Multiple fabrics in the Tavares pluton (NE Brazil): Insights on the interplay between emplacement mechanism, internal magmatic processes, and tectonic deformation

Abstract

Internal dynamic processes and contemporaneous deformation may produce multiple magmatic fabrics and structures during the crystallization of granitic plutons. Here we report the case of the Tavares pluton (Borborema Province, northeastern Brazil), a ∼160 km2 NE-SW elongate pluton bounded in the north and southeast by dextral and sinistral shear zones, respectively. A remarkable feature of this pluton is the occurrence of numerous elliptical flow-related structures delineated by mafic schlieren resulting from the interaction of dioritic and granitic melts. We analyzed airborne geophysical data, conducted a field and anisotropy of magnetic susceptibility (AMS) study at the pluton scale, and detailed the magnetic fabric of two elliptical structures. The pluton has a well-developed magmatic foliation defined by the shape preferred orientation of K-feldspar megacrysts and planar concentrations of mafic minerals. The foliation trajectories are concentric and inwardly dipping, indicating a lopolith shape for the pluton. NE-trending magmatic shear zones and faults attest to sinistral shearing before complete crystallization. The dominant AMS fabric at pluton scale, represented by NE-to NNE-trending magnetic foliations with steep dips associated with lineations of moderate to shallow plunges, is consistent with the sinistral strain regime. In contrast, the magnetic foliation and lineation in the two elliptical structures dip moderately to the east, with the magnetic foliation commonly transecting the orientation of the mafic schlieren. We relate this latter fabric to regional contraction, characterized by top-to-the-west tectonic transport in the country rocks, during pluton construction. We propose that (i) tectonic overpressure due to regional compression assisted progressive pluton inflation by floor subsidence, generating the concentric magmatic fabric, and (ii) a small component of noncoaxial shear during bulk contraction led to sinistral shearing in the late stages of crystallization. We found no evidence for shear zone-controlled pluton emplacement. Instead, (i) the SE-bounding sinistral shear zone is restricted to the pluton vicinity, suggesting nucleation due to thermal softening of country rocks by the pluton, and (ii) magmatic foliations and lineations that could be related to the N-bounding dextral shear zone are absent. Also, the available geochronological data suggest that emplacement of the Tavares pluton predated by several tens of millions of years the intrusion of well-documented syn-transcurrence plutons in the Borborema Province. Therefore, the spatial association of the Tavares pluton with strike-slip shear zones is not strong enough evidence to assume a shear zone control on pluton emplacement.