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Nature and composition of the lower continental crust in central Spain and the granulite-granite linkage: Inferences from granulitic xenoliths
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DOI：10.1093/petrology/40.10.1465.png)
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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 similar to 95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (similar to 0.01 and similar to 5%, respectively). Thermobarometric calculations based on mineral al paragenesis indicate equilibration conditions around 850-950 degrees 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. Sr-87/Sr-86 and epsilon(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.
Keywords:
felsic lower continental crust
granulite xenoliths
Si-Nd isotopes
Hercynian Iberian Belt
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