Successive mixing and mingling of magmas in a plutonic complex of Northeast Brazil

LITHOS 0

ELSEVIER Lithos, 34 (1995) 275-299

Successive: mixing and mingling of magmas in a plutonic complex of Northeast Brazil

S.P. Nevesa*b, A. Vaucheza “Laboratoire de Te,:tonophysique, Universitk des Sciences et Techniques du Languedoc, 34095, Montpellier Cedex 5, France

bDepartment of Geology, Federal University of Pernambuco, Recife, 50.000, Brazil

Received 7 July 1993; revised and accepted 27 April 1994

Abstract

Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Jape- ganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300-500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: ( 1) coarse-grained granite; (2) porphyritic granite and ( 3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sin&al, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite.

The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and major- elements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiOZ in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr.

The diorites conl.ain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Abundant enclaves of diorite in the porphyritic granite, despite their textural diversity, are typically devoid of chilled margins, and were therefore formed relatively early in the crystallisation history of the granite. They are interpreted as liqmd droplets separated from the heterogeneous hybrid magma through convection currents and incorporated in the crystallising granitic magma.

Subsequently, during the crystallisation of the porphyritic granite, mafic magma supply to the batholith contin- ued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of stromatitic-like structures, with alternating bands of mutually contaminated granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled liner-grained margins.

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276 S.P. Neves, A. Vauchez / Lithos 34 (199s) 275-299

1. Introduction

A wealth of data accumulated in the last two decades suggests that mixing and/or mingling of magmas are important petrologic processes during the magmatic history of many volcanic and plutonic bodies. Mixing of two (or more) magmas within a magma chamber to produce an homogeneous hybrid, sometimes followed by mingling and partial mixing of the hybrid magma with one of the remaining end-members during extrusion, has been demonstrated for volcanic/ subvolcanic rocks in several places (McGarvie, 1984; Vogel et al., 1984; Graham and Worthing- ton, 1988; Huppert and Sparks, 1988; Blake et al., 1992; Freundt and Schmincke, 1992), and various theoretical or experimental models have been proposed (Kouchi and Sunagawa, 1985; Koyaguchi, 1985; Campbell and Turner, 1986; Sparks and Marshall, 1986). In plutonic rocks, mingling is more frequently reported than mixing, although a more effective interaction between magmas of contrasting composition is expected in the deeper crust because the magmas can stay in contact in the liquid state during a longer time span. This apparent contradiction may be due to the easier identification of mingling than mixing but it may also reflect the lack of vertical cross-sections in most plutonic bodies. It is therefore possible that in some granitoid batholiths where magma mixing has actually occurred, this process was underestimated because the end-members and the product of their interaction are not simultaneously observable. By analogy with better documented volcanic evolution, it may be speculated that examples of mingling described in the literature represent new injections of mafic magma into a magma chamber where a mixing event is near to completion.

In this paper, we describe a plutonic igneous complex where favourable tectonic circumstan- ces exposed different levels of the intrusion. It will be shown that the major rock types present in the complex represent the crystallisation products of felsic and matic/intermediate magmas that mixed to generate a homogeneous hybrid, and that mafic magma was repeatedly in-

jetted into the hybrid magma during its solidification, leading to renewed mixing and mingling. In this context, the term “mixing” will be restricted to processes that lead to the production of an hybrid magma.

2. Geological setting

The Neoproterozoic (Brasiliano) erogenic evolution of the Borborema Province of northeastern Brazil (Almeida et al., 198 1) is characterised by the development of a continental-scale system of anastomosing transcurrent shear zones, and by a voluminous synorogenic granitic magmatism (Sial, 1986; Caby et al., 1991; Vauchez et al., 1992; Fig. 1). From a petrologic view- point, the better known granitoids are those exposed in the Central Structural Domain (CSD), where five igneous rock type associations have been recognised (Sial, 1986, 1987; Sial and Fer- reira, 1990). From these, the so-called Itapor- anga-type granite (Almeida et al., 1967) or Ita- poranga-type association (Sial, 1986, 1987)) which occurs in three batholiths bordering the northwestern portion of the Cachoeirinha/Sal- gueiro fold belt (Fig. 1) , shows clear evidence of commingling of mafic and felsic magmas (Mar- iano and Sial, 1988). The Itaporanga-type association consists of coarsely porphyritic, potassic talc-alkaline, quartz-monzonites to granodio- rites and K-diorites (biotite-rich diorites).

In the studied area (Fig. 2), the Itaporanga- type association of malic and felsic magmas characterises the Fazenda Nova batholith, from which a live-point whole-rock Rb-Sr isochron yielded an age of 630 ? 24 Ma (McMurry et al., 1987). In contrast with the plutons in the CSD, however, the Fazenda Nova batholith only represents a component of a larger granitoid complex that also comprises the Serra da Japeganga batholith, whose main facies is a coarse-grained granite (Fig. 2 ) .

The Fazenda Nova/Serra da Japeganga complex intrudes micaschists and orthogneisses metamorphosed under amphibolite facies conditions and is itself intruded by a fine- to me-

S.P. Neves, A. Vauchez /Lithos 34 (1995) 275-299 217

Fig. 1. Schematic ma.p of the Borborema province of northeast Brazil, showing the main strike-slip shear zones and granitoid batholiths. CSD - Central Structural Domain; SPSZ - Senador Pompeu shear zone; PASZ - Patos shear zone; PESZ - Pemam- buco shear zone.

dium-grained, locally slightly porphyritic, leucogranite. The complex is cross-cut by the sinistral, NE-trending Fazenda Nova shear zone (FNSZ), which largely separates the Fazenda

Nova and Serra da Japeganga batholiths (Fig. 2). Mineral stretching lineations in the FNSZ plunge systematically 1 O-30” northeastward, and sets of curved, synthetic minor shear zones, some with

218 S.P. Neves, A. vauchez / Lithos 34 (1995) 275-299

N

t

A 1000mr

El @YJ Late leucogranite dike

EEI Sheeted-dike complex

-k ,$ Dip and strike of magmatic foliation

$ ps Dip and strike of mylonitic foliation Stretching lineation

u b

Fig. 2. Northern part of the Fazenda Nova/Serra da Japeganga complex. (a) Sketch map showing the main petrographic facies and structural features. (b ) cross section; localization in (a); symbols represent the sense of shear for shear zones.

S.P. Neves, A. Vauchez / Lithos 34 (1995) 275-299 279

stretching lineations plunging up to 60 ‘, splay off the southern termination of the main fault. This kinematic pattern implies a component of vertical offset in addit.ion to the main strike-slip dis- placement. As the Faze&a Nova batholith is on the northwestern side of the FNSZ, it certainly represents a deeper level of the complex, uplifted during faulting.

The Serra da Japeganga and Fazenda Nova batholiths usually display a well-defined magmatic foliation marked by the planar disposition of tabular crystals of feldspars, hornblende prisms and biotite flakes,. In the Fazenda Nova batholith, the foliation mostly dips SSW to SE, and parallels the pluton’s boundaries, with dip in- creasing from 30-60” near its northwest margin, to subvertical neav the FNSZ (Fig. 2 ) . The diorites and related rocks mainly occur along the northwest boundiary of the pluton and therefore are geometrically below the porphyritic granite

(see cross-section in Fig. 2). In the eastern portion of the Serra da Japeganga batholith, where the magmatic fabric is preserved, the foliation generally strikes WNW-ESE to NNW-SSE and dips gently to moderately towards the NE or SW. This pattern suggests that the deepest part of the intrusion is exposed in the Fazenda Nova batholith, in agreement with the above conclusion that the Fazenda Nova batholith represents the root zone of the complex.

