r o m Crista de Galo, T r i n d a d e Island, S o u t h A t l a n t i c O c e a n (Almeida, 1 9 6 1 , t a b l e VI, analysis 3).

323

Among other designations, the former have been calle3 the 'Cr-diopside' suite and the latter the 'A1-Ti augite' (e.g. Irving and Wass, 1976); they are both higher in AI vx than the phenocrysts. These pyroxenes and their cogenetic phases will not be considered further here but are significant, along with the mantle lherzolites in demonstrating that the host lava has risen rapidly from mantle levels (i.e. faster than the rate of sinking of these inclusions). While the minerals of the Cr-diopside and A1-Ti augite suites have received much attention in the literature, other phases, which often occur in association with the green pyroxenes, are less well documented. These include reverse-zoned, Fe-rich kaersutite and biotite, apatite, magnetite, sphene and, in one case, haiiyne. Megacrysts of Na-rich feldspar, which have frequently been described, may also belong to this suite. The evidence that these phases are cognate with the green pyroxenes is that (a) they occur in the same rocks, {b) as noted above, they may occur as inclusions in the green pyroxenes, and (c) they have appropriate Fe/Mg ratios to have crystallized alongside the green pyroxenes but not from the magma represented by the groundmass of the rock in which they n o w occur. Iron-rich, reverse-zoned kaersutite was reported from the Canary Islands by Borley et al. (1971) from Tenerife, and by Fernfindez Santfn et al. (1974) from La Palma. Similarly it was observed in a complicated assemblage, which included reverse-zoned green pyroxenes, from West Greenland by Walton and Arnold (1970). Reverse-zoned mica is present alongside green pyroxenes in lamprophyre dikes from West Greenland (K. Hansen, personal communication) and in kimberlites from Kalix, northern Sweden (I. Printzlau, unpublished). It was also reported in kimberlites from Canada by Gittins et al. (1975), but no green pyroxenes were reported in this case. Anorthoclase and sodic plagioclase are frequently reported megacryst phases in basanites and related rocks and are usually regarded as being high pressure megacrysts of the host rock. However, an alternative explanation is that they are xenocrysts and are cognate with the green pyroxenes. Anorthoclase has not yet, to our knowledge, been encountered on the liquidus in a basaltic system, although Chapman (1976) reported it in a basalt with 10% added anorthoclase. It is clear that the bulk composition in Chapman's experiments was thus not basaltic b u t of an intermediate nature with respect to alkalies. In the 1971 eruption of Tenegufa volcano in the Canary Islands, Br~indle et al. (1974) reported sodic plagioclase xenocrysts with included hahyne and adhering trachytic material along with reverse-zoned kaersutites and green pyroxenes in the early, more evolved lavas. The conclusion that the green pyroxenes sometimes crystallized as part of an assemblage also containing Fe-rich kaersutite, anorthoclase, apatite and sphene finds support in an inclusion, possibly cognate, in a phonolite dike described by Borley et al. (1971). This inclusion consists of 90% anorthoclase along with kaersutite, clinopyroxene and sphene. The compositions of the kaersutite and clinopyroxene are similar to those encountered as reversezoned crystals in neighbouring rocks and which are therefore most simply explained as crystals detatched from syenites.

