Geochemical behaviour of spilitization in alkaline magmatism, Trias-Lias, of the Iberian Chain and Mallorca (Spain)

156 EVAL'UATION OF TRACE EL,EMENT MODEbS INVOLVING FRACTIONAL. CRYSTALL'IZATION AND MIXING PROCESSES FOR THE ERUPTIVE PRODUCTS OF THE NEVADO DEL' RUIZ VOLCANO, COLOMBIA Nicole VATIN-PERIGNON, Pierre GOEMANS (bab. of Geol., UA69 CNRS, Univ.J.-Fourier, 38031 Grenoble, France), Richard OL,IVER (ILL,, 38042 Grenoble, France). Concentrations of trace elements (such as Th, Ta, Hf, Cr and Y) and REE f o r pumices of the last eruption, as well as representative lavas of the compound volcano have been used in discrimination diagrams to t e s t f r a c t i o n a l c r y s t a l l i z a t i o n and p a r t i a l melting processes t h a t may have formed the 1985 eruptive magma. On a Th-Hf-Ta diagram the Ruiz series shows two d i s t i n c t crystal f r a c t i o n a t i o n trends from basaltic andesites which converge at the point where mixed 1985 pumices l i e . These two trends are confirmed on a Cr-Y diagram which also indicates that may have been produced by d i f f e r e n t rates of p a r t i a l melting of a plagioclase l h e r z o l i t e mantle source. On a Th-Ta diagram, modelling of fractional crystallization curves suggests a compositionally v a r i a b l e source (or two d i s t i n c t sources ?) with a subduction-zone component enriched in Th. The Th/Ta r a t i o is highly discriminant and shows that an important part of 1985 pumices has intermediate c h a r a c t e r i s t i c s and l i e s on a mixing line between the two f r a c t i c n a t i o n trends. Ce/Yb vs. Ta/Yb c l e a r l y shows two distinctive sources containing a probable subduction-zone component with d i f f e r e n t enrichment of Ta r e l a t i v e to primordial mant]e. Ta/Yb ratios are representative of these source compositions and in our view the pumices of the 1985 eruption are consistent with derivation from the less enriched mantle source together with a certain amount of mixing of two sourceS,

GEOCHEMICAL BEHAVIOUR OF SPILITIZATION IN ALKALINE MAGMATISM, TRIAS-LIAS, OF THE IBERIAN CHAIN AND MALLORCA (SPAIN). M. LAGO, D. ZACHMANN, R. VAQUER, and A. POCOVI. (Dept. of Earth Sciences, University of Zaragoza, 50009 Zaragoza, Spain.) -An alkaline magmatism of Trias-Lower Lias age is present in several parts of the Iberian Chain and the N Range of Mallorca. -The primary composition of basaltic sills of E Iberian Chain is intensely spilitized to albite, orthoclase, chlorite and carbonates. -On the contrary, similar basalts in Tarragona and Msllorcs are much less or not spilitized. Primary compositions are: Fo76_68, salite-type, Ti-rich augits and An73_62. Peridotitic xenoliths are common in lavas. -A geochemical comparison between basaltic rocks of both areas is made, emphasizing the Li and B behaviour in spilitization process. Water present in unconsolidated host sediments is assumed to be the main cause of spilitization.