3. The Fazenda NovalSerra da Japeganga complex

3. I. Petrographic facies

The Serra da Japeganga batholith contains an homogeneous coarse-grained granite whose main minerals are potassium feldspar, quartz, plagio-

Q = Sii3 - (K+Na+2Ca/3)

7 d -100

qfnz+

**@

qs

/Lq L -50

S 1 P = K - (Na+Ca)

-2:o I I , I

-200 -150 -100 -A0

Fig. 3. Chemical data for the rocks of the Fazenda Nova/Serra da Japeganga complex plotted on the Debon and LeFort’s ( 1988) chemical classification diagram for common plutonic igneous rocks. Crosses - porphyxitic granite; circles - coarse-grained granite; triangles - diorites. Curves labelled THUL, CALK and SALKD are typical trends for the tholeiitic, talc-alkaline and dark- coloured subalkaline magmatic associations. ad: adamellite; gr. granite; gd: granodiorite; to: tonalite; qmzd quartz monzodiorite; qmz: quartz monzonite; qs: quartz syenite; s: syenite; mz: monzonite; mzgb: monzogabbro, monzodiorite.

280 S.P. Neves, A. Vauchez / Lithos 34 (1995) 275-299

Fig. 4. Field photography of the porphyritic granite. Megacrysts are K-feldspar; smaller crystals are plagioclase and quartz; dark crystals are biotite and hornblende. Hammer’s head is about 12 cm wide.

clase, biotite and hornblende. Sphene and magnetite are important accessories, and zircon and apatite occur in trace amounts. The malic minerals commonly represent only 5 to 10 vol.% of the rock, biotite predominating over hom- blende. Modal analysis of this rock points to a quartz-syenitic to syenogranitic composition in the IUGS nomenclature (Streckeisen, 1976). On the Debon and LeFort’s ( 1988 ) chemical classification diagram, analysed samples of this facies (see Section 3.4. for more details) plot in or near the granite field (Fig. 3).

The Fazenda Nova batholith consists of two main rock-types that are intimately associated: relatively homogeneous porphyritic granites and finer-granted more mafic rocks, both of which are present in various proportions in most outcrops. The dominant facies is the porphyritic granite, which consists of K-feldspar megacrysts up to 8 cm long in a medium-grained matrix of plagioclase, quartz, K-feldspar, hornblende and biotite (Fig. 4). Sphene constitutes up to 3% of the ma-

trix. Epidote is another common malic mineral but appears to result from subsolidus reactions. Magnetite, alanite, zircon and apatite are the other accessories. It is difficult to estimate the modal composition of this rock due to its porphyritic nature. However, as megacrysts usually represent 15-30 vol.% of the rock (visual esti- mation on outcrops), and in the matrix plagioclase predominates over K-feldspar and quartz varies from 15 to 32 vol.%, the most common compositions are certainly quartz-monzonitic to monzogranitic. This rock is significantly richer in mafic minerals ( 1 O-20 vol.%) than the coarse- grained granite. Analysed samples plot in the quartz-monzonite field on the Debon and Le- Fort’s ( 1988) classification diagram (Fig. 3). Enclaves with various dimensions, geometry, textures and compositions are extremely common in this rock. They generally represent a few volume percent but may reach up to 30 vol.% (Fig. 5 ) . Their texture varies from equigranular, fine-grained to porphyritic and their composi-

S. P. Neves, A. Vauchez / Lithos 34 (I 995) 2 75-299 281

Fig. 5. Swarm of elongated mafic enclaves in the porphyritic granite.

tion from quartz-dioritic to granodioritic. The second main facies of the Fazenda Nova

batholith comprises relatively matic rocks. Two petrographic trends are recognised in this facies, one from diorites to progressively quartz-richer rocks, like in the enclaves, and the other from diorites to K-feldspar-richer rocks without con- comitant increa:se in quartz content. Members of the first trend contain only hornblende and biotite as matic minerals, whereas clinopyroxene is also present in the monzodiorites and monzonites belonging to the second. The latter ones are more common in the vicinity of a syenitic pluton that occurs northeast of the Fazenda Nova batholith. This paper will focus on the intermediate rocks of the first trend, by far the most abundant, and on their relationships with the porphyritic granite. Among these, fine-grained diorites to quartz-diorites with similar amounts of mafic and felsic minerals are the most mafic rocks observed. The most common varieties are porphyritic, containing sparse (0.5-2.5 cm) K-feldspar

megacrysts in a fine- to medium-grained matrix having a quartz-dioritic to granodioritic composition. Fine-grained mafic enclaves may be present in these latter. On the major-element classification diagram (Fig. 3) most samples of this facies plot in the monzodioritic/quartz-monzodioritic fields.

3.2. Mineral chemistry

Representative microprobe analyses of feldspars, amphiboles and biotites in porphyritic granite, coarse-g&ted granite and diorite are presented in Table 1.

Plagioclase compositions vary from AnZ6 to AnS5 in diorite, from AnZO to Arts2 in porphyritic granite, and from An13 to AI+. in coarse-grained granite, reflecting the composition of the magmas from which they crystallised.

Biotite shows little chemical variation, with Mg/(Mg+Fe) ratio ranging from 0.49 to 0.52 in diorite, from 0.54 to 0.58 in porphyritic gran-

282

Table la

S.P. Neves, A. Vauchez / Lithos 34 (1995) 275-299

Microprobe analysis of plagioclases and biotites from rocks of the Fazenda Nova/Serra da Japeganga complex

PklgioClase

Porphyritic granite Coarse-grained granite Diorite

Analysis No. 8 24 44 48 63 64 68 94 103 103 113 114

Rim Core Rim Core Core Core Rim Rim Rim Core Rim Middle

SiOz 63.56 59.01 61.4 60.53 63.71 63.19 63.97 65.51 61.23 59.75 62.32 60.10

Al& 25.08 26.11 26.29 26.47 23.55 23.77 23.23 22.12 25.14 25.61 24.23 26.14

Fe0 0.13 0.04 0.00 0.03 0.17 0.18 0.11 0.05 0.01 0.02 0.00 0.05

MgO 0.00 0.00 0.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

CaO 4.39 5.38 6.21 5.53 4.11 4.20 3.88 2.79 6.29 6.76 5.46 6.98

Na20 8.11 8.65 7.68 8.29 9.05 9.22 9.52 10.07 8.06 7.49 8.36 7.60

GO 0.07 0.12 0.12 0.08 0.37 0.12 0.22 0.06 0.10 0.33 0.06 0.07

Total 102.01 99.30 101.80 100.93 100.98 100.7 100.93 100.61 100.82 99.96 100.49 100.94

AB 77.98 73.91 68.60 12.75 78.23 79.31 80.62 86.41 69.45 65.46 73.22 66.08

AN 21.60 25.40 30.68 26.81 19.66 19.99 18.16 13.25 29.98 32.66 26.43 33.55

OR 0.42 0.69 0.72 0.44 2.11 0.70 1.22 0.33 0.58 1.88 0.35 0.37

Biotite


Analysis No. 3 3 5 5 55 56 95 96 100 101 108 109

Core Rim Rim Core Core Rim Core Rim Core Rim Rim Core

SiOz 37.67 37.32 38.19 37.95 35.36 35.26 34.85 34.66 36.08 36.16 36.24 36.30

Ti02 3.26 2.97 1.98 2.19 4.55 3.62 3.73 3.62 2.55 2.56 2.72 2.68

Al203 16.16 16.41 16.42 16.21 15.65 16.69 15.01 14.85 15.85 15.92 15.68 15.87

Fe0 18.08 17.80 18.27 17.48 28.25 28.47 28.92 28.31 19.51 18.89 19.64 20.36

MnO 0.01 0.25 0.36 0.29 0.03 0.30 0.35 0.35 0.19 0.25 0.018 0.26

MgO Il.97 12.3 13.29 13.11 4.60 4.62 4.45 4.38 11.07 11.25 11.07 11.38

Na,O 0.07 0.06 0.17 0.07 0.03 0.02 0.03 0.01 0.16 0.13 0.00 0.75

KzO 9.32 9.50 8.98 8.88 9.46 9.09 8.92 8.96 9.40 9.13 9.08 9.23

Total 96.54 96.62 97.68 96.20 97.93 98.08 96.25 95.19 94.8 94.29 94.81 96.82

MS/ (Mg+Fe)

