Magma mixing in the subvolcanic environment: petrology of the Gerena interaction zone near Seville,...

Contrib Mineral Petrol (1990) 105:9-26 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1990

Magma mixing in the subvolcanic environment: petrology of the Gerena interaction zone near Seville, Spain Antonio Castro 1' 2, Jesds D. de la Rosa 1, and W. Edryd Stephens 2

1 Departamento de Geologia y Mineria, Universidad de Sevilla, E-21819 La R/tbida, Huelva, Spain 2 Geology Division, University of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland

Received January 23, 1990 / Accepted July 2, 1990

Abstract. The mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high-level subvolcanic setting in the Spanish Hercynian at Gerena, near Seville. The compositions involved are calc-alkaline and the situation is one of tonalite-quartz diorite synplutonic dykes injected into a granitic magma chamber. The resulting hybrids include dykes, pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite. The present investigation is based on field and petrographic observations of hybridization textures, the identification of different stages in the crys- tallisation history of the tonalite through mineral textures, and the characterization of mineral compositions at these various stages. Proportions of the end-member magmas involved were obtained by major-oxide mixing models and tested satisfactorily with trace elements. A mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber. After high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions. It is argued that such high-energy flow conditions are a necessary re- quirement for effective hybridization in this environment in contrast to most large-scale magma chamber settings where mixing is driven by thermal and buoyancy con- trasts.

Introduction

The importance of magma mixing in the petrogenesis of calc-alkaline granitoid rocks is a matter of current

Offprint requests to . A. Castro

debate. While the principles of magma mixing were en- unciated early in the history of petrology (Bunsen 1851), Bowen (1928) later dismissed it as of little petrogenetic importance and consequently the process was largely disregarded by petrologists. Chemical (especially isotop- ic) studies have in the last decade revealed that mixing of magmas of contrasting composition is entirely consistent with available data and is the only consistent expla- nation of such data in some cases. This revival of interest in mixing has resulted in a large number of studies estab- lishing chemical principles (Langmuir et al. 1978), physical constraints (e.g. McBirney 1979; Blake 1981; Sparks and Marshall 1986), and petrographic criteria (e.g. An- derson 1976; Hibbard 1981). While "magma mixing" is once more widely regarded as an important process there is still considerable debate as to the quantitative contribution made by this process to the observed diversity of igneous rocks.

Calc-alkaline igneous activity at active plate margins is volumetrically dominant in crust-building and crustal- recycling processes. In volcanoes from such environments it is common to observe wide ranges in compositions of co-existing magmas (e.g. MacDonald and Kat- sura 1965) yet there is no consensus of opinion as to the extent to which mixing is significant in generating the calc-alkaline series. At least in volcanic series there are often textural criteria, usually glass inclusions (Anderson 1976) or disequilibrium mineral phases which disclose the mixing process (Gerlach and Grove 1982). In the plutonic environment such features rarely survive subsequent cooling and recrystallization, and whether magma mixing is important in generating the compositional diversity observed in cordilleran batholiths (as was suggested by Reid et al. 1983) is not yet known.

In the plutonic environment, where obvious mixed- magma features are rarely preserved, studies of magma interaction zones will help to establish criteria by which mixed parentage may be recognized. In order to study the effects in the calc-alkaline plutonic environment we have sought an example where there is a marked compositional contrast in a field situation of unambiguous

10

m a g m a to m a g m a con tac t a t a re la t ively high level. The selected area, the G e r e n a mass i f near Seville in the Her- cynian o f S W Spain, thus offers the o p p o r t u n i t y to inves- t igate tex tura l and c o m p o s i t i o n a l va r i a t ions o f hybr ids tha t m a y be expected to survive in the sha l low p lu ton ic s i tua t ion .

M a g m a mixing is the end resul t o f a process which br ings toge ther m a g m a s o f diverse c o m p o s i t i o n which in te rac t chemical ly and physical ly . The mos t c o m m o n l y descr ibed s i tua t ion is tha t o f a m o r e bas ic m a g m a injec ted into a felsic m a g m a . In such a case the expected m a g m a t i c t e m p e r a t u r e difference at the ins tan t o f injec- t ion m a y be o f the o rde r o f 200~400 ~ C and the conse- quences will involve marg ina l quench ing o f the ho t t e r m a g m a . C o n t i n u e d phys ica l in t e rac t ion m a y lead to in- c o r p o r a t i o n o f crystals , aggregates or clots in the hos t m a g m a , whereas chemica l in te rac t ion results in new growth , r e so rp t i on or r eac t ion textures a t the marg ins o f grains. In this p a p e r the t e rm " m i x i n g " is used for the process involved in b lend ing m a g m a s and phys ica l ly d i s t r ibu t ing xenogenous sol id par t ic les in m a g m a , " m i n - g l i ng" for the process o f mix ing tha t has n o t gone to c om p le t i on leaving a hyb r id rock with ident i f iable end- members , " h y b r i d i z a t i o n " as a process which m a y resul t in ei ther mixed or ming led rocks. Such usage is close to tha t def ined by Sparks and M a r s h a l l (1986) and Z o r p i et al. (1989).

The a im o f this p a p e r is to descr ibe the ma in pe t ro - logic fea tures o f one o f the in te rac t ion zones o f the Ger - ena massif , in o rde r to in te rp re t the d y n a m i c aspects o f m a g m a mix ing in a subvo lcan ic / sha l low p lu ton ic en- v i ronmen t and to define a set o f tex tura l and minera log i - cal pa t t e rns d i rec t ly re la ted to the mix ing o f tonal i t ic and grani t ic magmas . The genera t ion o f such pa t t e rns by the ming l ing a n d mix ing processes m a y have wider app l i ca t ion in the i n t e rp re t a t i on o f mix ing at the p lu ton- ic scale where of ten there is l i t t le bu t i so top ic evidence o f mixed pa ren t age remaining .

The Gerena massif

Geological setting

The Gerena massif is situated on the southern edge of the South Portuguese Zone (SPZ, Julivert et al. 1974) of the Hercynian chain of Iberia (Fig. 1). The SPZ is characterized by the presence of 10-20 km plutonic bodies, elongate NW-SE following the trend of regional structures. The plutonic rocks are epizonal and sometimes subvolcanic, and include granites, tonalites and gabbroic rocks alternating in elongated sheets intruding a pelitic-quartzitic sequence of Devonian-Lower Carboniferous age and occasionally intruding part of the Devonian-Carboniferous volcaniclastic complex (Simancas 1983). Due to the nearly simultaneous intrusion of both felsic and mafic magmas, the occurrence of interaction zones is very common. These are of two main types. (1) net-veined complexes, such as that described by Marshall and Sparks (1984) in which the felsic magma intruded a consolidated tonalite; (2) globular intrusions and synplutonic dykes of mafic magma intruding a felsic magma chamber often resulting in hybrid granodiorite rocks.

The Gerena massif is one of these interaction zones, in which magma-mingling, crystal-mingling and complete mixing occurred between a granitoid host and an intruding mafic magma. The zone of interaction appears on the southern part of the massif, along an E-W direction, and is particularly well exposed in several quar- ries. One of these outcrops near the village of Gerena has been chosen for detailed study as it is representative of the complete range of field relationships and rock types.

Rock types and field relations

Three main rock types have been distinguished in the Gerena interaction zone (GIZ), namely: (1) tonalites and quartz diorites; (2) biotite granitoids; (3)hybrid granoidiorites. The tonalites appear as pillow-like bodies, synplutonic dykes and fragments en- closed in the host granitoid. The hybrid rocks appear in two main settings, either in large homogeneous bands, or close to disrupted synplutonic dykes (Fig. 1).

Pillow-like bodies

There are two main types of pillowed bodies: (1) globules of variable size (Fig. 2 a); and (2) pillowed masses with crenulate contacts. In both cases the common characteristic is the presence of a darker rim with few or no phenocrysts. This rim is a finer-grained margin suggesting that the tonalite was intruded into the granitic magma and quenched. The trace of the contact is generally crenulate with convex lobes suggesting a low contrast in ductility between the two magmas during intrusion. The largest globules are usually fractured and invaded by granitic magma (Fig. 2b). The pillowed masses differ from the globules in their larger size and presence of both sharp and diffuse contacts. When the contact is sharp there is a finer-grained darker margin, but when the contact is diffuse the transition is defined by intermediate hybrid rocks.

Synplutonic dykes

Synplutonic dykes are prominent features of the GIZ. They are tabular bodies averaging ten m in length and one-two m in thick- ness (Fig. 2c). The lateral ends are mostly intrusive and sharp, and in some cases exploit synplutonic fractures which do not con- tinue beyond the dyke. The interpretation that contacts are intrusive is supported by the presence of a darker, fine-grained margin with a crenulate shape similar to that of the pillowed bodies. Nor- mally the dykes appear fragmented, the inner part being in contact with the host granitoid. This kind of contact is always transitional (Fig. 2d) throughout a hybrid granodiorite, suggesting that the dyke was broken during the magmatic stage, creating a situation for potential mingling.

