Synkinematic emplacement of the Pan-African Ngondo igneous complex (west Cameroon, central Africa)

Journal of African Earth Sciences. Vol.

Pergamon

Pll:SO899-5362(99)00038-X

28, No. 3. pp. 675691, 1999 1999 Elsevier Science Ltd reserved. Printed in Great Britain 0699.5362/99 $- see front matter

0

All rights

Synkinematic emplacement of the Pan-African Ngondo igneous complex (west Cameroon, central Africa) GABRIEL TAGNE KAMGA’,

ERIC MERCIER2, MICHEL ROSSY’ and

EMMANUEL N. N’SIFA3 ‘Laboratoire de Geosciences, Universite de Franche Comte, La Bouloie, 25030 Besancon Cedex, France 2DBpartement des Sciences de la Terre (ESA-CNRS 7072, groupe de Cergy), Universite de Cergy-Pontoise, Le Campus, 9501 1 Cergy-Pontoise Cedex, France 3Department of Earth Sciences, University of Yaounde 1, PO Box 812, Yaounde, Cameroon

ABSTRACT-The Ngondo Complex is one of the Pan-African plutons intruded in the West Cameroon Pan-African Orogenic Belt. The complex consists of three major groups of rocks: basic to intermediate rocks (diorites, granodiorites and minor gabbros), fined-grained granites and coarsed-grained granites successively emplaced in a metamorphic country rock of amphibolite-facies. Synkinematic emplacement of the complex, in relation with a ductile mega shear zone, is documented by a study of microstructures and foliation patterns which indicate a continuous transition from magmatic to high temperature solid-state deformation. The geometry of the internal foliation trajectories and the joint orientation in the complex suggest that the emplacement of the three groups of rocks was totally controlled by a N30° sinistral shear zone. Emplacement mechanisms, which are related in time and space to a continuum of deformation, may indicate a relative rheological change of the crust from ductile to brittle behaviour. o 1999 Elsevier Science Limited. All rights reserved. RESUME-Le complexe de Ngondo est un des complexes plutoniques appartenant a la chaine panafricaine de la partie ouest du Cameroun. II est forme de trois principaux ensembles mis en place successivement dans un encaissant gneissique de facies amphibolite: roches basiques a intermediaires (diorites, granodiorites, rares gabbros), granites a grain fin, granites a grain grossier. La mise en place syncinematique du complexe dans une zone de cisaillement ductile est demontree par I’analyse des microstructures et des foliations qui indique une transition continue de la deformation de l’etat magmatique a l’etat solide haute temperature. La geometric des trajectoires de foliation interne et I’orientation des joints dans le complexe suggerent que la mise en place des trois ensembles plutoniques est entierement controlee par le fonctionnnement d’une zone de cisaillement senestre N30°. Les mecanismes de mise en place, qui sont en relation dans le temps et dans I’espace avec un continuum de la deformation, refletent l’evolution du comportement rheologique, de ductile a fragile, de la croute. Q 1999 Elsevier Science Limited. All rights reserved. (Received

417196: revised version received

2713198: accepted

1O/3/98)

INTRODUCTION In deep parts of most erogenic belts, batholiths represent a major lithological formation. They are usually composite and made up of an association of several plutonic bodies. Their emplacement within the erogenic belt is one of the fundamental processes of magma transfer and crustal growth

(Wyllie et al., 1976; Patchett and Arndt, 1986). Magmas generated in the lower parts of the lithosphere rise up to the middle and upper crust, either through pre-existing fractures or by creating their own way up (ballooning, diapirism, doming, etc.). These general mechanisms (forceful

Journal

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Earth

Sciences

675

0

2OOkm

-

Figure 1. (a) Schematic geological map illustrating the relationships between the Pan-African North Equatorial Fold Belt and the principal tectonic units of Africa. A: West African Craton; B: Congo Craton; C: Kalahari Craton. 1: Trans-Saharan Belt; 2: Nigerian Fold Belt. 161 Location of the Ngondo Complex in relation to the Cameroon Line and Cameroon Central Shear Zone (CCSZl lineaments (from Nzenti et al., 1988). The Congo Craton is represented by the dashed area. (cJ Geological sketch map of the southern portion of the Pan-African North Equatorial Belt with the location of the Ngondo igneous complex (from Soba, 1989). 3: post-Pan-African cover; 4: Tertiary volcanism; 5: syntectonic granitoids 1600-580 Ma); 6: younger metasediments (1200-800 Ma)/calc-alkaline orthogneisses 1660-620 MaJ; 7: older metesediments and orthogneiss (2 100 Ma); 8: heterogenous migmatitic gneisses; 9: Yokadouma series I> 2 100 Ma?); 10: Ntem Complex (2800 MaMieactivated at 600 Ma.

Synkinematic

emplacement

of the Pan-African Ngondo igneous complex (west Cameroon)

emplacement), whereby room is made for large granitic magma bodies, are still a subject of debate because of their uncertainty. However, it is well established that, in erogenic belts, there is a temporal and spatial relationship between magma emplacement and regional deformations. A pluton could be pre-, syn- or late tectonic relative to regional deformations (Hutton, 1988; Hutton et al., 1990; Paterson et a/., 1989, 1991; Paterson and Fowler, 1993). A number of timing criteria have been enumerated, but most of them are still ambiguous (e.g. Paterson and Tobish, 1988). Recently, Miller and Paterson (1994) have demonstrated that, for syntectonic emplacement, the strongest criterion is probably the preservation of a continuous transition from submagmatic to high temperature solid-state deformation. This transition, when well documented, could provide a good insight into emplacement models, cooling and strain rates within the pluton and its wall rocks. The Ngondo Complex is one of the Pan-African plutonic complexes belonging to the west Cameroon erogenic belt. In this paper, a variety of structural elements present in this complex is described in order to determine the relationship between foliation development and pluton emplacement. Foliations and strain patterns will be used to determine the geometry and kinematics of the pluton deformation and to document a continuous transition from submagmatic (viscous) to high temperature solid-state (plastic) deformation during the syntectonic emplacement of the complex in a N30’ sinistral shear zone. The mechanism of intrusion and the evolution of the rheology of the crust with time will also be briefly discussed.

