Anisotropy of magnetic susceptibility, gravimetry and geochronology of the Cerne granite, Ribeira Belt, southern Brazil

Abstract

The Cerne granite is an elongated pluton in the NE-SW direction, parallel with the Ribeira Belt trend. This Neoproterozoic massif crosscuts supracrustal metamorphic rocks and has been considered late to post orogenic in the Ribeira belt evolution (Apiaí domain). The Lancinha and Morro Agudo shear zones are the major tectonic structures that frame the studied area. The southern border of the pluton is deformed by the dextral Cerne brittle-plastic shear zone, which is considered a Lancinha shear zone synthetic component. The intrusion consists mainly of fine-to medium-grained alkali feldspar granites and quartz alkali feldspar syenites. Conversely to its elongated shape, striking N20E, the rocks display isotropic structure. Analyzing field data, and integrating with petrography, gravimetry profiles, anisotropy of magnetic susceptibility (AMS) and geochronology techniques, we sought to contribute to understand the architecture of the Cerne granite and its emplacement mechanisms. The rocks display isotropic structure and very locally an incipient preferred orientation of biotite crystals. Quartz grains do not show preferred orientation but undulose extinction, and bulging-related recrystallization features are more commonly observed on low-temperature mylonites located close to the southern border of the pluton. The Cerne granite gravimetric data show two roots of two inverted teardrop-shaped plutons. ASM fabric is mostly controlled by coarse-grained magnetite, with a mean susceptibility of 7.15 mSI. The magnetic foliation (K3) shows a concentric arrangement shallowly dipping and the magnetic lineation (K1) displays subhorizontal plunging. AMS ellipsoids varies from neutral to oblate (T ≥ 0) in 75% of the sites. Concentric magnetic foliations and lineations seem to follow the plutons boundaries and reflect mostly magma chamber dynamics. The set of data suggests that the Cerne magma emplacement occurred in a place of local distension stresses associated with sinistral strike-slip system. Deep-seated tension gash probably controlled the Cerne granite ascent. SHRIMP U–Pb zircon dating provided an age of 614 ± 8 Ma attributed to the emplacement and crystallization of the granite.