| Abstrakt: |
The meta-igneous rocks in the Pianco-Alto Brigida foldbelt, Borborema Province (northeastern Brazil) comprises, in the studied area, greenschists, amphibolite, and felsic gneisses derived from basalt to rhyolite precursors. In chemical terms, the mafic rocks are divided into three subgroups. Group-A metabasites have MgO > 6% and TiO2 > 1.8%. All samples exhibit within-plate chemical characteristics, such as high Zr/Y (6 to 8) and enriched incompatible elements. They have a sloping REE-normalized pattern [(La/Yb)N = 8 to 20]. Group-B metabasic rocks have low TiO2 (< 1.8%) and low Nb/Y and Zr/Y ratios (0.05 to 0.5 and 2.5 to 4.0, respectively), which place them within the mid-oceanic ridge basalt field, with N-MORB characteristics. They show a less differentiated REE-normalized pattern [(La/Y)N = 0.5] than Group-A metabasites. Group-C metabasites have a Nb/Y ratio that is intermediate between Groups A and B, and lower Ti and Nb contents. The metafelsic rocks of Group D show a highly differentiated REE-normalized pattern, with (La/Yb)N varying from 4 to 200, and a pronounced negative Eu anomaly. Tectonically the meta-igneous rocks of the studied area are interpreted to be the products of the back-arc evolution of part of the Pianco-Alto Brigida foldbelt. |