0.54 0.55 0.56 0.57 0.23 0.22 0.22 0.22 0.50 0.51 0.50 0.50

ite, and from 0.2 1 to 0.24 in coarse-grained granite. The somewhat higher Mg/ (Mg + Fe) ratio in biotites from the porphyritic granite probably results from their crystallisation under higher oxygen fugacity than for the diorite that, however, has a higher whole-rock Mg/ ( Mg + Fe) ratio. This interpretation is supported by the occurrence of magnetite in the porphyritic granite but not (or very rarely) in the diorite.

Amphibole in the porphyritic granite is a mag-

nesio-hornblende (Leake’s 1978 nomenclature), whereas its composition ranges from has- tingsitic hornblende to ferro-tschermakite in the coarse-grained granite and from tschermakitic hornblende to magnesio-hornblende in the diorite. Application of the Schmidt’s ( 1992) formu- lation of the Al-in-hornblende geobarometer to amphiboles from the coarse-grained granite, porphyritic granite and diorites gives pressures of 540 to 695 MPa, 338 to 427 MPa and 441 to


Table lb

Microprobe analysis of amphiboles from rocks of the Fazenda Nova/Serra da Japeganga complex

Amphibole


Analysis 15 16 17 42 65 66 86 87 99 110 111 112

No. Rim Core Rim Rim Rim Core Rim Rim Rim Rim Core Rim

SiOl 45.90 45 23 45.76 46.68 40.8 40.55 40.32 39.2 42.8 43.75 44.52 42.86

TiOt 1.29 1 41 0.00 0.88 1.53 1.56 1.54 1.51 0.63 0.84 0.85 0.56

403 8.65 8 35 8.79 .8.36 9.64 9.92 9.43 10.6 10.68 9.54 9.29 10.10

Cr203 0.00 0.00 0.00 0.05 0.43 0.00 0.00 0.12 0.00 0.00 0.00 0.00

Fe0 5.96 15.42 16.38 16.15 27.12 27.54 27.69 26.46 19.03 18.28 18.26 19.57

MnO 0.37 0.00 0.00 0.35 1.11 0.64 0.46 0.35 0.35 0.38 0.78 0.38

MgO 11.44 11.89 11.88 11.8 3.68 3.89 4.14 3.65 9.85 10.00 10.36 9.55

CaO 11.78 11.78 11.91 1.86 11.03 10.84 10.9 10.97 12.1 12.16 11.81 11.96

Na,O 1.17 1.41 1.36 1.27 1.74 1.79 1.96 1.65 0.99 1.17 1.13 1.22

Kz0 0.83 0.85 0.88 0.72 1.29 1.30 1.28 1.54 0.97 0.93 0.88 0.91

Total 97.08 96.34 96.97 98.13 98.37 98.04 97.73 96.06 97.41 97.05 97.88 97.1

Cations

Si

AlIV

AlVI

Ti

Fe’+

Cr Fe*+

Mn

Mg Ca

NaM4

NaA

K

7.0126 6.7875 6.6852 6.7080 6.247 1 6.3786 6.1324 6.4028 6.9817 6.2997 6.4769 6.7396 0.9874 1.2125 1.3148 1.2920 1.7529 1.6214 1.8676 1.5972 1.0183 1.7003 1.5231 1.2604

0.3543 0.3062 0.1401 0.2367 0.3399 0.1549 0.048 1 0.1690 0.5400 0.1521 0.1411 0.3968 0.0727 0.1448 0.1564 0.2367 0.1467 0.1800 0.1519 0.1845 0.1190 0.0702 0.0933 0.0974 0.5781 0.5347 0.6025 0.5267 0.8046 0.8493 1.0793 1.0468 0.6972 1.032 0.8610 0.8471 0.0577 0 0 0.0032 0 0.0538 0 0 0 0 0 0

1.3103 1.4526 1.3034 1.4361 2.7437 2.6968 2.5106 2.6315 1.5715 1.3106 1.4025 1.4650

0.1015 0.0463 0 0.0032 0 0.0538 0 0 0.0447 0.0435 0.0479 0.1002

2.7718 2.5379 2.6207 2.5227 0.8553 0.8577 0.8915 0.9796 2.4496 2.1604 2.2078 2.3379

1.9114 1.8793 1.8665 1.8326 1.8576 1.8473 1.8589 1.8553 1.9950 1.9090 1.9294 1.916

0 0.0982 0.3105 0.2984 0.1781 0.2131 0.4059 0.0718 0 0.2822 0.3169 0

0.1891 0.2392 0.0933 0.0382 0.1304 0.3138 0.1503 0.5333 0.3060 0 0.0195 0.3323

0.1212 0’.1573 0.1303 0.0382 0.2896 0.2571 0.3001 0.2585 0.1510 0.1827 0.1757 0.1700

Total 15.4682 15.3965 15.2536 15.1939 15.4200 15.5709 15.4504 15.7918 15.8740 15.1428 15.1952 15.6626

P(MPa) 338 * 392 427 695 * 611 540 441 581 * 488

58 1 MPa, respectively (Table lb). Higher pressures for the diorite than for the porphyritic granite are in agreement with field relationships, but the highest values in the coarse-grained granite are not. Amphiboles in this latter are richer in AlI”, poorer in Si, and have similar Ca contents than the more matic porphyritic granite. Accord- ing to Giret et al.. ( 1980), amphiboles crystallised early in the magmatic history are richer in Ca and Alrv and we interpret the higher pressures in the coarse-grained granite as indicating lack of equilibrium between the amphiboles and the remaining liquid after their crystallisation.

3.3. Field relationships of mafic qnd felsic magmas

A number of structural and textural features indicate the coexistence of the magmas that subsequently crystallised to form the coarse-grained granite, porphyritic granite and diorite.

Although the coarse-grained and the porphyritic granites are usually in tectonic contact, re- licts of the original magmatic contact have been preserved at the southern tip of the Fazenda Nova batholith. There, the contact is gradational, the amount of matrix and the size of K-


Fig. 6. Irregular magmatic contact between the porphyritic granite and mafic rock in the Fazenda Nova batholith. Note K- feldspar megacrysts and granitic material almost surrounded by diorite along the contact and the presence of smaller K-feldspar xenocrysts inside the diorite.

feldspar megacrysts decreasing southward con- comitantly with an increase in quartz and K- feldspar contents. Combined with the absence of xenocrystic disequilibrium assemblages in these rocks, this suggests that they were intruded ap- proximately at the same time and interacted essentially when in a liquid state.