Fragments

Apart from a few minor globules like that of Fig. 2a, most of the small- to medium-sized tonalitic bodies (1-30 cm) are fragments, more-or-less reworked by the host granitic magma. Frag- ments are clearly recognized by the absence of fine-grained margins and non-crenulated contacts. The shape of the fragments is very variable, from angular, non-reworked fragments, to rounded, strongly reworked fragments. These fragments can be shown to derive from disrupted globules and synplutonic dykes in which part of the chilled margin is sometimes preserved (Fig. 2e). The angular fragments are generally arranged following flow structures around the major globules and pillowed masses (Fig. 2b, c). The rounded fragments may in some cases have transitional contacts throughout a hybrid aureole while in other cases they have sharp

11

oo Postectonic granitoid �9 Castilblanco granite #Quartz diorites Ill Diorites and gabbros

37~

? ~15'wT":'"T '~K:m' GERENA MASSIF

'7

9 ,'2

SCALE

; . + +

1meter , I Z . +

TONALITE

HOST GRANITOI D [;--7] HYBRID ROCK

(HOMOGENEOUS)

LEGEND:

I 1

.~'-, ~+ + ,

* + ~ § + + + +

§ + + + ~- § + + § + + . + + + + + § §

Fig. 1. Geological sketch of a vertical wall in the Gerena interaction zone. Note the pillowed form of most of the tonalitic bodies. Inset: setting of the Gerena massif in the south Portuguese zone (after Simancas 1983). Key to structural zones of the Iberian Massif:

HYBRID ROCK (HETEROGENEOUS)

SHARP C()NTACT . . . . . TRANSITIONAL CONTACT

CZ, Cantabr ian zone; WALZ, western Asturian-Leonese zone; CIZ, central Iberian zone; OMZ, Ossa-Morena zone; SPZ, south Portuguese zone

contacts, with no internal zoning. The fragments are considered to be enclaves, as they are solid material reworked within the granitic magma, though their origin is clearly related to the crystallization of a mafic magma intruded early into the host granitoid in which they now appear.

Hybrid zones

Another important feature of the GIZ is the presence of large hybrid zones related to flow structures (Fig. 2f). These zones are composed of a homogeneous hybrid granodiorite containing rounded, relict fragments of tonalites with diffuse contacts. These hybrid zones mainly appear at the base of the GIZ and locally at the transition zone between a tonalite dyke and host granitoid. The hybrid rocks vary from a nearly homogeneous facies to very heterogeneous rocks comprising a mixture of granite and milli- metre-scale enclaves of tonalite. The association of the more homogeneous facies with apparent flow-zones indicates that magmatic flow was an important factor promoting mixing and homogenization of the resulting hybrid magma. A detailed traverse to study the degree of chemical homogenization was carried out in the hybrid zone shown and the results are presented and discussed in a later section.

Petrography

The three main rock types of GIZ are: (1) tonalites and quartz diorites, representing the mafic magma; (2) biotite granitoids representing the host felsic magma; (3) hybrid granodiorites resulting from the mingling and mixing of (1) and (2).

Mafic rocks: tonalites and quartz diorites

These are fine-grained, slightly porphyritic rocks composed essentially of plagioclase, amphibole (green hornblende), quartz and subordinate biotite, apatite, alkali- feldspar, titanite, zircon and ore minerals. Modal abundances are listed in Table 1. Typical secondary minerals are chlorite, actinolite and epidote. Chlorite-epidote aggregates, with rounded shapes up four mm in diameter are features probably representing pseudomorphs of an early pyroxene. Also important are the presence of large (4-5 ram), rounded micro-xenoliths rimmed by a screen

13

Table 1. Modal analysis of representative samples of the Gerena interaction zone

Rock type To To To HG HG HG Gr Hy Hy Hy Hy sample GE-20 GE-19 GE-22 G E - 2 1 GE-23 GE-33 GE-37 GE-26 GE-29 GE-30 GE-34

Plagioclase 53.5 54.3 56.1 47 51.9 42.8 24.5 51.9 54 55.4 56 Quartz 11.7 15.1 18.5 27.7 24.1 38.5 41.3 30.8 27.2 23.9 23.2 K-feldspar 2.6 1.7 4.4 12.4 12.2 10.9 32.8 6.8 3.9 4 4.1 Amphibole 30.1 27.5 17.8 0.45 0 0 0 0 9.4 11.6 9.4 Biotite 1.5 1 3 11.9 11.3 7.6 1.1 10.3 5.2 4.8 7

To, tonalite; HG, host granite; Gr, granophyre; Hy, hybrid rocks

of small hornblende crystals; as well as leucocratic inclusions regularly distributed in several enclaves.

Plagioclase occurs as phenocrysts and in the fine- grained matrix. Two types of phenocryst are distinguished, namely: P L I is a large unzoned core with a high anorthite content (An55_65) rimmed abruptly by a more albitic plagioclase (An3o-An2o). These cores are interpreted as early phenocrysts indicating the fractionated character of the tonalitic magma. PL2 are euhedral crystals with a dendritic texture similar to that described by Hibbard (1981); these are the most common type of phenocryst. The dendrites have compositions of An3e~35 and their in-fillings have compositions of An2~25. Type PL2 includes hornblende, biotite and quartz of the matrix. Such dendritic texture has been interpreted by Hibbard (1981) to result from undercooling.

In the matrix, plagioclase is the most abundant mineral and is characterized by a tabular habit.

Hornblende occurs in the matrix along with plagioclase, quartz and biotite, and also forms fine-grained aggregates (Fig. 3a). These have a prismatic external shape suggesting an origin by reaction from pyroxene to hornblende during the magmatic stage; pyroxene re- licts have, however, been observed in only one thin section.

Quartz is invariably an interstitial, late phase appearing in the matrix and poikilitically including plagioclase, hornblende and biotite. Poikilitic quartz is best developed at the cores of dykes, pillowed masses and globules, forming large crystals that include matrix minerals at their edges.

Biotite is normally a subordinate phase with the ex- ception of the external part of the fine-grained margins

Fig. 2a-h. Different field relationships of the rocks and bodies appearing in the Gerena interaction zone: a mafic globule showing the chilled margin and lobate (liquid-liquid) contact; b major globule of tonalite magma pillowed and fractured; e synplutonic dyke and hybrid bands (bottom); d fragmented synplutonic dyke with location of the studied samples; e fragment of a synplutonic dyke showing a discontinuous chilled margin; f hybrid bands with tonalitic enclaves; the location of the hybrid-zone traverse samples is shown; g granitic ocelli in a tonalitic globule; h lobate granitic inclusions in a tonalitie fragment

(in which biotite is an essential phase) being more abundant than hornblende. It always appears in the matrix, with the same grain-size as hornblende and plagioclase.

K-feldspar is < 5% volume. It can appear as poikilitic crystals together with quartz.

Apatite is a typical accessory phase. Its habit is always acicular and it appears included in biotite, quartz and the external rims of the matrix plagioclases as well as in the rims of the plagioclase phenocrysts. The acicular habit is further support for an undercooling event during the crystallization of the tonalitic magma (Wyllie et al. 1962).

Micro-xenoliths are rounded bodies (1~4 mm) composed of a few crystals of quartz and K-feldspar or comprising a single crystal of quartz (Fig. 3b). They are characterized by the presence of a rim of hornblende crystals. Similar features were interpreted as micro-xenoliths or xenocrysts from the host granite (Vernon 1984) on which hornblende nucleated in fine crystals around the body. Crystals in the tonalitic magma continued to grow over the hornblende rim, and in optical continuity with the core. The rounded shape suggests that these bodies were resorbed in the tonalitic magma before the crystallization of hornblende.

Leucocratic inclusions are found in several of the tonalitic bodies. They are commonly concentrated near to the fine-grained margin of a fragment, and vary con- siderably in shape. Most are rounded or ocellar (2- 10 mm) (Fig. 2g). Others have irregular, lobate contacts with the host tonalite, are somewhat larger than the ocellar bodies (Fig. 2h) and are aligned parallel to the contact. These inclusions have concentric zoning with a core composed of quartz and a rim of K-feldspar and quartz. The leucocratic inclusions are interpreted as granitic liquid incorporated as droplets in the tonalitic magma; a similar interpretation was offered by Bussell (1988) for similar features in the Puscao Super Unit of the Andean Coastal Batholith. The variable shapes of these inclusions are attributed to the viscosity contrast of the two magmas and the different sizes of the inclusions.

