THE HIGH-GRADE UNITS OF THE IVREA ZONE IN THE OSSOLA VALLEY (PROVINCENOVARA, ITALY)

In an area of roughly 7 km(2) in the Ossola Valley some of the lowermo st crustal units of the Ivrea Zone are exposed. Virtually all rocks ar e of granulite facies grade. Metapelites and marbles, two types of bas ic and various ultramafic rocks occur. According to petrographic and s tructural criteria, the area studied has been divided into three units . These units are separated from each other by shear zones. Two large antiforms of different age are present within the area. Temperatures o f metamorphism range from 700 degrees C to 950 degrees C while the pre ssures were at 7 to 9.5 kbars. After some isobaric cooling from peak m etamorphic conditions, a decompressional phase was followed by another phase of isobaric cooling.