High-pressure minerals in mafic microgranular enclaves: evidences for co-mingling between lamprophyric and syenitic magmas at mantle conditions

• Published in: Contributions to mineralogy and petrology Vol. 145; no. 4; pp. 444 - 459
• Main Authors: Plá Cid, Jorge, Nardi, Lauro Valentim Stoll, Stabel, Larissa Zitto, Conceição, Rômmulo Vieira, Balzaretti, Naira Maria
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.07.2003
• Subjects: Crystallization; Geochemistry; Magma; Mineralogy; Petrology; Southern Brazil
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-003-0451-4

Mafic microgranular enclaves, composed of diopside and rare magnesium biotite phenocrysts in a groundmass of diopside, biotite, apatite, Fe-Ti-oxides, and alkali feldspar, are associated with Neoproterozoic Piquiri potassic syenite in southern Brazil. Co-genetic mica and clinopyroxene cumulates present inclusions of pyrope-rich garnet in diopside phenocrysts. Textural evidence, as well as the chemical and mineralogical composition, suggest that enclaves crystallized from a lamprophyric magma and co-mingled with the host syenitic magma. The contrasting temperature between both magmas and the consequent chilling was important for the preservation of some early-crystallized minerals in the mafic magma. Diopside groundmass grains contain micro-inclusions of K-rich augite and phlogopite, and some clinopyroxene phenocrysts and elongate groundmass crystals have potassium-rich cores. The pyrope-rich garnet have high #mg number (67-68), with appreciable amounts of Na 2O and K 2O comparable to pyrope synthesized at 5 GPa. The extremely high K 2O contents of K-rich augite micro-inclusions suggest non-equilibrium with the parental magma, whereas the other K-rich clinopyroxenes are similar to K-clinopyroxenes produced at 5-6 GPa. K-clinopyroxene and garnet in mafic microgranular enclaves suggest that lamprophyric magma started its crystallization at upper mantle conditions, and chilled clinopyroxenes with measurable amounts of K 2O are taken as evidence that co-mingling began still at mantle pressures. [PUBLICATION ABSTRACT]