Felsic rocks: host granitoids

These are leucocratic, medium-grained rocks. The texture is locally granophyric and the presence of graphic quartz is a common feature. Quartz, K-feldspar and plagioclase are the essential phases, with subordinate bio-

14

Fig. 3. a Typical hornblende clot in the tonalitic rocks, b Quartz xenocryst in the tonalite of the synplutonic dykes. Note the presence of the hornblende rim. e, d Plagioclases from the hybrid rocks.

Note in: e the corroded core crosscutting the previous zoning; d repeated resorption surfaces. Scale bar = 0.8 mm

tite. Allanite, apatite, hornblende and zircon are the commonest accessory minerals.

Plagioclase is generally unzoned or slightly zoned with a composition ranging from An24 at the cores to An20_15 at the rims. It generally includes quartz, biotite and apatite. Another form of plagioclase has an internal zone more anorthitic than the core. It is more abundant close to the hybrid facies and will be described later.

Quartz appears, forming large (2-3 mm), rounded crystals ending at the edges in a quartz - K-feldspar symplectite with graphic texture. Biotite appears as euhedral to subhedral crystals with inclusions of prismatic apatite, zircon and ore minerals.

Hybrid rocks

These are mesocratic, medium-grained rocks, locally highly heterogeneous in both texture and composition. They form large masses at the lower part (see Fig. 1) of the GIZ, with evident flow structures, and at the transitional contacts between the tonalite and host granitoids

of the fragmented synplutonic dykes. The texture is very heterogeneous being sub-ophitic and hypidiomorphic in the same thin section. Plagioclase, quartz, biotite, hornblende and K-feldspar are the essential components. Typical accessory minerals are apatite, zircon, titanite and ore minerals.

At least four types of plagioclase can be distinguished in the hybrid granodiorites: PL1 is identical to PL1 of the tonalite. The external rim may appear resorbed and replaced by a slightly more calcic plagioclase. PL2 is dendritic and identical to PL2 of the tonalite. PL3 is the more abundant and characteristic phase of the hybrid rocks. Several examples are shown in Fig. 3 c, d. They have unzoned or slightly zoned cores with identical characteristics to the plagioclases of the host granitoid. These cores are resorbed and regrown as a more calcic plagioclase ranging from An3c~45 to An20 25 towards the external rim. This rim also may be resorbed and regrown, sometimes repeatedly. Plagioclase in Fig. 3 d shows three stages of resorption suggesting that mixing is a dynamic process in which pulses of mafic liquid were injected several times (cf. Pearce et al. 1987).

15

A

Piag ioc la se phenocryst Pta9iodase matrix Quartz phenocryst

Quartz matrix

K-feldspar

Amphibole

Biotite

HOS~ANITOID HYBRID ROCK D1 0203040506 D7 D8 D9 OlO 0]1 D12

I I~ ~ ' - - ,

, ' ~ T +

0

CHILLED MARGIN B HYBRIDROCK TO.AUTE "~"OS.L~5

F1 F2 F3 F4 F5 F6B F6A

, [

i _ _ i ~ I

I

O~ percenfage scale : 0

I I

i

3O

Fig. 4a, b. Variations in modal abundances along: a the hybrid zone traverse; b the synplutonic dyke traverse. Acronyms refer to samples of Fig. 2d, f

PL4 appears in the matrix as tabular crystals similar to the plagioclase of the matrix of the tonalites. It may have been crystallized in part from the tonalitic magma and in part from the mixed liquid so that the rim has the same composition as the fillings of the resorbed zones of PL3.

The other minerals, namely quartz, biotite and hornblende, appear both as phenocrysts and in the fine- grained matrix.

Crystallization sequences

In a study of magma-mingling and -mixing the sequence of crystallization is important in constraining the timing of mixing events relative to cooling history. Classical criteria such as inclusion and moulding have been used to determine the sequences of crystallization in the GIZ rocks. This sequence is very difficult to establish and unravel for the hybrid rocks as most of their coarse crystals were derived from the tonalites and host granitoid. These crystals often have complex histories. Plagio- clase textures, for instance, indicate early growth followed by resorption and regrowth. By contrast, the relative crystallization sequence in the host granitoid is relatively straightforward and characterized by an early plagioclase-quartz assemblage followed by late crystallization of quartz and K-feldspar, giving rise to the granophyric intergrowth texture.

The relative crystallization sequence in the tonalite is more complex indicating an episodic cooling history during consolidation of the mafic magma. Three stages of crystallization have been distinguished on the basis of the appearance of minerals in the matrix or as phenocrysts, as well as the textural characteristics of phenocrysts:

1). The first is an early, low nucleation density stage in which an early, high-An plagioclase formed as phenocrysts along with a ferromagnesian phase (now altered to epidote-chlorite-titanite aggregates). Secondary hornblende aggregates probably formed initially as clinopy- roxenes at this stage.

2). The second is a high nucleation density stage in which most, but not all, of the matrix minerals were

developed. These are: (a) the tabular plagioclase (or at least their calcic cores); (b) hornblende; (c) biotite; (d) acicular apatite; (e)the dendritic stages of the plagioclase phenocrysts (PI2). The fine grain-size, the dendritic growth of plagioclase (Hibbard 1981) and the acicular habit of apatite suggest that this stage of crystallization involved quenching.

3). The final stage involved overgrowth in which the plagioclases (both phenocrysts and matrix) grew further towards more albitic compositions with poikilitic texture, as well as interstitial crystallization of large, poikilitic crystals of quartz and (less abundantly) K-feldspar. We designate this the overgrowth stage.

Field evidence suggests that the quenching responsi- ble for the high nucleation-density stage occurred when the mafic tonalitic liquid (carrying phenocrysts from the low nucleation-density stage) intruded felsic magma. The textural evidence suggests that after thermal equilibrium was attained between the tonalitic magma and the granitic magma a significant melt-fraction remained in the tonalite. Subsequent crystallization was presumably less rapid and took place under the cooling regime of the whole mass. Crystallization of this residual tonalitic melt, in thermal equilibrium with the host granitoid, gave rise to the poikilitic crystals of the overgrowth stage of crystallization of the tonalites. This textural pattern indicates a quenching episode which begins below the tonalite liquids temperature but ends before the solidus temperature is reached. It is a texture commonly observed in tonalitic enclaves in granodioritic rocks and may be an indication of synplutonic injection and crystallization of the tonalitic magma.

Variations across the hybrid zones and synplutonic dykes

A traverse of 12 core samples, extracted with a drill, was recovered along 54cm across a hybrid zone (Fig. 20. Rock type and mineral-abundance data are presented in Fig. 4a. The petrography of the rocks is similar to that described previously for both the host granitoid and the hybrid rocks. Changes observed from the granodiorite to the hybrid zone include:

16

/ J

f D.-1 "4"~

HOST GRANITOI0

, ~ / / / /

L 25, 22

g

/ \

/ 21

I i ,J t

/~;N 7 7 L 7 L

/ ( LL7

i

24

4 5

~.7 i

Fig. 5. Textures and compositions (in mo1% An) of representative plagioclase crystals with resorbed cores from within the hybrid-zone traverse of Fig. 2f. Stippled area indicates the filling around the resorption surface. Scale bar = 0.5 mm

(1) the matrix/phenocryst ratio increases; (2) the textural heterogeneity increases; (3) the biotite/hornblende ratio of the matrix decreases.

Plagioclase in the hybrid zone is characterized in general by the presence of a corroded core rimmed by a more anorthitic plagioclase which evolves outwards to a more albitic outer rim. The plagioclase in the hybrids first crystallized in the felsic magma and then suffered some dissolution when the tonalitic magma intruded the felsic magma. Some blending of the tonalitic (and presumably more calcic) magma with the felsic magma resulted in the crystallization of a more calcic rim over the resorbed zone. Finally a more albitic overgrowth resulted from subsequent cooling and fractionation of the hybrid magma. If plagioclase composition is a sensitive indicator of local-magma bulk-composition then the composition of such a blended liquid is clearly very heterogeneous, as revealed by the composition of the infilling plagioclases across the traverse (Fig. 5). Their normally zoned external rims developed after magma-mingling and consequently have nearly homogeneous rim- compositions. Such rims may be absent in several plagioclases mainly in the host granitoid (samples D1, D2, D3 of Fig. 5). The process of resorption and infilling may be repeated several times as revealed by the maxima and minima that characterize the compositional pattern of these plagioclases. A typical zoning pattern of a multi- ply resorbed crystal is shown in Fig. 3 d.

