AFM compositional reladons (+An, Qtz, HZO) of CFMASH amphibolites Sketch of reactions and compatibility relationships around tthree CFASH features that include stau¡olite, garnet, orthoamphibole, hornblende. Schematic perogenetic network NCFMASH (+llbl, An, Qrz, H2O) Geological map of the northeastern part of the Entia dome Sketch of the kyanite-garnet-biotite schist.
Chapter 1: Introduction
L Introduction
- Background
- Calculated phase relations in amphibolites
- Applications
- Aims of the thesis
- Outline of the thesis
- Aluminous rock-types and their constituent phases
- Chemographic relations in the FMASH system and the concept of singularities
- t Singularities
- Phase relations from real mineral assemblages
- Uncertainties in the calculated P-T projection
- Applications of the FMASH P-T projection
The internally consistent nature of the data used to calculate the phase diagrams (Figure 2.3) allows some confidence in the predictions of these diagrams (especially in the relative positions of the stability fields and the slopes of the univariate reactions that separate them). Small changes in the enthalpy of one of the staurolite endmembers (or in the thermodynamic data or a-X (activity-composition) relationships of garnet) can result in an exaggeration (or, less likely, a reversal) of differences in the calculated compositions.
FMASH
FMASH [Als] Cfd + Oam,Qtz,H2O
Discussion
However, Will and Powell (1992) have recently suggested that hard-to-obtain thermodynamic data can be determined from a combination of natural mineral pair data and better-constrained end-member thermodynamic data. This small change in the positions of the invariant points with the addition of ortlnedcnite and albite to the constituent phases suggests that the adjufinants made to the thermodynamic data to move the invariant point Ip1 high into the sillimanite stability field may not be necessary.
Chapter 3: Calculated phase relations in amphibolites
- Introduction
- An appropriate model system for calculations
- A CFMASH Phase diagram
The main chemical element of the remaining phases can be described in the model system NazO-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH)'. The FMASH invariant point, Ip1 (see Chapter 2), includes many of the phases of interest in the CFMASH system and will be the basis of this discussion.
CFASH + An, Qtz, H2O
The appearance of a new phase along the CFMASH univariant trace should reduce the coupling space by one, thus yielding an invariant point in "I/CFMASH (Fig. 3.3c). Applying Schreinemakers' principles to the calculated reactions shows that only 3 of the 6 possible invariant points are stable.
CFMASH +Qtz,An,H20
T'esting the calculated phase relations
- CFMASH invariant points and univariant reactions
- P-T constraints
- Phase diagrams developed by other workers
- SummarY
Further limitations on the relevance of the calculated CFMASH phase diagram for natural amphibolites can be expected due to evolving compatibility relationships. A problem encountered in comparing calculated phase relations with those determined from natural amphiborites is the persistent assertion that staurolite and hornblende assemblages occur under high-pressure conditions ) 6 kba¡ (eg Selverstone et al'' 1984 ;).
Extending the GFMASH system to include chlorite
- The effect of additional components
Such discrepancies are probably not surprising given the uncertainties associated with the location of the calculated reactions. The positions of the univariant reactions and invariant points in the (orthoamphibole-absent) CFMASH system involving these phases have been determined using the internally consistent data set of Holland and Powell pers. comm., the computer program TIIERMSçALC (version Z. Zbl Powell & Holland, 1988) and Schreinemaker's method.
CFMASH +An,Qtz,HzO
The effect of variabte activity of water
The effect of reducing the agrg on the CFMASH phase diagram is that the stable and metastable invariant points, shown in Figure 3.7, move toward lower pressures and . temperatures without changing the topology of the CFMASH equilibria fig. 3'11)' So.
The effect of uncertainties in the data and activity models
However, the effect of non-ideality in garnet is insufficient to allow the predicted phase relations to match those observed petrologically. This caused all CFMASH invariant points involving chlorite to shift toward the [Chl] inva¡iant (which is apparently not affected by changes in the data for amesite because it does not include the arnesira end-member). This adjustment allows the topology of the calculated phase diagram to be reversed such that the [Hbl, Oam] and [St, Oam] CFMASH invariant points become metastable with respect to the remaining non-orthoamphibole points, [Chl, Oam]' [Crd' Oam], [ Als, oam], [Grt, oam] (Figure 3.1a).
A nerv CFÌIIASH grid
An accurate solution of this problem is impossible given the poor constraints of currently available thermodynamic data. Lowering the enthalpy of hornblende (for example, by specifying that ÀlInU = -10 kJmol-1 or ÂH6 = AII¡6 - -10 kJmol-l) increases the stability field of staurolite hornblende. Although the difficulties in resolving the inconsistencies in the chlorite-containing CFMASH network are insurmountable, there is relatively good agreement between the calculated, modified network and the (chlorite-free) natural assemblages at higher temperatures. and the modified chlorite-bearing soil is strong and consequently any grid, except for chlorite-bearing assemblages with relatively low temperatures, can provide a basis for interpreting natural amphibolites and their reaction textures.