Irregular to cuspate contacts between the porphyritic granite and the diorites (Fig. 6), together with the occurrence of similar magmatic folds and concordant magmatic foliations in both

rock types, also support the conclusion that they coexisted as contemporaneous magmas. K-feldspar megacrysts cross-cutting the contact between the porphyritic granite and the diorites (Fig. 6) suggest mingling of malic, crystal-poor, and felsic, crystal-bearing, magmas (e.g., Hynd- mann and Foster, 1988; Vernon et al., 1988). Evidence of corrosion of K-feldspar megacrysts in the dioritic rocks suggests that most, if not all, of them were incorporated from the crystallising granitic magma. Mantled K-feldspar megacrysts


Fig. 7. Alternating bands of mylonitized diorite and porphyritic granite. Diorite often contains a large volume of K-feldspar xenocrysts, suggesting mechanical incorporation that probably began at the magmatic stage.

(rapakivi texture ) occasionally abundant in some outcrops and inverse zoning in some plagioclase grains fralm the porphyritic granite are commonly attributed to temperature increases due to the injection of a basic magma in a crystallising felsic magma chamber (Hibbard, 198 1; Wark and Stimac:, 1992). In contrast, the absence of clinopyroxene in the dioritic rocks may result from rapid cooling of the mafic magma down to the stability field of hornblende and plagioclase, inhibiting nucleation of higher-temperature phases (Blundy and Sparks, 1992). Alto- gether these observations suggest that the dioritic magma and the porphyritic granite interacted when this latter contained a considerable amount of crystals in suspension. This stands in contrast with the previous conclusion that porphyritic and coarse-grained granites interacted when in a magmatic, almost crystal-free, state.

Enclaves in the porphyritic granite are devoid of chilled margins, implying small temperature contrasts between coexisting magmas. Their variable shapes, dimensions, textures and com-

positions are probably controlled by the evolv- ing nature of interacting magmas and by dynamical factors. Megacryst-bearing enclaves with poorly-defined contacts and relatively coarse grain-size may represent the most contaminated mafic magma, whereas those with sharp, locally straight boundaries, usually finer-grained and megacryst-free, are thought to preserve a more primitive composition. An intermediate situation is represented by cuspate enclaves with sparse K-feldspar megacrysts..

Stromatitiolike structures, with alternating bands of mutually contaminated granite and quartz diorite (Fig. 7 ) , are common in the vicinity of the FNSZ. They may represent mafic magma emplaced into the porphyritic granite before the latter was crystalhsed enough to sus- tain a fracture. Swarms of extremely elongated enclaves (Fig. 5 ) may have resulted from the disruption of the mafic bands due to large shear in a laminar regime, and therefore may represent disrupted synplutonic dykes. In contrast, the

286 S.P. Neves, A. Vauchez /Lithos 34 (1995) 275-299

Fig. 8. Partial view of contorted synplutonic matic dyke up to 10 m wide cutting porphyritic granite. Leucocratic tonalitic veinlets complexly commingled with the mafic rock possibly reflects the occurrence of mixing deeper in the crust then freezing of the structure during dyke emplacement.

largest enclaves, up to 2 m in diameter, which are also those with the lowest axial ratios, may reflect rather slow motions at the interface of the two magmas. These enclaves are more common further away from the FNSZ. We apply the term mingling to processes responsible for the formation of the stromatitic-like structures and the enclaves because, although chemical and mechanical exchanges are important, the original compositions of the interacting magmas may still be recognised.

Contorted mafic dykes up to several meters wide crosscut the host granite (Fig. 8). These dykes probably developed through local fracturing of the still viscous porphyritic granite. They display gradational contacts, within a few centimetres, with the host and may grade into dis- membered blobs or mafic schlieren. Their internal structure reflects the dynamics of their emplacement and differs significantly from that of the porphyritic granite. Leucocratic tonalitic

veinlets in the dykes, and in some dioritic bodies, suggest that commingling of ma&z and felsic magmas may have occurred deeper in the crust.

Planar mafic dykes with sharp contacts at the mesoscopic scale but with walls deviating around granite megacrysts (some of which cross-cut the dyke-granite boundary), indicate fracturing of an almost, but not completely, crystallised magma. Clearlylate dykes, some with chilled finer-grained borders, marks the end of the mafic activity in the complex.

Less commonly, dykes of porphyritic granite cross-cutting rocks of intermediate composition are also observed. They locally form net-veined complexes (Fig. 9). This is probably due to higher crystal contents in dioritic than in granitic melts when they are at the same temperature. Therefore the dioritic melt may show a brittle behaviour when the felsic magma is still sufficiently mobile to promote fracturing and dyking.

The field relationships presented above sug-

S.P. Neves, A. Yauchez /Lithos 34 (1995) 275-299 287

Fig. 9. Magmatic enclaves resulting from fragmentation of almost solidified mafic material showing sharp contacts with phyritic granite.

gest that mafic magmas were injected continu- ously or episodically, but at a declining rate, during the crystallisat.ion of the porphyritic granite. Although mafic rocks predating the more felsic ones are not observed in the studied area, it remains possible that pervasive crustal melting was induced by the intrusion of mafic melts in the continental crust and that these magmas have in-. teracted to different extents with the crustal melt. This hypothesis will be discussed in more detail later.

3.4. Major-element geochemistry

Major elements of twenty-five samples from the Fazenda Nova/Serra da Japeganga complex have been analyseld (Table 2 ) . Some analysis are from mylonitized samples, but mass balance calculation using the isocon method of Grant ( 1986) shows that gain or loss of elements during mylonitization are not significant in most samples. All rocks are metaluminous (Al,O,<NazO+K,O+CaO) and have a high total alkali content. K,0/Na20 ratios are greater than unity in the coarse-grained and porphyritic

granites and close to unity in the more mafic rocks.

The porphyritic granite exhibits limited compositional variation (e.g., 61.2-64.9 wt.% SiOz, 4.13-4.97 wt.% K,O, 3-O-3.8 wt.% CaO). This is surprising due to the difficulty to obtain representative samples of porphyritic rocks. In spite of the limited number of analyses the uniform chemical composition of this rock suggests that it crystallised from a homogeneous magma. This conclusion is supported by a Rb-Sr reconnais- sance study carried out by McMurry et al. ( 1987 ). Five whole-rock analysis defined an isochron in a 87Sr/86Sr vs. 87Rb/86Sr plot, with a MSWD of 0.65 and an initial 87Sr/ 86Sr = 0.7065 +- 0.0005 (20 error). A pseudo-isochron may be produced by mixing of two end- members to varying degrees, in which case these values have no meaning. However, a scattering of isotopic ratios would be expected if homogenisation of the magma that formed the porphyritic granite had not occurred.

The coarse-grained granite is also relatively homogeneous in composition with SiOz contents ranging from 65.4 to 69.6 wt.% except for a sin- gle, strongly mylonitic, sample which reached

Tab

le 2

W

hol

e-ro

ck

maj

or-e

lem

ent

anal

ysis

for

roc

ks o

f th

e F

azen

da

Nov

a/S

erra

da

Jap

egan

ga c

ompl

ex

Coa

rse-

grai

ned

gr

anit

e A

nal

ysis

T

Q-

TQ-

TQ

- T

Q-

No.

17

7 21

2 33

3 33

6 TQ

- TQ

- TQ

- TQ

- T

Q-

TQ

- TQ

- 33

7 57

* 18

6B*

233*

34

0*

343*

35

6*

Si0

2 65

.42

66.1

1 65

.74

68.5

2 69

.65

69.3

5 67

.1

66.8

1 66

.46

73.3

4 68

.93

TiO

l 0.

63

0.64

0.

74

0.35

0.

33

0.51

0.

64

0.65

0.

52

0.26

0.

44

Al2

03

14.6

6 15

.19

15.2

6 15

.08

14.6

6 13

.89

15.0

9 14

.74

13.9

6 12

.26

13.3

7 F

ez03

6.

34

4.91

5.

38

3.71

3.