A second traverse of six core samples was made across a fragmented synplutonic dyke (see Fig. 2d). The dyke has a sharp contact and chilled margin on one side, and a transitional contact on the other side. The petrography of the tonalites, hybrid rocks and host gran-

itoids in this synplutonic dyke traverse is essentially similar to the descriptions of these types given above. Data for rocks types and mineral abundances are represented in Fig. 4b. Note the slight increase in the biotite content at the chilled margin (F5) mirroring a decrease in the hornblende content. Mineral-abundance varies gradually from the host granitoid to the inner part of the dyke.

Mineral chemistry

Plagioclases and ferromagnesian phases are sensitive to physico- chemical changes during mafic-felsic magma interactions. Plagio- clases were studied optically and the results were referred to above. Amphiboles and biotites were analyzed with a JEOL JCXA733 electron probe operating at 15 kV and 20 nA using a combination of pure metals, oxides and minerals as reference standards and with the application of ZAF correction procedures. Cores and rims of the same crystal were analysed in many cases. No significant zoning appeared in the analyses and therefore the analysis points are not distinguished in this study. The analysed thin sections come from the two traverses mentioned above in the petrography. In the tonalites, all analysed biotites and amphiboles are fine-grained crystals of the matrix and are likely to be informative about conditions during the main stage of crystallization and mixing.

Amphibole

Representative analyses of amphiboles from the GIZ are listed in Table 2 together with their structural formulae. The acronyms correspond to the samples of Fig. 4. Both the synplutonic dyke traverse and hybrid zone traverse are represented. Analyses which did not satisfy stoichiometric conditions and/or had unacceptable totals were

Tab

le 2

. R

epre

sen

tati

ve

mic

rop

rob

e an

alys

is o

f am

ph

ibo

les

Hy

bri

d z

on

e tr

aver

se

Syn

plut

. d

ike

trav

erse

Ro

ck t

ype

Hy

H

y

Hy

H

y

Hy

H

y

Hy

Hy

H

y

Hy

T

o T

o C

h.M

C

h.M

S

ampl

e D

-8

D-8

D

-8

D-8

D

-8

D-8

D

-11

D-1

1 D

-11

F-2

F

-5

F-5

F

-6b

F

-6b

an

alys

is

1 2

3 4

5 6

1 2

3 1

1 2

1 2

SiO

z 47

.23

46.7

7 47

.01

46.4

4 47

.39

47.7

8 46

.27

45.4

0 46

.34

47.4

7 46

.27

47.7

8 46

.48

46.1

4 T

iO2

1.00

1.

30

1.13

1.

28

0.93

0.

93

1.52

1.

59

1.48

1.

20

1.08

1.

28

1.18

1.

25

A12

03

5.97

6.

33

6.17

6.

60

5.45

5.

55

6.76

7.

25

6.81

5.

59

6.27

5.

49

6.33

6.

64

Fe

20

3 a

9.

85

7.73

8.

37

9.84

7.

05

6.86

7.

77

9.88

9.

40

7.19

8.

60

6.13

6.

09

8.40

F

eO

11.3

2 13

.12

12.5

5 11

.04

12.8

5 13

.19

12.7

4 11

.13

11.4

8 12

.51

13.0

4 12

.52

14.4

5 13

.48

Mn

O

0.60

0.

33

0.51

0.

51

0.70

0.

43

0.51

0.

35

0.44

0.

47

0.48

0.

47

0.59

0.

58

Mg

O

10.4

5 10

.16

10.1

1 10

.25

10.6

9 10

.80

10.1

6 10

.10

10.2

0 11

.03

9.91

11

.34

9.78

9.

47

CaO

10

.10

10.2

7 10

.22

9.80

10

.81

11.0

2 10

.30

9.85

9.

87

10.6

9 10

.30

10.9

3 10

.70

10.2

6 N

a20

1.

36

1.53

1.

38

1.52

1.

14

0.98

1.

58

1.64

1.

62

1.35

1.

60

1.10

1.

39

1.53

K

20

0.

46

0.55

0.

46

0.50

0.

53

0.53

0.

57

0.52

0.

47

0.45

0.

50

0.48

0.

60

0.55

Tot

al

98.3

5 98

.08

97.9

1 97

.78

97.5

5 98

.07

98.1

8 97

.71

98.1

1 97

.95

98.0

5 97

.51

97.5

9 98

.30

Str

uct

ura

l fo

rmu

lae

(0 =

23)

Si

6.97

8 6.

955

6.99

0 6.

898

7.07

3 7.

085

6.87

8 6.

766

6.86

7 7.

039

6.91

2 7.

091

6.98

0 6.

886

AI(

IV)

1.02

2 1.

045

1.01

0 1.

102

0.92

7 0.

915

1.12

2 1.

234

1.13

3 0.

961

1.08

8 0.

909

1.02

0 1.

114

S (T

) 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0

AI(

VI)

0.

018

0.06

5 0.

072

0.05

3 0.

032

0.05

5 0.

062

0.03

9 0.

056

0.01

6 0.

016

0.05

2 0.

100

0.05

4 T

i 0.

111

0.14

5 0.

126

0.14

3 0.

104

0.10

4 0.

170

0.17

8 0.

165

0.13

4 0.

121

0.14

3 0.

133

0.14

0 F

e 3+

~ 1.

107

0.87

0 0.

943

1.11

1 0.

797

0.76

9 0.

875

1.12

0 1.

059

0.80

7 0.

974

0.68

7 0.

691

0.95

1 F

e 2+

1.41

4 1.

642

1.57

3 1.

386

1.61

4 1.

644

1.59

4 1.

402

1.43

7 1.

560

1.64

3 1.

561

1.82

2 1.

695

Mn

0.

075

0.04

2 0.

064

0.06

4 0.

089

0.05

4 0.

064

0.04

4 0.

055

0.05

9 0.

061

0.05

9 0.

075

0.07

3 M

g

2.30

1 2.

252

2.24

0 2.

269

2.37

8 2.

387

2.25

1 2.

243

2.25

3 2.

437

2.20

6 2.

508

2.18

9 2.

106

S (C

) 5.

027

5.01

6 5.

019

5.02

7 5.

014

5.01

3 5.

017

5.02

7 5.

024

5.01

4 5.

021

5.01

0 5.

010

5.02

0

R 2

+b

0.

027

0.01

6 0.

019

0.02

7 0.

014

0.01

3 0.

017

0.02

7 0.

024

0.01

4 0.

021

0.01

0 0.

010

0.02

0 C

a 1.

599

1.63

6 1.

628

1.56

0 1.

729

1.75

1 1.

641

1.57

3 1.

567

1.69

8 1.

649

1.73

8 1.

722

1.64

1 N

a (M

4)

0.37

4 0.

347

0.35

2 0.

414

0.25

7 0.

236

0.34

3 0.

400

0.40

9 0.

287

0.33

1 0.

252

0.26

8 0.

340

S(B

) 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0

Na

(A)

0.01

5 0.

094

0.04

6 0.

024

0.07

3 0.

045

0.11

3 0.

074

0.05

7 0.

101

0.13

3 0.

065

0.13

7 0.

103

K

0.08

7 0.

104

0.08

7 0.

095

0,10

1 0.

100

0.10

8 0.

099

0.08

9 0.

085

0.09

5 0.

091

0.11

5 0.

105

S(A

) 0.

102

0.19

8 0.

133

0.11

9 0.

174

0.14

6 0.

221

0.17

3 0.

146

0.18

6 0.

228

0.15

6 0.

252

0.20

8

Tot

al

15.1

02

15.1

98

15.1

33

15.1

19

15.1

74

15.1

46

15.2

21

15.1

73

15.1

46

15.1

86

15.2

28

15.1

56

15.2

52

15.2

08

Hy,

hy

bri

d r

ock

s; t

o, t

on

alit

e; C

h.M

, ch

ille

d m

arg

in

" F

e 3 +

cal

cula

ted

on

th

e ba

sis

of

13 c

atio

ns,

ex

clu

din

g N

a, K

an

d C

a b

R2+

=F

EZ

+ +

Mg

+M

n

18

7 S , , 6S t t b3 7.0 .. �9 oo ,% o ~ ~o o ,.

�9 �9 �9 0

i I I

0.65 I I I

�9 �9 0 �9 O n 0 �9 �9 O~ �9 [] []

0 . 6 0 - . - -- ..

�9 o o

0.55 - ~- -

0.50 I I I 005 010 0.15 0.08 0.10

Na A K

e,a �9

0.12

Fig. 6. Plots of Mg 4~-alkalis and Si-alkalis for the hornblendes of the GIZ. Full circles = D8 ; full squares = D11 ; open squares = F2; open circles = F5 ; open triangles = F6B. Sample acronyms as in Fig. 4 and Table 2

rejected. Structural formulae were calculated using crystal-chemical constraints (Robinson et al. 1982). Ferric iron was calculated using the equation of Droop (1987) for calcic amphiboles, assuming total cations to be 13, exclusive of Na, K and Ca, the maximum value in unit formulae.