GEOLOGICAL

SETTING

The Cameroon North Equatorial Fold Belt is situated south of the Trans-Saharian Belt (Cahen et a/., 1984) and Eastern Belt of Nigeria (Fig. 1). This erogenic belt was formed through the West African and Congo Craton collision (Toteu et al., 1990; Nzenti et a/., 1988; Ngako et al., 1992; Penaye et a/., 1993). Two major phases of deformation (D, and DJ have been described, each of them accompanied by important magmatic activity. Pre- to syn-D, intrusions are basic to intermediate (diorites to granodiorites) dated at ca 630 Ma (U/Pb zircon age; Toteu et al., 1987). Syn- to late D, intrusions are represented by peraluminous anatectic granites with an age of ca 580 Ma (U/ Pb zircon age; Toteu et a/., 1987). The late D, structural evolution of the erogenic belt is dominated

by east-northeast shears, among which the Cameroon Central Shear Zone (CCSZ) is the most important. The CCSZ is a bulk N70° dextral wrench fault separating the Cameroon Pan-African Fold Belt into two main structural domains: one northern, dominated by north-south to northwestsoutheast structures; and the other southern, dominated by the east-west structures (Ngako et a/., 1991). This lineament shows an important structural control on the main geometrical features of the structures of the belt and could be an ultimate consequence of an important east-west to northwest-southeast crustal shortening during the Pan-African orogenesis. Between the two structural domains, the transition corresponds to a N30° mega shear zone. The Ngondo Complex is situated exactly within this intermediate zone. The Ngondo igneous complex (Fig. 1) is a high K talc-alkaline ‘I’ type intrusion and was emplaced within metamorphic rocks consisting mainly of banded gneisses of medium-grade (amphibolitefacies). The Ngondo Complex stretches over an area of about 1000 km*. It is elongated in shape and made up of three diachronous plutonic groups of rocks of unequal volumetric importance (Fig. 2a): (I) a basic to intermediate group (BIR); (2) a finegrained granites group (FGG); and (3) a coarsegrained granites group (CGG), emplaced in that order and in a short time span, as suggested by field observations and contact relationships between the magma sets. Rb/Sr data on the various groups are not fitted by ‘true’ isochrons but suggest emplacement ages towards ca 630 Ma (Tag& Kamga, 1994, inprep.).

LITHOLOGIES AND MICROSTRUCTURES Country rocks The metamorphic country rocks are mostly finegrained, biotite-amphibole gneisses characterised by a metamorphic layering and a granoblastic texture with a well-developed tectonic foliation defined by the preferred orientation and planar disposition of crystals (feldspar, biotite, quartz). Quartz grains, either grouped or isolated, present undulatory extinction, prismatic subgrains and are generally elongated and lobate in shape. Recrystallisations of quartz, which are evident in a few places, are of mosaic type (type 2 of Boullier and Bouchez, 1978). The quartz microstructure indicates that grain boundary migration was the dominant mechanism during the dynamic recrystallisation. The crystal deformations within the gneiss suggest a weak to high strain plastic deformation.

Journal of African Earth Sciences 677

et al.

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4 3 2 1 0 L rondo Complex and surrounding areas. Precambrian basement: 1: metamorphic Figure 2. (a) Principal lithologies of the . ,^^A. country rocks; 2: basic to intermediate rocks IHRI; 3: fine-grained granites IFGG); 4: coarse-gramed granites ICL~W (The dashed line indicates the dissymetrical zonation of the Sol& batholite). Post Pan-African rocks: 5: Cretaceous sandstones; 6: Nlonako Tertiary anorogenic complex; 7: Tertiary volcanic rocks lbasalts and trachytesl. 678 Journal of African Earth Sciences

Synkinematic

emplacement

of the Pan-African

Ngondo igneous complex

(west Cameroon)

-‘O

,

f /40

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60

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25Pd

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604 /&. foliation in metamorphic country rocks /- maematic and tectonic foliation ’ in &R and FGG b rg;c and tectonic foliation strectching lineation ,’ ,limits of myionitic foliation type m the plutomc rocks

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Figure 2. (b) Structural map showing the foliation and lineation patterns in the Ngondo Complex and surroundmg arEaa. The areas of high strain (mylonitic foliation) are delimited with dashed lines.

Journal of African Earth Sciences 679

G. TAGNE KAMGA

et al.

??

Basic to intermediate rocks (BIR)

0

Fine grained granite (FGG)

7

El Coarse grained granites (CGG)

J

5 A

65

lo

Figure 3. O-A-P classification (after Streckeisen,

Table 1. Summary

of the mineralogy

P

9o

1976) of the three Ngondo plutonic groups of rocks.

in the different

plutonic

groups

Mineralogy

Rock- Types BIR (Basic to Intermediate

Rocks)

auartz-monzogabbros

PI An7,-_45 + Hb + K’spar

+ 0 + Cpx

Quartz-diorites

PI An40_25 + Hb + Bi + 0 + K’spar

Granodiorites and Quartz-monzodiorites

PI An3O_25+ Bi + Hb + K’spar

+ 0

FGG (Fine Grained Granites) Granites

PI An,,,,

+ K’spar

+ Bi + 0

CGG (Coarse Grained Granites) Quartz-monzonites

PI An,,.,,

+ K’spar

Vlonzogranites

PI + K’spar

+ Bi + Hb + 0 f

Cpx

+ Bi + 0

PI:plagioclase;l-lb: hornblende;Bi: biotite; Cpx: clinopyroxene; K’spar: orthoclase or microcline; 0: quartz. Accessory minerals include zircon, apatite, opaques, sphene (up to 5% in the BIN and allanitelepidote, but their modal abundance is highly variable.

