Leucosome distribution in migmatitic paragneisses and orthogneisses: A record of self-organized melt migration and entrapment in a heterogeneous partially-molten crust

Abstract

The thickness and spatial distribution of foliation-parallel leucosomes in metric to decametric scale interlayered units of migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica are quantified along one-dimensional transects. This study demonstrates that leucosomes in stromatic metatexite migmatites have thickness and spacing distributions consistent with being sampled from a power-law (scale-invariant) distribution. However, leucosome distribution in the paragneisses and orthogneisses yields different scaling exponents and the largest leucosomes in orthogneiss are thicker than those in the paragneiss by approximately half an order of magnitude. The difference in the spatial distribution and maximum thickness of leucosomes between the two rock types is attributed to the decimetric scale of inherited compositional layering in migmatitic paragneiss that restricted the development of larger leucosomes compared with an absence of such heterogeneity in migmatitic orthogneiss that allowed thicker leucosomes to form. Phase equilibria modeling of the protoliths of the paragneisses and orthogneisses shows that at Cretaceous peak metamorphic conditions the spectrum of metasedimentary protolith compositions could have produced 8–48 vol.% melt and the range of igneous protolith compositions could have produced 3–17 vol.% melt at the crustal depth exposed, which is generally less than the volume of leucosome at outcrop (43–72 and 39–67 vol.%, respectively, in the paragneiss and orthogneiss). This discrepancy indicates that the Fosdick complex acted as both a source of melt production and a zone of melt entrapment, whereby some of the melt derived from deeper in the crust has partially crystallized during migration to shallower levels in the crust. The observed power-law behavior of leucosomes is consistent with the hypothesis that intracrustal differentiation by anatexis and granite magmatism is scale-invariant and represents a self-organized critical system. The interaction of this critical system with the compositional layering in the paragneisses and the interlayering between paragneiss and orthogneiss accounts for the three-dimensional distribution of leucosome in stromatic metatexite migmatites.