Journal of Petrology

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (23) Request Permissions Google Scholar Articles by ZEH, A. Articles by HOLNESS, M. B. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Journal of Petrology | Volume 44 | Number 6 | Pages 967-994 | 2003
© Oxford University Press 2003

The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

A. ZEH^1,* and M. B. HOLNESS²

¹ MINERALOGISCHES INSTITUT DER UNIVERSITÄT WÜRZBURG, AM HUBLAND, D-97074 WÜRZBURG, GERMANY
² DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK

Telephone: +49-931-888-5414. E-mail: armin.zeh{at}mail.uni-wuerzburg.de

RECEIVED JULY 15, 2001; ACCEPTED NOVEMBER 13, 2002

	ABSTRACT

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite, (B) andalusite–garnet–staurolite–muscovite–chlorite–magnetite and (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite. K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chlorite and (E) garnet–cordierite–chlorite ± staurolite. All assemblages contain quartz, plagioclase, biotite and ilmenite. P–T pseudosections calculated in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃ –SiO₂ –H₂O ± O₂ suggest peak metamorphism at 590 ± 20°C at 5 ± 0·5 kbar, followed by retrogression to 550°C at 3·0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P–T–X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of 80°C, with consequent rapid dehydration of chlorite- and muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusions—a ‘house of cards’ texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A–C resulted from prograde staurolite- and retrograde andalusite-forming reactions.

KEY WORDS: garnet microtexture; metapelite; THERMOCALC; pseudosection; reaction overstep; Nigeria

	INTRODUCTION

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
The correct interpretation of microtextures, domain assemblages and mineral compositions in metamorphic rocks is fundamental to deciphering pressure–temperature–time (P–T–t) paths in crustal terrains. In terms of equilibrium thermodynamics, bulk composition alone is sufficient to predict mineral assemblages, mineral compositions, intra-grain zonations and mineral modes, as well as the onset of mineral reactions along a certain P–T path in a chemically closed system. For a given bulk composition, such predictions can be made using P–T pseudosections calculated using an internally consistent thermodynamic dataset (e.g. Holland & Powell, 1990, 1998).

However, equilibrium thermodynamics alone is insufficient for predictions of microtexture, which is also controlled by kinetic factors involving both heat and mass transfer (Fisher, 1978; Walther & Wood, 1984; Lasaga, 1986; Ridley, 1986). In certain circumstances thermodynamics can also be insufficient to explain intra-grain compositional zonations (e.g. Loomis, 1983; Chernoff & Carlson, 1997). Assuming that the heat transfer is sufficient to trigger mineral reactions, reaction progress is controlled by the rates of mineral nucleation, surface reaction processes, and transport mechanisms (Loomis, 1983; Lasaga, 1986; Rubie, 1998). As shown by calculations and experimental results (Rubie & Thompson, 1985; Ridley, 1986; Brearley & Rubie, 1990; Cygan & Lasaga, 1992), mineral nucleation commonly requires a significant overstep of the pressure–temperature conditions. This holds true not only for solid–solid reactions but also for melting and dehydration reactions, with the consequence that mineral assemblages can persist metastably.

Another effect of reaction overstep is that intra-grain compositional zonations (e.g. of garnet) can be different within rocks of identical composition that experienced the same peak metamorphic conditions. As shown by Loomis (1983), metapelitic garnet porphyroblasts that nucleated and grew in equilibrium along a prograde P–T path will show a smoothly curved variation in composition across the grain. In contrast, the curvature of any compositional zonation is less pronounced if garnet nucleated and grew after reaction overstep along the same P–T path. In general, intra-grain compositional variation in garnets becomes flattened with increasing reaction overstep. This results from the fact that the garnet growth rate increases with increasing overstep, and as a consequence abundant garnet, of uniform composition, can be formed quickly. However, this happens only if the material supply during garnet growth is unaffected by fractionation (bulk, surface or Rayleigh fractionation) and refractory reactant phases.

In general, two growth rate controlling processes are suggested for metamorphic garnet: (1) a diffusion-controlled growth mechanism in which the incorporation of material into the growing grain is faster than the diffusive transport towards the site of growth; (2) an interface-controlled growth mechanism where the incorporation of material into the growing grain is slower than the diffusive transport towards it (e.g. Kretz, 1973, 1993; Carlson, 1989, 1991; Denison & Carlson, 1997; Daniel & Spear, 1999; Spear & Daniel, 2001). Each growth mechanism results not only in distinct patterns of chemical zonation, but also in distinct textures. Diffusion-controlled growth causes a diffusion aureole to be formed around garnet grains, causing a non-random porphyroblast distribution. If the diffusion aureoles overlap, bulk fractionation occurs, causing garnet zonation regardless of whether or not the P–T conditions change.

In contrast, interface-controlled growth results in a random porphyroblast distribution, with no correlation between grain size and nearest-neighbour distance (Kretz, 1973; Daniel & Spear, 1999). This growth mechanism not only allows the formation of garnet clusters, but also will lead to the formation of euhedral crystals (Kretz, 1993, p. 110).

Here, we present a detailed study of eight metapelitic rocks from a restricted area in the Ilesha Schist belt, Nigeria. These developed a range of mineral assemblages, compositions and modes, and contain unzoned garnet with different internal and external textures. P–T pseudosections in the model systems CNKTiMnFMASH ± O are employed to explain the observed mineral assemblages, compositions and modes, and to set constraints on the P–T evolution. Furthermore, discrepancies between P–T pseudosection predictions and observed textures are used to constrain the extent of reaction overstep (see Waters & Lovegrove, 2002). Finally, all our findings are combined to explain garnet microtextures observed in the different samples.

	FIELD RELATIONSHIPS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
The samples were collected from the westernmost part of the Ilesha Schist belt (Fig. 1), which is dominated by metapelitic biotite schists and gneisses prekinematically intruded by the Oshu granite and numerous mafic and ultramafic sills and dykes. Postkinematic pegmatite bodies are common [for more details see Rahaman (1988) and Ige et al. (1998)]. Rb–Sr whole-rock dating on two pegmatites yielded ages of 562 ± 12 and 550 ± 15 Ma (Matheis & Caen-Vachette, 1988). In the metapelites, early recumbent isoclinal D₁ folds are transposed during a second folding event (D₂), which led to the formation of upright to west-vergent antiforms and synforms with northward plunging fold axes (e.g. Boose & Ocan, 1991; Rahaman, 1992; Ige et al., 1998). The dominant mineralogical banding is parallel to the D₂ fold axis and strikes NNE–SSW with a dip of 70–90° ESE. At present little is known about the metamorphic evolution in the Ilesha Schist belt. P–T data are available only from amphibolites and meta-ultrabasites infolded with the Ilesha metaclastics, which yielded peak P–T conditions of about 550–620°C at 1·5–3 kbar (Ige et al., 1998).

View larger version (95K): [in this window] [in a new window]   Fig. 1. Geological map of the Ilesha region with sample localities, modified after Ige et al. (1998).

 

	PETROGRAPHY

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
All samples have a gneissose foliation parallel or sub-parallel to the axial planes of the D₂ folds, designated here as S₂. In some samples earlier and/or later foliations are evident from angular relationships between the foliation in the matrix and that defined by alignment of elongate quartz inclusions in staurolite and cordierite porphyroblasts (Fig. 2). Quartz and plagioclase in all rocks are commonly recrystallized, with textures suggestive of static crystallization. A few samples contain large xenomorphic plagioclase crystals and large quartz relicts with subgrains and undulose extinction. The former are commonly inter- or overgrown by new, statically recrystallized plagioclase. From mineral assemblages, modes and textures we distinguish five types of metapelites within the Ilesha Schist belt (Figs 2–5). Types A–C contain significant amounts of muscovite (K-rich metapelites) whereas types D and E are muscovite free (K-poor metapelites). Accessories in types A–C are magnetite and ilmenite, and in types D and E ilmenite and rare rutile.

View larger version (58K): [in this window] [in a new window]   Fig. 2. Synopsis of the textural and mineralogical evolution of rock types A–E (for further explanation, see text).

 

View larger version (193K): [in this window] [in a new window]   Fig. 3. Photomicrographs of type A (a–d) and type B samples (e, f). (a) Euhedral garnet with preferentially oriented biotite inclusions along garnet {110} planes, forming a ‘house of cards’ texture (sample a1). (b) Garnet with oriented biotite and tabular quartz inclusions. The large quartz grain (qtz) is surrounded by garnet facets, and tabular quartz inclusions generally become thinner towards the garnet rim and away from large, primary quartz inclusions (sample a1). (c) Cluster of euhedral garnets, which merge to form aggregates (sample a3). (d) Partially resorbed garnet with an internal ‘house of cards’ structure enclosed by staurolite (sample a1). (e) Resorbed garnet grains, biotite and muscovite enclosed by andalusite (sample b1). (f) Euhedral staurolite overgrowing S2 traced by opaque minerals and quartz. Matrix biotite defines S2a (sample b1).

 

View larger version (206K): [in this window] [in a new window]   Fig. 4. Photomicrographs of type C sample c1 (a–c) and type E sample e1 (d–f). (a) Prismatic sillimanite in association with garnet, biotite and minor muscovite. (b) Prismatic sillimanite with different orientation is overgrown by andalusite. The different orientation is evident from the varying grey scales of sillimanite. (c) Partially resorbed garnet overgrown by andalusite. (d) Cordierite with euhedral garnet inclusions overgrows the S2 foliation defined by elongate quartz grains and an early biotite generation. (e) A second biotite generation in cordierite perpendicular to S2. ( f) Angular staurolite and cordierite are separated by quartz. (Note muscovite intergrown with biotite and cordierite.)

 

View larger version (210K): [in this window] [in a new window]   Fig. 5. Photomicrograph of type D sample d1. (a) Euhedral garnet porphyroblasts are enclosed by staurolite, which is itself being resorbed by cordierite—typical of garnet-bearing domains. (b) Staurolite resorbed by cordierite and chlorite—typical of garnet-free domains. (c) Staurolite grain with a straight boundary in contact with intergrown biotite–chlorite aggregates. This assemblage is entirely enclosed by cordierite. (d) Euhedral garnet and biotite–chlorite aggregates entirely enclosed by cordierite, which replaces staurolite. (e) Biotite, zircon and euhedral garnet enclosed by cordierite. (f) Euhedral garnet grains of different size in contact with quartz and biotite, which is overgrown by chlorite. Individual garnet grains merge to form aggregates.

