Orthogneiss to migmatite: an analysis of partial melting from the Ulten Zone of NE Italy (2011)
Undergraduates: Sarah Mazza, Dr. Roberto Braga at the Universita' di Bologna, Italy
Faculty Advisor: Allen Glazner
Department: Geology
The Alps were formed by collision of the European and African plates during closure of the Mesozoic Tethyan Ocean. The Ulten Zone is a pre-Alpine tectonic unit formed of high-pressure metamorphic rocks. This study focuses on the transition from orthogneiss, a banded metamorphic rock rich in quartz and feldspar, to migmatite, a metamorphic rock formed by partial melting of orthogneiss. Partial melting drastically changes the rheology of the crust and therefore understanding it is vital for understanding the mechanics of subduction and continental collision zones.
Data collection involved measurements of mineral grain size, shape, distribution, and composition using a scanning electron microscope and electron microprobe; these measurements defined areas of crystallized melt. Small quartz grains within foliated biotite occur in all of the orthogneisses. Large plagioclase grains are present in the migmatites, consistent with the amphibolite grade of metamorphism required for melting. Film-type K-feldspar (~10um) is present in orthogneisses and probably marks channels through which melt traveled through migmatites.