THERMOBAROMETRIC ESTIMATES

The PT conditions of the high-grade schists during the M₁ and M₂ metamorphic events have been compared using plagioclase, biotite, and muscovite inclusions, zoned plagioclase, and matrix biotite (Fig. 8). Thermobarometric estimates have been performed using the Grt-Bt thermometer of Perchuck and Lavrent'eva (1983), the GASP barometer of Newton and Haselton (1981), and the Grt-Pl-Ms-Qtz (Hodges and Crowley, 1985) and Grt-Pl-Bt-Qtz (Hoisch, 1990) equilibria. The calibration by Perchuck and Lavrent'eva (1983) was preferred, because the (Al^VI+Ti)/(Al^VI+Ti+Fe+Mg) ratio of the biotite, ranging between 0.21 and 0.29, is higher than the value required for the application of the widely used Ferry and Spear (1978) calibration.

We obtained two different estimates using the M₁ mineral assemblage. The first one was attained using garnet core compositions (Fig. 5) and mineral inclusions. In this case the estimated PT conditions are 550°C and 5.8 kbar, within the kyanite stability field (Fig. 8A). Using garnet core and matrix plagioclase core compositions estimated P are slightly lower (5.2-5.6 kbar), but still within the kyanite stability field. This result has to be taken with some caution, because of the scarcity of data and the lack of kyanite in the Alboran basement rocks. The presence of rutile may be indicative of high pressures (Bohlen and Liotta, 1986), but this mineral is normally included in corundum and never associated with garnet in the high-grade schists.

The second estimate for the M₁ mineral assemblage was derived from compositions of garnet inner rims (Fig. 5), plagioclase cores, and biotite included within garnet. We obtained a large spread for the K_eq lines of the Grt-Bt thermometer (Fig. 8A), probably reflecting progressive biotite re-equilibration related to prograde or retrograde metamorphism. Combining the Grt-Bt K_eq lines with those obtained from the GASP barometer, we can conclude that the late stage of the M₁ metamorphism took place in a T range of 570°-640°C for a P ranging between 4 and 6 kbar. These PT values are consistent with the occurrence of both staurolite and sillimanite in the M₁ mineral assemblage.

PT results obtained for M₁ were checked by using Grt inner rims, and included Bt, Pl, and Ms for one Ms-bearing sample (161-976E-14R-3 [Piece 1A, 10-14 cm]). By combining the Grt-Pl-Ms-Qtz and Grt-Pl-Bt-Qtz barometers with the Grt-Bt thermometer we obtained consistent PT values of 605°-620°C and 4.8-5.7 kbar (Fig. 8A).

The M₂ PT conditions were defined using garnet outer rim (Fig. 5) matrix biotite, and plagioclase rim compositions (Fig. 8B). Similarly to M₁, the M₂ K_eq lines of the Grt-Bt thermometer show a wide dispersion. However, the presence of K-feldspar and abundant sillimanite along the S₂ foliation indicates that the breakdown of muscovite has been exceeded during the growth of the M₂ mineral assemblage. Therefore, we can neglect the PT estimates falling within the Ms + Qtz field, and we can relate them to progressive re-equilibration of biotite and garnet during the retrograde evolution. Summing up, the M₂ metamorphism should have occurred at 625°-710°C and 2.8-4.4 kbar.

The PT conditions obtained from the database of Holland and Powell (1990), for X_H2O = 1.0, yield systematically higher pressures (Table 3). In addition, the calculated temperature for the M₁ mineral assemblage is higher (617° ± 26°C) than the previous estimate for the same assemblage. This latter result can indicate that biotite and plagioclase inclusions were not in equilibrium with the host garnet, or that conditions of X_H2O = 1.0 are not appropriate for the Alboran rocks. However, the occurrence of staurolite and sillimanite in the M₁ assemblage, together with the lack of kyanite, indicates that pressures calculated with the database of Holland and Powell (1990) are probably too high.

PT conditions of the high-grade gneisses have been estimated only for the late M₃ assemblage (Fig. 9), using Grt-Bt thermometry (Perchuck and Lavrent'eva, 1983) and the Crd-Grt-Sil-Qtz barometer (Nichols et al., 1992). We obtained a T of 595-610 Kbar, for distinctly lower P of 1.25-1.75 kbar. This result is consistent with the occurrence of the reaction Bt + Al-silicate (Als) Grt + Crd in the high-grade gneisses.