 
Type A: Grt–St–Bt–Ms schists (samples a1, a2, a3)
Type A metapelites are dominated by garnet, staurolite, biotite, plagioclase, quartz and muscovite, with rare chlorite and accessory magnetite and ilmenite (Table 1). The following domain assemblages are present:

(A1) Grt + Bt + Ms ± Chl ± Pl + Mt + Ilm;

(A2) Grt + St + Bt + Chl + Ms ± Pl + Mt + Ilm;

(A3) Grt + St + Bt + Ms ± Pl + Mt + Ilm.

The S₂ foliation, traced by biotite, muscovite and rare chlorite, and overgrown by staurolite, transects randomly intergrown biotite–muscovite aggregates. Large chlorite flakes are common and overgrow the S₂ foliation at high angles. This fabric may be the early onset of a later foliation (S₃). Garnet occurs in biotite–muscovite-rich layers, where it may form grain clusters (samples a2 and a3), and is enclosed by staurolite. Ilmenite and magnetite are either enclosed by staurolite or in the matrix. Garnet grains are rarely euhedral, but commonly form irregular grains with straight boundaries in contact with quartz and biotite (Fig. 3a). Garnet grains in both the matrix and in the large staurolite porphyroblasts show two typical features: (1) a ‘house of cards’ structure enclosed in garnet; (2) partial resorption at porphyroblast rims and along inclusion arrays. The ‘house of cards’ structure, which we believe to be reported here for the first time, is formed by oriented inclusions of biotite ± tabular quartz (Fig. 3a, b and d) with three- and four-fold symmetry, suggesting that the inclusions are oriented parallel to garnet {110} planes. Sub-equant angular quartz inclusions in garnet are always bounded by garnet facets, and are invariably connected to tabular quartz grains parallel to {110} planes (Fig. 3b). The tabular quartz grains generally become thinner toward garnet rims and away from the angular quartz inclusions, and generally terminate with low-angle tips (Fig. 3b).

View this table: [in this window] [in a new window]   Table 1: Modes obtained by point counting thin sections of samples from the Ilesha Schist belt (in vol. %)

 
Because garnet enclosed by staurolite is resorbed (Fig. 3d), the ‘house of cards’ structure pre-dates staurolite growth. Resorption patterns of garnet in the matrix (i.e. those crystals not enclosed by staurolite) can be linked to two different garnet consumption events, the first during prograde staurolite growth, followed by a second retrogressive event. During the retrograde stage, garnet consumption commonly progressed most rapidly in the region of the biotite and quartz inclusions, resulting in an accentuation of the ‘house of cards’ array.

Type B: And–St–Grt–Bt–Ms schists (samples b1, b2)
In contrast to type A metapelites, andalusite is additionally found in type B metapelites. Sample b2 contains garnet (7·2 vol. %) both enclosed in staurolite and in the matrix, with only minor andalusite (Table 1). Sample b1 has minor garnet (0·9 vol. %), which either occurs in the matrix or is enclosed by abundant euhedral grains of andalusite (Fig. 3e). In sample b2 both the matrix garnets and the garnets enclosed by staurolite show similar resorption patterns to those observed in type A samples. They rarely show a ‘house of cards’ structure. Andalusite in all samples contains inclusions of biotite, muscovite, quartz, garnet, magnetite and ilmenite, but never staurolite. Garnet in andalusite always displays resorption textures (Fig. 3e), indicating that garnet grew before andalusite. Andalusite in both samples occurs with euhedral staurolite, but intergrowth of the two minerals is not observed. Staurolite of both samples overgrows an older foliation traced by quartz ± ilmenite ± magnetite (Fig. 3f ). In sample b1 the internal foliation is at a high angle to that in the matrix (Fig. 3f ), but in sample b2 it is nearly parallel. It is not clear which foliation is related to D₂. Here, we designate the internal foliation as S₂ and the external as S_2a (Fig. 2). Andalusite in sample b2 commonly overgrows patches of statically recrystallized quartz and plagioclase, indicating that deformation D_2a must have ceased before andalusite growth. Elongate chlorite grains, some perpendicular to S₂, occur in the matrix. In a few places chlorite is parallel to S_2a. In sample b1 chlorite replaces garnet and staurolite. In type B samples the following domain assemblages are present:

(B1) Grt + Bt + Ms ± Chl ± Pl + Mt + Ilm (b1, b2);

(B2) Grt + St + Bt + Chl + Ms ± Pl + Mt + Ilm (b2);

(B3) Grt + St + Bt + Ms ± Pl + Mt +Ilm (b2);

(B4) St + Bt + Ms ± Mt + Ilm (b1);

(B5) And + (Grt) + Bt + Ms + Mt + Ilm (b1, b2);

(B6) And + Bt + Ms + Chl + Mt + Ilm (b1, b2).

Type C: Sil–And–Grt–Bt–Ms–(Chl) schist (sample c1)
Type C contains prismatic sillimanite (Table 1) and no staurolite. The S₂ foliation is traced by quartz and mica. Garnet is heterogeneously distributed throughout the rock, always occurring in mica-bearing domains. Garnet grains in sillimanite-rich domains are commonly euhedral, whereas those in andalusite-rich domains are partially resorbed (Fig. 4a and c). Some garnet grains in both the matrix and within andalusite are preferentially consumed along parallel-sided angular cracks (Fig. 4c). The ‘house of cards’ structure is absent. Sillimanite grains overgrow S₂ but are themselves overgrown by andalusite (Fig. 4b) together with biotite, muscovite, garnet, magnetite and ilmenite, and clusters of well-recrystallized quartz grains. A few prismatic grains are homotactically intergrown with andalusite parallel to the sillimanite [010] direction. The random orientation of sillimanite in andalusite suggests that sillimanite was stable before andalusite growth (see Kerrick & Woodsworth, 1989). Late chlorite grows perpendicular to S₂. Domain assemblages observed in sample c1 are

(C1) Sil + Grt + Bt + Ms + Ilm + Mt;

(C2) And + Sil + Grt + Bt + Ms + Ilm + Mt;

(C3) And + (Sil) + (Grt) + Bt + Ms + Chl + Ilm + Mt.

Type D: Crd–St–Grt–Bt schist (sample d1)
Type D is muscovite and magnetite free, but contains abundant cordierite (Table 1). Sample d1 shows a weak foliation traced by the alignment of elongate quartz and biotite. Garnet invariably forms clusters of euhedral grains (Fig. 5d–f ), similar to those in sample a2 ( Figs 3c and 5f). Garnet is enclosed by staurolite and/or cordierite and is in contact with biotite, chlorite and quartz (Fig. 5a and d–f ). Garnet inclusions in staurolite are always euhedral (Fig. 5a) and smaller than in the matrix (Fig. 6e–h), indicating that matrix garnet growth continued after staurolite growth ceased.

View larger version (100K): [in this window] [in a new window]   Fig. 6. Element distribution maps of garnet. (a–d) Type A, sample a1: garnet, with a ‘house of cards’ structure shows no zonation of Mn (c) or Fe (d). Ca (b) increases slightly at some garnet rims, whereas Mg (a) decreases. (e–h) type D, sample d1: small euhedral garnets enclosed by staurolite and larger euhedral matrix garnets are unzoned and show uniform Mg (e), Ca (f ), Mn (g) and Fe (h) contents.

 
Two types of staurolite grains are observed: type StI occurs in garnet-rich domains and contains garnet (Fig. 5a); type StII occurs in garnet-free domains and has no garnet inclusions (Fig. 5b). Staurolite is always surrounded by replacive cordierite (Fig. 5a and b) and shows planar crystal faces only where it is in direct contact with biotite and chlorite (Fig. 5c). Euhedral biotite flakes occur in contact with garnet (Fig. 5f ), often intergrown with chlorite (Fig. 5c, d and f ). Ilmenite is either in the matrix or enclosed by garnet and staurolite. The following domain assemblages are present (all + quartz + plagioclase):

(D1) Grt + St + Bt + (Chl) + Ilm;

(D2) Grt + St + Bt + Crd + Chl + Ilm;

(D3) St + Bt + Chl + Crd + Ilm;

(D4) Grt + Bt + Crd + Ilm;

(D5) Grt + Bt + Chl + Crd + Ilm.

Type E: Crd–Grt–Bt–(St) schist (sample e1)
Type E has the same assemblage as D, but with more abundant cordierite and very little staurolite (Table 1). A pronounced foliation (S₂) is traced by quartz and early biotite and overgrown by large cordierite porphyroblasts (Fig. 4d and e) containing a second biotite generation perpendicular to S₂ forming a weak S₃ fabric (Fig. 4e). Euhedral garnet grains are enclosed by cordierite (Fig. 4d and e). A few garnet grains have a core rich in randomly oriented ilmenite and rare rutile, and a nearly inclusion-free rim. Garnet may be in contact with euhedral staurolite, quartz, fibrolitic sillimanite and cordierite. Staurolite is surrounded by quartz, plagioclase and cordierite (Fig. 4d). Textural evidence for the stability of the staurolite + garnet + cordierite assemblage is ambiguous. Chlorite enclosed by cordierite is locally parallel to the second biotite generation. No reaction was observed between chlorite and garnet. A very few muscovite flakes (0·1 vol. %) are present, mostly intergrown with chlorite and sometimes with biotite in cordierite, and rarely in contact with staurolite (Fig. 4f ). The following predominant domain assemblages are present:

(E1) Grt + Crd + Bt + Chl + Pl + Ilm ± Rt;

(E2) Grt + Crd + Bt + Pl + Ilm.

Rarely observed domain assemblages are

(E3) St + Bt + Pl + Ilm ± Ms ± Crd ± Chl;

(E4) Grt + St + Sil + Bt + Pl + Ilm ± Crd;

(E5) Crd + Bt + Chl + Ms + Pl + Ilm.

