Geologia e geocronologia da suíte metamórfica colorado e suas encaixantes, SE de Rondônia: implicações para a evolução mesoproterozóica do SW do cráton Amazônico

Abstract

Combined geochronological (U-Pb, 40Ar/39Ar, and Rb-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondônia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: 1) porphyritic monzogranite associated with amphibolite (bimodal magmatism), 2) interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation), 3) muscovite-garnet leucogranite, and 4) mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as undeformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0.18). The porphyritic ortogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 + 45 Ma, and 87Sr/86Sr i = 0.7040 + 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age of 1360 + 13 Ma. The 40Ar/39Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 + 3 Ma (grain 1), and 1303 + 2 Ma/1305 + 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 + 2 Ma. Hornblende from two amphibolite samples yielded 40Ar/39Ar plateau ages of 1313 + 4 Ma; 1313 + 6 Ma and 1312 + 3 Ma (RO-18), and 1325 + 3 Ma; 1326 + 2 Ma; 1330 + 3 Ma (RO-19). The weighted-mean age is 1319 + 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondônia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilândia Terrane (1215 - 1110 Ma). However, the unequivocal characterization of an orogeny refered to the time interval 1.36 - 1.32 Ga is still uncertain due to scarcity of geochronological data in the region. Similar ages are reported for correlatable intrusive rocks which cut the polydeformed basement rocks of central Rondônia. Thus, these ages are compatible with available Mesoproterozoic tectonic models for the SW Amazonian Craton.