High‐Grade K‐Feldspar Veining in Granulites From the Ivrea‐Verbano Zone, Northern Italy: Fluid Flow in the Lower Crust and Implications For Granulite Facies Genesis

Abstract

We present observations of extensive networks of K‐feldspar microveins associated with quartz and plagioclase in granulite facies rocks from the Ivrea‐Verbano Zone, northern Italy. Back‐scattered electron images of the granulite facies samples show a continuous system of K‐feldspar veins along quartz/plagioclase and plagioclase/plagioclase grain boundaries that can be traced across the entire length of a thin section. Quartz grain rims in contact with these veins show evidence of extensive corrosion. In contrast, contact between the veins and the orthopyroxene and garnet grains is very clean, with no signs of secondary alteration. K‐feldspar + quartz ± plagioclase, of the same compositions as the veins, are commonly found as near‐rim inclusions within garnet. Microprobe traverses from the interiors of the K‐feldspar veins to their rims show increasing Ba, indicating a metasomatic origin. Some veins display a boudinaged texture, possibly related to the D2 deformation event in the Ivrea‐Verbano Zone and therefore related to the emplacement of the mafic underplating responsible for initiating granulite facies metamorphism. The K‐feldspar also exists as patches of variable size in plagioclase (5–50% of the grain), superficially resembling exsolution antiperthite. The composition of the K‐feldspar “lamellae” in the antiperthite is close to that of the veins, especially with regard to the Ba content. Approximately half of the plagioclase grains contain little K‐feldspar. Remarkably, the K‐feldspar veins and patches diminish in both scope and size with decreasing metamorphic grade and eventually disappear once amphibolite facies is reached. We postulate that the K‐feldspar veins in the Ivrea‐Verbano Zone granulites are evidence of pervasive, high‐temperature, low‐H2O activity brines and that these brines were an important dehydration mechanism from amphibolite grade to granulite grade. Alkali‐ and volatile‐rich mafic underplating, represented today by the basal Mafic Formation in the Ivrea‐Verbano Zone, is the most likely source of the heat and fluids responsible for the genesis of these granulites. This mechanism may have application to other granulite facies terranes where similar K‐feldspar microveining has been observed.