Crystallization–deformation history
The earliest crystallization–deformation history of metamorphic rocks is generally recorded by mineral inclusions in other phases (e.g. Figs 2–5). In contrast to many other examples of regional metamorphism (e.g. Bell & Hickey, 1999; Spear & Daniel, 2001) the inclusion patterns in garnet in all samples described here never provide evidence for overgrowth of an earlier fabric. Microtextures (Fig. 2) indicate that the dominant foliation S₂ is statically overgrown by staurolite and andalusite in rock types A and B, and by staurolite and cordierite in rock type E. Evidence for the ‘rotation’ of staurolite is found only in sample b1, which displays angular relationships between the foliation enclosed in staurolite and that in the surrounding matrix (S_2a). Sillimanite in sample c1 overgrows S₂ with random orientations. Late chlorite and a second biotite generation (sample e1) locally form a weak S₃ fabric perpendicular to S₂, suggesting that the stress field changed after S₂ formation. In summary, the available evidence suggests that the penetrative deformation associated with the D₂ folds ceased before the metamorphic peak, with essentially static retrogression.

	MINERAL CHEMISTRY

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Mineral compositions were measured in all the observed domain types (Tables 2–5). Biotite, chlorite, cordierite, staurolite, muscovite, ilmenite and magnetite show only minor compositional variation within individual thin sections, although significant differences occur between samples (Table 5).

View this table: [in this window] [in a new window]   Table 2: Mineral compositions of samples from the Isaobi area used for P–T calculations (see Table 6)

 

View this table: [in this window] [in a new window]   Table 3: Mineral compositions of samples from the Mokuro area (see Fig. 1)

 

View this table: [in this window] [in a new window]   Table 4: Mineral compositions of samples from the locations e1 and a1 (Fig. 1)

 

View this table: [in this window] [in a new window]   Table 5: Compositional range of garnet, staurolite, cordierite, biotite, chlorite, muscovite, plagioclase and ilmenite of samples from the Ilesha Schist belt

 

View this table: [in this window] [in a new window]   Table 6: Results of P–T calculations of samples from the Ilesha Schist belt using THERMOCALC v.275 (Holland & Powell, 1998) and mineral compositions shown in Tables 2–4

 
Generally garnet is unzoned, with grains of different size (even resorbed fragments) showing no significant compositional variation (Figs 6 and 7). Calcium contents are low with Ca/(Fe + Mg + Mn + Ca)^Grt of 0·03–0·06, and Mg is fairly constant with Mg/(Fe + Mg + Mn + Ca)^Grt between 0·10 and 0·14. In contrast, Mn and Fe are highly variable [Mn/(Fe + Mg + Mn + Ca)^Grt = 0·09–0·20, and Fe/(Fe + Mg + Mn + Ca)^Grt = 0·64–0·80]. A very slight zonation of iron and magnesium was observed in garnet enclosed by cordierite in type D (sample d1; Fig. 7), and a slight increase of Ca occurs at the rim of garnets in sample a1 (Figs 6 and 7).

View larger version (42K): [in this window] [in a new window]   Fig. 7. Garnet composition and profiles in different domains of samples of rock types A–E. St, Crd, And denote profiles across garnet grains included by staurolite, cordierite or andalusite, respectively, Mx, matrix garnet; r, rim; c, core; FMMC = Fe + Mg + Mn + Ca (for further explanation, see text).

 
In sample a1 a steep increase of Fe/(Fe + Mg) was detected in the outermost 1–2 µm of some matrix garnet grains, probably caused by limited diffusive Fe–Mg exchange with surrounding biotite and/or chlorite. This increase was not detected in garnet grains enclosed by staurolite and/or cordierite (Figs 6 and 7). As shown in Fig. 7, the garnet composition in samples belonging to the same rock type can be different (e.g. a1, a2, a3). On the other hand, garnets in different rock types may have similar compositions (e.g. a2 and b1).

The Fe/(Fe + Mg) ratio varies between 0·79 and 0·83 in staurolite, between 0·42 and 0·52 in biotite, between 0·41 and 0·50 in chlorite, and between 0·27 and 0·29 in cordierite (Table 5). The Fe/(Fe + Mg) of biotite and chlorite is generally higher in muscovite-bearing samples, and lower in muscovite-poor or -free cordierite-bearing samples.

Biotite generally has a Ti^vi content between 0·08 and 0·09 p.f.u., although biotite in sample b2 contains up to 0·13–0·14 p.f.u. Muscovite in all samples has silica contents between 2·99 and 3·03 p.f.u., and contains significant amounts of Na in the interlayer position with Na/(Na + K)^Ms ranging between 0·15 and 0·23. Plagioclase is virtually unzoned in all samples and ranges from An_0·25 to An_0·39 (Table 5). Types D and E contain nearly pure, end-member ilmenite (Table 5), whereas in all other samples ilmenite contains up to 11 mol % hematite. In all samples, the pyrophanite component is insignificant (Table 5).

	GEOTHERMOBAROMETRY

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
P–T conditions for the various samples were estimated using the compositions of minerals assumed to be in chemical equilibrium. Determination of the equilibrium assemblage at different stages during the P–T evolution is not straightforward because some samples obviously contain minerals dating from several different stages. This is clearly apparent in the case of samples b1, b2 and c1, where andalusite and foliation-perpendicular chlorite formed late in the metamorphic evolution, whereas the coexisting sillimanite, garnet and plagioclase represent an earlier stage. Furthermore, the present-day mineral compositions may not reflect their compositions during growth.

To solve the latter problem we assumed that in samples with minor (late) andalusite, the compositions of biotite, muscovite and chlorite did not change significantly during andalusite growth (e.g. sample b2). In contrast, in samples with abundant late andalusite, the compositions of biotite, muscovite and chlorite probably changed significantly during retrogression (e.g. sample b1 and c1). Consequently, estimated peak P–T conditions of these samples should be treated with caution.

Retrograde resetting of Fe–Mg in biotite is probably not problematic given the abundance of biotite (Table 1, Fig. 12). Garnet shows almost no retrograde diffusive Fe–Mg exchange patterns, and the persistence of euhedral grains shows that resorption during retrogression was minor.

View larger version (21K): [in this window] [in a new window]   Fig. 12. Predicted evolution of mineral modes in sample a1 (a), sample b2 (b) and sample d1 (c), assuming thermodynamic equilibrium during temperature rise at 5 kbar. Mole proportions* = mole proportions with quartz and H2O in excess.

 
P–T calculations were carried out using the ‘average pressure–temperature calculations’ feature of the software package THERMOCALC (Holland & Powell, 1998). Activities of the mineral end-members were calculated with the program Ax of T. J. B. Holland (personal communication, 2001, available at http://www.esc.cam.ac.uk/astaff/holland/index.html). Details of the mixing models used to calculate the activities of end-members are given in the caption to Table 6. The mineral analyses and mineral end-members used for P–T calculations are shown in Tables 2–4. We used mineral end-members in the system CNKFMASH, with Mn, Ti and Fe³⁺ excluded because of large errors in the calculated activities and/or lack of knowledge about the amount of Fe³⁺ in the aluminosilicates (Table 6). Results are presented in Table 6 and in Fig. 10a.

View larger version (18K): [in this window] [in a new window]   Fig. 10. (a) P–T conditions obtained for different samples using the program THERMOCALC (Holland & Powell, 1998), shown in Table 6. (b) Synopsis of the individual P–T path sections obtained from P–T pseudosections of the individual samples.

 
The P–T estimates involve errors in the thermodynamic data of the mineral end-members and errors of end-member activities [for details see Powell & Holland (1994)]. ‘Geological errors’ resulting from retrogression are not considered. The mean P–T values, quoted below for simplicity, are not the best P–T estimates, but all P–T values within an error ellipse (Fig. 10) have the same significance.

P–T calculations using mineral compositions of types A and B (a2, a3, b1, b2) yielded identical (within error) average P–T conditions of 585 ± 25°C at 6 kbar (Fig. 10a). Similar P–T conditions were estimated for sample e1. In contrast, samples a1 and d1 yielded temperatures of 616 ± 17°C at identical or slightly lower pressures (sample d1: 5·1 ± 0·4 kbar). Sample c1 gave lower P–T conditions of 577 ± 31°C at 3·8 ± 1·1 kbar. These differences (Fig. 10a) might indicate that rocks in the Ilesha Schist belt represent different stages during the P–T evolution.

	P–T PSEUDOSECTIONS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
To obtain additional information about the metamorphic evolution, P–T pseudosections in the model system CNKTiMnFMASH were constructed for the magnetite-free samples of types D and E (Fig. 8). Pseudosections for the magnetite–ilmenite-bearing samples of types A–C were calculated in the model system CNKTiMnFMASHO (Fig. 9). Calculations were carried out using the software package THERMOCALC v.TC3mac (Powell & Holland, 1988; Holland & Powell, 1990, 1998) with the internally consistent thermodynamic dataset HP98 and activity models described in Appendix B. Bulk compositions were reconstituted from mineral modes and compositions (Tables 1–4). In all pseudosections the error ellipses of the P–T estimations are shown for comparison.

View larger version (60K): [in this window] [in a new window]   Fig. 8. P–T pseudosections for samples of types A (a), B (b) and C (c) constructed in the model system CNKMnTiFMASHO. The isopleths show the calculated Mn/(Fe + Mg + Mn + Ca)Grt composition of garnet in each phase field. The ellipses represent the P–T conditions calculated with THERMOCALC using the observed mineral assemblages. The labelled black dots denote the mineral assemblages observed in each of the samples (see Fig. 2 for details). The bold black line marks the section of the P–T path that can be inferred for each sample. The bold grey line in (a) and (b), labelled 90, denotes garnet-in for the assumption that in the respective sample the garnet mode was overestimated by 90% (for further explanation, see text).