Pseudosections and continuous reactions in the CFMASH system
The main difference in the topology of the pseudo-sections developed for phase relations without chlorite and phase relations with cloite is in the low-temperature equilibria. The compositions predicted for the other P-T ranges are similar, except that the complex evaluation part of the chlorite-bearing CFMASH system (enlarged in Figure 3.19) is located about 3 kbar higher than that of the non-chlorite system, due to different locations. of invariant equilibria in both systems (see Fig. Schematic pseudo-sections for the CFMASH system without chlorite (Fig. 3.4) with an excess of homblende, anorthite, quartz and water vapor, showing the changing topology of deviant and trivariate fields with decreasing X¡". a) with iron-rich groups; b, c and dare for .. successively lower Xp" total composition; e) Mg-rich rocks; l) CMASH end member system.
Introduction
Geological setting and background
Figure 4'l A geological map of the north-eastern porlion of the Entia Dome, afrer Foden et al. Supracrustal assemblages are found as lenses within the volumetrically dominant intrusive felsic magmas of the Entia Dome. Of particular interest is a band of supracrustals occurring in the northeastern quadrant of the Entia dome (Fig. 4.1).
Rock chemistry, petrography and mineral chemistry
- Biotite-rich rocks
- Magnesium-rich rocks
- Amphibolites
Crossing of the connecting lines results from the different preformed reaction stages in the different samples. Amoeboid inclusions of staurolite occur in the nucleus of the very thick gamete (Fig. a.zg) which has faint traces of inclusion. It generally has an Xpe smaller than that of coexisting garnet and staurolite, similar to orthoamphibole, and greater than coexisting biotite and chlorite, in the range 0.21-0.71 (Table 4.3 and Ag.l-7). Orthoamphibole in amphibolites of the Harts range is generally gedrite (except for cordierite-rich samples containing anthophyllite), which exhibits considerable replacement of edenite and tschermakite (0-0.53 Na per formula unit, Aliv 0.62-I .536 and Table .
Conditions of metamorphism
- Geothermometry
- Ãverage pressure calculations
Theoretical phase relations
Theoretical phase relationships in model compositional systems provide another avenue for interpreting complex textural relationships in metamorphic rocks.
KFMASH
F)MASH
The determination of changing physical conditions from metamorphic reaction textures in amphibolites
- Fe-rich amphibolites
- Aluminous, intermediate Xps amphibolites
- Magnesium-rich amphibolites
- Temporal sequence of reactions
- P-T evolution of the Harts Range amphibolites
The transverse nature of some of the matrix gedrite grains and the absence of any disequilibrium texture suggests that gedrite was made. The foliated-parallel nature of the staurolite inclusions in the garnet, together with the length of the foliations in the surrounding matrix of gamete-gedrite-hornblende-plagioclase-quartz, suggest that the staurolite-gamete was stable in the earliest preserved stages of the deformational history. Table 4.6). However, the poorly developed nature of the leaves in these samples suggests that these phases developed during relatively weak deformations, in the decline phases of an intense.
Discussion
The Entia gneiss complex (ie 240,900 and 50 or more million years) is comparison with the thermal time constant of the lithosphere (the time required for geothermal relaxation, which is believed to be of the order of 80 million years)'. Since the isotopic systems with high closure temperatures preserve an age of about 1400 Ma and are correlated with the regional penetrant, this age probably corresponds to the peak pressure and temperature conditions represented by garnet-hornblende equilibria. This increase may correspond to the growth of coarse-grained cordierite developed between þanite and hornblende and/or orthoamphibole and sillimanite replacing kyanite in pelites.
Conclusion
It is likely that at least some of these isotopic events reflect distinct thermal episodes. A further thermal disturbance at 450 Ma sufficient to restore Sm-Nd mineral systematics can be recorded in the mineral assemblages in the staurolite-anorthite coronas developed between earlier hornblende and kyanite. The eastern Arunta Inlier (e.g. the Srangways and Harts ranges) thus experienced a meømorphic history that is at least slightly distinct from many other northern Australian terranes and this causes considerable significance for the Harts range in future interpretations of the northern Australian Proterozoic terranes.
Introduction
Hbl Pl Bt Grt jedro Cyrt Grt rob Bt StftPlEP Pl Pg Hbl l{bl jedro. Grt Hbl HbI St Hbl St EP rob EP jedro Hbl Pl Grtrim KY EP EP Br l{bl.
