RAPAKIVI TEXTURE FORMATION VIA PLAGIOCLASE MUSH ARMORING: EVIDENCE FROM THE ORESTES MELT ZONE, ANTARCTICA

The formation of rapakivi texture, that is plagioclase mantles on K-feldspar cores, has been the subject of numerous studies. Consequently, numerous mechanisms for their origin have been proposed (e.g. exsolution, sub-adiabatic ascent, magma-mixing, etc.). Ultimately, the question of formation reduces to “growth outwards” (i.e. epitaxial growth of plagioclase on K-feldspar) or “growth inwards” (i.e. plagioclase replacement of K-feldspar). However, it may not be a question of either/or, but rather the mechanism of formation could be a bit of both. Here, we present results of LA-ICP-MS analyses of rapakivi feldspars from the Orestes Melt Zone of Antarctica. The parameters of the system are well-constrained, and unlike classical rapakivi granites, the comparatively small scale of the Orestes Melt Zone ensures that much of the original textural and chemical signatures are preserved. Major and trace element analyses were performed on plagioclase phenocrysts, rapakivi feldspars, and granophyre. Preliminary results are not entirely consistent with either a pure growth-outward or pure growth-inward model. A key finding is that the chemical source for the plagioclase mantles is distinct from that of the interior K-feldspar core and the exterior granophyre. The chemical signature of the plagioclase mantles on the K-feldspar cores is more akin to the separate plagioclase phenocrysts. Towards the core-mantle contact the chemical signature of the plagioclase mantle takes on more of a K-feldspar core trace element signature; suggesting a mixing relationship. We conceptualize these results as the product of reactive-wetting by a creeping plagioclase mush. During high-temperature melting, plagioclase out of equilibrium with the melt will begin to partially melt, forming essentially a plagioclase mush. In this model, the mush behaves rheologically as a fluid and begins to mechanically enclose the K-feldspar cores. This armoring effect can be thought of as a growth-outward mechanism. As the plagioclase mush comes into contact with K-feldspar, a peritectic-like reaction ensues. This diffusive reaction is a growth-inward mechanism.