 

View larger version (81K): [in this window] [in a new window]   Fig. 9. P–T pseudosections for sample d1 (a) and for sample e1 (b) constructed in the model system CNKMnTiFMASH. The isopleths in (b) show the calculated Mn/(Fe + Mg + Mn + Ca)Grt composition of garnet in each phase field. The labelled black and white circles are the observed mineral assemblages (see Fig. 2 for details). Only the assemblages labelled with black dots agree with mineral modes and compositions (see text). The ellipses represent the P–T conditions calculated with THERMOCALC using the observed mineral assemblages.

 
Mineral assemblages, compositions and modes
Types A and B
Because types A and B show identical mineral assemblages, with the exception of (minor) andalusite, their phase diagrams will be discussed together. The two CNKMnTiFMASHO pseudosections in Fig. 8a and b account for the staurolite–garnet–biotite–muscovite–chlorite–magnetite–ilmenite assemblage observed in samples a1 and b2. The calculated Mn/(Fe + Mg + Mn + Ca)^Grt isopleths agree with the measured Mn/(Fe + Mg + Mn + Ca)^Grt in the phase fields (A1, B1) chl–ms–bt–grt–pl–mt–ilm, (A2, B2) chl–ms–bt–grt–st–pl–mt–ilm and (A3, B3) ms–bt–grt–st–pl–mt–ilm between 575 and 590°C at 5 kbar (Fig. 8a and b).

For sample b2 the calculated phase fields of the three assemblages (B1), (B2) and (B3) are the same within error as the P–T calculations (Fig. 8b). For sample a1 the estimated temperatures are slightly higher and not consistent with the calculated mineral assemblages. The absence of sillimanite in types A and B sets an upper temperature limit of between 590 and 600°C at 5 kbar (depending on bulk composition). The presence of andalusite and absence of cordierite in sample b2 suggests formation of assemblage (B5) at P–T conditions of 550°C at 3 kbar (Fig. 8b). As shown in Table 7, the compositions of plagioclase, biotite, muscovite and staurolite, and the mineral mode obtained from thin sections of sample a1 agree with those in the P–T pseudosections at 580°C and 5 kbar (Table 7). Significant differences are observed only for garnet—the calculated Fe/(Fe + Mg + Mn + Ca)^Grt is lower and the Ca/(Fe + Mg + Mn + Ca)^Grt higher than the measured compositions. The same agreements and discrepancies are obtained for sample b2 (not shown).

View this table: [in this window] [in a new window]   Table 7: Comparison between calculated and measured mineral composition and mode

 
Type C
The pseudosection accounts for the observed peak metamorphic assemblage (C1) sil–grt–bt–ms–pl–ilm–mt. Parity between measured and calculated Mn/ (Fe + Mg + Mn + Ca + Ca)^Grt in the phase field (C1) requires a minimum temperature of 580°C at 4·5 kbar. These conditions are similar to those obtained from types A and B (Figs 8c and 10). The upper and lower pressure limits are constrained by plagioclase-out and cordierite-in, respectively. As shown in Table 7, at 590°C and 5 kbar there is good agreement between the observed and calculated composition of garnet, biotite and muscovite, and of the mineral mode. The only exception is the plagioclase composition, for which the calculated anorthite content is lower (0·17) than the estimated content (0·32). Reasons for that could be a slight underestimation of the bulk CaO.

As in sample b1, the observed assemblages (C2) and–bt–ms–(grt)–pl–mt–ilm and (C3) and–bt–ms–chl–pl–mt–ilm are formed during the retrograde evolution at 550°C at 3 kbar. The Mn/(Fe + Mg + Mn + Ca)^Grt isopleths in Fig. 8c predict that the manganese content of resorbed garnet in andalusite should be higher than that of unresorbed garnet. This is indeed the case (Fig. 7), but not to the extent predicted. The discrepancy results from the fact that garnet resorption in sample c1 was not as complete as required by the phase diagram calculations, which predict total equilibrium [see discussion by Zeh (2001)]. Combining the P–T data, sample c1 provides evidence for retrogression from 590°C at 4·5 kbar to <550°C at <3 kbar.

Types D and E
For samples d1 and c1, pseudosections were calculated in the model system CNKTiMnFMASH (Fig. 9). The pseudosection for type D predicts the observed domain assemblages (D1) grt–st–bt–chl–pl–ilm, (D2) grt–st–bt–chl–crd–pl–ilm, (D4) grt–crd–bt–pl–ilm and (D5) grt–crd–bt–chl–pl–ilm, but not (D3) st–crd–bt–chl–pl–ilm (Fig. 9a). Assemblages (D2)–(D5) could be explained by a continuous pressure and temperature drop from 610°C at 5 kbar to 570°C at 3·5 kbar, in agreement with the retrograde P–T paths obtained for types B and C (Fig. 8b and c). However, if the observed and calculated mineral compositions and the mode are taken into account this particular interpretation is probably incorrect.

Importantly in all assemblage domains, (D1)–(D5), the mineral compositions are identical with no zonation. Furthermore, garnet is invariably euhedral. Finally, the measured mineral mode and compositions of all minerals agree well with those calculated at P–T conditions of 610°C and 5 kbar (Table 7, Fig. 11e–h, D2), and are identical to the P–T estimates made above. These factors point to a complete chemical and textural equilibrium in sample d1 established at constant P–T conditions, rather than by continuous formation of the different domain assemblages during a P–T evolution.

View larger version (82K): [in this window] [in a new window]   Fig. 11. (a–d) Effects on P–T pseudosection topology for sample d1 as a result of variations of system components. (a) KFMASH; (b) KMnFMASH; (c) KMnTiFMASH; (d) CNKMnTiFMASH. The ellipse shows the P–T conditions during which the domain assemblages observed in sample d1 were inferred to have formed. The bold dark grey line marks the position of garnet-in. (e–h) Isopleths of (e) spessartine, (f) grossular, (g) almandine and anorthite, and (h) Fe-staurolite and Fe-cordierite, which result from calculations in the model system CNKMnTiFMASH related to diagram (d).

 
The resorption of staurolite by cordierite in sample a1 (Fig. 5a and b) apparently contradicts this, but this discrepancy can be explained by considering the evolution of the mode. As shown in Fig. 12c, significant staurolite will be resorbed during a very small T rise from 608 to 610°C at 5 kbar, as the assemblage changes from st–grt–chl–bt–pl–ilm to st–grt– chl–crd–bt–pl–ilm, and finally to st–grt–crd–bt– pl–ilm. Staurolite resorption is accompanied by growth of cordierite and garnet and loss of chlorite, in agreement with thin-section observations (Figs 5 and 6e–h). The chlorite (3 vol. %) in sample a1 can be interpreted as prograde relics armoured by cordierite. Notably, despite the marked assemblage change between 608 and 610°C the compositions of garnet, staurolite, biotite, chlorite, cordierite and plagioclase remain nearly unchanged (Fig. 11e–h).

The upper temperature limit for the formation of the equilibrium assemblage in sample d1 is at 615°C, because of the absence of sillimanite (Fig. 9a).

For sample e1 (type E) the dominant assemblages (E1) crd–grt–bt–pl–ilm and (E2) crd–grt–bt–chl–pl–ilm are consistent with the CNKMnTiFMASH phase diagram (Fig. 9b). The rarely observed muscovite, chlorite and rutile [assemblages (E1) and (E5)] may be relics from the prograde evolution. In contrast, the rare assemblage (E4) st–crd–chl–bt–pl–ilm–(sill) cannot be accounted for, perhaps as a result of thin-section-scale bulk-rock heterogeneities, kinetic effects, or pseudosection errors (see below). The assemblages (E1), (E2) and (E5) are consistent with a prograde temperature rise from 575 to 600°C at 4·5 kbar (Fig. 9d), similar to the path obtained for types A–C. At 600°C and 4·5 kbar there is an excellent agreement between the calculated and observed compositions and modes of garnet, chlorite, cordierite, biotite and plagioclase (Table 7). Thus, we are confident that these minerals were in equilibrium at the estimated metamorphic peak.

P–T path
The fragments of the P–T path inferred from the P–T pseudosections provide evidence that different samples from the Ilesha Schist belt preserve different sections of the pro- and retrograde P–T evolution (Fig. 10b). The evidence points to a heating event from about 575 to 610°C at 5 ± 0·5 kbar, followed by retrogression to 550°C at 3 kbar (Fig. 10). Taking into account that the P–T calculations of all samples incorporate a ‘geological error’, and that the P–T pseudosections also involve errors (see below) we argue that the peak assemblages of all samples were formed at nearly identical metamorphic conditions of 590 ± 20°C at 5 ± 0·5 kbar.

The lack of geochronological data for the metamorphic peak in the Ilesha Schist belt means that it is not possible to distinguish between a single metamorphic event and polymetamorphism. Although the agreement between the peak P–T conditions of all samples suggests they were formed during a regional metamorphic event, rather than by contact metamorphism, it is possible that late andalusite growth in type B and C samples was triggered by intrusion of the pegmatites (Fig. 1). The P–T path derived above suggests that the P–T conditions of 550–625°C at 1–3 kbar inferred by Ige et al. (1998) from the metabasic and meta-ultrabasic rocks reflect the retrograde P–T evolution rather than the metamorphic peak in the Ilesha Schist belt.

	REACTION OVERSTEP

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Generalities
A general feature of all CNKTiMnFMASH(O) P–T pseudosections is that garnet-in reactions are predicted to occur at temperatures as much as 80°C lower than those either obtained by the P–T estimates (Table 7, Figs 8 and 9) or required by the measured and calculated garnet compositions and modes (Table 6, Figs 8 and 9). Furthermore, all phase diagrams imply that garnet-in should occur in chlorite-bearing assemblages (± muscovite, ± biotite, ± staurolite, ± sillimanite). However, in chlorite-bearing phase fields the garnet composition isopleths are steep (Figs 8 and 9), meaning that garnet produced during a temperature rise should show a pronounced compositional zonation similar to that observed in many natural garnets (e.g. Hollister, 1966) or constrained by numerical simulations (e.g. Loomis, 1982). The absence of garnet zonation may possibly be explained by either growth at constant P–T–X or annealing of any zoning by diffusion. The latter can be excluded for various reasons. First, it would result in a pronounced and curved increase of Fe/(Fe + Mg) toward the rim of garnet in contact with biotite and chlorite. At the very least, garnet of all samples would show pronounced zonation of Ca, which has a relatively low diffusion coefficent (Chakraborty & Ganguly, 1992). Furthermore, if growth zonation had formerly been present, enclosed garnet grains of varying size should have different compositions. The absence of these features suggests that the lack of chemical zonation in the Ilesha garnets is primary. We conclude that garnet grew under constant P–T–X conditions. The corollary of this is that the discrepancies between the predicted garnet-in temperature derived from the pseudosections and that from the P–T estimates reflect a significant reaction overstep.

