The Campinas Jaguariúna Sill, NE Paraná Continental Flood Basalt Province, Brazil: insights on the mechanisms of emplacement and differentiation from geochemical and magnetic data

Abstract

The Paraná Continental Flood Basalt Province (PCFB), one of the largest magmatic provinces in Planet Earth, is composed of lava flows, dike swarms and sills that occupy thousands of kilometers across central-eastern South America. The Campinas-Jaguariúna Sill (CJS) is a large (<50 m thick and >100 km 2 ) diabase intrusion showing frequent medium to coarse-grained gabbroic segregations and was emplaced at the interface between the Precambrian basement and sedimentary rocks in the northeastern Paraná Basin. This paper applies aeromagnetic and geochemical modeling to help understand the emplacement mechanisms and petrological evolution of the CJS. Magnetic field filtering techniques demonstrates that the CJS continues for tens of kilometer in the subsurface; analogous features are shown to be associated with other neighbor diabase occurrences, creating lobe-shaped structures that seem to freeze their original geometry and are not fully exposed by the current erosion levels. Petrological modeling indicates that the parental magma of the CJS was emplaced at ~1 kbar, 1000 ppm H 2O and oxygen fugacity near FMQ-1. The gabbroic segregations, exposed as cm-to dm-sized sheets and eyeshaped lenses, are more evolved than the host diabase and can be shown by geochemical modeling to correspond to residual melts formed by crystal fractionation in a closed-system. The parent magma of the CJS is akin to the more primitive Paranapanema magma-type of the PCFB and we tested the possibility that sills intruding upper sedimentary layers may be part of a single sill complex. For this purpose we used geochemical data from the well-studied Limeira Sill, which is akin to the more evolved Pitanga magma-type. Our modeling shows that under crystal fractionation in the representative system condition a CJS starting compositions cannot generate the Limeira rocks.