Detailed studies have shown that the rocks are high-Si[O.sub.2] adakites (HSA) and that the parental magma was generated as a result of subduction and partial melting of an oceanic slab and its overlying sediments and have a post-collisional affinity (Kohnavard, 2015).
The phenocryst population can be identified as crystals inherited from one of the parental magmas, but fine grains were inherited from one of the parent magmas or must have formed after the mixing event.
Parental magma likely comprised a plagioclase-phyric basaltic melt, containing ~15-45 vol% plagioclase phenocrysts.
Lens-like bodies of Archaean megacrystic anorthosite (e.g., Fiskenoesset, Bad Vermillion Lake), for example, likely formed through flotation of plagioclase phenocrysts during in situ fractional crystallization of basaltic parental magma (Windley et al.
Furthermore, the estimated ~45:55 crystal to melt ratio of the parental magma lies well above the ~35:65 ratio required to form rigid crystal frameworks, suggesting that plagioclase crystal frameworks formed shortly after magma emplacement.
The bulk composition of the Marginal Zone (~74% plagioclase and ~26% pyroxene plus accessory minerals) suggests that the crystal to melt ratio of parental magma was only ~15:85, well below the ~35:65 threshold for developing crystal frameworks.
Just as important is that these processes are consistent with the crystallization of a viscous plagioclase-phyric parental magma.
Since then, no attempt has been made to integrate the field characteristics with the newer petrogenetic models for massif-type anorthosite, leaving some uncertainty in regards to how viscous plagioclase-phyric parental magmas could have crystallized under apparently dynamic conditions.
As such, the following discussion on parental magmas and magmatic processes may be relevant to massif-type anorthosite in general.
Parental Magmas: Crystal to Melt Ratio and Rheology