Metasomatic reactions triggered by localized and dynamically evolving fluid flux record multistage intrusion history: An example from the syntectonic Caçapava do Sul Granitic Complex, Southern Brazil

In this study, we report field, microstructural and geochronological evidence for localized and dynamically evolving fluid flux associated with the syntectonic intrusion of the Caçapava do Sul Granitic Complex into the dolomitic marbles of the Passo Feio Metamorphic Complex (Dom Feliciano Belt, southern Brazil). The marbles were intruded by mafic intrusions between 600 and 580 Ma followed by felsic intrusions from 578 to 557 Ma, triggering a set of metasomatic reactions in the host and intrusive rocks. Skarns and hydrothermal veins were formed at different stages of pluton assembly and represent useful indicators of the pathways used by the magmatic fluids when infiltrating the dolomitic marbles.

The first metasomatic stage resulted in the formation of diopside and forsterite skarns from fluids released from felsic apophyses. Fluid pathways, recorded by reaction textures of the skarns, were highly localized along pre-existing areas of high permeability such as lithological boundaries, foliation planes as well as in fold hinges and axial planes. Fluid flux in this first stage must have occurred at high temperatures (590 to 630 °C) in which the changes of the skarn assemblages were controlled by either variable fluid fluxes or constant flux and variable chemical properties of the fluid phase (e.g., silica activity, XCO2). Zircon Usingle bondPb age of c. 578 Ma obtained for a felsic apophysis directly associated to forsterite skarns defines the age of the first metasomatic stage.

The second metasomatic stage is mainly characterized by serpentinization of previous skarn silicate assemblages, chloritization of mafic and felsic apophyses and formation of calcite-chlorite-sulfide veins and breccias at ∼300 °C. The fluid flow regime at this stage was controlled by the porosity and permeability created by fracturing related to the cooling of the granitic complex. Metasomatic apatite and titanite from mafic apophyses record consistent Usingle bondPb ages of c. 557 Ma that mark the second metasomatic stage associated with the thermal resetting effect caused by late felsic intrusions and subsequent cooling of the Caçapava do Sul Granitic Complex. The intrusive record of the magmatic complex lasts for at least 21 My.

Our study shows that metasomatic reactions commonly observed in the host rocks and apophyses of a magmatic complex provide useful insights on timing of the pluton assembly, deformation and associated fluid activity.