660 Journal of African Earth Sciences

Synkinematic

emplacement

Table 2. A summary viscous deformation

BIR

of the Pan-African Ngondo igneous complex (west Cameroon)

of typical microstructures or plastic deformation

observed in the plutonic

groups and relevant

either to

magma tic stage

solid-state stage

preferred orientation of feldspar and biotite crystals parallel to the compositional layering

preferred orientation of strained quartz crystals and ribbon-like microstructures

rotation of feldspars of the phenocrysts

recrystallisation of quartz into equant or elongated mosaic patterns

and “tiling”

recrystallisation and “microclinisation” of plagioclase recrystallisation of biotite abundant myrmekites FGG

alignment

CGG

strong

asymetric recrystallisation tails on feldspar porphyroclasts quartz microstructures as in BIR

of K’spars and enclaves

alignment

of K’spar megacry ts

Plutonic groups Nomenclature and mineralogy of the three plutonic groups are given in Fig. 3 and Table 1. Basic to intermediate rocks (BIRI The BIR are diorites, quartz-diorites, quartz-monzodiorites, quartz-monzogabbros and granodiorites with heterogranular grain size ranging from 1 mm to 1 cm. Quartz-monzodiorites and granodiorites are by far the most abundant types. In the field, these rocks are intimately associated and form two separate parallel bands with a north-northeast-south-southwest direction, one lying to the eastern side of the complex and the other to the western side. Contacts between the different rock types are generally sharp, rectilinear and concordant, which could be interpreted as an indication of their synchronous emplacement in a magmatic state. On a cartographic scale the most mafic types (diorites, quartz-diorites and monzogabbros) appear as scattered bodies dissociated within intermediate types (monzodiorites, granodiorites), which are volumetrically more abundant. These intermediate rocks also contain enclaves (0.5 to 2 m length) of microdiorites and xenocrysts

deformed twins and ovoidal shape of feldspars

of felspar and quartz. Finally, metre to hectometresized blocks of gneisses from the country rocks are also present in the BIR. The BIR generally show a compositional layering marked by alternance at the millimetric to pluricentimetric scale of layers enriched either in quartz and feldspar or biotite and/ or amphibole. They display a constant north-northeast-south-southwest penetrative foliation defined by an alignment of tabular feldspar phenocrysts and/ or flattened microdioritic enclaves. Fine-grained granites IFGGI The FGG compositions are those of monzogranites and syenogranites. They crop out as small stratoid intrusions in the BIR. They appear equally as veins or dykes (20 cm to 1 m thick) either concordant or discordant on the BIR foliation. The FGG are leucocratic, typically fine-grained (grain-size less than 1 mm) rocks, while locally medium-grained. Coarse-grained granites ICGG) The CGG unit, which makes up the core of the complex (Fig. 2a) is elongated in a north-northeast-south-southwest direction over about 40 km; its width varies between 12 km (southern

Journal of African Earth Sciences 68 1

G. TAGNE KAMGA part) and 6 km (northern part). The body is asymmetrically zoned and consists of two main petrographic facies; quartz monzonites in the southwest and monzogranites towards the north and east borders. The CGG group represents the latest PanAfrican magmatic pulse in the area; it is clearly discordant on the BIR and FGG groups, which are cross-cut by veins or dykes (2 cm to 1 m thick) of CGG and found as enclaves along the contacts. The CGG display a well-developed magmatic foliation defined by strong alignment of K-feldspar megacrysts (up to 8 cm) and flattened microdiorite enclaves (5 cm to 1 m). On the whole, microstructures observed in rocks from the Ngondo Complex record the evolution of deformations from a viscous, magmatic or submagmatic stage to a plastic solid-state stage (Table 2). Magmatic microstructures include a preferred orientation of early crystallised minerals; plagioclase and biotite in the BIR, K-feldspars in the FGG and K-feldspar megacrysts and biotite in the CGG; as well as orientation of microdioritic enclaves which are present in all the plutonic groups. Crystal rotation figures and ‘tiling’ of phenocrysts are frequent in the BIR and are ascribed to crystal rotational flow in a magma (Blumenfeld, 1983; Blumenfeld and Bouchez, 1988). The transition between magmatic and solid-state deformation is mainly shown by incipient deformation of quartz, depicted at the microscopic scale by undulatory extinction and by grain shape modifications. The solid-state, plastic deformation overprint in the BIR and FGG is clearly shown by the preferred orientation of elongated and flattened, lobate quartz crystals or aggregates, inducing a weak to strong foliation which is always parallel to the magmatic flow planes. Ribbon-like aggregates (type 2 of Boullier and Bouchez, 1978) appear in the most deformed rocks, particularly in the outer parts of the complex where the strain of plastic deformation is higher. Recrystallised medium-sized grains (0.2-0.5 mm) form an elongated or equant mosaic pattern, suggesting that the grain boundary migration was the dominant process either during the dynamic recrystallisation (Nicolas and Poirier, 1976; Bouchez, 19771, or during post-dynamic, static recrystallisation (Simpson and Paor, 1991) at relatively high temperature (Gapais and Barbarin, 1986). These microstructures, as well as the ovoid or sigmoidal shape of feldspar porphyroclasts, abundant myrmekites in many deformed rocks and recrystallisation of biotite, could be related to plastic deformation under relatively high temperatures (e.g. >3504OO’C). In the CGG the crystals are generally

682 Journal of African Earth Sciences

et al.

less deformed than in the BIR, except in local shear bands. Quartz occurs as large groups of overlapping crystals (0.5 mm to 1 cm) showing welldeveloped subgrains. The crystals are weakly elongated and lobated. Recrystallisation is relatively rare along the grain boundary and displays mosaiclike features. Biotite is generally euheudral and rarely kinked. These microstructures suggest that plastic deformation in the CGG has a lower strain intensity than in the BIR and FGG.

MACROSTRUCTURAL

ELEMENTS

As already noted, the structural relationships between the lithological groups indicate that the emplacement of the Ngondo Complex into the gneissic country rocks took place in three stages: il the basic to intermediate rocks; ii) the fined-grained granites; and iii) the coarse-grained granites. Moreover, the absence of a contact aureole around the complex suggest that the latter was emplaced in conditions close to those of the country rocks (amphibolite-facies conditions). Likewise, the shape of the magmatic enclaves and the contact relationship between the groups suggest that there is no significant time difference between the emplacement of the three plutonic groups. In the following, the tectonic conditions of the magma emplacement will be discussed in the light of macrostructural evidence found in the complex. The tectonomagmatic history of the studied zone is revealed by an abundance of diachronous sets of structural elements, among which some (the more recent) are ubiquitous. In this section, the description of these structural elements will proceed, starting from the more recent (retro-tectonic analysis). Fractures and acid dyke-filled joints (Fig. 4) These structures identically affect both country rocks and the whole complex. Fractures and diaclases appear in three principal sets of directions: N120-1 50°, N30-50° and N70-80° (Fig. 4). The first direction, which is the most represented, is sub-perpendicular to the complex elongation (transverse fractures); its dip average is about 70° towards the southwest or northeast. The two other sets of direction are, respectively, close and oblique to the elongation direction of the complex. Their dip varies from 40-80° southeast or northwest. Felsic aplite and pegmatite dykes correspond to late intrusions related to the CGG magmas. They rarely exceed two metres in thickness and display a constant direction around N 10 to N20 all over

