Fluid and Solid Inclusions in Host Minerals of Permian Pegmatites from Koralpe (Austria): Deciphering the Permian Fluid Evolution during Pegmatite Formation

• Main Authors: Kurt Krenn, Martina Husar, Anna Mikulics
• Format: Article
• Language: English
• Published: MDPI AG 2021-06-01
• Series: Minerals
• Subjects: fluid inclusions; magmatic and metamorphic fluid; spodumene-garnet-tourmaline; in-situ mineral reactions; Permian pegmatites; Eastern Alps
• Online Access: https://www.mdpi.com/2075-163X/11/6/638

Fluid inclusions (FIs) and associated solids in host minerals garnet, tourmaline, spodumene, and quartz from six pegmatite fields of Permian origin at Koralpe (Eastern Alps) have been investigated. Although pegmatites suffered intense Eoalpine high-pressure metamorphic overprint during the Cretaceous period, the studied samples originate from rock sections with well-preserved Permian magmatic textures. Magmatic low-saline aqueous FIs in garnet domains entrapped as part of an unmixed fluid together with primary N₂-bearing FIs that originate from a host rock-derived CO₂-N₂ dominated high-grade metamorphic fluid. This CO₂-N₂ fluid is entrapped as primary FIs in garnet, tourmaline, and quartz. During host mineral crystallization, fluid mixing between the magmatic and the metamorphic fluid at the solvus formed CO₂-N₂-H₂O–rich FIs of various compositional degrees that are preserved as pseudo-secondary inclusions in tourmaline, quartz, and as primary inclusions in spodumene. Intense fluid modification processes by in-situ host mineral–fluid reactions formed a high amount of crystal-rich inclusions in spodumene but also in garnet. The distribution of different types of FIs enables a chronology of pegmatite host mineral growth (garnet-tourmaline/quartz-spodumene) and their fluid chemistry is considered as having exsolved from the pegmatite parent melt together with the metamorphic fluid from the pegmatite host rocks. Minimum conditions for pegmatite crystallization of ca. 4.5–5.5 kbar at 650–750 °C have been constrained by primary FIs in tourmaline that, unlike to FIs in garnet, quartz, and spodumene, have not been affected by post-entrapment modifications. Late high-saline aqueous FIs, only preserved in the recrystallized quartz matrix, are related to a post-pegmatite stage during Cretaceous Eoalpine metamorphism.