Pseudosection errors
Before accepting this hypothesis we consider the errors involved in calculating the pseudosections. THERMOCALC does a full error propagation using the errors associated with the activities of end-members and with the thermodynamic data. Typically, the uncertainties on lines separating areas of stability of different mineral assemblages are ±10°C. However, the position of the separating lines is dependent on the thermodynamic dataset, the compositional system, the activity models used for solid-solutions (Appendix B), and the bulk composition.

The thermodynamic calculations presented in this paper all used the internally consistent thermodynamic dataset HP98. Use of the outdated dataset HP90 (with its much simpler activity models) results in the same assemblages at any temperatures but at 0·5–1·0 kbar higher pressures. Generally, the choice of dataset has little effect on phase diagram topology in the P–T range of 550–650°C and 4–8 kbar, but at lower pressures there are important differences. For example, the coexistence of andalusite and biotite can be explained using only HP98 and our chosen activity models. The usefulness of our chosen activity models is reflected in the good agreement between the calculated and measured mineral assemblages, compositions and modes (Table 7).

To illustrate the effects on topology resulting from the use of different compositional systems, a series of P–T pseudosections in the model systems KFMASH, MnKFMASH, MnTiKFMASH and CNKMnTiFMASH were constructed for sample d1 (Fig. 11a–d). In all four phase diagrams the assemblages (D1) grt–st–bi–chl (+ ilm + pl) and (D2) grt–st–bi–chl–crd (+ ilm + pl) fall within the error ellipses obtained by the P–T estimates. In the system KFMASH, assemblages (D1) and (D2) are separated by the univariant reaction chl + st = cd + grt. In the systems MnKFMASH and MnTiKFMASH the change from (D1) to (D2) is effected by a change from a trivariant to a divariant field, and in the system CNKMnTiFMASH from a quadri- into a trivariant phase field. In general, as the system becomes more complicated, more realistic information can be obtained. For example, both the amount and composition of plagioclase and the garnet composition can be predicted only using CNKMnTiFMASH (Table 7).

Importantly, the predicted temperature of garnet formation in the KFMASH phase diagram falls by 50°C if Mn is included (Fig. 11a and b), as a result of the strong fractionation of Mn into garnet (Symmes & Ferry, 1992; Mahar et al., 1997). The predictions of the MnKFMASH system are substantially unchanged if Ti, Ca and Na are included (Fig. 11c and d). The phase diagrams in Fig. 11a–d also show that even if the garnet stability field is extended to lower temperatures, the stability fields of staurolite, biotite, cordierite and chlorite remain essentially unchanged, with important implications for the assessment of reaction overstep, as outlined below.

Variations of bulk composition also influence phase diagram topology (Zeh, 2001). In this study, bulk-rock variations can result from point counting and electron microprobe errors. As errors in electron microprobe analyses are small, important variations in bulk composition are likely to result only from point counting. We estimated the mode by point counting 2000 points per thin section, resulting in mode errors no larger than 5–10% for the abundant minerals (>10 vol. %), 20–30% for minor minerals (<10 vol. %), and 50% for very minor minerals (<3 vol. %). All samples are relatively fine grained, and thus overestimation of a porphyroblastic phase can be excluded. Bulk variations as a result of mineral zonations can also be excluded, as minerals of all samples are unzoned and the compositions of the minerals throughout the thin section are nearly constant (Table 5). A potential source of error could be the heterogeneous distribution of phases in close proximity to the piece taken for thin-section preparation. If minerals with a low Fe/(Fe + Mg) ratio (e.g. cordierite or chlorite) are overestimated relative to minerals such as staurolite and garnet with higher Fe/(Fe + Mg), the boundaries between di-, tri- and higher-variant phase fields will shift to higher temperatures. This is demonstrated for sample a1 in Fig. 13, where we assume that the amount of garnet is overestimated by either 50% or 90%. A reduction in garnet mode reduces the bulk Fe/(Fe + Mg) and reduces the phase field grt–st–bi–ms. An overestimation of Fe³⁺ will have the same effect (see White et al., 2000). Overestimation of Ti shifts rutile-bearing fields to higher temperatures and lower pressures (Fig. 9a and b), and the first appearance of biotite occurs at lower temperatures. Overestimation of plagioclase increases the Na and Ca bulk composition and would only shift the plagioclase-out reaction to lower pressures (Figs 8b, c and 9a, b), but permit garnet-in at slightly lower pressures and temperatures. In summary, variations of Fe²⁺, Fe³⁺, Mg, Ca, Na, Ti, Al and K resulting from 10% variation in the dominant minerals change steep phase field boundaries by ±10–20°C and shallow boundaries by ±0·5 kbar.

View larger version (37K): [in this window] [in a new window]   Fig. 13. KMnFMASH pseudosection for sample a1 showing the location of the garnet-in reaction (Grt-in = 100) and the change of the size and position of the phase field grt–st–bi–ms, caused by variations of the bulk composition owing to incorrect estimation of garnet mode. An overestimation by 50% results in Grt-in = 50, and by 90% results in Grt-in = 90. Whereas the topology of the phase field grt–st–bi–ms changes only slightly, garnet-in shifts to higher temperatures for underestimations of garnet mode.

 
However, the field of garnet stability is highly sensitive to variations in Mn, with minor overestimations resulting in a significant decrease in garnet-in temperatures. As nearly all Mn in our samples is concentrated in garnet, overestimates of reaction overstep are linked to overestimation of the garnet mode. For example, the amount of reaction overstep in sample a1 will be 180°C if we assume that the garnet mode is estimated correctly, to 100°C if garnet is overestimated by 50%, and to 20°C if garnet is overestimated by 90% (Fig. 13). The amount of overstep is only slightly higher if Ca, Na, Ti and Fe³⁺ are included (Fig. 8a and b). We consider garnet overestimation of >50% unrealistic and argue that garnet in all samples was formed after reaction overstepping by more than 50°C.

We should emphasize that reaction overstep in types A, B, C and E is constrained only by the difference between the pseudosections and the P–T estimates. In type D, however, reaction overstep is additionally constrained by staurolite, which was formed together with garnet at 610°C, which is 80°C higher than the temperatures predicted for staurolite-in (Fig. 9a). As staurolite-in in sample d1 is nearly independent of both bulk-rock compositional variations and the compositional system (Fig. 11a–d), a reaction overstep of 80°C is plausible.

	GARNET TEXTURES

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
‘House of cards’ garnet
The observed garnet–staurolite intergrowth relationships in sample a1 (Fig. 3d) provide evidence that garnet with the ‘house of cards’ array was formed before staurolite in the assemblage (A1) chl–bt–ms–grt–pl–mt–ilm, under nearly constant P–T–X conditions, with a reaction overstep of >50°C. To explain the ‘house of cards’ texture we have to answer the following questions: Which reaction led to garnet formation? Why are biotite and quartz included in garnet? What caused the preferential orientation of biotite and quartz parallel to garnet {110} planes?

From the reactants and products predicted by the phase diagrams and observed in thin sections (Figs 3, 8a and 12a), it is most likely that garnet formation is predominantly controlled by a disequilibrium reaction between Chl and Ms in the quadri-variant CNKTiMnFMASHO assemblage (A1) (Figs 8a and 12a). Calculations using sample a1 indicate that the modes of ilmenite, magnetite and plagioclase are nearly constant in the phase field chl–bt–ms–grt–pl–ilm–mt and will not seriously influence balancing of reaction (R1). Excluding these phases, we have to consider a reaction in the trivariant MnKFMASH assemblage chl–bt–ms–grt–qtz–H₂O (Appendix C):

(R1)

which explains the contemporaneous formation of biotite and garnet, and release of H₂O. Furthermore, (R1) is consistent with the fact that staurolite can subsequently be formed by reaction (R2) in assemblage with chlorite and muscovite. The volume balance of reaction (R1) is (all numbers in vol. %)

As sample a1 contains 3·8 vol. % garnet (Table 1), this predicts that 3·6 vol. % H₂O would be formed. This H₂O volume is a maximum estimate as Fe³⁺ in chlorite will decrease the H₂O volume slightly. The reaction results in the release of all water in chlorite, whereas the amount of water in muscovite will be consumed by biotite. If reaction (R1) occurred at constant P–T–X conditions the H₂O will be released instantaneously, resulting in a large pore volume containing essentially non-overpressured H₂O. Such a situation is mechanically unstable and gravitational forces will drive the rock to compact, expelling the fluid upwards along fractures. The time scale for such compaction will be of the order of tens of years for a 10 m thick horizon of reacting pelite (Graham et al., 1997). If we assume rapid chlorite breakdown with a large reaction overstep it seems likely that this time scale for compaction and expulsion of H₂O will be long compared with that for garnet growth, with two important consequences.

First, H₂O will transport ions quickly from the reactants to garnet nuclei. This will smooth out compositional gradients at garnet interfaces and, if necessary, the H₂O in the (transient) porosity will store ions until they are needed. These are ideal conditions for interface-controlled garnet growth.

Second, there will be, at least transiently, hydrostatic pressure conditions. Biotite flakes beside garnet will be rotated parallel to temporary garnet surfaces during garnet growth, as suggested by Ferguson et al. (1980). The question is: Why are these biotite flakes incorporated in garnet? According to the volume balance, there would be enough ‘space’ (pore volume) around garnet to push biotite flakes ahead of the growing garnet surface to create mica domes around garnet (see Ferguson et al., 1980). A possible explanation is that biotite was directly precipitated at growing garnet surfaces with the lowest free energy.

