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5.5.3.1 Garnet zoning profile

A transect of composition points across a garnet porphyroblast from sample LH 255 reveals a maximum value of grossular in the core of the crystal, an increase of grossular component in the centre of the crystal followed by a sharp decrease towards the edge (figure 5.19a). The spessartine and pyrope components show no regular pattern of zoning, except that one side of the transect records a significant drop in spessartine content which is accompanied by a

corresponding rise in almandine. Apart from this fluctuation, the almandine content and the Fe/(Fe+Mg) ratio both show a ‘saucer’ shaped curve with minimum values at the core and maxima at either side. There is a gradual decrease of almandine and Fe/(Fe+Mg) toward the rims.

The contours of garnet composition for the MnNCKFMASH system plotted by Spear et al

(1990) (figure 5.20) may be used to interpret the observed patterns of Fe/(Fe+Mg) and grossular variation. The peak of grossular component at the core of the crystal indicates that the pressure decreased, or the temperature increased, from a maximum at the onset of garnet growth (figure 5.20d). The increase of Fe/(Fe+Mg) at the ‘rim’ in the centre of the crystal may imply a decrease in temperature, while the decreasing trend of Fe/(Fe+Mg) towards the rim of the crystal may indicate the growth of staurolite in the assemblage (Spear, 1993). The drop in grossular and spessartine at one edge of the profile that is accompanied by an increase in almandine is likely to signify a drop in pressure near the end of porphyroblast growth.

Sequential points of composition across the diameter of a garnet porphyroblast from sample LH 154 shows the typical compositional variation that is seen in garnets from the

gamet/staurolite-andalusite grade rocks (figure 5.21). No significant variation is seen in the grossular, almandine, spessartine or pyrope components and, consequently, the Fe/(Fe+Mg) ratio shows no zoning. From the X(Fe) and M(Grt) contours shown in figure 5.5 and figure 5.6 it seems most likely that garnet growth followed a P-T path sub-parallel to the X(Fe) contours and with a slightly negative gradient, tracing a path of increasing temperature and decreasing pressure.

5.5.3.2 Pressure estimates - Introduction

In some sillimanite-absent assemblages, garnet porphyroblasts are locally equant and do not show extensive evidence of resorbtion at the rims. This texture suggests that the composition of the garnet porphyroblast rims may be in equilibrium with minerals in the matrix of the samples.

5.5.3.3 Application o f the GASP barometer

In sample LH 255 (VF 42880675), garnet porphyroblasts and plagioclase crystals exhibit curved inclusion trails that are oblique to the main fabric in the sample (figure 4.21). These textures suggest that both mineral phases grew during the development of the main fabric (Ss) in this area (section 4.5.2). A transect of composition across a garnet porphyroblast from this rock is seen in figure 5.19a.

Mineral pairs of corresponding core and rim locations in both the garnet and plagioclase porphyroblasts were used in the GASP barometer (Ganguly & Saxena, 1984). The locations at which the garnet compositions were measured is shown in figure 5.22a; composition data for the plagioclase crystals in the matrix is presented in figure 5.22b.

Results from the application of the GASP barometer are shown in figure 5.23.

5.5.3.4. Application o f the G RAIL barom eter

Intergrowth textures between ilmenite and rutile were identified as inclusions in garnet porphyroblasts in samples LH138 and LH311 (gamet/staurolite-andalusite grade). A plot of the compositional variation across a garnet porphyroblast in sample LH 311 is seen in figure 5.24; no significant variation of the major components is identified.

The elongate rutile/ilmenite intergrowths were aligned parallel to the internal foliation of the garnet porpyroblasts (figure 5.25 and figure 5.26). Spear (1993) suggests that intergrowths between ilmenite and rutile are a consequence of early mtile converting to ilmenite prior to the growth of garnet. Alignment of the rutile needles to the internal foliation within the garnet is concordant with rutile growth predating the growth of garnet. Although these minerals (garnet, ilmenite, rutile) did not, then, grow simultaneously, their close association was used as the basis for assuming equilibrium and the GRAIL barometer was applied. In this case, the pressures obtained from GRAIL will constrain the minimum pressure value for a given temperature.

Keq plots for GRAIL assemblages in samples LH 138 and LH 311 are shown in figures 5.27a and 5.27b, respectively.

At 550°C, the Keq lines from the GRAIL barometer estimates a pressure of 2 kbar less than those estimated from GASP at the same temperature (e.g. compare figure 5.22 and figure

5.27). This variation appears to confirm that the rutile/ilmenite intergrowths included within

the garnet porphyroblasts in samples LH138 and LH311 did not grow in equilibrium with the garnet; as a consequence of this, results from the GRAIL barometer do not generate results that can be reliably interpreted.

