Geoquímica del granito de dos micas de A Espenuca: origen y evolución

Abstract

[Abstract] The A Espenuca two-mica granite, situated in the northern sector ofthe Central Iberian Zone, was variably deformed by the Valdoviño Fault. Deformation intensity is observed to increase from W to E, also being more intense in the northern half. Results of the geochemical study of the less deformed granite facies, known in the regionalliterature as the «weakly deformed facies», are presented. Chemical variations are faint (DSi02 z 4%) and reflect a whole composition close to the granite minimum. Analyzed samples correspond to alkali-dch(Na20+K20, x = 8.4 %), CaO-depleted rocks « 0.9 %), showing very low values for the B (=Fe+Mg+Ti) parameter x = 30, and a marked peraluminous character, A/CNK, x =1.16 (range 1.1-1.26), which from the mineralogical point of view is traduced in the occurrence of muscovite (± garnet). The rocks studied are depleted in REE (LREE =75.77-189.83; x = 117.4), notably in the HREE. They exhibit considerably fractionated pattern (La/Lu)n x = 50 as well as important Eu negative anomalies (Eu/Eu*, x = 0.36). These rocks can be linked by fractionated crystallization processes. Massbalance calculations indicate crystallization ratios < 30%, the fractionated minerals being Kfs, Bt, PI. These values to explain the trace element variations observed in the different samples. The chemical-mineralogical composition of analyzed facies suggests an origin by partial melting of crustal rocks (either pelites, schists, meta-greywackes or orthoderived materials) at considerable depths, where garnet constitutes a restite stable phase. Nevertheless, calculated models show that not aH these materials equally adjust as source to the granite magma. The best fits are obtained considering a source similar to 0110 de Sapo gneisses. Key Words: Two-mica granite, fractional crystallization, partial