Large-scale silicate liquid immiscibility: a possible example from northeastern Brazil

Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widespread in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic inclusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only in different proportions. Rare 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, characterized by high SiO₂, low Al₂O₃ and TiO₂ contents and low Mg#.

The three rock types have similar REE chondrite-normalized patterns, with negative slopes and lack of Eu anomaly, with the total REE in the pyroxenite greater than that of the syenite. Trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite. Major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs.

The rocks have similar high δ¹⁸O values (avg. w.r.+ 8‰_SMOW, corrected from pyroxene), high initial ⁸⁷Sr/⁸⁶Sr ratios (about 0.710), and low ¹⁴³Nd/¹⁴⁴Nd (avg. 0.51104).

Field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.