The amphiboles are Mg-hornblendes following Leake's (1978) classification; Si ranges 6.766 7.091. The wider range corresponds with the hornblendes of the hybrid-zone traverse which also shows more variability in whole-rock compositions. Compositional homogenei- ty corresponds with the nearly constant optical proper- ties observed in the analysed amphiboles. A minor difference is noted between the hornblendes of the tonalites and that of the hybrid rocks as deduced from the content of Na in the A-site (Fig. 6). This variation may be due to differences in NazO activity which may be greater in the mafic magma. This supports the idea that some of the hornblende crystallized during the hybridization process, but subsolidus exchange cannot be excluded. However the Si content is nearly constant for a wide range of (Na + K)A from tonalites to hybrid rocks. The SiO2 content of both rocks types is very similar (see Table 4).

Alkalis in the A-site show a negative correlation with Mg ~ (Mg/Mg + Fe 2 +, Fe 2 + deduced by stoichiometry) values (Fig. 6). The higher Na and K values correspond to hornblendes from the tonalites of the synplutonic dyke traverse. These probably crystallized at lower oxygen fugacity (fo2), as suggested by Yamaguchi (1985). The lowest Mg ~ value corresponds to an analysis from the chilled margin of the synplutonic dyke traverse. Vol- atile transfer to the host granite magma may account for a decreasing of fo2 in the tonalitic magma before the quenching event.

Figure 7 shows the TA1 (total A1) vs WA1 variation for analysed hornblendes. The pressure range calculated from the Hollister et al. (1987) equation based of Ham- marstrom and Zen (1986), varies from 2.6 to 0.7 kbar. These data agree with the epizonal-subvolcanic nature of the GIZ. The barometer was applied to all analysed hornblendes. Textural criteria indicate that hornblende is in equilibrium with plagioclase in the early stages of crystallization and with quartz in the late stages. How-

J

1.3

1.1

0.9 m

0 t 7 -

0 5 05

I I I I I

& O �9

D ~ 1 7 6 dl,

0 1 2 I I I

3 4 K b a r I I -

P( t l Kbar)=-476+B 6L TAt

I I I I I 07 0.9 1.1 1.3 1.5 1.7

Fig. 7. TA1 (total A1) IVA1 diagram for the hornblendes of the GIZ. Pressure scale is after Hollister et al. (1987) based on Hammar- strom and Zen (1986). Symbols as in Fig. 6

ever the results obtained are only indicative, because not all the analysed points are within a few microns of the contact of hornblende with quartz, the ideal rela- tionship to be used when applying the barometer (Zen 1988, personal communication). The apparent wide range of pressure for the hornblendes of the hybrid rocks may be a function of not meeting all the constraints of this geobarometer or may reflect a real variation in pressure at the time of amphibole growth. Certainly, the lower pressures are consistent with the field evidence for the shallow level of emplacement.

Biotite

Representative microprobe analyses of biotites from selected samples of the GIZ are listed in Table 3. Analysed biotites come from the synplutonic-dyke traverse and the hybrid-zone traverse described in the petrography section. For both traverses biotite displays rather homogeneous compositions as indicated in the M g - F e - - T i diagram (Fig. 8 b). All the analyses are well grouped in the field of biotites in the classification diagram (Fig. 8 a) of Deer et al. (1966). As biotite probably crystallized

Tab

le 3

. R

epre

sen

tati

ve

mic

rop

rob

e an

alys

is o

f bi

otit

es

Hy

bri

d z

on

e tr

aver

se

Sy

mp

lut.

dik

e tr

aver

se

Ro

ck t

yp

e H

G

HG

H

y H

y

Hy

Hy

H

y H

y

Hy

H

y

Hy

H

y

To

Ch

.M

HG

H

G

Sam

ple

D-3

D

-3

D-6

D

-6

D-6

D

-7

D-7

D

-7

D-8

D

-8

D-I

1

F-2

F

-5

F-6

b

F-6

a F

-6a

anal

ysis

1

2 1

2 3

1 2

3 1

2 1

1 1

1 1

2

SiO

2 34

.97

35.7

3 35

.07

35.3

4 36

.11

35.6

2 35

.25

35.6

6 36

.09

35.4

2 36

.01

35.9

5 36

.40

35.8

6 35

.40

35.1

2 T

iO2

4.65

4.

59

4.53

4.

41

4.56

4.

20

4.53

4.

60

4.69

4.

58

4.85

4.

73

4.73

4.

86

4.61

5.

52

Alz

O3

13.8

3 13

.90

14.1

5 13

.86

14.1

6 13

.70

13.9

2 13

.86

13.3

5 13

.40

13.3

1 13

.85

13.4

6 14

.31

13.6

3 13

.58

FeO

t 23

.52

23.9

6 23

.92

23.4

8 24

.04

23.5

4 23

.23

22.9

3 21

.84

21.6

3 22

.66

22.3

0 21

.87

23.6

1 23

.75

23.2

9 M

nO

0.

16

0.20

0.

15

0.22

0.

17

0.27

0.

15

0.19

0.

21

0.17

0.

11

0.21

0.

20

0.19

0.

28

0.13

M

gO

8.

60

8.62

9.

00

9.05

9.

28

9.20

9.

38

8.93

9.

40

9.33

9.

11

9.08

9.

68

7.76

8.

43

7.90

C

aO

0.00

0.

00

0.02

0.

00

0.03

0.

02

0.01

0.

02

0.00

0.

01

0.06

0.

04

0.04

0.

10

0.05

0.

05

Naz

O

0.12

0.

07

0.17

0.

16

0.14

0.

09

0.07

0.

17

0.06

0.

10

0.13

0.

07

0.05

0.

06

0.13

0.

12

KzO

8.

93

9.10

8.

91

9.27

9.

21

9.34

9.

16

9.49

9.

13

9.13

8.

94

9.89

9.

68

9.43

9.

66

9.86

Tot

al

94.7

8 96

.17

95.9

2 95

.79

97.7

0 95

.98

95.7

0 95

.85

94.7

7 93

.77

95.1

8 96

.12

96.1

1 96

.18

95.9

4 95

.57

Str

uct

ura

l fo

rmu

lae(

0=

22

)

Si

5.49

7 5.

509

5.48

4 5.

477

5.50

5 5.

539

5.48

5 5.

539

5.62

4 5.

586

5.60

4 5.

559

5.60

5 5.

556

5.53

0 5.

504

AI(

IV)

2.50

3 2.

491

2.51

6 2.

523

2.49

5 2.

461

2.51

5 2.

461

2.37

6 2.

414

2.39

6 2.

441

2.39

5 2.

444

2.47

0 2.

496

Z s

ite

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

AI(

VI)

0.

073

0.10

2 0.

039

0.06

4 0.

049

0.05

1 0.

039

0.07

7 0.

077

0.07

8 0.

046

0.08

3 0.

049

0.17

0 0.

040

0.01

3 T

i 0.

550

0.54

5 0.

519

0.53

1 0.

529

0.49

1 0.

530

0.53

7 0.

550

0.54

3 0.

568

0.55

0 0.

548

0.56

6 0.

542

0.65

1 F

e 2+

3.09

2 3.

090

3.12

8 3.

043

3.06

5 3.

061

3.02

3 2.

979

2.84

6 2.

853

2.94

9 2.

884

2.81

6 3.

059

3.10

3 3.

053

Mn

0.

021

0.02

6 0.

020

0.02

9 0.

022

0.03

6 0.

020

0.02

5 0.

028

0.02

3 0.

015

0.02

8 0.

026

0.02

5 0.

037

0.01

7 M

g

2.01

5 1.

981

2.09

7 2.

090

2.10

9 2.

132

2.17

5 2.

067

2.18

3 2.

193

2.11

3 2.

092

2.22

1 1.

792

1.96

2 1.

845

Y s

ite

5.75

1 5.

744

5.80

2 5.

758

5.77

3 5.

771

5.78

7 5.

685

5.68

3 5.

689

5.69

0 5.

637

5.66

0 5.

613

5.68

3 5.

579

Ca

0.00

0 0.

000

0.00

3 0.

000

0.00

5 0.

003

0.00

2 0.

003

0.00

0 0.

002

0.01

0 0.

007

0.00

7 0.

017

0.00

8 0.

008

Na

0.03

7 0.

021

0.05

2 0.

048

0.04

1 0.

027

0.02

1 0.

051

0.01

8 0.

031

0.03

9 0.

021

0.01

5 0.

018

0.03

9 0.

036

K

1.79

1 1.

790

1.77

7 1.

833

1.79

1 1.

853

1.81

8 1.

881

1.81

5 1.

837

1.77

5 1.

951

1.90

2 1.

864

1.92

5 1.

971

X s

ite

1.82

8 1.

