RECONNAISSANCE STUDY OF TRACE-ELEMENT ZONATION IN GARNET FROM THE CENTRAL STRUCTURAL DOMAIN, NORTHEASTERN BRAZIL - AN EXAMPLE OF POLYMETAMORPHIC GROWTH

Trace-element and major-element zoning in garnet from regionally metamorphosed rocks of the Serido Formation, Central Structural Domain, Northeastern Brazil, was measured with an ion- and electron-microprobe. The distribution of major and trace elements is different in the three garnets analyzed. Two of them (garnet SS-21 and SV-2) show a zoning profile with respect to some major elements, whereas the third garnet (SS-40) presents a fairly flat profile. Trace-element zoning is also different in these garnets and can be divided into: I) elements which show an approximately Bat profile; II) elements which show an approximately flat profile, but present a sharp step-like increase at boundary core/rim; III) elements which show a continuous increase or decrease from core to rim; and III) elements which show an irregular behavior. The discontinuities in Ti, V and Eu/Eu* profiles are interpreted as indications of overgrowth of garnet during a second episode of metamorphism. The inflections (Y and Cr) and hump (Sc) probably represent a partial re-equilibration of these elements during the second metamorphic event. The flat profiles in major elements indicate re-equilibration (diffusion) during the second metamorphism, at P/T conditions of high amphibolite facies.