Synkinematic

n= 1401

1

T-

emplacement

of the Pan-African

-R2

cl

Figure 4. Rose diagrams of fractures (a) and dyke-filled joints (6) in the studied area and their structural interpretation in terms of a Riedel shear model (c). The model is consistent with the emplacement and deformation of the Ngondo Complex in a large N30° sinistral shear zone (see text for explanationl.

the complex with a dip varying from 40-70” towards the west or east (Fig. 4). Joint orientation and density distribution indicate that both the fractures and the acid dyke-filled joints are dependent. The geometrical disposition of these principal joint directions could be compared and interpreted in terms of Riedel secondary fractures (Tchalenko, 1970). Thus in this Riedel shear model, the N30°-80° joints are the synthetic Riedel fractures (RI 1, those of the N70°-80’ their symmetric (P), and the transversal N120-150° joints, the antithetic Riedel fractures (R2). The Nl 0-20° dyke-filled direction is representative of the extensional direction. The previous geometrical disposition of fractures and joints is consistent with a N30° sinistral strike-slip shear with a maximal compressive stress to the north-northwestsouth-southeast. These structures are posterior to the emplacement of the three plutonic groups. However, the injection of aplites and pegmatites sealing the extensional fractures indicates a quick evolution of the mechanical behaviour of the crust, from plastic to brittle conditions, at the end of the magmatic activity. Brittle-ductile

shear bands

In the whole complex,

brittle to ductile shear bands of various widths (centimetre to metre) are observed

Ngondo igneous

complex

(west

Cameroon)

(Fig. 5). They cut across magmatic foliation and induce a foliation curvature and re-orientation. Ductile shear bands are well-developed, especially in the CGG. They are responsible for a ductile deformation in the rocks which is attested by distorted minerals (feldspars, biotite and quartz aggregates), rounded porphyroclasts and grain size reduction. The feldspar porphyroclasts in these ductile shear bands often displays crystallisation tails at their extremity. One could also notice a change in the orientation of magmatic foliation along the shear bands (Fig. 5) which is sometimes folded in the BIR. The inference is that this magmatic reorientation or deformation occurred under submagmatic and high temperature conditions. The strongest evidence for this interpretation is the overall high ductility of the affected rocks and the infilling of some shear bands by felsic mineral crystallisation. Some authors (Gagny and Cottard, 1980) interpreted such ductile shear bands as diastrophic and indicated that they correspond to a late magmatic stage of deformation. Locally in the BIR, brittle shears are conjugate. In another zone near the northwestern edge of the complex, a shear band with typical S-C foliations can be seen (Berthe et a/., 1979). The kinematic indicators present in this shear band (quartz, feldspars) indicate a N30° sinistral shear. The presence of S-C foliations shows that the plutonic rocks have still been deformed by a N30° sinistral shear after their emplacement and consolidation. Penetrative metamorphic

structures

in plutonic

and

rocks

Magma tic structures in the coarse-grained granites The magmatic foliation is mostly defined by a planar preferred orientation of euhedral feldspar megacrysts, as well as hornblende and biotite. A statistical analysis of the feldspar megacryst orientation reveals two main domains in the pluton: i/ one domain in the south characterised by a northwest-southeast to north-northwest-southsoutheast foliation direction; and ii) another domain in the north with a dominant northeast-southwest direction of foliation. In these two domains, other secondary directions of foliations could be also found. They are generally well-developed along intragranitic microscale shears bands. In the southwest unit, the concentration of the pole of planar fluidality around a great circle (Fig. 61 suggests a preferential orientation of crystals during magmatic flow. The pole of the circle (104OE, 62O) could be interpreted as representing the magmatic flow vector (magmatic lineation). Thus it is

Journal of African Earth Sciences 683

G. TAGNE KAMGA et al.

2oaTl

a

b

Figure 5. Brittle and ductile deformations in the plutonic rocks drawn from field observations. la) Tension fractures in dioritic rocks. (b) Brittle shears in granodioritic rocks. (c-e) Reorientation of the magmatic foliation in the CGG along ductile shear bands. In ld) and (e) the shear bands are infilled by N30°-45OE acidic veins.

possible that the quartz-monzonite magmas, located in the southwest, were emplaced first, as it is generally accepted that the formation of the zoned plutons starts with the most mafic rock types. Specific structures in the other plutonic lithologies Fine-grained granites. The main structure visible in the FGG are magmatic and tectonic foliations. The magmatic foliation is defined by the planar orientation of K-feldspar (but in places plagioclase or biotite) and also the orientation of some microdioritic enclaves. This foliation is not well-developed when the grain-size is too small. The tectonic foliation is concordantly superposed to the magmatic foliation and defined by an elongation and flattening of quartz aggregates. Its orientation, similar to that in the BIR varies from NIOO-N40° and dips from 30-75O towards the southeast and northeast (Fig. 6).

684 Journal of African Earth Sciences

Basic to intermediate rocks. The structures in these rocks are both magmatic and tectonic. The magmatic foliation is defined by the planar preferred orientation of some euhedral crystals of feldspars and micas in the magmatic layering. This layering is of compositional type and defined by changes in the percent of igneous minerals (the thickness of the layers ranging from 1 cm to 1 m). The magmatic origin of this structure is attested by the presence within the layers of well-orientated and non-deformed igneous minerals such as amphibole, biotite and feldspar. Mafic enclaves, when present, are also orientated parallel to the plane of the magmatic fluidity. The tectonic foliation is concordantly superposed to the magmatic foliation and proceeds from plastic (solid-state) deformation. It is mostly defined by feldspar and quartz aggregate flattening. This tectonic foliation is not

foliation in different

of the tectonic

parts of the complex

Figure 6. Interpretation

foliations

and the metamorphic

and magmatic

rocks.

in the Ngondo country

Complex

in terms of finite strain trajectories.

