Recycling and hybridization of incremental episodes of magma intrusions: Pinheiro Machado Complex, Southeastern Dom Feliciano Belt, Brazil

Abstract

The Pinheiro Machado Complex extends for over 10,000 km2 as part of the Brasiliano-age Dom Feliciano Belt in Rio Grande do Sul, Brazil. In the Monte Bonito region, the complex is exposed in three quarries, which record multiple and complex magmatic structures revealing competing magmatic processes. Older magmatic rocks are eroded by younger ones, evidenced by several stages of disaggregation of magmatic xenoliths into fragments and schlieren, and by their deformation by ductile dragging. Hybridization is evidenced by mafic microgranular enclaves in granitic rocks close to contacts with diorite intrusions, and by diffuse, gradational boundaries between different magma mafic and felsic batches. Syenogranites represent the latest magma intrusion episode and recycle host mushes and rocks intruded earlier. In this process, zircons were recycled forming antecrysts. Finally, as the body evolved, some rocks underwent anatexis evidenced by patchy leucosomes with peritectic hornblende and/or titanite or magnetite, sometimes forming stictolithic or fleck texture. U–Pb SHRIMP ages obtained for zircons from an early diorite and a late-intrusive magnetite granite yielded crystallization ages of 616 ± 3 Ma and 608 ± 4 Ma, respectively. High U zircon rims yield Concordia ages around 600 Ma in both rock types, which can be related to crystallization of highly fractionated melts, late anatexis evidenced for example by fleck texture in rocks of both stages, or Pb loss. Thus, the geology and magma crystallization ages obtained for the Monte Bonito region of the Pinheiro Machado Complex indicate that batholith growth was punctuated by intervals of melt influx separated by periods of relative quiescence. The magmatic structures resulted from a variety of recycling and hybridization processes, that evolved as a function of changing rheological contrasts between intruding magmas and previously intruded batches at different stages of crystallization.