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 metam orphosed rocks of the Serido Formation, Central Structural Domain, Nor theastern Brazil, was measured with an ion- and electron-microprobe. T he 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 pr ofile with respect to some major elements, whereas the third garnet (S S-40) presents a fairly flat profile. Trace-element zoning is also dif ferent in these garnets and can be divided into: I) elements which sho w an approximately Bat profile; II) elements which show an approximate ly flat profile, but present a sharp step-like increase at boundary co re/rim; III) elements which show a continuous increase or decrease fro m core to rim; and III) elements which show an irregular behavior. The discontinuities in Ti, V and Eu/Eu profiles are interpreted as indic ations of overgrowth of garnet during a second episode of metamorphism . The inflections (Y and Cr) and hump (Sc) probably represent a partia l re-equilibration of these elements during the second metamorphic eve nt. The flat profiles in major elements indicate re-equilibration (dif fusion) during the second metamorphism, at P/T conditions of high amph ibolite facies.