The stereograms

show

the orientation

rcle pole

> 14%

72814%

8A12%

4Zi8%

< 2 %

of

G. TAGNE KAMGA homogenously distributed in the complex. Along the margin, it is strongly developed (mylonitic type) with small-scale shear bands, whereas towards the core it becomes moderate or weak. This variation in the intensity of the foliation illustrates a decrease of strain (negative strain gradient) from the edge towards the core of the complex. Both tectonic and magmatic foliations are penetrative in the BIR and display a constant NlOO-N40° direction (Fig. 6). West of the complex, they dip 30-60° towards the southeast, while in the east they are often reversed and vary from 40-90° to the southeast and northwest. A sub-horizontal stretching lineation is associated with these foliations. It is defined by a strong alignment of elongated and undeformed crystals of amphibole and also by stretched quartz aggregates and/or microdioritic enclaves and xenoliths. Its direction varies from N20°-N40° and its dip from IO-25O towards the north. The igneous origin of the amphibole and enclaves suggests that the lineation was formed first in a magmatic state, then later developed after magma cooling during plastic deformation. Gneiss country rocks. The gneiss foliation is defined by the planar preferred orientation of flattened crystals (quartz, felsdpar and biotite). Its direction along the western margin of the complex varies from NOO-N20° and the dip from 40-75O to the southeast (Fig. 6). In the large gneiss screen situated in the northwest part of the complex, this same foliation varies from N30°-N45O. In the east margin of the complex, the direction of foliation is more constant and varies from N30°-N50°, while the dip is reversed and varies either towards the northwest or southeast. A subhorizontal stretching mineral lineation defined by quartz aggregates is sometimes visible trending N30°-N40° (Fig. 6). On the whole, the gneiss foliations are generally concordant to the magmatic foliations.

MESOSCOPIC

EVIDENCES OF A DUCTILE SHEAR ZONE

In the Ngondo Complex, the orientation of the foliation could be thus interpreted in terms of finite strain trajectories. The geometry of foliation trajectories indicates two structural domains: i) One in the core of the complex, within the CGG, characterised by a non-penetrative foliation. This structural domain displays in its southern area a sigmoidal pattern with a general northwestsoutheast to north-northwest-south-southeast direction of foliation, which is discordant on the

686

Journal

of African

Earth

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edge of the CGG pluton. North of this domain, the foliation trajectories are more constant and orientated north-northeast-south-southwest. id Another domain, located on both sides of the complex, is that of the BIR and FGG. The foliation trajectories are penetrative with a northnortheast-south-southwest direction and a subhorizontal stretching lineation. The foliation trajectories in the southern part of the CGG pluton cannot only account for a magmatic flow. Indeed, Blanchard et al. (1979) have shown that in an intrusion, when the foliation orientation is of magmatic flow origin, its general tendancy is to be parallel to the margin or wall of the intrusion or to show a ‘pinching’ angle (‘angle de pincement’) symmetrical to the flow injection axis (shear direction). In the case of the Ngondo Complex, foliation trajectories in the CGG are sigmoidal. This geometrical pattern suggests a synkinematic emplacement of the pluton in an active strike-slip sinistral shear. The shear direction inferred is therefore N30° and almost parallel to the edge of the batholite (Fig. 8). The foliation trajectories in domain 2 are compatible with a shear deformation which implies a sub-horizontal stretching and northeast-southwest shortening (Sanderson’s model, 1982). It is considered that the foliation is orientated perpendicular to the direction of maximum finite shortening. Therefore, the subhorizontal lineation of the BIR and FGG could be of tectonic origin. Such subhorizontal lineations have been described elsewhere in magmatic rocks and ascribed to a tectonic origin (e.g. Gouanvic, 1983; Courrioux, 1984; Castro, 1986). In the BIR and FGG, the lineation is marked by elongate quartz and amphibole aggregates and indicates the direction of the major axis of finite strain elipsoid. Many authors have considered that in the highest strain portions of shear zones, the stretching lineation in the mylonitic foliation approximates the shear direction (e.g. Berthe et a/., 1979). As already stated, the stretching lineation in the BIR and deformed FGG plunges northnortheast about 1 O-25 O. The localisation of zones of strong foliation and lineation (mylonitic foliation) near the contacts of the complex with country rocks indicates the presence of high strain ductile wrenching along the complex. At the east of the complex synkinematic indicators, such as asymmetric porphyroclasts, reflect a sinistral displacement along the contact. This wrench orientation almost parallel to the east edge of the complex is similar to that inferred from the CGG foliation trajectories. Although mylonitic foliation is restricted only to the edges of the complex, it could be

Synkinematic

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of the Pan-African Ngondo igneous complex (west Cameroon)

concluded that the shear movement during the BIR emplacement was dominantly sinistral strike-slip with a minor dip-slip component uplifting the western side of the shear zone relative to the eastern. The use of the foliation trajectories for determining shear sense in sheared rocks is not unusual (e.g. Simpson and Schmid, 1983; Simpson, 19861, but this should be done with caution. The geometrical similarity between shear orientation displayed by foliation trajectories in plutonic rocks and that of brittle Riedel structures (fractures and veins) suggest a continuum of deformation controlled by the same N30° sinistral shear from ductile to brittle state. This infers that the whole Ngondo plutonic complex was emplaced in a large active N30° sinistral ductile shear zone. This shear should be initiated with the emplacement of the BIR, followed by that of the FGG and CGG. Both the main phase and late phase of the magmatic emplacement were structurally controlled and deformed under the same shear kinematic conditions.