811

1.83

2 1.

881

1.83

8 1.

883

1.84

1 1.

935

1.83

3 1.

869

1.82

4 1.

979

1.92

3 1.

899

1.97

3 2.

016

Tot

al

15.5

79

15.5

55

15.6

35

15.6

39

15.6

11

15.6

54

15.6

28

15.6

21

15.5

17

15.5

59

15.5

14

15.6

15

15.5

84

15.5

12

15.6

56

15.5

95

HG

, h

ost

gra

nit

oid

; H

y, h

yb

rid

ro

cks;

To,

to

nal

ite;

Ch

.M,

chil

led

mar

gin

20

A

§ g'O,60

I..l_ v

s

0.55

11 BIOTITE

~

02 2.5 3 WA [

~. I

•

[3

ml~ cl �9

O

I .35 255

\

BI

,D3] D6

�9 D7 HZT �9 D8 �9 Dll [3F2 [ oF5 ]SDT /'F6B X F6A

HZT v v v v v v ~/

Mg Fe2(t)

Fig. 8. a Plot of biotites from the GIZ on the Fe~/Fet + Mg vsWA1 diagram, b On the T i - M g - F e t diagram. HZT: hybrid zone traverse. SDT: synplutonic dyke traverse. Acronyms corresponds to samples of Fig. 4 and Table 3. Fet = total iron

through a wider T interval than hornblende in the cooling history, a greater compositional range might be expected. The relatively homogeneous composition of the GIZ biotites implies that minor compositional differences were lost during cooling, suggesting that perhaps biotite is more susceptible to re-equilibration than hornblende. In the absence of FezO3/FeO determinations the

information available from analysed biotites may be limited. Cation-site variations in biotites with distance along traverses are considered below.

Compositional variations across the traverses

Mean values and ranges of hornblende and biotite have been plotted against distance for the synplutonic-dyke and hybrid-zone traverses. The selected cations and ra- tios plotted in the diagrams (Fig. 9) are those in which abundance is controlled by different activities of melt components. The parameters rA1, WA1, (Na+K)A, and Mg # [Mg 4~ =Mg/(Mg+Fe2+)] have been plotted for hornblendes; TA1, WA1, s and 2;X for biotites. The following points are noted:

The hornblendes and biotites richer in XA1 and WA1 come from the chilled margin of the symplutonic-dyke traverse. However, in the hybrid-zone traverse there is contrasting behaviour of elements in biotites and hornblendes. There is a decrease in TA1 and IVA1 from the host granitoid to the hybrid rocks richer in mafic components. For D8 to D l l TA1 and ~VA1 increase for hornblende (Dl l is more mafic) and are nearly constant for biotite. These observations are consistent with our view that hornblende in the more mafic hybrids crystallized early, prior to mixing, and is of generally similar composition to early hornblendes in the chilled margins. In the hybrid-zone traverse biotite has similar TA1 and WA1 to the biotites from the host granitoid (F6A) of the synplutonic-dyke traverse. Most of the biotite components come from the felsic magma. Biotites are modified slightly during mixing, the A1 content decreasing gradually in the biotites of the more mafic hybrids. Oxygen fugacity variations can be qualitatively assessed from the Mg ~ of hornblende (Yamaguchi 1985; Pe-Piper 1988). The lowest value of Mg # is in the chilled-margin hornblendes of the synplutonic-dyke traverse, and Mg # has

A B

1.20

1.1C

1.00

1.2

1.0

0.8

0.60

0,55

F2 F5

-'vA" H

# M g

02O 0.16 0.12

0 10

-HI

B more

_/ID8

--/t

F-4

--HI

i _ / / i ~ 1 20 30 30 40

i t

260 -TAI

2s0-F - { =6A

F5 //

~vAi 2 .5 -

2 4 - ~ I!

5.65

I I

1.9 •

1.8

] t ,tl D(cm) 0 10 20 30

~ ~..~-~-~ ~fic D3

0 07 8 D,, I

I I 20 40

Fig. 9a, b. Variations of cation sites for: a

hornblende, b biotite for the synplutonic-dyke and hybrid-zone traverses. Means and ranges are represented against distance D. Acronyms correspond to samples in Fig. 4 and Tables 2 and 3

Tab

le 4

. W

hole

roc

k an

alys

is o

f re

pres

enta

tive

sam

ples

fro

m t

he G

eren

a in

tera

ctio

n zo

ne

Roc

k ty

pe

FG

To

FG

To

T

o T

o T

o T

o T

o T

o T

o T

o H

G

HG

H

G

Gr

Hy

Hy

Hy

Hy

Sam

ple

GE

-24

GE

-20

A-9

G

E-1

9 G

E-2

2 G

E-2

5 G

E-2

8.1

GE

-28.

2 G

E-3

1 G

E-3

2 G

E-2

1 G

E-2

3 G

E-3

3 G

E-3

7 G

E-2

6 G

E-2

9 G

E-3

0 G

E-3

4

SiO

2 60

.76

59.1

3 59

.62

58.7

4 58

.88

58.2

1 59

.18

58.9

8 58

.41

58.9

4 68

.31

67.1

0 67

.69

73.5

2 67

.22

63.0

0 60

.85

65.0

8 T

iO2

0.97

1.

00

0.96

1.

02

0.96

0.

92

0.95

0.

96

0.99

0.

84

0.43

0.

52

0.47

0.

18

0.50

0.

65

0.71

0.

62

A12

03

16.8

9 17

.31

16.8

4 16

.72

17.3

1 16

.99

17.7

1 17

.58

16.9

8 17

.84

16.2

9 16

.59

17.0

5 13

.27

16.3

6 17

.06

17.3

9 16

.98

FeO

t 6.

44

6.65

6.

19

6.48

6.

40

6.46

6.

14

6.14

6.

41

5.71

3.

19

3.96

3.

25

2.08

3.

97

Mn

O

0.12

0.

13

0.13

0.

13

0.13

0.

12

0.11

0.

12

0.13

0.

12

0.04

0.

04

0.04

0.

05

0.05

M

gO

2.68

2.

99

2.88

2.

87

3.04

2.

98

2.76

2.

69

2.98

2.

52

0.84

0.

88

0.88

0.

28

0.98

C

aO

5.30

6.

17

5.96

6.

23

6.15

6.

06

5.84

5.

87

6.37

6.

29

3.36

3.

50

3.76

1.

16

3.45

N

a20

4.

19

4.42

3.

99

4.13

4.

12

4.02

4.

30

4.02

4.

22

3.83

4.

49

4.37

4.

42

3.64

4.

25

K2

0

1.85

1.

95

2.23

1.

88

2.17

2.

05

2.40

2.

36

1.92

2.

08

2.61

2.

50

2.51

4.

15

2.51

P

~Os

0.09

0.

10

0.12

0.

10

0.10

1.

10

0.11

0.

11

0.10

0.

12

0.10

0.

14

0.13

0.

05

0.12

I.

L.

1.00

0.

60

0.60

1.

40

0.80

1.

80

1.00

1.

00

1.20

1.

60

0.40

0.

20

0.60

1.

00

0.80

4.60

5.

13

4.67

0.

08

0.09

0.

07

1.52

1.

94

1.71

4.

81

5.28

4.

24

3.80

3.

90

4.15

2.

30

2.26

2.

31

0.11

0.

12

0.13

2.

00

2.20

0.

80

Tot

al

100.

29

100.

45

99.5

1 99

.7

100.

06

99.7

1 10

0.53

99

.80

99.7

1 99

.89

100.