324

ORIGIN OF THE GREEN PYROXENES AND RELATED PHASES The large number of megacryst phases present in many alkaline basic lavas cannot all have been in equilibrium at the same time with the magma represented by the groundmass in which they are now found. As stated in the previous section, it is generally accepted that the minerals of the Cr-diopside suite are derived from mantle lherzolites while those of the A1-Ti augite suite are probably cognate phases precipitated from the magma at depth. What was the nature of the liquid from which the green pyroxenes and their probable cognate minerals formed? There appear to be three possibilities: (a) they are cognate but formed under a different pressure-temperature regime from the other minerals, (b) they are xenocrysts derived from a different magma and (c) they are xenocrysts derived from the wall rocks traversed by the host magma. We believe that the second possibility is the most acceptable although the last may be valid in some instances. The first possibility has been most frequently invoked previously. Thus, for example, Frisch and Schmincke (1969) suggested that the mantling by more Mg-rich pyroxene was a consequence of oxidation caused by the resorption of kaersutite. Wilkinson (1975) also suggested that it might be a consequence of oxidation, which also caused a simultaneous precipitation of magnetite. These explanations do not explain the other chemical peculiarities of the green pyroxenes, i.e. their high Na, Mn and Fe 3+, which suggests that they formed in an alkali-rich environment (see also discussion in Carmichael et al., 1974). In contrast, Borley et al. (1971) suggested that the Fe-rich cores of clinopyroxene and kaersutite grew in the upper regions of magma chambers while the mantling with more Mg-rich material t o o k place after the crystals settled into hotter, more Mg-rich lower parts. This hypothesis does not seem to us very likely on a worldwide basis. If the anorthoclase megacrysts are accepted as being cognate with the green pyroxenes, isotopic studies show they have often grown in an environment with a different ~Sr/~6Sr ratio (Stuckless and Irving, 1976; I. Printzlau, in preparation) and cannot therefore be cognate with the enclosing rock. On the other hand, Chapman and Powell (1976} argued on the basis of density estimates that the anorthoclase megacrysts found in basic rocks had in fact grown from magmas of intermediate composition and it seems to us extremely unlikely that the green pyroxenes can have precipitated from the same magma as the groundmass or A1-Ti augite megacrysts. Thus, Larsen (1977) pointed out that his green pyroxenes had a lower Cr content than even the groundmass pyroxenes and were therefore unlikely to have formed prior to them from the same magma. Also, many green pyroxenes are extremely Fe-rich compared to the groundmass (e.g. Table 2, column 2). Thus, although uncertainties are involved in calculating the Fe/Mg ratio of the coexisting magma from experimental studies (Thompson, 1974; Duke, 1976), largely due to the unknown effect of minor pyroxene components and lack of precise

325

knowledge of the oxidation state of Fe, it is difficult to envisage conditions which would favour such an Fe-rich composition forming. On the basis of these considerations, we regard hypotheses involving a cognate origin for the green pyroxenes as being discredited. The green pyroxenes apparently formed from intermediate to salic ('evolved') magmas and were later incorporated into the more basic magmas in which they are n o w found, i.e. possibilities (b) and (c}. While some of the described occurrences may arise by mechanism (c), this does not appear to be the most important, otherwise polymineralic xenoliths would be expected to be frequent and this is not the case. We believe that the most reasonable explanation is that they grew from an evolved magma and, while the crystals were still in suspension, this was overtaken and became mixed with a substantial volume of more primitive magma carrying minerals of the Cr-diopside and A1-Ti augite suite. Thompson (1977} arrived at a similar conclusion and was able to estimate from the relative abundance of the two types of pyroxene in a lava from Colli Albani, Italy, that the basic magma had comprised about 90% of the total. Likewise, it has been suggested that the nepheline hawaiites in southern Sweden might have formed by mixing of the commoner basanitic magma with a b o u t 30% phonolite of the composition represented by the glass (Table 4) found in some of the nodules (I. Printzlau, in preparation). Two features are to be expected in products of magma mixing: linear variation and occasional inhomogeneity. It might be argued that non-linear variation in many alkaline suites does not support the view that mixing has played a significant role in their genesis. However, many alkaline suites show only minor departure from linearity, e.g. the lavas of Roccamonfina, Italy, which were described as 'exhibiting a linear trend similar to the calc-alkaline t~end' (Ghiara and Lirer, 1976--1977} and in other cases non-linearity may be superimposed by other processes (accumulation of crystals, abundance of mantle xenocrysts). In other, well-established cases of magma mixing examples are found where the process has been arrested giving an inhomogeneous rock (e.g. the 'emulsion lavas' of Iceland, Blake et al., 1965). Alkaline rocks of this nature apparently do occur; we infer the example shown in Fig. 2 to be such. Similar textures are described and depicted from the early stages of the Teneguia eruption of La Palma, Canary Islands, ( F e m ~ d e z Santin et al., 1974, Fig.l) and were explained by Ibarrola (1974} as due to the assimilation of trachytic or phonolitic material to give the earlier, more evolved lavas, which also contain green pyroxene and reverse-zoned kaersutite. These features are just as readily explained as being due to magma mixing. Of course, it is extremely difficult to decide if such textures have arisen b y mixing or unmixing, b u t it appears that the named examples are essentially different from the immiscibility textures described by Philpotts (1971) as t h e y are diffuse and do n o t apparently contain minerals of the same composition in both phases as immiscibility requires. However, more work needs to be done on these textures in the light of our suggestion, including experimental work to show that the phases are indeed capable of mixing.

