A Clinopyroxene-Orthopyroxene-Plagioclase Symplectite Formed by Garnet Breakdown in Granulite Facies, Guaxupé, Minas Gerais, Brazil

In mafic granulites, garnet can form by reactions such as Opx + Pl = Cpx + Grt + Qtz; Opx + Pl = Grt + Qtz. As a result of isothermal decompression (ITD), garnet can then break down to a characteristic orthopyroxene-plagioclase symplectite. Mafic, iron-rich garnet-pyroxene granulite from the Guaxupé Massif has symplectite that formed by near-isothermal decompression, as a consequence of uplift of the granulite facies terrane. This symplectite was found to consist of vermicular clinopyroxene-orthopyroxene-plagioclase, with clinopyroxene clearly growing from the garnet that is breaking down, modal amounts of clinopyroxene being less than orthopyroxene. Electron probe analyses show clear differences between core (Cpx1), rim, and symplectite clinopyroxene (Cpx2). Considering also the presence of magnetite in the symplectite texture, garnet breakdown is thought to be better represented by a reaction such as Cpx1 + Grt + O2 = Cpx2 + Opx + Pl +Mt + Qtz.