GENESIS OF THE SOUTH-IBERIAN PYRITE BELT IN A PALAEOACCRETIONARY PRISM - PETROLOGICAL ARGUMENTS

The South-Iberian pyrite belt was emplaced during an episode of extens ional tectonics of Upper Famenian to Visean age, subsequent to a major episode of accretion of oceanic exotic terranes, in the southern part of the Iberian Variscan orogen. It shows the association of calc-alka line acidic rocks (lavas and intrusives) and tholeiitic basalts. Inter mediate rocks are scarce, the magmatism being considered as bimodal bu t with a large predominance of acidic products (similar to 70%). Three successive acidic volcanic episodes are described. In the Spanish par t of the belt, the basic lavas show geochemical characteristics which cover those of resent continental tholeiites and back-are basin basalt s, but are clearly distinct from those of island are tholeiites, The a cidic rocks are heterogeneous but there is no systematic difference be tween the lavas of the three episodes. The acidic rocks show geochemic al characteristics distinct from those of within-plate lavas and pluto ns, but also from those of subduction- or continent/continent collisio n-related lavas and granites. Similar rocks are described in plutonic complexes intruding palaeo-accretionary prisms, e.g. the Shimanto belt (SW Japan) or the accretionary prism of Alaska. Some samples with hig h Zr contents are comparable to the granites of the British Volcanic T ertiary Province. Geochemical differences between the South-Iberian ba sic and differentiated rocks preclude the derivation of the acidic mag mas by fractional crystallization of the basic ones. Lead isotopes com position of the acidic lavas are close to that of the h orogen a reser voir of Stacey and Kramers [1975] and Doe and Zartman [1979]. Trace el ements show that the acidic magmas could have derived from the melting of a greywacke-type reservoir, outside the garner stability field (i. e. at low to medium pressure). On the other hand: the involvement of a n evolved source comparable to the upper continental crust is preclude d. In agreement with recent models for the geodynamic evolution of the South-Iberian Variscan orogen, we propose that the Pyrite belt was em placed over an accretionary prism, subsequent to its deformation, meta morphism and partial melting.