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Fig.2. Photograph of a polished slab (ca. 4 cm long) of a lamprophyre dike from Kangerdlugssuaq, East Greenland, showing heterogeneous texture interpreted as an arrested stage in the mixing of two components (specimen CKB 70-21, analysed and briefly described by Brooks and Platt, 1975). Although analyses of the two components have not been made, the light part is composed predominantly of alkali feldspar (~Or~2 A.b2s An3) and is trachytic while the dark part is basanitic. This rock contains bright green cores to the microphenocrysts with ca. 13% acmite and a high Fe/Mg ratio as well as abundant fragments of apatite. We conclude on the basis of the above discussion that the green pyroxenes and their cognate phases originate as phenocrysts in intermediate to salic magma which has later mixed, on occasions incompletely, with a basic magma. The rocks in which they are now f o und must accordingly be hybri d rocks and at least some of the variation observed in alkaline rock suites might be caused by mixing of this type. INTERMEDIATE AND SALIC MAGMAS IN THE MANTLE While the basic members of alkaline suites are probably derived f r o m the mantle with little modification (they have an appropriate composition and carry nodules of mantle material), the salic members of these suites are usually regarded as being near-surface differentiates of the basic magmas. In this section we will assemble the evidence to show t hat some salic magmas may arise by direct partial melting in the mantle. This evidence, although perhaps n o t conclusive, is nevertheless persuasive, and an awareness o f this possibility will hopefully lead t o further evidence being uncovered. The evidence is twofold: (a) presence of salic glasses in mantle nodules, and (b) presence of mantle nodules in salic lavas. Nodules f r o m southern Sweden, which contain phonolitic glass and green pyroxenes, have already been referred to in the section 'Nature of the green p y r o x e n e s ' (p. 318). The nodules and mineral compositions are similar to those described by Lloyd and Bailey (1975) and are believed t o be fragments of anomalous mantle, i.e. mantle metasomatized by various c o m p o n e n t s derived f r o m deeper levels of the mantle, for which much evidence has recent-

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ly been accumulating (e.g. Boettcher et al., 1977; Erlank and Richard, 1977). Textures in the Swedish nodules suggest that the phonolitic melt is in the process of forming at the expense of the nodule minerals and thus indicates the possibility of melts of this composition forming from mantle materials. Such small quantities of melt might find it difficult to segregate into descrete bodies but would be available for mixing with later, more basic and more voluminous fractions. In other cases, segregation and upward migration might occur with either eruption at the surface or pauses in magma chambers at various depths, where green pyroxenes and other minerals would crystallize, before being overtaken and flushed out by basic melts, with which mixing would occur. From our work it is not possible to say if the anomalous mantle is regional in extent or has arisen locally around magma pockets, but other studies (e.g., Lloyd and Bailey, 1975) suggest that the former is the case. That salic magmas can originate in the mantle is also shown by reports of mantle inclusions in hawaiite (Wilkinson and Binns, 1969), trachyte (Wright, 1969a, b) and phonolite (Frechen, 1948, p. 401; Wright, 1966; Price and Green, 1972). Reviews of this subject with reference to the Australian/Tasmanian province have recently been given by Green et al. (1974) and Sutherland (1974). Of course, such magmas may be the products of crystal fractionation at mantle depths but this seems to require a rather complicated thermal regime. Direct derivation of salic magmas in the mantle would also be a convenient explanation for the existence of 'Daly gaps' (Chayes, 1977). Thompson (1977) has been able to show that potassium-rich rocks from Colli Albani, Italy, were generated at mantle depths although their composition is quite incompatible with their having been in equilibrium with normal mantle. In this case, the 'anomalous mantle' was apparently a subducted crustal slab. CONCLUSIONS Green pyroxenes are present in many alkaline, basic lavas along with a number of other minerals which are incompatible with the host rock. Several lines of evidence lead us to conclude that these minerals probably are phenocrysts derived from intermediate to salic magmas, which have undergone mixing with the basic magma. Although other explanations are possible in individual cases, we find the evidence for magma mixing to be generally convincing. More controversially, we suggest that the evolved magmas may be generated in the mantle and their high Fe/Mg ratios and low Ni content (features which are generally judged to be inconsistent with a mantle origin), are due to melting of an anomalous, possibly metasomatized mantle, for which there is n o w abundant evidence. The following model pictures events which may possibly occur in alkaline provinces. Influx of volatiles and incompatible elements from deeper levels of the mantle locally or regionally generates an upper mantle of anomalous

