High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites

Abstract

The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite-massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedralto-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4-7.6 wt% Al 2 O 3), through the rims (1.3-3.6 wt%), into the host rock (0.5-1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En~6 0-70) and rims (En~5 0-70) of the HAOMs than in the host rock (En~4 5-60) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65-1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas.

Equilibrium crystallization of massif-type anorthosite residual melts: a case study from the 1.64 Ga Ahvenisto complex, Southeastern Finland

“…The 1.64-Ga Ahvenisto complex The 1.64-Ga Ahvenisto AMCG complex (Savolahti 1956(Savolahti , 1966Johanson 1984;Alviola et al 1999;Heinonen et al 2010bHeinonen et al , 2015Heinonen et al , 2020 belongs to the Fennoscandian rapakivi suite (e.g., Rämö and Haapala 2005;Rämö et al 2014) and is located northwest of the Wiborg batholith in southeastern Finland (Fig. 1).…”

“…Nd-Sr isotopes suggest a comagmatic origin for the anorthositic (initial Ɛ Nd − 0.9 to − 0.5 and Sr 0.7037 to 0.7041) and monzodioritic rocks (initial Ɛ Nd − 1.1 to − 0.2 and Sr 0.7028 to 0.7040; Heinonen et al 2010b). Al-in-opx geobarometry suggests that the anorthositic rocks crystallized polybarically at three different levels: (1) high-P conditions in lower crust (~ 1100 MPa), (2) mid crustal levels (~ 500 MPa), and (3) upper crustal levels (~ 200 MPa) (Heinonen et al 2020).…”

“…Large (up to 30 cm long) orthopyroxene (high aluminum orthopyroxene megacrysts = HAOM) and associated plagioclase megacrysts usually ~ 10 cm (up to 50 cm) long are found in several locations throughout the anorthositic arc ( Fig. 2d, e.g., Savolahti 1966;Alviola et al 1999;Heinonen et al 2020).…”

“…Some HAOMs were carried to shallower crustal levels by the ascending mushes and record mineralogical evidence of low-pressure re-equilibration (e.g., exsolved plagioclase lamellae; Emslie 1975). The compositional zoning of HAOMS suggest that continuous crystallization took place during ascent and composition of the matrix orthopyroxene in the host anorthositic rocks indicates that final crystallization happened at upper crustal levels (Heinonen et al 2020). The continuous crystallization of plagioclase and mafic phases from the mushes left behind a residual magma, usually considered to be monzodioritic in composition (e.g., Emslie et al 1994;Markl and Frost 1999;Markl 2001;Heinonen et al 2010b;Charlier et al 2010).…”

“…1) is one of the key locations for Fennoscandian massif-type anorthosite studies (Alviola et al 1999;Heinonen et al 2010aHeinonen et al , b, 2015Heinonen et al , 2020Heinonen 2012;Fred et al 2019), as it is the best representative of AMCG suite rocks at the current level of exposure in Finland (e.g., Rämö 1991). Recent studies in the Ahvenisto complex area have focused on the mantle vs. crustal sources of the complex (Heinonen et al 2010b(Heinonen et al , 2015, the physical and chemical interactions of the associated monzodioritic and granitic magmas (Fred et al 2019), and the polybaric crystallization history of the Ahvenisto anorthosite (Heinonen et al 2020). Heinonen et al (2010b) and Fred et al (2019) preliminarily suggested that the monzodioritic rocks may approximate melt compositions and form trends that could be interpreted as a liquid line of descent (LLD) of the residual melt left after fractional crystallization of the anorthositic cumulates.…”