MELT INFILTRATION TEXTURES IN MIGMATITES OF THE LITTLE COTTONWOOD CONTACT AUREOLE, UTAH, USA

Partial melt generation and migration within the highest grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole was produced by a combined muscovite/biotite dehydration reaction in the sillimanite + k-feldspar stability field. Pelites and quartzites are interlayered on a scale of few cm to tens of meter. Migmatites were locally strongly deformed during intrusion emplacement due to strain partitioning into these rheologically weak rocks. Melt extraction from the pelites resulted in restites (magnetite + cordierite + alumosilicate ± biotite) surrounded by feldspar enriched quartzite zones. Feldspar is interstitial to quartz grains with acute angles and its amount decreases with distance from the restite. This texture is the result of infiltration of partial melts into the quartzite. Larger, discrete melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins, and ductile shear zones. Transition between melt pods and quartzite is gradual. Pure quartzite gives way to quartz crystal clusters and finally individual quartz crystals, all suspended in the feldspar rich matrix of the pod. Hence infiltration of the partial melt resulted in decompaction of the quartzite. The connectivity of this feldspar network can be illustrated in 3D using computed micro X-ray tomography. Similar textures can be observed at the south side of the LC stock, where quartzite xenoliths enclosed in a dioritic magma were intensively infiltrated by melts and partially molten. Here cauliflower textures between the xenoliths and the dioritic magma indicate that both had very similar rheology. The partial melts generated in the BC formation are very rich in SiO2 (75-80 wt %). They are similar in composition to first melts produced in our hydrothermal partial melt experiments on high grade pelites cores. The experimental melts are water rich and in disequilibrium with the melting rock. We speculate, that the BC partial melts were also very water rich. They are similar to pegmatoidal melts, and hence were able to easily infiltrate the surrounding quartzites.