70

4.37

4.

67

5.37

6.

51

3.79

5.

80

Mn

O

0.09

0.

08

0.07

0.

05

0.06

0.

05

0.07

0.

06

0.08

0.

05

0.08

M

gD

0.59

0.

94

0.88

0.

27

0.26

0.

61

0.91

0.

86

1.55

0.

20

0.41

C

aO

2.16

2.

19

2.66

1.

66

1.33

1.

81

2.15

2.

52

2.94

0.

84

1.59

N

a10

3.73

3.

60

3.52

3.

66

3.52

3.

42

3.95

3.

47

3.37

2.

84

3.15

K

zO

5.43

5.

46

5.21

6.

21

6.08

5.

15

4.64

0.

98

3.99

5.

63

5.50

P

ZO

S

0.18

0.

23

0.21

0.

08

0.08

0.

15

0.20

0.

19

0.18

0.

05

0.11

Tot

al

99.2

3 99

.37

99.6

9 99

.59

99.6

6 99

.4

99.4

1 99

.61

99.5

7 99

.25

99.3

8

Por

phyr

itic

gr

anit

e D

iori

te

An

alys

is

TQ

- TQ

- T

Q-

TQ

- TQ

- TQ

- TQ

- T

Q-5

9 T

Q-6

2 T

Q-

TQ

- T

Q-

TQ

- T

Q-

No.

52

B

131A

22

8 26

4 15

9*

229*

52

A

239

265

65*

185*

21

2*

Si0

2 64

.63

61.4

3 62

.3

61.2

64

.91

61.6

7 60

.58

52.6

8 61

.05

60.8

6 56

.38

58.1

3 60

.52

60.8

8 T

iOl

0.85

0.

85

0.96

0.

91

0.67

0.

77

0.95

1.

42

0.96

0.

90

1.30

1.

05

0.76

0.

81

f&o3

15

.90

15.9

0 15

.02

15.1

5 15

.68

16.3

5 15

.39

16.7

5 15

.36

14.5

4 17

.81

13.8

3 15

.71

13.9

5

Fe2

03

4.69

4.

69

6.05

6.

34

4.84

6.

14

7.06

9.

96

7.12

6.

51

7.75

8.

43

7.55

7.

16

Mn

O

0.07

0.

07

0.09

0.

09

0.08

0.

08

0.09

0.

13

0.09

0.

09

0.10

0.

14

0.13

0.

10

Mgo

1.

91

1.91

2.

99

3.35

1.

89

2.51

3.

56

4.39

3.

29

4.53

2.

69

5.83

2.

65

4.23

C

aO

3.09

3.

09

3.84

3.

81

3.03

3.

77

4.37

6.

48

4.29

3.

98

5.51

5.

28

4.70

3.

98

Naz

O

3.64

3.

64

3.57

3.

23

3.93

3.

73

3.49

3.

78

3.49

3.

14

3.92

3.

43

3.81

2.

94

K,O

4.

72

4.72

4.

21

4.97

4.

13

4.45

3.

35

2.73

3.

36

4.39

3.

35

2.81

3.

12

4.54

p205

0.

26

0.26

0.

39

0.36

0.

27

0.31

0.

38

0.62

0.

37

0.40

0.

44

0.35

0.

27

0.40

Tot

al

99.5

7 99

.47

99.4

2 99

.40

99.4

3 99

.78

99.2

4 99

.44

99.7

8 99

.37

99.3

4 99

.3

99.3

1 99

.37

*Myl

onit

ized

sa

mpl

es

S.P. Neves, A. Vauchez /Lithos 34 (1995) 275-299 289

50 55 60 65 70 75 SiO

2

6: *

50 55 60 65 70 75 SiO

2

10-r

50 5.5 60 65 70 75 SiO

2

21 50 55 60 65 70 75

SiO .?

10 ,'d,'....'....'....'.,.,

a- AA A b

: 6-

qc ++ . .

#-. F* :

4- . . *

50 55 60 65 70 75 SiO

2

Fig. 10. Geochemical variation (oxide versus Si02) for porphyritic granite (crosses), coarse-grained granite (circles), and diorite (triangles).


73.3 wt.% Si02 (Table 2). In contrast, the malic/ intermediate rocks of the complex show a larger variation in wt.% Si02 from 52.7 to 6 1 .O (Table 2).

Major-element variations are displayed in Harker diagrams in Fig. 10. Important aspects are: ( 1) major-element abundances in the porphyritic granite are intermediate between the coarse-grained granite and dioritic rocks, partly overlapping, respectively, with the silica-poorer and silica-richer samples of these rocks and (2 ) a good linear correlation exists between SiOZ and most oxides. Negative correlation is observed between SiOZ and Ti02, Fe203, MgO, CaO and P205, and positive correlation between SiOZ and K,O. A poor negative correlation exists between A1203 and SiOZ but no obvious relationship is observed between Na,O and silica contents.

4. Discussion

Northeastward plunging stretching lineation in the Fazenda Nova sinistral shear zone implies that the northwestern block of the fault was uplifted. This suggests, when combined with field data which show that the three main rock facies in the Fazenda Nova/Serra da Japeganga plutonic complex resulted from the crystallisation of coexisting magmas (e.g., gradational or cuspate contacts, concordant magmatic foliations, synmagmatic folds), that the Fazenda Nova and Serra da Japeganga batholiths represent different levels of the same batholith. In the following, we will discuss petrologic processes likely to have been involved in the genesis of the different rock types present in the complex.

4. I. Origin of the porphyritic granite

Field evidence and chemical data highlight a progressive transition from the porphyritic granite to the overlying coarse-grained granite. Gra- dational contacts between different rock types may result from several distinct petrologic processes, e.g.: ( 1) roof melting above a high-temperature intermediate magma; (2) restite un- mixing (White and Chappell, 1977; Chappell and White, 1987); (3) crystallisation in a composi-

tionally zoned magma chamber resulting from thermogravitational stratification of an initial homogeneous magma (Hildreth, 198 1); (4) fractional crystallisation of an intermediate magma with segregation of a differentiated granitic liquid fraction; (5) successive intrusion of magmas with different composition from a progressively depleted and/or heterogeneous source and (6) magma mixing. In the case of the porphyritic and coarse-grained granites, hypothesis ( 1) can readily be discarded because roof melting results in the production of a high-silica, minimum-melt magma and the coarse-grained granite does not have such a composition. With the exception of rather rare xenoliths, all enclaves in the porphyritic granite show the classi- cal features of drops of more malic magma in a crystallising granite (Vernon, 1984); combined with the absence of xenocrystic minerals and re- sorbed calcic plagioclase cores, this militates against the differentiation of a restite-richer magma as the origin of the coarse-grained granite (hypothesis 2 ) . Thermogravitational frac- tionation (hypothesis 3 ) has been invoked to explain the formation of subvolcanic zoned silicic magma chambers (Hildreth, 1981) but it is un- likely in deep seated plutons, where the temperature gradient between the magma and its country rock is much smaller. Hypothesis (4) may hardly explain both the linear trend shown on the chemical variation diagrams (Fig. 10) and also the absence of igneous matic cumulates in the porphyritic granite. Hypothesis (5) is attractive considering the similarity of the porphyritic and coarse-grained granite mineralogy, that differs only in mineral proportions. However, if these rocks represent two successive melts extracted from an heterogeneous source, a larger scattering in chemical variation diagrams would be expected. In the case of a progressively depleted homogeneous source, the objection already raised against fractional crystallisation will apply, because curvilinear trends should be observed on the Harker diagrams.