DISCUSSION Fluctuations of foliation dips in the BIR and FGG The variations of the orientation of plane foliations in the Ngondo Complex could be explained by an interaction between magma intrusion and ductile wrench (‘effet de toit’). To the east, the dip fluctuation in the plutonic rocks results from a northwestsoutheast tectonic transpression and northeastsouthwest stretching contemporaneously with the magma emplacement. The foliation orientations in the BIR and FGG indicate a flower sheet-like shape. Contacts between plutonic groups and tectonic setting As previously stated, the field investigations have shown that the Ngondo igneous complex is intrusive into gneiss country rocks. It was formed by successive emplacement of three plutonic groups of rocks: basic to intermediate; fine-grained granite; and coarse-grained granite. The relationship between these three plutonic groups suggests that there is no significant time difference in their emplacement. Moreover, many structural and microstructural evidence illustrates the synkinematic emplacement of the complex in an active ductile shear zone: i. the elongated shape of the whole complex parallel to the foliation in the country rocks; ii) the geometrical concordance and continuity between the internal plutonic foliation and the country rock foliation; iii) the geometrical concordance between mag-

matic state foliation (viscous deformation) and solid-state foliation (plastic deformation). This implies a similarity in orientation of strain ellipsoids of both deformations. Everywhere, magmatic foliation and solid-state foliation are well visible; iv) the presence of a positive strain gradient from the core complex to the margins: solid-state foliation is strongly developed at the contacts with the wall rocks and weak to moderate in the interior. This strain gradient variation is coupled to microgranitoid enclave shape evolution. At the edge of the complex, in the basic to intermediate rocks, the foliation is of high intensity (mylonitic) and the enclaves quite elongated (X/Z = 4 to 6). On the contrary, in the CGG at the centre of the complex, the foliation is generally weaker (except along micro-shears bands) and the enclaves elliptic in shape (X/Z = 2 to 4 only). This enclave shape variation illustrates well the strain variation intensity in the Ngondo Complex; vl sigmoidal foliation trajectories in the CGG. Microstructural evolution in the plutonic groups is also consistent with the variation of the strain gradient. Microstructures record a transition from magmatic through submagmatic to high temperature solid-state deformation during syntectonic emplacement of the Ngondo Complex. This high temperature state deformation is attested by mineral recrystallisation (biotite, quartz and feldspar), transformations of plagioclase into microcline along crystal microfractures, formation of perthites and myrmekites, all together with quartz deformation-recrystallisation mechanism processes (grain boundary migration). These obsevations suggest relatively high temperatures that can be estimated at about 450-550°C and are consistent with the stability of hornblende and the local recrystallisation of biotite. According to Arzi (1978) and van der Molen and Paterson (19791, the transition between the viscous (magmatic) deformation and the plastic (solid state) deformation takes place at a critical melt percentage of 70%. Moreover, the infilling of feldspar microfractures by a quartzofeldspathic assemblage proves beyond doubt that the melt was still present and that plastic deformation began before magma consolidation. These latest criteria strongly document the syntectonic emplacement of the Ngondo Complex. From field data and numerical modeling, many authors (Brun and Pons, 1981; Hutton, 1988) have made a detailed description of strain patterns related to the interference between granite emplacement and regional deformation. Granites emplaced during regional wrenching have typical strain patterns with sigmoidal or helicoidal foliation trends and

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Metamorphic

Rocks

Figure 7. Block diagram showing the asymmetrical dome pattern of the basic to intermediate rocks. The dip foliation pattern illustrates a westward p&on expansion. The FGG are flat-lying concordant intrusions into the BIR magmatic complex. The tectonic context is weakly transpressive and the shape of the pluton mainly controlled by the regional foliation pattern.

slightly plunging stretching lineations. Moreover, their internal foliation shows a geometrical continuity with the foliation in wall rocks and foliation triple-points are observed at the extremities of the pluton. Although foliation triple-points have not been observed at the margins of the Ngondo Complex, which are poorly exposed and covered by recent volcanic and sedimentary formations, the foliation trend patterns and lineations within the complex are compatible with an emplacement in a regional wrench system. Indeed, the foliation trajectories in the complex indicate a large N30° sinistral ductile shear zone. Emplacement mechanism The emplacement mechanism of the Ngondo Complex could be subdivided into three main stages, each of which corresponds to the magmatic emplacement of a plutonic group. First stage: the basic to intermediate rocks (BIR) The foliation orientation and lineation in these rocks suggest that the plutonic group is a N30° thick, blade shaped, elongated intrusion. This geometry indicates that the intrusion was emplaced and essentially controlled by the regional ductile shear zone. Indeed, in the western part of the intrusion, the internal foliation gently dips towards the southeast, while in the east it is steep and dips towards the northwest and southeast. This

688 Journal of African Earth Sciences

foliation dip variation suggests an asymmetrical dome pattern laterally expanding in a west-northwest direction and stretching in a north-northeastsouth-southwest direction (Fig. 7). The lateral westward expansion of this pluton cannot be related to a simple mode of magma ascent such as diapirism @run et a/., 1990) or an extensional tectonic setting (Lister and Davis, 1989). Indeed, the regional tectonic context of the Ngondo Complex is transpressive. The subhorizontal stretching lineation in the earlier basic to intermediate rocks indicates that these plutonic components underwent a north-northwest-south-southeast shortening. On the other hand, the laterally expanded geometry of the body indicates a major control exerted by the regional foliation pattern. The best explanation is that the expansion took place at a side where the shear stress in the shear zone was quite low and so emplacement was controlled more by the pre-existing structure of the country rocks than by the ductile wrenching. The presence of gneiss screens in these plutonic rocks indicates that much of the emplacement was achieved by localised stoping with the stoped blocks subsequently rotated and deformed parallel to the foliation in the granitoids. Second stage: fine-grained granites The FGG appear as flat-lying concordant intrusions in the Blf?. Their elongated shape indicates that

Synkinematic

emplacement

of the Pan-African Ngondo igneous complex (west Cameroon) in the the magmatic unit or by a clockwise rotation (about 10-20°) of the main stress direction leading transtension along a normally transpressional shear zone. Variations of regional stress orientation during the Pan-African Orogeny, especially during the transition between D, and D, phases, are well documented in the northern Cameroon Pan-African Belt (Ngako et al., 1992). Finally, during the complex cooling, the N30° sinistral deformation was still active and controlled fracturation and late vein geometry.

