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

Aku Heinonen,Radoslaw M Michallik,Heli Kivisaari

doi:10.1007/s00410-019-1648-5

Aku Heinonen, Radoslaw M Michallik + Show 1 more

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https://doi.org/10.1007/s00410-019-1648-5

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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, euhedral-to-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% Al2O3), 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~60–70) and rims (En~50–70) of the HAOMs than in the host rock (En~45–60) 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.

Highlights

Proterozoic massif-type anorthosites are large intrusions of anorthositic rocks commonly associated withCommunicated by Chris Ballhaus.Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Several petrogenetic models imply a significant mantle contribution to the parental magmas of both the anorthosites and the associated ferroan granites
There is no significant correlation observed between the host rock composition and high-aluminum orthopyroxene megacrysts (HAOMs) type
Two texturally different but compositionally similar HAOM types were recognized from the Ahvenisto complex of SE Finland

Summary

Introduction

Several petrogenetic models imply a significant mantle contribution to the parental magmas of both the anorthosites and the associated ferroan granites. The “all-crustal models” explain all rock types in AMCG suites as consanguineous fractionates of a common crustal parental melt of evolved jotunitic composition (e.g., Duchesne et al 1999; Longhi et al 1999). Balancing between these two extremes are the classical “two-source models” that distinguish a mantle origin for the anorthositic rocks and a dominant lower crustal source for the granitic rocks (e.g., Rämö 1991; Emslie et al 1994). Isotope data indicate that contamination of mantle-derived magmas by lower crust material is an important process in AMCG petrogenesis (e.g., Bybee et al 2014; Heinonen et al 2015)

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