LARGE-SCALE SILICATE LIQUID IMMISCIBILITY - A POSSIBLE EXAMPLE FROM NORTHEASTERN BRAZIL

Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widesp read in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic in clusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apa tite, blue amphibole, magnetite), but only in different proportions. R are inclusions of a ''mixed'' rock (about 60% syenite + 40% pyroxenite in an emulsion-like texture) are also present. Pyroxenes in the three units are all only slightly zoned, silica-saturated and extremely low in Al2O3 (0.2-1.4%). Amphiboles are mostly K-rich richterite, charact erized by high SiO2, low Al2O3 and TiO2 contents and low Mg#. The thre e rock types have similar REE chondrite-normalized patterns, with nega tive slopes and lack of Eu anomaly, with the total REE in the pyroxeni te greater than that of the syenite. Trace element patterns for the mi xed rock are intermediate between those for the pyroxenite and syenite . Major element partitioning between pyroxenite and syenite has the sa me sense as that one observed between immiscible liquids in volcanic l avas and trace element partitioning is similar to the experimentally d etermined partition of immiscible liquid pairs. The rocks have similar high delta(18)O values (avg. w.r. + 8 parts per thousand(SMOW), corre cted from pyroxene), high initial Sr-87/Sr-86 ratios (about 0.710), an d low Nd-143/Nd-144 (avg. 0.51104). Field and geochemical characterist ics indicate chemical equilibrium among the three rock types and sugge st liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.