On all variation diagrams (Fig. 10) a line con- necting the composition of the most mafic samples with that of the average coarse-grained granite adequately describe the composition of the porphyritic granite. The good linear correlation

S. P. Neves, A. Vauchez / Lithos 34 (I 995) 27%299 291

observed in most of these diagrams argue for an origin of the porphyritic granite through mixing of felsic and malic magmas. Moreover, the uniform chemical composition and lack of disequilibrium xenocryst assemblage in the porphyritic granite suggest complete mixing of two almost crystal-free magmas. Lack of correlat,ion between SiOZ and Na20 and greater scattering in the A1203 and MgQ vs. SiOZ plots (Fig. 10) when compared with the other oxides, may be attributed either to end-member compositions variable for these components, to limited fractional crystallisation subsequent to mixing, or to slight post-crystallisation chemical modifications associated with the mylonitization of some samples. In addition to chemical data, the modal composition of the porphyritic granite, intermediate in ma& and quartz contents between those of the coarse-grained granite and the dioritic rocks, suggests that the felsic end-member is cur- rently represented by the coarse-grained granite and that the malic end-member was similar to the more malic rocks present in the complex.

4.2. Physical conditions of mixing

Density and viscosity of magmas Formation of a lhybrid magma from two coex-

isting magmas is critically dependent on the density and viscosity contrasts between them, which, in turn, are a function of temperature and water content. Therefore:, to evaluate the probability of the porphyritic granite resulting from mixing of felsic and malic magmas, these parameters should be estimaterd. Hornblende and biotite are commonly the only mafic minerals present in the most mafic rocks of the Fazenda Nova batholith; this indicates that these rocks crystallised from a relatively water-rich magma. More than 3 wt.% HZ0 is necessary for early crystallisation of amphibole from a basaltic melt (Eggler, 1972) and water-undersaturated basaltic to andesitic magmas typically have liquidus temperatures of 1050’ C to 1150’ C at crustal pressures (Green, 1982). Therefore, a minimum water content of 3 wt.% and an emplacement temperature of ca. 1100°C represent reasonable values for the maflc end-member.

The temperature and water content of the fel-

sic end-member are more difficult to constrain. However, it was previously stated that the magma that crystallised as the coarse-grained granite was probably close to the liquidus at the time it interacted with the mafic magma. The coarse-grained granite has strong mineralogical (it is a hom- blende, magnetite and sphene-bearing granite) and geochemical (agpaitic numbers < 1.0) af- finities with I-type granites. Taking into account the liquidus temperature in the Ab-An-Or-H,0 system ( > 940°C; see, e.g., review in Nekvasil, 199 1) and the temperature of partial melting ex- perimentally determined for tonalitic to amphi- bolitic rocks at pressures appropriate to the lower crust (850”-1000°C; see, e.g., Carrol and Wyl- lie, 1990; Vielzeuf et al., 1990; Rushmer, 1991; Skjerlie et al., 1993), it may be considered that the initial temperature of the felsic magma was certainly higher than 850°C. On the other hand, Merzbacher and Eggler ( 1984) and Rutherford and Devine ( 1988 ) reported phase relationships in melts of dacitic composition, similar in SiOZ contents to the coarse-grained granite, suggesting that a minimum of 4 wt.% HZ0 at 300-400 MPa is needed to crystallise amphibole. Naney’s ( 1983 ) experiments on granodioritic compositions also showed that more than 4 wt.% HZ0 is necessary to stabilise hornblende. These values considerably exceed the 1.9 wt.% Hz0 (at 500 MPa) estimated by Holloway ( 1973). As the high tschermakite component of amphibole in the coarse-grained granite suggests relatively low water contents in the magma (Poli and Schmidt, 1992), it seems reasonable to consider water contents for the felsic end-member between 3 and 5 wt.%.

Using these temperature and H20 content estimates, densities and viscosities were calculated following Bottinga et al. ( 1982, 1983) and Per- sikov ( 199 1) respectively. To model the composition of the felsic and mafic magmas involved in the mixing event we used the coarse- grained granite sample TQ-177 and the silica poorest diorite sample (TQ-59) respectively (Table 2). It was assumed that the mafic magma contained 3 wt.% HZ0 and had a temperature of 1100 o C. For the felsic magma, a temperature of 900’ C and a water content of 3 wt.% ( felsic end- member 1 in Table 3 ), have been considered as


Table 3 Calculated Reynolds numbers and wd/vz values using diorite and coarse-grained granite samples to model end-member compositions of interacting magmas in the Fazenda Nova/Serra da Japeganga complex. A crustal density of 2.77 g.cms3 was used in the calculations. v is dynamical viscosity (v= q/p). See text for other assumptions involved in the calculations

Matic end-member Rel Felsic end-member wd/ Felsic end-member wd/v2 Felsic end-member wd/v2

v2

d(cm) 1 (1) 2 (2) 3 (3)

100 94 2.6 0.1 0.02 T=1100”C T=900”C T=850”C i’-= 800°C

300 490 13.4 1.7 0.38 3 wt.% Hz0 3 wt.% H20 4 wt.% Hz0 5 wt.% Hz0

500 1053 28.9 6.2 1.3 0=0 0= 15% 0=30%

1000 2979 81.8 34.1 7.6 p=2.37 g.cm-’ p=2.31 g-cm-’ ~~2.29 gcm-3 ~~2.28 g-cmm3

2000 8460 252.3 78.0 17.0 v= 785 cm2.s-’ v=28,584 v=85,146 v=391,587

cm2-s-r cm2-3-l cm2-s-r 3000 15,542 426.8 143.3 31.6

the most realistic values. However, computa- tions have also been performed with lower temperatures and higher water abundance (felsic end-members 2 and 3 in Table 3 ) . At 900’ C, a granitic magma is almost crystal-free but at 850’ and 800°C the presence of crystals may significantly affect its viscosity. From a rheological point of view, crystal-bearing magmas may be treated as dispersed systems, whose viscosity is mostly dependent on the viscosity of the liquid phase and the particle concentration (Einstein, 19 11). The effects of crystals on the viscosity of melts are generally calculated using the Ein- stein-Roscoe relation:

q=qi( 1 -R0)-2.5

where 1 is the viscosity of the suspension, Q the viscosity of the liquid phase, 0 the percentage of crystals and R a parameter related to packing geometry. Marsh ( 198 1) argued that the appropriate value of R for magmas is 1.67. We used this value in the above relation to calculate the viscosity of the felsic magma at 850 ’ and 800 ’ C assuming 15 and 30 vol.% solids, respectively, which are typical values for granitic compositions at these temperatures (see, e.g., review in Whitney, 1988).

Table 3 summarizes the calculated densities and viscosities. The density difference between

the magmas is less than 5% and the viscosity contrast is less than three orders of magnitude even in the case of the more viscous felsic end- member. These values are conservative ones because the water content used in the calculations are minimum values, and overheating of the granitic magma in contact with the dioritic magma is unavoidable. A water content in the matic magma higher than that assumed here does not significantly affect its viscosity. In contrast, the viscosity of the granitic melt will be reduced by almost an order of magnitude, at 900 o C, if its water content is increased from 3 to 5 wt.%, resulting in a viscosity contrast with the mafic magma less than one order of magnitude. Al- though these calculations merely represent estimates, it is clear that density and viscosity contrasts between felsic and malic magmas are rather smaller in hydrous conditions than in the anhy- drous case and therefore favour mixing.

Mechanical mixing The possibility of two magmas having the

physical characteristics defined above having undergone mechanical mixing may be discussed on the grounds of fluid dynamics estimates. The fluid dynamics calculations are derived from experimental and theoretical considerations only. The results, therefore, do not represent a dem-

S.P. Neves. A. Vauchez / Lithos 34 (1995) 275-299 293

onstration that a physical process occurred in the natural case, but are merely an evaluation of the probability for the process having been active.