06

99.8

0 10

0.8

Tra

ce e

lem

ents

(pp

m)

Nb

3

2 1

2 3

4 2

3 1

Zr

108

113

114

96

98

103

109

107

105

Y

31

29

22

24

30

29

32

29

25

Sr

245

272

264

280

268

260

271

266

279

Pb

95

80

92

82

92

95

101

101

86

Th

13

16

16

18

15

18

11

16

9 P

b 14

15

12

15

10

17

12

14

13

Z

n 55

60

62

64

51

65

55

59

64

C

u 18

6

5 11

6

9 22

22

6

Ni

6 11

12

8

11

11

12

9 7

Cr

88

73

88

99

91

102

90

92

91

V

213

209

195

208

210

201

190

182

204

Ba

672

514

456

460

502

466

574

571

442

Hf

3 3

3 2

3 2

3 3

3 C

e 45

42

35

12

39

40

41

38

32

L

a 12

11

15

3

6 14

11

16

5

99.3

8 10

0.21

99

.93

99.8

7 10

0.76

1 5

8 4

5 3

7 4

6 10

4 17

7 27

0 19

9 14

2 24

0 15

0 15

4 17

8 28

19

22

20

40

24

37

37

40

28

4 20

5 21

6 23

4 68

21

4 25

2 25

4 23

2 92

10

1 96

98

21

1 10

0 93

10

0 11

0 15

16

21

14

21

15

14

19

10

7

16

16

16

29

14

14

12

17

52

24

24

26

23

20

36

51

41

8 38

20

23

13

12

44

38

40

10

9

11

7 15

11

10

11

10

98

89

14

0 91

15

4 14

4 10

9 93

91

16

7 60

52

67

21

55

10

6 12

9 10

6 49

1 69

9 81

3 71

3 56

2 73

6 62

2 60

5 70

5 2

5 6

5 4

6 4

4 4

47

67

94

58

58

84

46

57

66

9 24

42

25

29

40

26

19

29

FG

To,

fin

ed-g

rain

ed t

onal

ite;

To,

ton

alit

e; H

G,

host

gra

nito

id;

Gr,

gra

no

ph

yre

; H

y, h

ybri

d ro

cks

22

a nearly constant evolution through the hybrid-zone traverse. These hornblendes crystallized early at lower fo2 than the hornblendes of the tonalites (F5) and hybrid zone. However, hornblendes from the chilled margin (F6B) are beyond the range of hornblendes from the hybrid-zone traverse which itself is very similar to the range of hornblende in the tonalite (compare F2, F5 with D8, D11). This may indicate that the tonalite trans- ferred volatiles (probably mainly H20) to the host granitoid and in so doing decreased the fo2 in the interface before quenching occurred.

The content of Y-site cations in biotite may reflect a diffusion-controlled process such that biotites from the chilled margin are poorer in Y-site cations than those of the inner part of the dyke. However, only one crystal was analysed in sample F6B (chilled margin) and the point is not proven. It may be noted from this Y-site diagram that biotites of the host granitoid (F6A) are poorer in Y-site cations than biotites from the granitoid inside the hybrid-zone traverse (D3). These biotite compositions appear to have been influenced by their prox- imity to the hybrid zone with which the D3 granite has a transitional contact over a few centrimetres. Biotites from the granite enclosing the synplutonic dyke (F6A) were not affected by diffusion from the mafic magma as it quenched rather suddenly, whereas in the hybrid zone the two magmas reached thermal equilibrium and ionic diffusion was probably more effective.

Alkalis (A-site) in hornblende show a pattern op- posed to that of Mg 4t=, increasing towards the chilled margin of the synplutonic-dyke traverse. They display a wide range in the hybrid rocks of the hybrid-zone traverse. The enrichment in the (Na + K)A content may be related to alkali transfer from the felsic magma to the intruding tonalite. Biotites of the synplutonic-dyke traverse are less affected by this alkali-migration process as deduced from the cation content in X-site (Na + K + Ca). Biotite crystallized later than hornblende and probably after the early-quenched margin was developed, preventing ionic diffusion between dyke and host. This interpretation is supported by the fact that biotites of the host granitoid (F6A) have a range in X-site content beyond that of biotites of the granitoid related to the hybrids (D3). These biotites have X-site cations similar to those of the hybrid-rocks traverse, suggesting that ionic diffusion was possibly involved in the hybrid zone in which the two magmas where in thermal equilibrium.

Whole rock compositions

The major- and trace-element concentrations of representative samples from the GIZ (Table 4) were analysed by X-ray fluorescence in order to characterize the chemical changes related to the mingling and mixing processes and as the basis of a mixing test for generating the hybrid granodiorites. X-ray fluorescence analysis was per- formed on a Philips PWI212 spectrometer using glass beads fused with Spectroflux 105 for major oxides and pressed-powder pellets for trace elements. The tech-

J 1[ z,.

Q5

17 ~ , []

15

i[ -,-~ t ,

A

~ a , , o

0,10 a

0,04.[

4,2 �9 �9 � 9

Lx

3,8 �9

*d~* a

0,121 ~ ~�9 a 0,08 [ []

s's

TiO 2

At203

A A

A

A

~t15

A

~'mm

z~ m m ~

~t [ ]

A

,& [ ]

Si02 wt~

FeO

MgO

MnO

CaO

Na20

K20

P205

75

Fig. 10. SiO2 variation diagrams for whole-rock samples in the GIZ. Full triangles =normal tonalites; open squares= fine-grained tonalites; open triangles = hybrid granodiorites; half filled squares = host granitoid; ful l squares = granophyric granitoid beyond the interaction zone

10 ..Q

i,.J I 5 ',,,t3 (",4

k.J o

-5

..o 1~

k,J I o, 5 (',4

k_t 0

-~C_~o

-5

15

~I0 03 < rn

~ s > - T

15

M10 03 <

> - "1"-

-IO

GE26

R=I,O0 a=0,88

0 10

GE29 ~~

R=Q99 a= 0.47

10 ..s

U I 5 o

u 0

-5

-10 40

0 10 -10 Ca-C b

g

c_)

m

z~

GE-26 /

a=0,88 t~?

NbZr Y $r RbThPbZnOa Ni Cr V BaHfCe ka

GE-30 a=0,25

A

/

L i i t I i r i i I I I I

NbZr Y Sr RbThPbZnCuNi Cr V BaHfCe ka

GE 30

R=0,93 a=0,25

10

.~ ~~ I [ GE 3L, [

Ca-C b lb

15

10 GE-29 z~

a= 0.47 /

Ao S NbZr Y Sr RbTh PbZnCu Ni Cr V BaHf Ce La

is f 10 GE-34 t

F a=O,6S /

5]-- ~/u

~ Lx ~ cl

0 l , . . . . "7 , @ ~, , ~" . . . . NbZr Y Sr RbTh PbZnCu Ni Cr V BaHfCe L

23

Fig. 11. Mixing test for four hybrid granodiorites (GE-26, GE-29, GE-30, GE-34). a = fraction of felsic magma involved in the mixing; R = correlation coefficient

Fig. 12. Rock/GEl9 tonalite- normalised plots showing the composition in trace elements for the hybrids of the GIZ (squares) compared with the calculated compositions (triangles) using the mixing test for major elements

niques and correction procedures employed are essentially those described by Norrish and Chappell (1977).

Figure 10 shows silica variation diagrams for the analysed samples. Most of the major oxides display good linear trends on Harker diagrams. The exceptions are Na20 and P2Os which show scattered distributions, though it should be noted that precision for these elements is somewhat poorer than for the other major elements. The good linear trends, in which the hybrid granodiorites have intermediate positions between tonalites and host granitoids, are consistent with the magma- mixing process as deduced from field and petrographic relationships. The sample richer in SiO2 (GE-37, Ta- ble 4) and poorer in MgO is a granophyre from the Ger-

ena massif 1 km beyond the GIZ. The variable silica content of the four hybrid samples may be a simple indicator of the variable degrees of mixing between tonalite and host granitoid.

Mixing test

The relative participation of both felsic and mafic magma in the generation of the hybrid rocks, has been esti- mated using a mixing model (Langmuir etal. 1978; Fourcade and All6gre 1981). Samples GE-21 and GE-19 (Table 4) were chosen to represent the compositions of the felsic and mafic end-members respectively. Figure 11

24

shows four mixing tests for the hybrid granodiorites of the GIZ plotting the composition of sample GE-21 minus GE-19 against the composition of each hybrid (GE- 26, GE-29, GE-30, GE-34) minus GE-19. The linear cor- relations are close to + 1.0. The respective slopes reflect the fraction of felsic magma participating in the mixture. This fraction varies from 0.25 in GE-30 to 0.88 in GE-26. The results of the test are consistent with a mixing process as deduced from field and petrographic data. These results were tested independently using the trace-element abundances (Table 4). The values of the slope have been used to calculate the proportions of the felsic magma end-member, namely 25%, 65%, 47% and 88% (see Fig. 11). Using these results, model abundances of trace elements in four hybrid rocks were calculated. These calculated compositions are compared with the real compositions of the four hybrid rocks (GE-26, GE-29, GE- 30, GE-34) as rock/tonalite-normalised plots (spider diagrams) in Fig. 12. The correlation is good for most of the analysed trace elements. However, Cu, Ce and La are not well correlated. The Cu anomaly may be due to the presence of Cu-sulfides as late phases in the hybrid rocks and tonalites, where they are probably related to late hydrothermal processes after mixing. Cerium and La are richer in the hybrid rocks compared with the tonalites. It is evident on these plots (Fig. 12) that these anomalies are greater for samples with high proportions of the felsic end-member (GE-26, GE-34). These elements are mainly provided by the felsic magma and the discrepancy with the calculated composition may be due to an anomalously low content of Ce and La in the tonalite GE-19 (see Table 4) chosen for the normaliza- tions.

