Geologia, geoquímica e mineralogia dos corpos anfibolíticos de Água Azul do Norte: condições metamórficas e implicações tectônicas para o Domínio Sapucaia - Província Carajás

Abstract

The metamafics bodies identified in the Água Azul do Norte area, located in the south-central portion of the Carajás domain, crosscut the TTG basement and include two distinct varieties: (i) actinolite amphibolite, an extensive elongated body (~17 km long) with N-S orientation and inflection to NE, composed essentially of plagioclase and amphibole, with relicts of igneous pyroxene crystals and plagioclase, defining a subophitic texture; and (ii) diopside amphibolite, with occurrence restricted to the extreme east portion of the area, outcropping as small lenticular anastomosed bodies of NW-SE orientation. The latter shows nematoblastic and porphyroblastic textures, mylonitic foliation, and S-C pairs. The mineral paragenesis recognized in this variety include: Plg+Amph+Di+Ilm, which represent the metamorphic peak, while Plg+Amph+Ep+Clz+Tit+Ap+Qtz+Ser were generated during the retrometa-morphism. The plagioclase from the actinolite amphibolite has a broad compositional spectrum, ranging from calcic oligoclase to calcic labradorite (An28-65) with calcic compostions representing igneous inheritances. The plagioclase from the diopside amphibolite has more homogeneous composition and was classified as sodic andesine (An31-35). The amphibole from the actinolite amphibolite shows compositional zoning with Mg/Fe ratio slightly higher than those of the diopside amphibolite, and can be classified as Mg-hornblende, tschermakite, actinolite and edenite. In addition, the amphibole of the diopside amphibolite has AlVI of ~0.4 and Fe3+ of 0.7 to 0.8 contents, which allows its classification as Mg-hastingsite. Taking into account the chemical data, these bodies had a protolith with composition compatible with tholeiitic basalts, multielement standard of continental tholeiites (diopside amphibolite) and tholeiites low K (actinolite amphibolite), incompatible elements (HFSE) ratios suggest a source derived from primitive mantle, with significant changes in the magma composition due to interaction with the continental crust and/or subcontinental lithosphere. The chemical-mineralogical and textural evidences indicate that the protolith of actinolite amphibolite underwent deformation in the submagmatic stage and later a deformation in solid state at shallow depths. In contrast, the diopside amphibolite was submitted to ductile deformation regime in relatively higher depths. The metamorphic path of the actinolite amphibolite reveals isothermal decompression (metamorphic peak at 2.7 kbar and 430 °C; and retrometamorphic equilibrium at 1.2 kbar and 425 °C), associated with its exhumation and/or emplacement of leucogranite bodies, whereas the diopside amphibolite indicates amphibolite facies metamorphism in intermediate crustal level conditions (5 kbar; 540oC). These data indicate the exposure of relatively deep crustal levels in Água Azul do Norte (9-16 km).