According to Campbell and Turner ( 1986 ), a denser mafic magma may be injected fast enough in a felsic magma. chamber to form a fountain and mix with the resident magma, provided flow within the fountain is fully turbulent, when the velocity (w) of the inflowing magma satisfies the relation:

wd/v2 > k

where d is the width of the fissure, v2 the dynamic viscosity of the granitic melt and k is a constant. Campbell and Turner (1986) suggested that the application of the above relation is independent of scale or nature of fluids and proposed that flow within a fountain is fully turbulent if Re,, the inflow Reynolds number, is greater than 400, and that complete mixing will occur for wd/v,:> 70 and partial mixing for 7<wd/v,<70.

wd/v, is calculated from:

WV, = Q/VZ and Re, from:

Re, = Qh where v, and v2 are the dynamic viscosities of the input and resident magmas and Q is given by:

Q= [kdp)lcfp,) l”2d3’2 where g is gravity acceleration (980 crn.~-~), dp is the density difference between crust and input magma, fis a friction coefficient dependent upon the roughness of tlhe conduit walls (usually 0.03; Campbell and Turner, 1986; Blake et al., 1992 ), and pm is the density of the input magma.

Table 3 summarises the parameters used in the calculations and the resulting Reynolds’ numbers and wd/v2 values. For an input magma with composition similar to sample TQ-59, the flow is fully turbulent when the width of the fissure is larger than 3 m. If the composition of the granitic magma is represented by the felsic end-members 1 or 2, which probably better describe the physical conditions of the felsic magma, as discussed earlier, complete mixing will be possible

for fissure widths larger than ten or twenty meters. In the studied area, synplutonic dioritic dykes up to ten meters wide (Fig. 8 ) suggest that the feeders to the Fazenda Nova batholith may have had widths similar to the calculated ones. These results do not imply that the fountain model is the only possible mechanism of mixing, however the consistency between field observations and theoretical arguments strengthen the likelihood for mechanical mixing in the production of the porphyritic granite.

Chemical mixing According to fluid dynamic calculations de-

veloped above, a mafic magma with a composition similar to sample TQ-59 would be readily dispersed when injected in a higher volume of felsic magma similar in composition with the coarse-grained granite. Subsequent formation of a homogeneous hybrid magma involves chemical mixing between the dispersed mafic material and the resident magma through diffusion of the components of one magma into the other. How- ever, because heat diffuses much more rapidly than do chemical components (Sparks and Mar- shall, 1986), thermal equilibrium is probably reached before substantial chemical exchanges occur between the coexisting magmas. In the studied case, the temperature difference between the two magmas probably was in the range 200 ‘- 300°C (850”-900°C and 1050”-1150°C in the granitic and hydrous mafic magmas respectively), allowing an equilibrium temperature of around 1000’ C, sufficiently high to prevent an appreciable percentage of crystallisation in the matic component and to ensure efficient chemical mixing.

The inclusions of one magma in the other ini- tially take the form of ribbons, which may break up to form isolated droplets (Campbell and Turner, 1986). Assuming that the droplets of matic magma have spherical shape, we may estimate the time necessary to homogenise the composition of the two magmas by applying the equation of diffusion for spherical bodies (e.g., Crank, 1975, chapter VI):

6C/&=D[SZC/6r2+ (2/r)JC/&]

where D is the diffusion coefficient and C the


1 l;i Granitic magma

Homogeneous hybrid magma

m Mafic magma

Heterogeneous hybrid magma

- Chilled mafic magma

/ Droplets of mafic magma

q .m Crystals q

S. P. Neves, A. Vauchez / Lithos 34 (I 995) 2 75-299 295

concentration of a diffusing component in a sphere of radius r. Assuming that D remains constant, this equation can be simplified (Philpotts, 1990, p. 265) to:

(Dt)l’2=0.75r

t is the time needed to raise the concentration of the diffusing component at the centre of a sphere of radius r to more than 98% of its concentration in the material surrounding the sphere. The degree of homogenisation is controlled by the slow- est-moving magmatic components (Si and non- alkalis), for whiclh typical diffusivities in hydrous silicate mehs at 900’ - 1000 ’ C vary from lo-l2 to lo-l4 m2.sm1 (Baker, 199 1; Chekhmir and Epel’baum, 199 1) . Using these values in the above equation, shows that, if the dimensions of the dispersed mafic material did not exceed some centimetres, homogenisation may be achieved after only 1 O2 to lo4 years. Even if we consider particles with diameters of several decimetres (which are typical sizes for microgranular enclaves in granitoids) the time for complete homogenisation remains short compared with geological time scale. For example, considering a particle with a rad.ius of 50 cm (a size certainly overestimated regarding the usual size of the enclaves), the time necessary for the composition to be homogenisecl, calculated using a mean diffusion coefficient of lo-l3 m2-s- ‘, is less than 50,000 years. This time span remains small enough to prevent a substantial temperature de- crease in the intrusion due to heat loss to the country rock.

4.2. Origin of the dioritic rocks

According to field relationships and petrographic evidence, the dioritic rocks from the Fa-

zenda Nova batholith were intruded during the crystallisation of the porphyritic granite. This suggests that the mafic end-member may have been completely consumed during the mixing process that resulted in the production of the porphyritic granite. Using the same arguments discussed in the section dealing with the origin of the porphyritic granite, any process other than mixing fails to explain the compositional variation of the diorites. This supports the occurrence of new malic pulses during the crystallisation of the porphyritic granite, leading to repeated mixing. In the meantime, the nature of the malic magmatism did not change, or changed only slightly, otherwise a linear trend from the dioritic rocks to the porphyritic granite but not up to the coarse-grained granite would be expected in the Harker diagrams.

The first episode of mixing might have resulted in a stable, density stratified magma chamber containing an upper, slightly contaminated or uncontaminated layer, and a lower hybrid layer. An evaluation of the time elapsed before this lower layer was involved in a new hybridisation process may be obtained by considering estimated crystal growth rates of K-feldspar, which range from 10-lo-lO-l’ cm*s-’ (Cashman, 1990) to lo-l4 cm-s-’ (Christen- sen and DePaolo, 1993). Taken a mean growth rate of lo-l2 to lo-l3 cm*s-‘, would result ap- proximetely in 6 x 1 O4 to 6 x 1 O5 years necessary for a given crystal to reach 2.0 cm, a typical size for K-feldspar xenocrysts in the diorites. Chem- ical and petrographic data (presence of xeno- trysts in the diorites, compositional variation correlated with increased quartz contents, etc.) are strong evidence that mixing also involved, at this time, mechanical exchanges of crystals al-

Fig. 11. Model showing the proposed scenario for the successive mixing and mingling events in the Fazenda Nova/Serra da Japeganga complex. (a 1) A differentiated mafic magma is injected in a silicic magma chamber. The dispersed matic material and the host granitic magma readily reached thermal equilibrium. This induced the formation of a hybrid magma through diffusive chemical exchanges that began to collect on the chamber floor. (b) Situation after complete mixing, resulting in the development of a stable density stratification in the magma chamber. (c) When crystallisation was sufficiently advanced to allow precipitation and growth of K-feldsp,ar, a new matic input arrived in the magma chamber, spreading as a “lava flow” on its floor (heavy lines represent partially chilled margin). (d) The heat and water supplied by the mafic magma to the overlying granitic magma triggered the convection and stirring necessary to induce the disruption of the chilled margin and to promote vigorous mechanical mixing, resulting in the production of a xenocryst-bearing hybrid magma. The lower temperature under which this second mixing event took place impeded complete homogenisation. (e) Continuing convective flow within the chamber led to mingling of the xenocryst-free and xenocryst-bearing hybrid magmas.


ready present in the porphyritic granite and new crystals resulting from cooling of the malic magma. This resulted in the production of a tex- turally and compositionally heterogeneous, crystal-bearing, hybrid resulting from variable contamination by the granitic magma.

