Evolução petrológica e estrutural das rochas metabásicas e granitóides da Serra Leste, Província Mineral de Carajás

Abstract

In the Serra Leste, located to the northeast of Curionópolis village, next to the Serra Pelada, one has identified mafic/ultramafic rocks belonging to the Luanga Complex, metavulcano-sedimentary rocks, metagabbros and deformed granitoids. Quartzites, banded iron formations and micaschists are associated with metabasalts. Post-Archaean diabase and gabbro dykes occur subordinately. The metabasalts display subophitic texture, partially modified subophitic texture and nematoblastic/granoblastic texture. These rocks underwent three phases of metamorphism. The former one (M0) is represented by hydrothermal alteration developed in seawater environment under greenschist conditions. The second phase (M1) has thermal nature and developed in response to the emplacement of 2.85 Ga granitoids. Deformation related to the granite emplacement and thermal metamorphism produced a localized foliation (N40ºE). The third metamorphic phase (M2) is marked by the thermal effects of hornblende hornfels facies, promoted by the 2.76 Ga granites. Locally one can describe a foliation (S2) that was developed synchronously to the M2 phase. Metabasites from the Serra Leste area have subalkaline signature comparable to that of the iron-rich tholeites. The chemical characteristics of these metabasalts are similar to those of the greenstones from the Munro Township (Canada). Trace elements behavior permits tentatively to compare these rocks to the basalts from island arc or oceanic crust. Biotite-quartz diorites, biotite trondhjemites, hornblende-biotite trondhjemites, biotite granodiorites and biotite monzogranites are identified in the Serra Leste area. These rocks have medium-K calc-alkaline signature, comparable to the I-type and CA-2 rocks from volcanic arcs. The adopted petrogenetic model to explain the origin of these magmas is the partial melting of metabasalts in a subduction zone context. Modification of magmas composition could be explained by some degrees of fractioning crystallization of feldspar, amphibole and minor pyroxene. These granitoids show layering and preferred orientation of minerals, both developed during the magma emplacement and crystallization. The presence of metabasite xenoliths in the 2.85 Ga granitoids as well as the thermal effects promoted by the granitoids on the metabasites lead to conclude that the supracrustal rocks are older than 2.85 Ga.