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c o m p o s i t i o n w h i c h w o u l d cause the u p l i f t o f t e n o b s e r v e d in rift areas ( L l o y d and Bailey, 1975). Partial melting, w h i c h m a y be d e l a y e d f o r long p e r i o d s ( B r o o k s et al., 1976), will first cause the p r o d u c t i o n o f evolved m e l t s rich in i n c o m p a t i b l e e l e m e n t s a n d volatiles. T h e early evolved m e l t s either rise t o the surface w i t h o u t f u r t h e r m o d i f i c a t i o n , collect in i n t e r m e d i a t e d e p t h m a g m a c h a m b e r s ( w h e r e green p y r o x e n e s a n d o t h e r phases m a y begin to p r e c i p i t a t e b e f o r e being o v e r t a k e n b y later m e l t fractions, which m i x w i t h the evolved m a g m a ) , or m i x e s with t h e m o r e basic m a g m a s at t h e site o f g e n e r a t i o n . In this w a y , a wide c o m p o s i t i o n a l and t e x t u r a l range o f green p y r o x e n e s can be a c c o u n t e d for, as can t h e i r various m o d e s o f o c c u r r e n c e , i.e. in m a n t l e nodules, as relict p h e n o c r y s t s a n d as occasional c u m u l a t e s w i t h apatite and magnetite. This p a p e r p r e s e n t s a review o f t h e s e green p y r o x e n e s and t h e i r related phases, w h i c h h i t h e r t o have a t t r a c t e d little a t t e n t i o n . We a d m i t t h a t s o m e o f t h e a r g u m e n t s p r e s e n t e d h e r e are c o n j e c t u r a l b u t we h o p e t h a t o u r suggestions will s t i m u l a t e o t h e r s to critically appraise f u r t h e r o c c u r r e n c e s o f this interesting a s s e m b l a g e o f minerals. ACKNOWLEDGEMENTS We t h a n k K. H a n s e n , P.R. K y l e and J.G. Larsen f o r access to u n p u b l i s h e d i n f o r m a t i o n . T h e m a n u s c r i p t has b e n i f i t t e d f r o m t h e c o m m e n t s o f A.T. A n d e r s o n , B.H. Baker, P.R. Kyle, H.U. S c h m i n c k e a n d R.N. T h o m p s o n . REFERENCES Almeida, F.F.M. de, 1961. Geologia e Petrologia da Ilha da Trindade. Div. Geol. Min. (Rio de Janeiro) Monogr., 18. Anderson, A.T., 1974. Evidence for a picritic, volatile-rich magma beneath Mt. Shasta, California. J. Petrol., 15: 243--267. Anderson, A.T., 1976. Magma mixing: petrological process and volcanological tool. J. Volcanol. Geotherm. Res., 1:3--33 Anderson, A.T. and Wright, T.L., 1972. Phenocrysts and glass inclusions and their bearing on oxidation and mixing of basaltic magmas, Kilauea volcano, Hawaii. Am. Mineral., 57: 188--216. Babkine, J., ConquerS, F. and Vilmonot, J.C., 1968. Les caract~res particuliers du volcanisme au nord de Montpellier: l'absarokite du Pouget; la ferrisalite sodique de Grabels. Bull. Soc. Ft. Mineral. Cristallogr., 91: 141--150. Best, M.G. and Brimhall, W.H., 1974. Late Cenozoic alkali basaltic magmas in the western Colorado Plateau and the Basin and Range transition zone, U.S.A. and their bearing on mantle dynamics. Geol. Soc. Am. Bull., 85: 1677--1690. Blake, D.H., ElweU, R.W.D., Gibson, I.L., Skelhorn, R.R. and Walker, G.P.L., 1965. Some relationships resulting from the intimate association of acid and basic magmas. Q. J. Geol, Soc. London, 121: 31--49. Boettcher, A.L., O'Neil, J.R., Windom, K.E., Stewart, D.C. and Wishire, H.G., 1977. Metasomatism and the genesis of kimberlites and alkali basalts. Extended Abstracts, 2rid Int. Kimberlite Conf., 3 pp. Borley, G.D., Suddaby, P. and Scott, P., 1971. Some xenoliths from the alkalic rocks of Teneriffe, Canary Islands. Contrib. Mineral. Petrol., 31: 102--114.

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