Journal of Petrology | Volume 40 | Number 10 | Pages 1465-1496 | 1999
© Oxford University Press 1999
Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite–Granite Linkage: Inferences from Granulitic Xenoliths
1 Departamento De Petrología Y Geoquímica, Facultad De Ciencias Geológicas, Universidad Complutense 28040 Madrid, Spain
2 Department of Geology, Birkbeck College Malet Street, London WC1E 7HX, UK
3 CNRS UMR 6524, Département Des Sciences De La Terre, Université Blaise Pascal 5 Rue Kessler,F-63038 Clermont-Ferrand, France
4 Departamento De Geologa, Facultad De Ciencias Del Mar, Universidad De Cádiz 11510 Puerto Real (Cádiz), Spain
Received July 22, 1998; Revised typescript accepted April 29, 1999
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Abstract |
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Xenolith-bearing alkaline ultrabasic dykes were intruded into the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal xenoliths in the dykes can be divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form 
95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (0.01 and 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the alkaline magma. Felsic metaigneous and metapelitic xenoliths exhibit clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal xenoliths. 87Sr/86Sr and 
Nd of the felsic xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0.706–0.712, and –1.4 to –8.2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non-underplated crust in this region of the Hercynian orogenic belt.
KEY WORDS: felsic lower continental crust; granulite xenoliths; Sr–Nd isotopes; Hercynian Iberian Belt
* Corresponding author. Telephone: +34–913944910. Fax: +34–915442535. e-mail: granito{at}eucmax.sim.ucm.es
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