Discussion

Most petrological studies of magma mixing in plutonic environments find little field evidence to support a kinematic model for mixing (e.g. Reid et al. 1983; Frost and Mahood 1987; Zorpi et al. 1989; Dorais et al. in press). The reason for this is that most of these studied areas probably represent the final stages of the hybridization process in which a hybrid granodiorite or tonalite appears as a host to fragments of mafic magma as enclaves. Many attempts have, however, been made to construct kinematic models mostly based on fluid dynamic consid- erations (Eichelberger 1981; Koyaguchi 1985; Blake and Campbell 1986; Huppert et al. 1982). In the GIZ the interaction process between the two magmas stopped at early stages leading to field relationships that help us to understand the chemical and kinematic processes of magma mixing.

Field relationships in the GIZ indicate that a mafic magma was injected into a felsic magma chamber at the present level of emplacement. Intrusion took place mainly in the form of synplutonic dykes and liquid (pillow-like) globules. Most of the dykes were disrupted by magmatic flow. Extensive areas of hybridization exist between these magmas, and, as described above, there is evidence in the mineral textures and compositions,

of episodic crystallization histories accompanied by compositional blending during the hybridization stage. The first conclusion is that the process of mixing occurred episodically in response to the intermittent input of mafic magma into the felsic chamber. This is supported by the appearance in the same place of evolved- hybrid facies resulting from complete mixing together with non-mixed synplutonic bodies with sharp contacts and chilled margins. A wide variety of intermediate stages exist in the case of partially disrupted synplutonic bodies which are locally hybridized. Non-hybridized portions of mafic magma from the synplutonic bodies appear homogeneously scattered inside the hybrid facies which are, in turn, associated with high-flow zones as indicated by the presence of conspicuous flow structures. This is the usual case in homogeneous granodiorites in which mixing processes have been proposed to explain the principal chemical and mineralogical variations. A feature of the enclaves in the GIZ, and many others studied in granodiorites from elsewhere, is the slightly hybrid character of the enclave magma (Vernon 1983, 1984).

The mineral chemistry of the tonalites from the GIZ clearly indicates that the system of mafic magma-felsic magma was open before quenching of the mafic bodies near the interface. Mixing by ionic diffusion probably occurred at least locally in the melt fractions as deduced from variations in the compositions of hornblende and biotite. In a static situation the extension of such ionic diffusion is very limited as it acts only for short time during crystallization of the chilled margin. However the hybridization process will have been much more efficient in the dynamic situation of dyke-emplacement. A further process contributing to the contamination of the tonalite is the capture of crystals (xenocrysts) from the felsic magma, quartz and plagioclase being the more common xenogenous phases. The presence of felsic xenocrysts is a very common feature of tonalitic enclaves appearing in homogeneous plutonic rocks (Vernon 1983; Zorpi et al. 1989). It seems likely from the field evidence that the slightly hybrid character of the marie magma developed during the intrusion of the synplutonic dykes.

Thermal equilibrium mixing

A significant feature of the GIZ is the presence of homogeneous hybrid zones associated with flow-structures. Homogenization subsequent to initial mixing is the main problem with interpreting a magma-mixing origin for many intermediate plutonic rocks. Thermal equilibrium is a necessary condition for two magmas to mix completely. As suggested by Sparks and Marshall (1986) this condition can easily be satisfied as the rate of thermal diffusion is orders of magnitude greater than the rate of chemical diffusion. However, once thermal equilibrium is reached the two magmas must contain a significant melt-content for mixing to occur. That is, the equilibrium temperature must be significantly above the solidus temperature of the higher-T mafic magma. In the tonalites of the synplutonic dykes petrographic evidence

25

indicates that these conditions were satisfied, particularly by the unusual, multistage crystallization history of the inner parts of the dykes.

During quenching a stage of high nucleation density crystallization occurred giving rise to the fine-grained matrix. This stage occurred until crystallization of the chilled margins was complete but stopped in the cores of the dykes in which a stage of low nucleation-density crystallization occurred. This final stage took place in thermal equilibrium with the host granitoid indicating that the equilibrium temperature was above that of the tonalite solidus. Although these two main conditions seem to have been satisfied in the GIZ, it is important to have some constraints on the melt content of the tonalite magma at the equilibrium temperature. Firstly, if this melt-content is less than the rheological critical melt-percentage (RCMP, Arzi 1978) melt is trapped between the crystals preventing the occurrence of mixing. In this case highly energetic flow is necessary to disaggre- gate the tonalite magma. Secondly, for the two magmas to mix completely the two remaining melts, after thermal equilibrium is reached must have similar viscosities and thus broadly similar compositions.

It is clear from the experimental data of Wyllie (1977) for tonalite, and Maaloe and Wyllie (1975) for biotite granite, that both magmas may be in thermal equilibrium over a wide range of temperatures. A tonalite magma with a moderate crystal content will be in thermal equilibrium with granitic liquid. The melt fraction of the tonalite magma is probably not of granitic composition, and these differences in composition imply viscosity con- trasts which may be sufficient to inhibit commingling and mixing. However, at greater crystallinities the residual melt of the tonalite will approach granitic compositions thus reducing the viscosity contrast at lower temperatures and consequently favouring mixing. Tonalites in the GIZ have low modal K-feldspar (<5%) and it is only at high crystallinity that the residual melt will approach ternary minimum granitic liquid compositions, presumably similar in major-oxide composition to the granitic liquids with which it was in intimate contact in the interaction zone, and at the same temperature. It is clear from petrographic information that the magma will have had a high proportion (probably >70%) of crystals when the remaining melt-fraction approached granitic compositions. This is wholly consistent with our contention that hybridization in the GIZ took place between a highly crystalline tonalitic magma and a largely liquid granite in thermal equilibrium. At around 70% crystallinity, Arzi (1978) has shown that the relative viscosity (grnagma/gmelt) tends to extremely high values and the 30% remaining melt-fraction represents the minimum porosity in an idealised framework of spherical crystals. This rheological critical melt-percentage (RCMP, Arzi 1978) may vary depending on other parameters such as crystal shape, size-distribution, etc. but the RCMP is broadly a band 70-90% crystals for most magmatic situations. Thus if thermal equilibrium occurs at low temperatures, as discussed above, the tonalite magma will have a high crystal content and a high viscosity. A high-energy regime would be necessary to dis-

aggregate such a relatively rigid network of principally mafic crystals to effect mingling with the co-existing granite.

Quantitative modelling of the process described above may be possible when crystallinity-temperature data become available from experimental studies of granite-tonalite systems, analogous to those published by Marsh (1981) on more basic compositions. With such data, the proportion of melt of each end-member in a mixed-magma situation in thermal equilibrium will al- low calculations of composition and viscosities and pro- vide a test of the process proposed for the GIZ.

In summary there are two main factors governing magma-mingling and -mixing in thermal equilibrium. On the one hand, the residual melts must have similar compositions and viscosities. This is favoured by low temperatures and high crystallinities in the tonalite. On the other hand, the high relative viscosity of the mafic magma requires energetic flow in the composite magma chamber for effective mingling, disaggregation and mixing to occur. These combined conditions were achieved in the Gerena interaction zone where the hybrid zones are closely associated with flow-structures.

Conduit mixing (Koyaguchi 1985; Blake and Camp- bell 1986) and mixing in stratified magma chambers (Ei- chelberger 1981; Huppert et al. 1982) are the principal models for magma-mingling and -mixing. Both models require highly energetic flow. However, field relationships described in the GIZ are not entirely consistent with these model-requirements but rather indicate an episodic process of mixing in thermal equilibrium mainly related to the intrusion of synplutonic dykes, and the disruption and mingling of two magmas in local thermal equilibrium leading to hybridization.

Conclusion

This study of the Gerena interaction zone has attempted to identify the controls on the processes of magma-mingling and -mixing in the subvolcanic environment, where a quenched system has permitted detailed field, petrographic, mineralogical and whole-rock geochemical studies. For this particular example it is evident that a set of circumstances must coincide before mixing is effective. Thermal equilibrium must be acheved between the more basic magma and the acid magma before complete crystallization and this must be followed by disruption, disaggregation and dispersion of the crystalline- and melt-fractions of the more mafic magma in a highly energetic flow environment. Whether these processes are ap- plicable to magma-mingling and -mixing situations and the generation of intermediate hybrid magmas on a large scale cannot be stated from this study as many textural characteristics are frequently casualties of recrystallization in the plutonic environment. Nevertheless such studies of the arrested stages are important in understand- ing the dynamics of magma-mingling and -mixing.

Acknowledgements. Jon Blundy provided many useful suggestions for an earlier version of the manuscript. This work was in part supported by a grant (PS87-0125) from the Spanish CICYT. WES

26

is grateful for financial support from the University of St. Andrews for fieldwork in Spain. Donald Herd assisted with electron microprobe analyses and Angus Calder with XRF analyses.

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