Evidence of Protolith Contamination in the Generation of an Anatectic Complex: Peña Negra, Central Spain

ABSTRACT

The Peña Negra complex, in central Spain, provides a superb opportunity for the study of Variscan partial melting processes. The heat source for the anatexis has been controversial. The presence of mafic rocks in the vicinity has given rise to the suggestion that mafic magmas of mantle origin provided the requisite heat for the generation of the anatectic granites. Experimental petrology can be used to support this concept, but the volumes of mafic magma necessary for the anatexis appear improbably large. Geophysical models for Central Iberia do not support the presence of such large basic complexes at depth. Furthermore radiometric dating of the scarce mafic outcrops indicates that they are too old to have been involved in the granitic magma genesis. The alternative interpretation is that melting of the protolith was promoted by the radioactive decay of K, U and Th. Anatexis was enhanced by the existence of a fertile protolith and by the presence of shear structures that permitted volatile fluxing. The situation is believed to have been analogous to that in other European Variscan terrains.

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