?’ / 1’ .?

a0 /

01

b’I/ @

Figure 8. Schematic diagrams illustrating the 2 stage emplacement of the CGG in a pull-apart structure controlled by the N30° sinistral shear. a,: regional stress; +’ normal component of regional stress; aK’hydrostatic pressure.

their emplacement has been tectonically controlled by the same active ductile shear zone. On the other hand, we can observe that these granitic intrusions form north-northeast-south-southwest symetric alignments on both sides of the coarsegrained granite. This may imply that the FGG intrusions have been guided by localised openings in the BIR plutonic set. Their emplacement could also have been influenced by again, preexisting foliation. Thus, the FGG mechanism of intrusion suggests a rheological change of the crust which becomes more brittle with time. Third stage: coarse-grain ed granites After the BIR and FGG plutons were emplaced, in the same continuum of deformation, the N30° sinistral shear zone evolved into a pull apart structure creating an opening which allowed the CGG magma ascent (Fig. 8). The presence of subvertical planar foliations in the southern part of the pluton may indicate the root of the intrusion. Then, during the magma ascent and synchronously with the shearing displacement, the shear deformation induces sigmoidal foliation trajectories in the CGG. The magma records the regional deformations during cooling by superposed foliations and by late-stage micro-shears. At this stage of the evolution, the shear movements are greater and the pluton shape mainly controlled by the shear zone. Following this, in the northern part of the CGG pluton, the releasing bend opens and the magma flows by simple shear along the active N30° strikeslip fracture (Fig. 8). This could be explained either by a relative increase of the hydrostatic pressure

CONCLUSION The petrographic, structural and microstructural investigations have shown that: i) The Ngondo igneous complex is syntectonic and made up of three unequal plutonic groups of rocks: basic to intermediate rocks which are by far the most abundant; fine-grained granite; and coarse-grained granite. They were successivelly emplaced and deformed in a large active N30° sinistral ductile shear zone. ii) The structuration of the complex was made from magmatic (viscous deformation) to high temperature solid-state (plastic deformation). This high temperature state is documented by quartz microstructures and mineral recrystallisation such as biotite, microcline, myrmekite and perthite. The estimated temperature condition is about 450~55OOC. iti) The emplacement of the three plutonic group of rocks is related in time and space to a continuum of ductile to brittle deformation, which indicates a change in the rheology of the crust evolving from ductile to brittle state. iv) The Pan-African Ngondo Complex displays characteristics of erogenic-related granitoids emplaced in a regional transpressive tectonic environment. The presence of xenoliths, not as common as microgranitoid enclaves, implies that it is not of high-level origin. The transtension occuring along a normally transpressional shear zone at the final stage of the complex emplacement could be attributed to a clockwise rotation (about lo20°) of the regional stress field after collision. The kinematic and orientation of the shear zone controlling the Ngondo Complex emplacement is similar to that of the Cameroon Line. The activity of this lineament, which has been attributed to the Tertiary (Tchoua, 1974; Moreau et a/., 19871, must have started since the Precambrian. It is outlined by many other syntectonic plutons and anorogenic complexes. For example, in the Air Belt, the magmatic activity (between 620 and 570 Ma) along the Trans-Saharan north-south wrench is

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G. TAGNE KAMGA et al. linked to the oblique collision between the West African Craton and the reactivated eastern mobile belt (Black and LiBgeois, 1993). The wrenching occurred parallel to the suture zone of the two areas in collision. In the same way, the Cameroon Line, which is an intracontinental mega-shear, would be at proximity and subparallel to the limits of the plates in collision. Works in north Cameroon interpret the Massenya-Ounianga gravimetric anomaly as a Pan-African N50° suture line (Ngako eta/., 1992). In this hypothesis, the difference of about 20° between this later direction and the N30° Cameroon Line direction could be explained by a clockwise rotation of the two plates in collision in relation to the initial convergence direction. This explanation agrees with the conclusion of Ngako et a/. (I 992) on the variation of strain direction during the Pan-African tectogenesis.

ACKNOWLEDGEMENTS The authors are greatly indebted to the late Professor J. Marre for his field contribution and discussions while he was at the University of Yaounde (Cameroon). They wish to thank A. M. Boullier and J. P. Karche for discussions and their critical readings of early drafts of the manuscript. D. Hutton is gratefully aknowledged for his constructive comments. Finally this paper has been greatly improved by the constructive reviews of H. Schandelmeier and an anonymous reviewer. This work is a part of the senior author’s Ph. D thesis carried out at the University of Franche-Comt6 (France). Editorial Handling - L. Tack

REFERENCES Arzi, A. A. 1978. Critical phenomena in the rheology of partially melted rocks. Tecronophysics 44, 173-I 84. Berthe, D., Choukroune, P. and Jegouzo, P. 1979. Orthogneiss, mylonite and non coaxial deformation of granites: the example of the South Armorican Shear Zone. Journal Structural Geology 1, 31-42. Black, R. et LiBgeois, J.-P. 1993. Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimonies. Journal Geological Society London 150, 8998. Blanchard, J. P., Boyer, P. et Gagny, C. 1979. Un nouveau critere de mise en place dans une caisse filonienne : le “pincement” des min&aux aux Bpontes. Tectonophysics 53, l-25. Blumenfeld, P. 1983. Le “tuilage des m6gacristaux”, un critbre d’6coulement rotationnel pour les fluidalit& des roches magmatiques. Application au granite de Barbey-SBroux (Vosges, France). Bulletin SociM G&ologiqoe France (7). XXV, 309-318. Blumenfeld, P. and Bouchez, J. L. 1988. Shear criteria in granite and migmatite deformed in the magmatic and solid states. Journal Structural Geology IO, 361-372.