5. Interpretations

Production of large volumes of granitic magmas in the continental crust is commonly attributed to intrusions of basalt that trigger partial melting in the lower to middle crust (e.g. Hil- dreth, 1981; Huppert and Sparks, 1988). In northeastern Brazil, gabbroic to dioritic rocks represent a significant component in most granitoid batholiths, pointing to mantle involvement in magma genesis. This is the case in the Fa- zenda Nova/Serra da Japeganga plutonic complex, characterised in the field as well as on geochemical grounds by a close association of granites and malic/intermediate rocks, and we therefore speculate that it originated by partial melting of the lower crust, due to intrusion of a mantle-derived mafic magma. Based on field, geochemical, petrologic and fluid dynamics arguments, the following scenario is suggested for the subsequent evolution of the Fazenda Nova/ Serra da Japeganga igneous complex (Fig. 11) .

After the granitic magma reached the intrusion level, but before it started to crystallise, a maIic magma was injected into the magma chamber (Fig. 11 a). The mafic magma probably was relatively evolved and had a high water content. This would have resulted in low viscosity and temperature contrasts with the granitic magma, inhibiting chilling and leading to exten- sive physical mixing of the two magmas. The dispersed matic particles were small enough to ensure efficient chemical exchanges with the host granitic magma, resulting in the production of an homogeneous hybrid (Fig. 11 b).

Subsequent influxes of mafic magmas were clearly intruded in the partially crystallised hybrid magma. This suggests that the time elapsed between the end of mixing and arrival of new malic batches was large, so that the temperature and viscosity contrast between the resident and

malic magmas had became too high to allow injection of the latter as a fountain. Because the ar- riving malic magma was also more dense than the hybrid magma, it might have spreaded out onto the chamber floor as a “lava flow” (Blake et al., 1992; Fig. 1 lc). At this time, the temperature of the hybrid magma would be close to the solidus temperature of the ma& magma and, as a consequence, the malic magma near the surface of the “lava flow” might have been partially quenched against the cooler granitic pluton without considerable contamination (Fig. I 1 c). With continuous or episodic malic injections, temperature was increased in the granitic magma immediately adjacent to the “lava flow”, reduc- ing its viscosity and favouring the occurrence of convective instabilities. This eventually disrupted the partially quenched margin (Fig. 11 d ) , allowing mixing of the two magmas (Fig. 11 e ) . Subsequent malic inputs would more easily as- cend to higher levels in the chamber and mixed with the crystallising felsic magma. Repeated mixing episodes of this nature produced the range of composition and texture observed in the intermediate rocks of the complex.

It is thought that most enclaves in the porphyritic granite were ripped away from the newly formed hybrid magma and dispersed through convection currents at different levels of the magma chamber (Fig. 1 le). The formation of these enclaves therefore represents a stage intermediate between the main mixing events and the attainment of high crystal contents in the porphyritic granite.

Enough melt was still present in the porphyritic granite to prevent fracturing even when K- feldspar megacrysts have reached a size of several centimetres. At that time, laminar flow of mafic magma through the still viscous granite would favour mingling of the two magmas. This is evidenced by stromatitic-like structures (Fig. 7) with complete gradational contacts between mafic and felsic bands and presence of large K- feldspar megacrysts in the malic bands that highlight the importance of chemical contamination and mechanical exchange of crystals. With further cooling, contorted synplutonic dioritic dykes (Fig. 9), still displaying gradational contacts with the porphyritic granite, were intruded, subse-


quently followed by late planar synplutonic dykes and, finally, by post-crystallisation dykes.

We have so far considered the evolution of the Fazenda Nova/Serra da Japeganga complex without taking into account its tectonic setting. We estimated that the first mixing event may have lasted less than 50,000 years. Even if this value is underestimated, it remains small compared with typica strain rates. Therefore, the stratigraphy of the: Fazenda Nova/Serra da Ja- peganga complex, with the coarse-grained granite on top of the porphyritic granite, certainly reflects the original situation in the magma chamber. Due to thle density contrast between the two magmas, the original stratification, once es- tablished, tends to stay stable. However, con- temporaneity of magmas crystallisation and strike-slip faulting is documented by stromatitic- like structures, that clearly show the imprint of deformation before the porphyritic granite be- haved as a solid. This suggests that the already active Fazenda Nova shear zone may at least have chanelled the transport of the late matic batches. Furthermore, the similarity of magmatic and mylonitic fabrics in both the porphyritic granite and diorite shows that deformation began before the attainment of the critical melt percentage in the magmas. Therefore, the FNSZ may also have acted as a conduit for the ascent of the mat% batchLes involved in the whole second mixing event. Shear deformation might even have favoured the production of dioritic hybrids because stretching at the interface between the melts increases the surface of contact, facilitat- ing chemical exchanges. The internal structure of the Fazenda Nova /Serra da Japeganga complex therefore reflects lboth the dynamics of magma emplacement, responsible for the production of a stable stratigraphy early in the magmatic history, and the superimposed deformation, that largely controlled the development of the magmatic fabric.

6. Conclusion

Intrusion or extrusion of magmas is appar- ently controlled by rheological contrasts in the crust, rather than .the attainment of the point of

neutral buoyancy (Clemens and Mawer, 1992; Parsons et al., 1992; Hooft and Detrick, 1993) or the crystal content of the magma. This supports the idea that granitic magmas, like most silicic volcanics, are emplaced essentially in the liquid state (e.g., Clemens and Mawer, 1992). The extent to which a mafic magma injected into a silicic system will interact with the resident magma is critically dependent of the crystallisation degree of the felsic magma. If a relatively evolved mafic magma reaches a silicic magma chamber when the resident magma is still crystal-free or poor, a homogeneous hybrid magma can be generated. If the resident magma contains an appreciable concentration of crystals but is still sufficiently mobile, a compositionally heterogeneous, xenocryst-bearing hybrid magma, will result. On the other hand, if the crystal content is high enough to prevent efficient dispersal of the mineral phases, even taking into account some dissolution due to temperature increases as a result of heat transfer from the mafic magma, mingling processes will be more probable.

All three cases above are documented in the Fazenda Nova/Serra da Japeganga complex and highlight that mixing of magmas may be a petrogenetic process as efficient in plutonic as in volcanic contexts and that homogeneous as well as heterogeneous hybrids can be produced in both environments. This suggests that some granitoids interpreted as end-member magmas in mingling events may indeed represent hybrid rocks and, in other cases, that a zone of hybrid rocks may lie beneath some enclave-bearing granitoids.

Acknowledgements

This paper benefited from helpful comments by R.S. D’Lemos and an anonymous reviewer. We are indebted to A.N. Sial for the discussion in the field, and to Majorie Wilson for major-element chemical analysis carried out at the Uni- versity of Leeds. Microprobe analysis were carried out at the University of Montpellier. This research was funded by European Community Project CI l-0320-F-CD. SPN acknowledges doctoral scholarship from Conselho National de

298 S.P. Neves. A. Vauchez /Lithos 34 (1995) 275-299

Desenvolvimento Cientifico e Tecnoldgico ( CNPq-Brazil ) .

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Successive mixing and mingling of magmas in a plutonic complex of Northeast Brazil