If we assume that garnet growth occurs in an originally homogeneous fluid, K will be concentrated at the garnet–fluid interface by surface fractionation (Loomis, 1983) (Fig. 14a and b). When the K concentration becomes high enough, nuclei of biotite will form near the garnet surface and grow in equilibrium with garnet. As a result of simultaneous garnet growth, such biotite flakes will be overgrown after they are rotated parallel to garnet surfaces.

View larger version (122K): [in this window] [in a new window]   Fig. 14. (a and b) Synopsis showing the suggested mechanism of garnet growth with a ‘house of cards’ structure. (a) A pre-existing quartz grain is incompletely overgrown by garnet, which displaces quartz as it continues to grow. Biotite is nucleated at the garnet surface in a region of high K concentration. (b) After further garnet growth, quartz and biotite inclusions are preferentially oriented parallel to garnet surfaces. (c–e) Textural evolution in sample d1 (rock type D). Muscovite domains in (c) are transformed into staurolite-bearing, garnet-free domains (D3') in (d), whereas formerly muscovite-free areas of the rock are transformed in staurolite-free domains containing garnet (D4'). Between these, garnet- and staurolite-bearing domains occur (D1). (e) Further textural development results in staurolite being replaced by cordierite, which now grows with garnet.

 
The formation of tabular quartz crystals in garnet is assumed to occur during incomplete overgrowth of older quartz grains by garnet (Figs 3b and 14a). Once overgrowth has begun, the garnet–quartz boundary continues to migrate to minimize the surface free energy, forming the facets bounding the quartz inclusions (Fig. 3b). For this to occur, Fe, Mg, Mn, Ca and Al must be transported to, and Si away from, the migrating interface along the grain boundary between the partially enclosed quartz and the encroaching garnet. A compositional gradient will be established between the interior and exterior of the garnet. The diffusion distance becomes larger with continued garnet growth, resulting in a reduction in the efficiency of mass transfer (Fig. 14a). The silica displaced by migrating garnet will be precipitated along the nearby transport paths. The crystallographic control of the orientation of the quartz inclusions (Fig. 3b) probably results from the fact that the diffusion pathways themselves are affected by grain boundary migration to obtain low-energy garnet surfaces. Because more material (Fe, Mg, Mn, Ca and Al) is supplied to garnet–quartz interfaces near the external garnet surface as a result of diffusion transport, coupled quartz displacement and garnet migration will be faster here. This is a plausible explanation for the observation that tabular quartz inclusions are generally thinner at the garnet rims (Figs 3a, b and 14b), and why completely overgrown tabular quartz inclusions terminate with low-angle tips.

Resorbed garnet in staurolite
Following hydrostatic garnet growth in sample a1, staurolite joins the assemblage (A2) grt–chl–bt– ms–pl–ilm–mt (Fig. 7a), via the garnet-consuming reaction

(R2)

with quartz, plagioclase, ilmenite and magnetite being in excess, resulting in the garnet resorption observed in types A and B (Fig. 2). Because garnet consumption continues until chlorite is exhausted, the amount of garnet consumption depends on the original amount of chlorite (Fig. 12a and b).

Garnet resorption in andalusite
The observed garnet resorption in andalusite (Fig. 4c) is in good agreement with a transition from 5 kbar and 590°C to 3 kbar and 550°C. However, it could also be explained by a temperature rise in the phase field grt–chl–bt–ms–and–mt–ilm (Fig. 8c). In the absence of additional petrological information the interpretation of garnet resorption in andalusite is ambiguous. However, the presence of randomly oriented sillimanite inclusions in andalusite (Fig. 4b) of sample c1 clearly shows that garnet grew prior to retrogression.

Euhedral unzoned garnet and garnet clusters in type D
We have shown that minerals of sample d1 achieved complete equilibrium at P–T conditions of 610°C at 5 kbar, after a reaction overstep of 80°C. Such a large overstep may account for the formation of abundant unzoned garnet (17·7 vol. %), and garnet clusters in sample d1. As discussed by Loomis (1983), the growth rate of product phases will significantly increase with the amount of reaction overstep. Thus, if the overstep is very large, as suggested above, reactant phases such as garnet will grow quickly. Furthermore, if the physico-chemical conditions (P–T–X ) are constant during growth, the product phases will be unzoned, provided that no fractionation (bulk, surface or Rayleigh fractionation) occurred, and that all elements needed for the growth of the product phases were steadily transported toward the products. The latter point implies the absence of refractory reactant phases or other kinetic effects, which can control the material supply [examples have been provided by Chernoff & Carlson (1997) and Waters & Lovegrove (2002)]. The euhedral garnet habit in sample d1 (Figs 5 and 6) and presence of clusters of garnet grains of various sizes (Figs 5f and 6) strongly suggest interface-controlled growth (Kretz, 1993, p. 110; Daniel & Spear, 1999) of garnet crystals in close proximity, without any competition among growing garnet grains. The latter was obviously achieved by an instantaneous release of significant amounts of H₂O during garnet growth, as a result of reactions (R3) and (R4), which allowed a fast intragranular material transport and storage. From the observed textures (Fig. 5), phase diagram predictions and mass balance constraints (Figs 9a and 12c), it seems most likely that staurolite, garnet and biotite were initially formed at the expense of metastable muscovite and chlorite via the reaction (Appendix C)

(R3)

which has the volume balance (all numbers in vol. %)

(R3v)

Reaction (R3) ceased on consumption of muscovite. Subseqently, staurolite was resorbed, whereas garnet growth continued together with cordierite growth (Fig. 5a and b) via the reaction (Fig. 11a)

(R4)

which has the volume balance (all numbers in vol. %)

(R4v)

Reaction (R4) ceased after chlorite reacted out, or was armoured by cordierite (Fig. 5). The previous occurrence of muscovite in sample d1, as needed for reaction (R3), is well constrained by the fact that muscovite is the only potential K-bearing phase, which is stable before biotite-in, along the prograde P–T path (Fig. 12a). Thus, to form biotite in sample d1 muscovite must have reacted. According to the thermodynamic calculations in Fig. 12c, 15 mole prop.* (mole prop.* = mole proportions with quartz and H₂O in excess) muscovite must have been present in sample d1 before biotite-in, assuming a closed-system behaviour for sample d1 (with quartz and H₂O in excess).

Garnet-bearing and garnet-free domains in type D
Although reactions (R3) and (R4) can plausibly account for many garnet, staurolite and cordierite textures observed in sample d1, as well as the release of significant amounts of H₂O, the question as to the heterogeneous distribution of garnet and staurolite remains unanswered. We suggest that this could be due to the heterogeneous distribution of muscovite before reaction (R3), resulting in different domain assemblages (Fig. 14c–e) formed by irreversible sub-reactions (e.g. Foster, 1981).

As demonstrated for sample a1, garnet will be consumed in domains containing the assemblage grt–st–chl–ms as a result of the progress of reaction (R2). Thus, garnet nucleation and growth will be suppressed in ms–chl–st domains, whereas staurolite nuclei will grow by the sub-reaction

(R3a)

In contrast, in muscovite-free domains garnet will be formed at the expense of staurolite, as a result of the sub-reaction (biotite and ilmenite are in excess)

(R3b)

The two partial reactions (R3a) and (R3b) are end-members, leading to the formation of domain assemblages (D3') and (D4') (Fig. 14d). Nucleation and growth of staurolite adjacent to garnet, to form assemblage (D1), was possible where the relative proportions of nearby chlorite and muscovite permitted a reaction similar to (R3).

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
(1) K-rich (types A–C) and K-poor (types D and E) metapelites from the Ilesha Schist belt show a wide range of mineral assemblages, compositions and modes, resulting from variable bulk compositions. Garnet in the K-rich samples occurs in the assemblages (A) grt–st–ms–pl–chl–mt–ilm, (B) and–grt–st–ms–pl–chl–mt–ilm and (C) sil–and–grt–ms–pl–chl–mt–ilm, and in the K-poor samples in the assemblages (D) grt–st(much)–crd–bt–chl–pl–ilm and (E) grt–st (little)–crd–bt–chl–pl–ilm. In the K-rich metapelites, garnet enclosed by staurolite (types A and B) and by andalusite (types B and C) is resorbed, whereas garnet in all domains of the K-poor samples (types D and E) is euhedral. Furthermore, garnet in type A samples has a characteristic ‘house of cards’ texture, formed by oriented inclusions of biotite and tabular quartz parallel to {110}. Garnet in types B and D commonly forms clusters. All phases are unzoned, showing nearly identical compositions on a thin-section scale.

(2) P–T pseudosection calculations in the systems CNKTiMnKFMASH(O) predict all observed predominant mineral assemblages, and also, for most samples, the observed mineral compositions and modes (Table 7). The P–T pseudosections and P–T estimates provide evidence that garnet nucleation and growth in the K-rich metapelites (types A and B) occurred between 575 and 590°C at 5 ± 0·5 kbar. Mode evolution calculations additionally constrain that the observed garnet resorption in types A and B result from prograde garnet consumption during staurolite formation. In the andalusite-bearing samples (types B and C) garnet resorption additionally occurred during retrogression at P–T conditions of 550°C at 3 kbar. For K-poor metapelites (types D and E), parity between P–T pseudosection calculations and thin-section observations (assemblages, mode and composition) provides evidence that the observed domain assemblages represent total equilibria, which were formed at 590–610°C at 5 ± 0·5 kbar.

(3) Combined thin-section observations and P–T pseudosection calculations indicate that garnet growth in all samples resulted from reaction of chlorite and muscovite with an overstep of >50°C. Although assessment of the overstep using garnet alone involves significant errors, in particular related to the correct evaluation of the Mn content, the consideration of staurolite can be used to refine the estimates of overstep.

(4) Significant overstep of the garnet-in reaction, as a result of chlorite and muscovite breakdown, can account for the lack of garnet zonations, the presence of garnet clusters, euhedral garnet habits, and the observed ‘house of cards’ texture. These characteristics point to rapid interface-controlled garnet growth at constant P–T–X conditions, in the absence of fractionation and refractory reactant phases.

