Mode of emplacement of shallow igneous bodies in the Malargüe fold and thrust belt, Mendoza

Abstract

IT is considered that the rheological contrast between strata and tabular igneous bodies exert an important control on the emplacement and shape of magmatic bodies (Menand, 2011; Thomson and Schofield, 2008). Field observations and laboratory experiments regarding the location of sills and laccoliths indicate that some of these are formed when the feed channel reaches a layer whose rigidity does not allow to progress, thereby causing the lateral spread of magma (Kavanagh et al., 2006). There are several publications studying emplacement models of these bodies in compressive environments (Galland et al., 2007; Montanari et al., 2010; Menand, 2011; Ferre et al., 2012; Walker et al., 2016) which reveal that the bodies are constructed from amalgamation of successive pulses of tabular bodies, that are emplaced through fault systems under compressive stresses.The Malargüe FTB is a suitable place to study the mode of emplacement of neogene shallow intrusives likes sills, dykes and laccoliths. Figure 1 shows the distribution of the main bodies exposed in our study area.In the NE area (Fig. 1) the Laguna Amarga body and other shallow intrusive rocks are emplaced over a wide area coincident with a backthrusts system affecting basement rocks located in the hanging wall of the Carrizalito thrust (Turienzo et al., 2012). This relationship was also observed in seismic lines and exploration wells where it is possible to observed sills intruded in Late Jurassic-Early Cretaceous rocks at the footwall of the thin-skinned Sosneado and Mesón thrusts. This implies that the feed channels of intrusive bodies must be deeper than those thrusts. Therefore we consider that the emplacement of the Laguna Amarga laccolithic body may be associated with the underneath backthrust system generated in the hangingwall of the thick-skinned Carrizalito thrust (Araujo et al. 2013, 2019).To the south of the Atuel river (Fig. 1) the Cerro Chivato body seems to be emplaced benefitting from the weakness of the Auquilco Formation gypsum. In its eastern edge the body shows structural complexity in the contact with La Manga and Lotena Formations, disrupting and folding the strata. In the western side is possible to recognize the Tordillo Formation clearly resting on the intrusive in a concordantmanner. We interpret that this intrusive could havebeen emplaced through a north-south thrust fault within the Auquilco gypsum. This fault can be observed within the strata located of the intrusive, verging to the east.