Petrology, Vol. 6, No. 6, 1998, pp. 555–563. From Petrologiya, Vol. 6, No. 6, 1998, pp. 606–614.
Original English Text Copyright © 1998 by Dolores Pereira.
English Translation Copyright © 1998 by
/Interperiodica Publishing (Russia).
The Pe a Negra anatectic complex (PNAC) is
located in the middle-western part of the Avila
batholith, in central Spain (Fig. 1) (Bea and Pereira,
1990; Pereira, 1992; Pereira and Bea, 1994). It crops
out over 350 km
, and it is made up of high–medium
grade metamorphic rocks, mainly migmatites, and ana-
tectic products derived from the melting of those mig-
matites during the Hercynian orogeny. Generation and
coexistence of the different products is mainly the
result of the locally composition variation of a highly
heterogeneous source. A fertile lithology is the main
feature to take into account, although the presence of
shear structures, controlling the role of volatile ele-
ments, is a very important factor in triggering the par-
tial melting of the migmatites (Pereira and Shaw, 1996,
Migmatites have a nonvariable mineralogy, charac-
teristic of low pressure conditions: quartz + plagioclase +
alkali feldspar + biotite + cordierite
main phases. Ilmenite, apatite, zircon, pyrite, chalcopy-
rite and tourmaline are common accessory minerals.
This paragenesis does not contain an association that
can be used as a good thermobarometric sensor. How-
ever, there is a restricted and narrow band of migma-
tites, close to the high part of the complex, where we
have identified garnet as an accessory phase (Pereira,
1992, 1993). This fact will be crucial when applying
some geothermometric calculations, calibrated by dif-
ferent authors on the pairs cordierite–garnet (Bhatta-
., 1988), biotite–garnet (see Spear and Pea-
cock, 1990), and on the association garnet–plagio-
conditions (Hodges and Spear, 1982).
The purpose of this paper, therefore, is the study of
pressure and temperature conditions under which the
rocks from the PNAC were generated and to compare
these results with the ones obtained by other authors for
the generation of similar complexes.
Mesocratic migmatites are the most abundant mate-
rial in the PNAC, and sheets of granodiorite and small
lens-shaped bodies of cordierite leucogranite can be
found interlayered with them (Pereira, 1992; Pereira
and Bea, 1994; Pereira and Shaw, 1996, 1997). These
migmatites are high-grade, mostly neosomatic, and can
be called diatexites (in the sense of Mehnert, 1987).
They are made up of: (1) a granodioritic leucosome,
with hypidiomorphic texture, containing quartz, pla-
gioclase, alkali feldspar, cordiente, and biotite as main
phases; and (2) a restitic melanosome, which appears as
small enclaves or as schlieren within the leucosome,
and it is made up by sillimanite, cordierite, and biotite,
with very abundant ilmenite and very rare alkali feld-