5.5.3.5. Temperature estimates - Introduction

No examples of biotite inclusions in garnet were identified within the gamet/staurolite - andalusite assemblages. Where staurolite was absent, and little evidence of garnet resorption at the edges of the porphyroblasts was present, it was assumed that due to the absence of other Fe / Mg phases in the rock the Fe:Mg ratio of the matrix biotite would have maintained a constant value since the end of garnet growth. Based on this assumption, compositions of garnet taken from the rim of the garnet porphyroblasts and average biotite compositions from the matrix were used in the gamet-biotite thermometer of Ferry & Spear with Berman (1990). These calculations will yield estimates of the ambient temperatures prevailing near to the end of garnet growth.

5.5.3.6. Application o f the GARB thermometer

The gamet-biotite thermometer was applied, with the assumption discussed above, to three samples - LH61, LH 154 and LH349. Plots of the Keq lines for these samples are presented in

figures 5.28 - 30.

The Keq lines generated from the application of GARB to each sample cluster close together, and a comparison of the results from the three individual samples reveals that the Keq lines of each sample all plot in similar positions on the P-T plot. For a pressure of 600 kbar, the GARB barometer indicates that the gamet/staurolite-andalusite grade rocks were in the temperature range 600-650°C.

5.5.3.7 Application o f the Garnet-amphibole thermometer

Sample LH255 (VF 42880675), from the gamet/staurolite-andalusite grade rocks, contains large porphyroblasts of garnet and amphibole, both of which exhibit curved inclusion trails. These textures indicate that both garnet and amphibole grew during the Ds deformation event (section 4.5.2) and it is reasonable to assume that these phases were in equilibrium with each other during their growth. Sequential point analyses, parallel to the curved inclusion trails in the porphyroblasts, have been made across the diameter of both the garnet and amphibole crystals in this section. The interpretation of the garnet profile is discussed above (section 5.5.3.1); the plot of composition across the amphibole porphyroblast shows no variation of the Fe/(Fe4-Mg) ratio (figure 5.31).

The gamet-amphibole thermometer of Graham & Powell (1984) has been applied to mineral pairs taken from the core, intermediate areas within the crystals and from rims of the garnet and amphibole porphyroblasts. Kgq lines for the thermometer, obtained from the program of Spear (1991) are shown in figure 5.32. It appears from these results that no change in temperature occurred during the growth of the garnet porphyroblasts; this is concordant with the absence of compositional zoning in the porphyroblasts noted above (figure 5.19).

5.5.4 Summary

The results obtained from the application of barometry and thermometry to the sillimanite- bearing and gamet/staurolite-andalusite assemblages from the Alpujarride Complex help to constrain the P-T history of the rocks of different metamorphic grades. Relationships of minerals to the regional foliations Ss and St have been explored in sections 4.2 - 4.5; a

summary of the results from the micro-structural studies is presented in table 4.1. The results from thermobarometry are most usefully interpreted in terms of the stmctural evolution of the rocks.

In the kyanite-sillimanite grade rocks, garnet growth ended prior to the generation of the Ss foliation (table 4.1); the pressure-temperature conditions of 7.5 - 10.5 kbar and 650 - 750°C predicted from figure 5.33 relate to the conditions prevailing before Ds affected these rocks. In kyanite-sillimanite-andalusite grade rocks, garnet growth continued during the Ds

deformation (table 4.1); the pressure-temperature conditions constrained on figure 5.34 for these rocks (i.e. 7.5 - 9.5 kbar and 600-700°C) indicate the ambient conditions during the Ds

phase of deformation. The results for both the kyanite-silliimanite and kyanite-sillimanite- andalusite grade rocks, considered together, suggest that these rocks experienced similar conditions of pressure and temperature during the early part of their metamorphic history.

Gamet/staurolite-andalusite grade rocks were at conditions of approximately 9 kbar and 650°C (figure 5.35) after the end of the Ds phase of deformation (table 4.1). The gamet/staurolite-andalusite grade rocks probably followed a path of lower pressure and temperature than the sillimanite-bearing rocks; this is compatible with other studies in the Alpujarride Complex that suggest rocks of lower metamorphic grade originate from structurally higher positions in the metamorphic pile (e.g. Argles et al, 1999).

Andalusite grew in the kyanite-sillimanite-andalusite and gamet/staurolite-andalusite rocks during and after the Dt deformation (table 4.1); these rocks must have experienced a rapid period of decompression between the Ds deformation and the onset of Dt.

P

2

D

5.6

Introduction

Three methods of geochronology have been applied to rocks within this area of study and have yielded useful results that help to constrain the timing of the metamorphic and structural evolution of the Alpujarride Complex described in previous sections. A map showing the location of the samples used for geochronology is presented in figure 5. 36; the techniques used, and the results arising from the application of these techniques are set out in the following sections.

5.7

U-Pb technique