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

摘要: Abstract 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 syenite have same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only different proportions. Rare a “mixed” rock (about 60% syenite+40% an emulsion-like texture) also present. Pyroxenes three units all slightly zoned, extremely low Al2O3 (0.2–1.4%). Amphiboles mostly K-rich richterite, characterized by high SiO2, TiO2 contents Mg#. The types similar REE chondrite-normalized patterns, negative slopes lack Eu anomaly, total greater than that syenite. Trace element patterns for mixed intermediate between those Major partitioning has sense one observed immiscible liquids volcanic lavas trace is to experimentally determined partition liquid pairs. rocks δ18O values (avg. w.r.+ 8‰SMOW, corrected from pyroxene), initial 87 Sr 86 ratios 0.710), 143 Nd 144 0.51104). Field geochemical characteristics indicate chemical equilibrium among suggest immiscibility pyroxenite, representing original magma composition.