Irregular igneous and structural patterns in La Chinchilla pluton (NW Argentina): a case of asymmetrical body?

MACCHIOLI GRANDE, MARCOS; ALASINO, PABLO H.; ROCHER, SEBASTIÁN; LARROVERE, MARIANO A.; DAHLQUIST, JUAN A.

General Roca

Simposio; 3° Simposio sobre Petrología Ígnea y Metalogénesis Asociada; 2015

Universidad Nacional de Río Negro

New mapping and structural studies of a magmatic body in Sierra de Velasco, NW Argentina, named La Chinchilla pluton along with previous geochemical data (Grosse et al., 2009; Dahlquist et al., 2010) suggest an asymmetrical tabular shape for this single pulse and provide strong evidence to propose a shallow emplacement level where magmatic stoping dominated (e.g. Pinotti et al., 2002; Paterson et al., 2008).La Chinchilla pluton is a Lower Carboniferous monzogranite (Grosse et al., 2009) with a simple mineral assemblage of K-feldspar, quartz, plagioclase and biotite. The pluton exhibits a textural gradual variation, ranging from a porphyritic unit in the southeastern region to an equigranular unit in the northwestern region, resulting in a textural asymmetrical pattern (cf. Salvatore et al., 2013). The host rock is Huaco pluton, a porphyritic syeno- to monzogranite, also from Carboniferous age. Felsic aplitic and aplopegmatitic dikes intrude the host rock.Structural analysis of dikes revealed the presence of subhorizontal dikes in the northwestern area, where the contacts are shallow-dipping, and subvertical dikes intruding the host rock nearby steep-dipping intrusive contacts. This is compatible with an asymmetrical tabular arrangement for La Chinchilla pluton (Vigneresse et al., 1999; Gudmundsson, 2006).Moreover, a dominant stress field is inferred based on strike and dip measures of magmatic foliations defined by alignment of K-feldspar phenocrysts in both the pluton and the host rocks, with a shortening direction oriented SW-NE, consistent with the general NW-SE strike of the body and its tabular geometry (Paterson et al., 1998; ?ák et al., 2007, 2008). Considering crystallization ages for both plutons (Huaco pluton ~ 350-358 Ma; La Chinchilla pluton ~ 345 Ma), this strain field may have been active at about 12 Ma.Field evidence such as polygonal and stepped contacts, diking and host rock displacement, supports brittle fracturing as the dominant deformation mechanism, and thus suggests that magmatic stoping dominated during the late stage of the evolution of the chamber at upper-crustal levels.