The role of xenoliths and flow segregation in the genesis and evolution of the Paleoproterozoic Itapema Granite, a crustally derived magma of shoshonitic affinity from southern Brazil

Abstract

The Paleoproterozoic Itapema Granite (IG) is a large granitic body that intrudes orthogneisses and amphibolites, altogether representing part of the basement for Neoproterozoic granitic magmatism in southern Brazil. It is composed of fine- to medium-grained hornblende-biotite monzogranites to granodiorites that host abundant country-rock xenoliths and irregularly distributed mafic aggregates. Interaction between magmatic flow and low flow-velocity zones related to xenolith-rich portions resulted in flow segregation and mineral fractionation as the main mechanisms that led to compositional and textural diversity, expressed also in geochemical trends. The geometry and distribution of flow foliation suggest that the Itapema Granite is a subhorizontal, sheet-like intrusion and that its emplacement was controlled by thrusting under upper-amphibolite facies conditions. Many geochemical characteristics of the Itapema Granite are similar to those of shoshonitic granitoids. However, lower alkali contents, lack of basic or intermediate associated rocks and intimate temporal association with high-grade gneisses and migmatites suggest that it was crustally derived. Its origin is compatible with melting of amphibolitic orthogneisses derived from continental tholeiitic or shoshonitic sequences under high geothermal gradient conditions, where temperatures reached at least 850 °C.