Crystal transfer from magma to wallrock during syntectonic granite emplacement

Abstract

Field, microstructural and mineral compositional evidence from the Mesoproterozoic K-feldspar megacrystic Red Granite at Oncócua Platform (southwestern Angola) is consistent with crystal transfer from magma to wallrock during syntectonic intrusion. K-feldspar megacrystic Red Granite intruded during folding of wallrock tonalite. Enclaves of the wallrock tonalite are elongated parallel to Red Granite intrusive contacts, a K-feldspar megacryst and hornblende-defined magmatic foliation, and a gneissosity in the Red Granite. Stromatic layering present in the tonalite is crosscut by the Red Granite intrusive contacts or is isoclinally folded with fold axial planes and hinges filled with Red Granite. K-feldspar megacryst clusters and curved grain boundaries (i.e. contact melting), as well as thin Red Granite fold axial planar sheets containing K-feldspar megacrysts that are typically wider than the sheets themselves, are all consistent with melt loss and crystal accumulation during solidification. The wallrock tonalite also hosts K-feldspar megacrysts and hornblende phenocrysts that are interpreted to be the same population as those in Red Granite, on the basis of their size, shape, nature of inclusions, compositions and compositional zoning. We propose that these crystals were transferred from the intrusive Red Granite magma to the wallrock tonalite via magmatic conduits that subsequently collapsed due to external stresses, leaving behind the larger crystals. The pristine preservation of intrusive relations at Oncócua Platform may mean that crystal transfer from magma to wallrock is more common in incrementally assembled granitoid plutons than previously thought.Supplementary material: Mineral chemical data are available at https://doi.org/10.6084/m9.figshare.c.5448664