THE ROLE OF HYBRIDIZATION IN THE GENESIS OF HERCYNIAN GRANITOIDS IN THE GREDOS MASSIF, SPAIN - INFERENCES FROM SR-ND ISOTOPES

The Gredos massif is one the better exposed granitoid complexes of the Iberian massif. It is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes. The massif also contains mega-enclaves of migmatites with which the granitoids show transitiona l contacts. Two major magmatic associations have been distinguished in this study: (1) One comprises the granitoids with microgranular encla ves, the enclaves, and basic rocks; (2) the other is formed by leucogr anites, intrusive into the former series and free of microgranular enc laves. Field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process. The Sr-Nd isotopic study reveals that this process is complex, relatin g different end-members of mantle and crustal affinities, and occurred around 295 Ma ago, late with respect to the main deformation phases o f the Hercynian orogeny. The granitoids with microgranular enclaves (G ME) are part of an overall mixing trend involving Palaeozoic mantle-de rived magma and melts of older crustal material. Amphibole-bearing GME , in general, contain greater proportions of the mantle-derived compon ent than the cordierite-bearing GME. The actual mixing processes took place on a variety of scales, sometimes between melts which were thems elves hybrids. On a local scale this hybridization process can be mode lled by simple binary mixing as documented in the case of a composite dyke. The isotopic signatures of the basic rocks are probably, to a la rge degree, the result of interaction with crustal melts, though addit ionally the presence of an enriched mantle source cannot be eliminated . Microgranular enclaves and their immediate hosts have differing init ial Sr and Nd isotopic signatures, indicating that isotopic equilibriu m was not attained. This suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system. The enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees. Late-stage p eraluminous leucogranites have similar initial Nd isotopic composition s to the evolved GME; a crustal source with a radically different Nd i sotopic composition or age does not need to be invoked in their petrog enesis.