Total 15.233
- Petrography and mineral chemistry of the low variance kyanite- staurolite-hornblende garbenschiefer
- Geological setting
- Petrography and mineral chemistry
- Representation of amphibolites in compositional space
- Compatibility relations in amphibolites from the Zillertal
In the Eastern Alps the metamorphism is generally low-medium pressure greenschist to amphibolite facies, but the sparsely exposed Penninian units and Austroalpine complexes (e.g. Koralm and Öutal) provide some evidence of higher pressure metamorphism in the Eastern Alps (e.g. This basemenr includes three tectonic plates (Fig. 5.1), (i) the pre-Mesozoic Zenual gneiss (Cliffl . 1981), a felsic orthogneiss at the base of the section, (ii) the Palaeozoic lower (inner) Schieferhülle and (iii) the permo- Mesozoic Upper (Perifer) Schieferhülle (Ackermand et al., 1978) Ilmeníte often includes inclusion paths in garnet and hornblende, but the dominant oxide in m?Fx is rutile.
- Amphiboiites from the Post Pond volcanics, vermont, usA
- Kyanite, staurolite and garnet amphibolites from the Zillertaler Alpen, Austria
Although Spear (1932) does not report any reaction textures of carbonate-free amphibolites from the Post Pond Volcanics, crossline relationships and apparent univariate equilibria are evident in the rhechemographic projection used to show their equilibria (Fig. 6.1). Many of the equilibria observed in the Post Pond Volcanics are intermediate with respect to these two compatibility diagrams. Compatibility relationships for assemblages from Post Pond Vo'Ícanics, Vermont, USA (+ hornblende, plagioclase, quartz and aq.u€olts uãpóur).
- Kyanite-, staurolite- and gedrite'amphibolites from Georgia, USA
Overlapping bond triangles and the presence of the unique kyanite-staurolite-garnet-chlorite assemblage suggest that" like the Post Pond Volcanics, these assemblages may have equilibrated under the influence of variable fluid compositions (chapter s). These include: kyanite-staurolite-- chlorite-gedrite-hornblende-biotite-plagioclase (+ quartz and rutile + pyrrhotite, sample L-112-N, Helms et al., 1987), garnet-chlorite-gedrite-hornblende-plagioclase (sample L- LI2-D), staurolite-garnet-chlorite-homoblende-plagioclase (sample L-100), staurolite-garnet-chlorite-gedrite-hornblende (sample L-7g), þanite- A mafic-ulramafic slice (Laurel Creek Amphibolite) sourced from the Blue Ridge Metamorphic Province of the Southern Appalachians contains several low variances. The intermediate Xpg of the Laurel Creek amphibolite and its abundance of low-variance assemblages with complex phase relationships make the Georgia Blue Ridge amphibolites a potentially useful tool for understanding compatibility relationships in kyanite and staurolite amphibolites. The amphibolites of the Ha¡ts Mountains define a relatively complete compatibility diagram shown in Figure 6.7, in which the Xp" of phases decreases in the order staurolite .. gt; garnet > cumingtonite > hornblende ) orthoamphibole > cordierite. Kyanite-staurolite garnet from the Sölden region of Oaøl, Austria (+ hornblende, plagioclea, quartz and hydrous vapor) apparently equilibrated at 34 kba¡, < 670oC (Purtscheller and Mogessie, 1984). 1980) report that Quartenschiefer often contains coarse hornblende, which is scablike (garben) in habit and is part of the assemblage: homblende-þanite-staurolite-garnet-biotite--chlorite-plagicclase--epidote-quartz-ilmenite. Petrographic summaries of the Frodalera homblende garbenschiefe are given in Table 6.1 and selected mineral composition information is summarized in Table 6.2. Garnet is typically richer in Fe than staurolite, the same relationship preserved in samples from the Eastern European Alps and other areas where metamorphism apparently did not exceed about 7 kbar. Gibson (1979) suggests that the second stage of metamorphism evident in these samples resulted in the consumption of hornblende and kyanite to form margarite chlorite and plagioclase al - 550-720oC, with pressures up to 9.5 kbar. The trivariate kyanite-staurolite assemblage (in the absence of corundum, apparently saturated with silica) is shown in a compatibility diagram similar to that developed in the previous chapter, with quartz, plagioclase, hornblende and excess steam (Fig. 6.11 ). The spatial ordering of the phases is believed to be due to the diffusion-controlled nature of the reaction, where the location of the mineral was determined by the relative mobility of its released constituents (Schumacher & Robinson, 1987). The diffusion-controlled nature of the reactions resulted in the achievement of local equilibrium, while the sample as a whole retained up to several nonequilibrium assemblies. The compatibility relationships of the assemblies before and after this decompression are depicted in Fig. A d siil Very Mg-rich amphibolites from the southwestern Pamirs of Tajikistan include a variation of the "white shale" assemblage first reported by Schreyer, describing inclusions of talc, cordierite, qrrartz, and apatite in kyanite. The higher-pressure assemblage reported from the Pamirs occurs at the Mg-rich extreme of the compatibility diagram in Fig. Among the most useful compatibility relationships are those presented by Spear (1982) from the Post Pond volcanics.
XZP xz10
Hornblende amphibolites from New Hampshire
A sill
