Pereira, 1998, PT anatectic complex spain

Pereira, 1998, PT anatectic complex spain - Petrology, Vol....

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555 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 åÄàä ç‡Û͇ /Interperiodica Publishing (Russia). INTRODUCTION 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 2 , 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, 1997). Migmatites have a nonvariable mineralogy, charac- teristic of low pressure conditions: quartz + plagioclase + alkali feldspar + biotite + cordierite ± sillimanite as 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- charya et al ., 1988), biotite–garnet (see Spear and Pea- cock, 1990), and on the association garnet–plagio- clase–sillimanite–quartz to calculate pressure conditions (Hodges and Spear, 1982). n ˜ 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. THE SAMPLES 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- spar.
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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Pereira, 1998, PT anatectic complex spain - Petrology, Vol....

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