The origin of rapakivi texture by sub-isothermal decompression

Abstract

The rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.64–1.55 Ga) anorogenic rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680–720°C) and high P (5–6 kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780°C) and intermediate to low P (3.5–1 kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1 kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760–780°C) from the lower-middle crust (5–6 kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep-seated rapakivi granites formed by slow cooling.