	APPENDIX A: ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Mineral compositions for samples a2, a3, b1 and e1 were obtained using a Cameca SX-50 electron microprobe in the Department of Earth Sciences, University of Cambridge. Spectra were recorded with an energy-dispersive (ED) Si(Li) detector system manufactured by Link Analytical Ltd, and processed with the ZAF-4FLS software provided. An accelerating voltage of 20 kV was used, with beam currents of 3 nA. A spot size of 2–3 µm was used for all analyses except those of feldspars, for which a defocused beam of 50 µm was used. Electron microprobe analyses of minerals in samples a1, b2, c1 and d1 were carried out with a CAMECA SX-50 microprobe in the Mineralogical Institute, University of Würzburg, with three independent wavelength-dispersive crystal channels. Instrument conditions were 15 kV acceleration voltage, 15 nA specimen current, and 20 s integration time for all elements except for Fe (30 s). Natural and synthetic silicates and oxides were used for reference, and matrix corrections were carried out by the PAP program, supplied by CAMECA. Point analyses were performed with a 5 µm beam diameter for plagioclase and muscovite, and a 1 µm beam diameter for all other minerals. Detection limits (1) for a typical silicate analysis were 1 wt % relative for each element. Qualitative element maps of garnet were performed with a JEOL microprobe at the University of Frankfurt (Department of Mineralogy) using instrument conditions of 15 kV acceleration voltage, 3 nA specimen current, and 100 ms integration time for all elements. A grid of 3 µm x 3 µm was used for the sample shown in Fig. 6a–d and 2 µm x 2 µm for that shown in Fig. 6e–h.

	APPENDIX B: ACTIVITY MODELS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Activity expressions assuming ideal mixing on sites for cordierite, staurolite and chloritoid were used in the formulation of Mahar et al. (1997). For muscovite the activity model of Holland (available from the THERMOCALC website; www.earthsci.unimelb.edu.au/tpg/thermocalc/) was employed, and for plagioclase the activity models of Worley & Powell (1998). For magnetite we used the formulation of White et al. (2000). Non-ideal contributions were calculated for staurolite, biotite, muscovite, garnet, magnetite, ilmenite and plagioclase using the symmetric formalism method outlined by Powell & Holland (1993, 1999) and Holland & Powell (1996a, 1996b). For staurolite the Margules parameter W_mst,st = -8 was used (T. Holland, personal communication, 2001).

Biotite: K(Fe²⁺,Mg,Mn, )^M2(Al,Ti,Fe³⁺, Fe²⁺,Mg,Mn)₂^M1(Al,Si)^T1(Si)₂^T2(OH)₂O₁₁
Biotite in the component system (CN)KTiMnFMASHO was modelled using the following compositional variables:

and the order–disorder parameter N_bi = 3(x_bi - x_Fe²⁺,M2).

The site distributions in biotite are

where indicates a vacancy. The ideal mixing on sites activity expressions for the biotite end-members are

The end-member proportions _i are given by

Margules parameters: W_phl,ann = 9, W_phl,east = 10, W_phl,obi = 3, W_phl,tbi = –10, W_ann,east = –1 W_ann,obi = 6, W_ann,tbi = 12, W_east,obi = 10 (White et al., 2000). All other Margules parameters are set to zero. For calculations in the model system (CaN)KTiMnFMASH, all f_bi terms in the equations above were omitted.

Chlorite: (Mg,Fe,Mn,Al)^M1(Mg,Fe,Mn, Al)^M2,M3 ₄(Mg,Fe,Mn,Al)^M4(Al,Si)^T2₂Si^T1₂O₁₀(OH)₈
Chlorite is modelled using the following compositional variables:

and the order–disorder parameter Q_chl = (x_Al,M4 – x_Al,M1)/2.

The site distributions in chlorite are

The ideal mixing on sites activity expressions for the chlorite end-members are

The end-member proportions _i are given by

Margules parameters W_afchl,clin = 18, W_afchl,daph = 14·5, W_afchl,ames = 20, W_clin,daph = 2·5, W_clin,ames = 18, W_daph,ames = 13·5 (T. J. B. Holland: THERMOCALC website). All other Margules parameters are set to zero.

Garnet: (Fe²⁺,Mg,Ca,Mn)^X₃(Fe^{3 +} ,Al)^Y₂Si₃O₁₂
Garnet in the model system (NKTiH)MnCFMASO is modelled using the following compositional variables:

The site distributions in garnet are

The ideal mixing on sites activity expressions for the garnet end-members are

The end-member proportions _i are given by

Margules parameters: W_py,alm = 3·0, W_py,spss = 4·5, W_py,gr = 33, W_py,andr = 73, W_alm,andr = 60 (see White et al., 2000, p. 498). For calculations in the model system (CaNKTiH)MnFMAS the variable f_g and the Margules parameters for W_py,andr and W_alm,andr were excluded.

Ilmenite: (Mn,Fe²⁺,Fe³⁺)^A(Ti,Fe³⁺)^BO₃
Compositional variables:

Site distributions in ilmenite are

Activity models are

The end-member proportions _i are given by

Margules parameters: W_ilm,hem = 26·6 and W_ilm,pnt = 1·76 (Pownceby et al., 1987; White et al., 2000). For calculations in the model system CaNKMnTiFMASH, pure ilmenite was used (for further explanation, see text).

	APPENDIX C: MASS AND VOLUME BALANCE CALCULATIONS

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
The mineral volumes were calculated using the partial volumes of the end-members amesite, clinochlor, daphnite and Mn-chlorite for chlorite; muscovite, celadonite and Fe-celadonite for muscovite; annite, phlogopite, Mn-biotite and eastonite for biotite; almandine, pyrope and spessartine for garnet; Fe-staurolite, Mg-staurolite and Mn-staurolite for staurolite, Fe-cordierite, Mg-cordierite and hydrous cordierite for cordierite. The volume data used are from the internally consistent thermodynamic dataset of Holland & Powell (1998) at standard conditions.

View this table: [in this window] [in a new window]   Mineral compositions used to calculate reaction (R1)

 

View this table: [in this window] [in a new window]   Mineral compositions used to calcultate reaction (R3)

 

View this table: [in this window] [in a new window]   Mineral compositions used to calculate reaction (R4)

 

	ACKNOWLEDGEMENTS

 
A.Z. thanks Uli Schüssler (University of Würzburg, Germany) for help with the electron microprobe analyses, Heidi Höfer (University of Frankfurt, Germany) for producing the garnet element maps, and O. A. Ige (Natural History Museum, Obafemi Awolowo University, Ife, Nigeria) for use of the samples. We are indebted to T. J. B. Holland for help with the geothermobarometry and P–T pseudosection calculations, although any remaining misconceptions are our own. We also thank Kurt Bucher, John Schumacher and an anonymous reviewer for critical comment on an earlier version of the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION FIELD RELATIONSHIPS PETROGRAPHY MINERAL CHEMISTRY GEOTHERMOBAROMETRY P-T PSEUDOSECTIONS REACTION OVERSTEP GARNET TEXTURES SUMMARY AND CONCLUSIONS APPENDIX A: ANALYTICAL METHODS APPENDIX B: ACTIVITY MODELS APPENDIX C: MASS AND... REFERENCES

 
Bell, T. H. & Hickey, K. A. (1999). Complex microtextures preserved in rocks with a simple matrix: significance for deformation and metamorphic processes. Journal of Metamorphic Geology 17, 521–535.[CrossRef][Web of Science]

Boose, S. & Ocan, O. (1991). Geology and evolution of the Ife–Ilesha Schist belt, southwestern Nigeria. In: Benin–Nigeria Geotraverse. International Meeting on the Proterozoic Geology and Tectonics of High Grade Terrain. IGCP 215, 123–129.

Brearley, A. J. & Rubie, D. C. (1990). Effects of H₂O on the disequilibrium breakdown of muscovite + quartz. Journal of Petrology 31, 925–956.[Abstract/Free Full Text]

Carlson, W. D. (1989). The significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallisation. Contributions to Mineralogy and Petrology 103, 1–24.[CrossRef][Web of Science]

Carlson, W. D. (1991). Competitive diffusion-controlled growth of porphyroblasts. Mineralogical Magazine 55, 317–330.[Web of Science]

Chakraborty, S. & Ganguly, J. (1992). Cation diffusion in aluminosilicate garnets: experimental determinations in the spessartine–almandine couples, evaluation of effective binary diffusion coefficients, and applications. Contributions to Mineralogy and Petrology 96, 78–92.

Chernoff, C. B. & Carlson, W. D. (1997). Disequilibrium for Ca during growth of pelitic garnet. Journal of Metamorphic Geology 15, 421–438.[CrossRef][Web of Science]

Cygan, R. T. & Lasaga, A. C. (1992). Crystal growth and the formation of chemical zoning in garnets. Contributions to Mineralogy and Petrology 79, 187–200.

Daniel, C. G. & Spear, F. S. (1999). The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north-west Connecticut, USA. Journal of Metamorphic Geology 17, 503–520.[CrossRef][Web of Science]

Denison, C. & Carlson, W. D. (1997). Three-dimensional quantitative textural analysis of metamorphic rocks using high-resolution computer X-ray tomography: Part II. Application to natural samples. Journal of Metamorphic Geology 15, 45–57.[CrossRef][Web of Science]

Ferguson, C. C., Harvey, P. K. & Lloyd, G. E. (1980). On the mechanical interaction between growing porphyroblasts and its surrounding matrix. Contributions to Mineralogy and Petrology 75, 339–352.

Fisher, G. W. (1978). Rate law in metamorphism. Geochimica et Cosmochimica Acta 42, 1035–1050.[CrossRef][Web of Science]

Foster, C. T. (1981). A thermodynamic model of mineral segregation in the lower sillimanite zone near Rangeley, Maine. American Mineralogist 66, 260–277.[Abstract]

Graham, C. M., Skelton, A. D. L., Bickle, M. J. & Cole, C. (1997). Lithological, structural and deformation controls on fluid flow during regional metamorphism. In: Holness, M. B. (ed.) Deformation-enhanced Fluid Transport in the Earth's Crust and Mantle. Mineralogical Society Series 8, 196–226.

