High-K calc-alkaline to A-type fissure-controlled volcano-plutonism of the Sao Felix do Xingu region, Amazonian craton, Brazil: Exclusively crustal sources or only mixed Nd model ages?

摘要

In the Sao Felix do Xingu region, southern portion of Amazonian craton, voluminous well-preserved Late Paleoproterozoic volcanic and plutonic rocks are grouped in the Sobreiro and Santa Rosa formations. The basal Sobreiro Formation (1.88 Ga) is composed of massive andesitic to rhyodacitic flows and volcani-clastic facies that are high-K and metaluminous, geochemically similar to calc-alkaline granitoids of volcanic arcs. Fractional crystallization of magnetite + augite + magnesiohastingsite (up to 20 vol. %) and zircon + plagioclase in most evolved rocks controlled the magmatic evolution of this unit, under oxidizing condition. The Santa Rosa Formation (1.87 Ga) comprises: 1) massive, bedded, and foliated rhyolitic lava flows, and large dikes of banded rhyolite and ignimbrite; 2) felsic ignimbrite associated with thin unwelded ash-fall tuff and highly rheomorphic felsic ignimbrite; 3) felsic crystal tuff, lapilli-tuff, and breccia; and 4) stocks and dikes of granitic porphyry, and subordinate equigranular granite intrusions. This unit was in part erupted along major fissures and has A-type intraplate geochemical affinity, peraluminous composition, and transitional subalkaline to alkaline characteristics. The magmatic evolution was predominantly controlled by fractional crystallization of zircon + apatite + Fe-Ti oxides + feldspars (up to 30 vol. %) with subordinate biotite, probably under intermediate oxidizing conditions. Nd model ages (3.0-2.49 Ga) for the Sobreiro Formation strongly suggest its generation by mixing of mantle-derived and anatectic melts of Archean rocks beneath the volcanic sequences in the Sao Felix do Xingu region. The Santa Rosa Formation could have been originated from several Archean crustal sources (T_(dm) 3.12-2.56 Ga), involving assimilation/contribution of juvenile mantle components. The integrated data point to a narrow transition from Andean-type subduction to a dominantly exten-sional tectonic setting for the volcanic and plutonic event of the Sao Felix do Xingu region. A stage of flattening in the subduction angle of the subducted plate during the Paleoproterozoic ocean-continent orogenesis in the Tapajds Gold Province might explain the volcanism and plutonism in the S3o Felix do Xingu region as consequence of the arc migration, resulting in the ~ 2.0-1.87 Ga geochronological and metallogenetic zoning observed in southern Amazonian craton.