Recycled crystal population proving complex plumbing systems: an archetypal example from Cretaceous alkaline magmatism in SE Brazil

Porphyritic alkaline dykes from the Serra do Mar province (SE Brazil) carry clinopyroxene macrocrysts with a complex magmatic history. Clinopyroxene is the main macrocryst phase from the Bom Repouso dyke, representing different populations. Based on petrographical features, mineral chemistry and equilibrium relationships, the clinopyroxene populations are classified as antecrysts (green cores and colourless cores and mantles) and phenocrysts (Ti-augite mantle/rims). Calculated compositions for the liquid in equilibrium with the macrocryst cores indicate mafic (colourless cores) and felsic (green cores) alkaline melts. These were used as poles for mixing-model curves, revealing a hybrid origin for the host matrix. Clinopyroxene barometry reveals key insights into the plumbing systems of the Serra do Mar province, with the Bom Repouso dyke as an archetypal example of these transcrustal systems. All target bodies record a crystallization history over a continuous range of pressures (>8 kbar up to the surface). Some preferred levels for magma storage are observed: c. 0–0.5, 1–2.5, 3–4 and 5.5–7 kbar. Our work highlights the complex magmatic history of the Serra do Mar alkaline occurrences, encompassing several open-system effects. A thorough petrographical examination is necessary to enhance our comprehension of these magmas, while petrogenetic models based on whole-rock compositions may lead to biased interpretations.