Studying the kinematics of magma emplacement in fabric-weak, epizonal plutons: The example of the Sauce Guacho and Santa Rosa plutons, eastern Sierras Pampeanas, Argentina

Abstract

This work presents emplacement kinematics for two plutons emplaced in a crustal level dominated by brittle conditions as a useful methodology in the study of shallow, small intrusive bodies where the field evidence of deformation is scarce. The study was carried out in the Sauce Guacho and Santa Rosa plutons, both located on NO sector of Sierra de Ancasti. They are zoned syeno-monzogranitic, strongly peraluminous intrusive bodies representing typical Carboniferous intracontinental magmatism in the Eastern Sierras Pampeanas of Argentina. Both plutons present ellipsoidal external shapes, and sharp and discordant contacts with the host rocks suggesting an emplacement with high rheological contrast between the intrusive magma and the metamorphic host rock. The geophysical study highlights an elongated, arcuate gravimetric low that coincides and is aligned with the long axis of both plutons and with a zone characterized by brittle structures. Regarding structural measurements, the main penetrative structure of the host rocks (Ancasti Formation) shows an averaged NNW-SSE orientation and presents a slight but significant variation close to the plutons. Magmatic fabrics were observed in the porphyritic units (K-feldspar megacrysts) that constitute part of the igneous bodies. All these pieces of evidence suggest a possible structural control during the emplacement of the igneous bodies and constitute the data used in the structural analysis that tests five different conceptual models simulating the shear zone influence for the emplacement of the plutons. According to our kinematic analysis, the distinct structural features and external shapes found in the Sauce Guacho and Santa Rosa plutons and their metamorphic envelope were imposed by the curved deformation zone. Thus, magma ascent and emplacement were probably assisted by the nucleation and growth of a dextral transtensional structure, with a predominance of simple shear (Wk ≫ 0.81) in its southern, NNE-SSW-striking segment (Santa Rosa pluton area), and with a predominance of pure shear (0.6 > Wk > 0.1) in its northern, NE-SW striking segment (Sauce Guacho pluton area). In addition to a feasible emplacement model, our structural analysis shed light on the barely known Carboniferous tectonic scenario in this intracontinental setting of SW Gondwana. The common divergence angle estimated for both brittle-ductile shear zone segments is oriented 157°±9°. Therefore, this structure could have acted as a dextral transtensional structure with predominance of the coaxial component (extension) in NE-SW segments and dominated by simple shear in oblique NNE-SSW branches, interpreted as R-type fractures.