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Bouchez, J. L. 1977. Plastic deformation of quartzites at low temperature in an area of natural strain gradient. Tectonophysics 78,23-50. Boullier, A. M. et Bouchez, J. L. 1978. Le quartz en rubans dans les mylonites. Bulletin SociM GBologique France (7). XX, 253-262. Brun, J. P. and Pons, J. 1981. Strain patterns of pluton emplacement in a crust undergoing non-coaxial deformation, Sierra Morena, Southern Spain. Journal Structural Geology 3. 219-229. Brun, J. P., Gapais, D., Cogne, J. P., Ledru, P. and Vigneresse, J. L. 1990. The Flamanville Granite (NW France): an unequivocal example of syntectonically expanding pluton. Geological Journal 25, 271-286. Cahen, L., Snelling, N. J., Delhal, J. and Vail, J. R. 1984. The geochronology and evolution of Africa. 5 12~. Oxford Science Publication, London. Castro, A. 1986. Structural pattern and ascent model in the Central Extremadura batholith, Hercynian belt, Spain: Reply. Journal Structural Geology 8, 633-645. Courrioux, G. 1984. Etude d’une Bvolution magmatique et srrucrurale dans le contexte d’une zone de cisaillement ductile active: exemple du linbament granitique hercynien de Puentedeune (Galice, Espagne). Thbe 3&me cycle 217~. Universitb de Nancy 1, France. Gagny, C. et Cottard, F. 1980: Proposition de signes conventionnels pour la representation de certaines structures magmatiques acquises au tours de la mise en place et de la cristallisation. 1058 Congr& National Soci&& Savantes, Sciences, fast. /I, ~~37-50. Caen, France. Gapais, D. and Barbarin, B. 1986. Quartz fabric transition in a cooling syntectonic granite (Hermitage Massif, France). Tectonophysics 125, 357-370. Gouanvic, Y. 1983. MBtallogen&se ?I TungstBne-Etain et Or dans le lineament granitique de Monteneme (NW Galice, Espagne): Un exemple d’6volution dans une zone de cisaillement ductile hercynienne. These 38me cycle 249p. Universite Nancy 1, France. Hutton, D. H. W. 1988. Granite emplacement mechanisms and tectonic controls: inferences from deformation studies. Earth Science 79, 245-255. Hutton, D. H. W., Dempster, T. J., Brown, P. E. and Becker, S. D. 1990. A new mechanism of granite emplacement: intrusion in active extensional shear zones. Nature 343, 452-455 Lister, G. S. and Davis, G. A. 1989. The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A. Journal Structural Geology 11, 65-94. Miller, R. B. and Paterson, S. R. 1994. The transition from magmatic to high-temperature solid-state deformation: implications from the Mount Stuart batholith, Washington. Journal Structural Geology 16, 853-865. Moreau, C., Regnoult, J. M., Deruelle, B. and Robineau, B. 1987. A new tectonic model for the Cameroon Line, Central Africa. Tectonophysics 141, 317-334. Ngako, V., Jegouzo, P. et Nzenti, J. P. 1991. Le cisaillement Centre Camerounais. Rble structural et gbodynamique dans I’orogen&e panafricaine. Comptes Rendus Acadbmie Sciences Paris 313. 457-463. Ngako, V., Jegouzo, P. et Nzenti J. P. 1992. Champ de raccourcissement et cratonisation du Nord Cameroun du Proterozo’ique sup&ieur au Pal4ozoique. Comptes Rendus Acadbmie Sciences Paris 315, 371-377. Nicolas, A. and Poirier, J. P. 1976. Crystalline plasticity and solid state flow in metamorphic rocks. 444~. WileyInterscience. Londres. Nzenti, J. P., Barbey, P., Macaudiere J. et Soba, D. 1988. Origin and evolution of the late Precambrian high-grade Yaounde gneisses (Cameroon). Precambrian Research 38, 91-l 09.

Synkinematic

emplacement

of the Pan-African Ngondo igneous complex (west Cameroon)

Patchett, P. J. and Arndt, N. T. 1986. Nd isotopes and tectonics of 1.9-l .7 Ga crustal genesis. Earth Planetary Science Letters 78, 329-338. Paterson, S. Ft. and Fowler, T. K. 1993. Re-examining pluton emplacement processes. Journal Structural Geology 15, 191-206. Paterson, S. R. and Tobisch 0. T. 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-l 11 I. Paterson, S. R., Vernon, R. H. and Tobisch, 0. T. 1989. A review of criteria for the identification of magmatic and tectonic foliations in granitoids. Journal Structural Geology 11, 349-363. Paterson, S. R., Vernon, R. H. and Fowler, T. K. 1991. Aureole tectonics. Reviews Mineralogy 26, 673-722. Penaye, J., Toteu, S. F., van Schmus, W.R. and Nzenti, J. P. 1993. U-Pb and Sm-Nd preliminary geochronologic data on the Yaounde Series, Cameroon: reinterpretation of the granulitic rocks as the suture of a collision in the “Centrafrican” belt. Comptes Rendus Academic Sciences Paris 317, 789-794. Sanderson, D. J. 1982. Models of strain variation in nappes and thrust sheets: a review. Tectonophysics 88, 201-233. Simpson, C. 1986. Determination of movement sense in mylonite. Journal Geological Education 34, 246-261. Simpson, C. and de Paor, D. 1991. Deformation and kinematics of high strain zones. AnnualGSA Meeting, StructuralGeology Tectonic Division, pp29-52. San Diego. Simpson, C. and Schmid, S. M. 1983. An evaluation of criteria to deduce the sense of mouvment in sheared rocks. Geological Society America Bulletin 94, 1288-l 291.

Soba, D. 1989. La serie du Lom: Etude geologique et geochronologique d’un bassin volcano-sedimentaire de la chaine panafricaine a I’Est du Cameroun. These doctorat es sciences 198p. Universite Paris VI, France. Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Science Reviews 12, l-33. Tagne Kamga, G. 1994. Le complexe plutonique Panafricain de Ngondo (Ouest Cameroun): Structure et Petrogenese. These doctorat 224~. Universite Franche-Comte, Besancon, France. Tchalenko, J. S. 1970. Similarities between shear zones of different magnitudes. Geological Society America Bulletin 81, 1625-1640. Tchoua, F. 1974. Contribution a I’btude geologique et petrologique de quelques volcans de la ligne du Cameroun (Monts Manengouba et Bambouto). These doctorat es sciences 347~. Universite Clermont-Ferrand, France. Toteu, S. F., Michard, A., Bertrand, J. M. and Rocci, G. 1987. U/Pb dating of Precambrian rocks from northern Cameroon, erogenic evolution and chronology of the Pan-Africain Belt of Central Africa. Precambrian Research 37, 71-87. Toteu, S. F., Macaudiere, J., Bertrand, J. M. and Dautel, D. 1990. Metamorphic zircons from North Cameroon; implications for the Pan-Africain evolution of Central Africa. Geologische Rundschau 79, 777-778. Van der Molen, I. and Paterson, M. S. 1979. Experimental deformation of partially melted granite. Contributions Mineralogy Petrology 70, 299-318. Wyllie, P. J., Huang, W. L., Stern, C. R. and Maaloe, S. 1976. Granitic magmas: possible and impossible sources, water contents and crystallization sequences. Canadian Journal Earth Sciences 13, 1007-1019.

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