Holland, T. J. B. & Powell, R. (1990). An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K₂O–Na₂O–CaO–MgO–MnO–FeO–Fe₂O₃–Al₂O₃–TiO₂–SiO₂–C–H₂–O₂. Journal of Metamorphic Geology 8, 89–124.[Web of Science]

Holland, T. J. B. & Powell, R. (1992). Plagioclase feldspars: activity–composition relations based upon Darken's Quadratic Formalism and Landau theory. American Mineralogist 77, 53–61.[Abstract]

Holland, T. J. B. & Powell, R. (1996a). Thermodynamics of order–disorder in minerals. 1: symmetric formalism applied to minerals of fixed composition. American Mineralogist 81, 1413–1424.[Abstract]

Holland, T. J. B. & Powell, R. (1996b). Thermodynamics of order–disorder in minerals. 2: symmetric formalism applied to solid solutions. American Mineralogist 81, 1425–1437.[Abstract]

Holland, T. J. B. & Powell, R. (1998). An internally consistent thermodynamic data set of petrological interest. Journal of Metamorphic Geology 16, 309–343.[CrossRef][Web of Science]

Holland, T. J. B., Baker, J. & Powell, R. (1998). Mixing properties and activity–composition relationships of chlorites in the system MgO–FeO–Al₂O₃–SiO₂–H₂O. European Journal of Mineralogy 10, 395–406.[Abstract/Free Full Text]

Hollister, L. S. (1966). Garnet zoning: an interpretation based on Rayleigh fractionation model. Science 154, 1647–1651.[Abstract/Free Full Text]

Ige, O. A., Okrusch, M., Schüssler, U., Schmädicke, E. & Cook, N. J. (1998). The metamorphosed mafic–ultramafic complex of Mokuro, Ilesha Schist Belt, southwestern Nigeria. Journal of African Earth Sciences 26, 593–618.[CrossRef]

Kerrick, D. M. & Woodsworth, G. J. (1989). Aluminum silicates in the Mount Raleigh pendant, British Columbia. Journal of Metamorphic Geology 16, 309–343.

Kretz, R. (1973). Kinetics of the crystallisation of garnet at two localities near Yellowknife. Canadian Mineralogist 12, 1–20.

Kretz, R. (1993). A garnet population in Yellowknife schist, Canada. Journal of Metamorphic Geology 11, 101–120.[Web of Science]

Lasaga, A. C. (1986). Metamorphic reaction rate laws and development of isograds. Mineralogical Magazine 50, 359–373.[Web of Science]

Loomis, T. P. (1982). Numerical simulation of disequilibrium growth processes of garnet in chlorite bearing, aluminous rocks. Canadian Mineralogist 20, 411–423.[Web of Science]

Loomis, T. P. (1983). Compositional zoning of crystals: a record of growth and reaction history. In: Saxena, S. K. (ed.) Kinetics and Equilibrium in Mineral Reactions. Advances in Physical Geochemistry. Berlin: Springer, pp. 1–60.

Matheis, G. & Caen-Vachette, M. (1988). Rb–Sr isotopic study of rare-metal bearing and barren pegmatites in the Pan-African reactivation zone of Nigeria. In: Precambrian Geology of Nigeria. Kaduna. Nigerian Geological Survey, pp. 291–299.

Mahar, E. M., Baker, J. M., Powell, R., Holland, T. J. B. & Howell, N. (1997). The effect of Mn on mineral stability in metapelites. Journal of Metamorphic Geology 15, 223–238.[CrossRef][Web of Science]

Powell, R. & Holland, T. J. B. (1988). An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program. Journal of Metamorphic Geology 6, 173–204.[Web of Science]

Powell, R. & Holland, T. J. B. (1993). On the formulation of simple mixing models for complex phases. American Mineralogist 78, 1174–1180.[Abstract]

Powell, R. & Holland, T. J. B. (1994). Optimal geothermometry and geobarometry. American Mineralogist 79, 120–133.[Abstract]

Powell, R. & Holland, T. J. B. (1999). Relating formulations of the thermodynamics of mineral solid solutions: activity modelling of pyroxenes, amphiboles and micas. American Mineralogist 84, 1–14.[Abstract]

Pownceby, M. I., Wall, V. J. & O'Neill, H. C. (1987). Fe–Mn partitioning between garnet and ilmenite: experimental calibration and applications. Contributions to Mineralogy and Petrology 97, 116–126.[CrossRef][Web of Science]

Rahaman, M. A. (1988). Recent advances in the study of the basement complex of Nigeria. In: Precambrian Geology of Nigeria. Kaduna. Geological Survey of Nigeria, pp. 11–44.

Rahaman, M. A. (1992). The Precambrian geology of Nigeria. In: GEOTRAVERSE, Colloquium on West African Geology. UNESCO (Nigeria) Publication 3, 52–70.

Ridley, J. (1986). Modeling of the relations between reaction enthalpy and the buffering of reaction progress in metamorphism. Mineralogical Magazine 50, 375–384.[Web of Science]

Rubie, D. C. (1998). Disequilibrium during metamorphism: the role of nucleation kinetics. In: Treloar, P. J. & O'Brien, P. J. (eds) What Drives Metamorphism and Metamorphic Reactions? Geological Society, London, Special Publications 138, 199–214.

Rubie, D. C. & Thompson, A. B. (1985). Kinetics of metamorphic reactions at elevated temperatures and pressures: an appraisal of available experimental data. In: Thompson, A. B. & Rubie, D.C. (eds) Metamorphic Reactions: Kinetics, Textures and Deformation. New York: Springer, pp. 27–79.

Spear, F. S. & Daniel, C. G. (2001). Diffusion control of garnet growth, Harpswell Neck, Maine, USA. Journal of Metamorphic Geology 8, 179–195.

Symmes, G. H. & Ferry, J. M. (1992). The effect of whole-rock MnO contents on the stability of garnet in pelitic schists during metamorphism. Journal of Metamorphic Geology 10, 221–237.[Web of Science]

Walther, J. V. & Wood, B. J. (1984). Rate and mechanism in prograde metamorphism. Contributions to Mineralogy and Petrology, 88, 246–259.[CrossRef][Web of Science]

Waters, D. J. & Lovegrove, D. P. (2002). Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld Complex aureole, South Africa. Journal of Metamorphic Geology 20, 135–149.[CrossRef][Web of Science]

White, R. W., Powell, R., Holland, T. J. B. & Worley, B. A. (2000). The effect of TiO₂ and Fe₂O₃ on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃. Journal of Metamorphic Geology 18, 497–511.[CrossRef][Web of Science]

Worley, B. & Powell, R. (1998). Singularities in NCKFMASH (Na₂O–CaO–K₂O–MgO–Al₂O₃–SiO₂–H₂O). Journal of Metamorphic Geology 16, 169–188.[CrossRef][Web of Science]

Zeh, A. (2001). Inference of a detailed P–T path from P–T pseudosections using four metapelitic rocks of variable composition from a single outcrop, Shackleton Range, Antarctica. Journal of Metamorphic Geology 19, 329–350.[CrossRef][Web of Science]

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				C. Wei, W. Wang, G. L. Clarke, L. Zhang, and S. Song Metamorphism of High/ultrahigh-pressure Pelitic-Felsic Schist in the South Tianshan Orogen, NW China: Phase Equilibria and P-T Path J. Petrology, October 1, 2009; 50(10): 1973 - 1991. [Abstract] [Full Text] [PDF]

				C. Groppo, F. Rolfo, and B. Lombardo P-T Evolution across the Main Central Thrust Zone (Eastern Nepal): Hidden Discontinuities Revealed by Petrology J. Petrology, June 1, 2009; 50(6): 1149 - 1180. [Abstract] [Full Text] [PDF]

				T.C. Chudy, A. Zeh, A. Gerdes, R. Klemd, and J.M. Barton Jr. Palaeoarchaean (3.3 Ga) mafic magmatism and Palaeoproterozoic (2.02 Ga) amphibolite-facies metamorphism in the Central Zone of the Limpopo Belt: New geochronological, petrological and geochemical constraints from metabasic and metapelitic rocks from the Venetia area South African Journal of Geology, December 1, 2008; 111(4): 387 - 408. [Abstract] [Full Text] [PDF]

				S. Chakraborty, S. Dasgupta, and S. Neogi Nucleation kinetics controlled by chemical overstepping and its tectonic implications: an example from the Sikkim Himalaya European Journal of Mineralogy, December 1, 2007; 19(6): 791 - 803. [Abstract] [Full Text] [PDF]

				A. ZEH Calculation of Garnet Fractionation in Metamorphic Rocks, with Application to a Flat-Top, Y-rich Garnet Population from the Ruhla Crystalline Complex, Central Germany J. Petrology, December 1, 2006; 47(12): 2335 - 2356. [Abstract] [Full Text] [PDF]

				C. A. Zuluaga, H. H. Stowell, and D. K. Tinkham The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths American Mineralogist, October 1, 2005; 90(10): 1619 - 1628. [Abstract] [Full Text] [PDF]

				A. ZEH, R. KLEMD, and J. M. BARTON JR Petrological evolution in the roof of the high-grade metamorphic Central Zone of the Limpopo Belt, South Africa Geological Magazine, May 1, 2005; 142(3): 229 - 240. [Abstract] [Full Text] [PDF]

				A. ZEH Crystal Size Distribution (CSD) and Textural Evolution of Accessory Apatite, Titanite and Allanite during Four Stages of Metamorphism: an Example from the Moine Supergroup, Scotland J. Petrology, October 1, 2004; 45(10): 2101 - 2132. [Abstract] [Full Text] [PDF]

				T. JOHNSON and M. BROWN Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany--a Complementary Pseudosection Approach J. Petrology, June 1, 2004; 45(6): 1237 - 1259. [Abstract] [Full Text] [PDF]

				A. ZEH, I. L. MILLAR, and M. S. A. HORSTWOOD Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating J. Petrology, May 1, 2004; 45(5): 949 - 973. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (23) Request Permissions Google Scholar Articles by ZEH, A. Articles by HOLNESS, M. B. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Online ISSN 1460-2415 - Print ISSN 0022-3530

Copyright © 2009 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics