The strongly peraluminous A-type granites of the Characato suite (Achala batholith), Sierras Pampeanas, Argentina: Evidence of Devonian-Carboniferous crustal reworking

Abstract

The largest igneous body in the Sierras Pampeanas (the Achala batholith) is composed by several magmatic intrusions. How many plutons form this batholith and its intrusive history remains unclear. The emplacement of the batholith began at ca. 380 Ma; the Characato suite, in the northern extreme of the body, represents a relatively young intrusive phase in its construction (ca. 360 Ma). This suite, composed mainly of two-mica monzogranites, correspons to a strongly peraluminous A-type magma (A/CNK > 1.1) with fractionated compositions (high Rb/Sr and Rb/Ba). The parental magma corresponds to a high temperature melt generated by biotite dehydration melting, which was then differentiated to form the Characato monzogranites. These monzogranites have crustal Nd and Srisotope signatures (εNdt −6.0 to −6.1 and 87Sr/86Srt 0.7074 to 0.7131) and abundant inherited zircons (ca. 90%). The inherited ages are mainly Early Cambrian, and to a lesser extent Mesoproterozoic, Neoproterozoic and Ordovician, all they recognized in the basement of the Sierras de Córdoba. The Nd-isotope composition, added to the inherited zircon ages of the Characato suite, indicate that these magmas derived from partial melting of a heterogeneous crust composed by the S-type Pampean granites and migmatites, Famatinian granitoids, and probably the low-to medium-grade metamorphic rocks of the Puncoviscan Series. The high variability and positive εHft values of the inherited Cambrian zircons found in the Characato suite could have been transferred from the Neoproterozoic grains of the Puncaviscana Series to the Early Cambrian S-type granites and migmatites during the Pampean orogeny.