Late carboniferous intracontinental magmatism in the northernmost Sierras Pampeanas, Argentina: The case study of the Tres Cerritos pluton

Abstract

The Tres Cerritos pluton, a composed, shallow intrusive body, represents the northernmost Late Carboniferous intracontinental magmatism emplaced in the Eastern Sierras Pampeanas of Argentina. Our geochronological study has yielded a U–Pb zircon age of 318 ± 1.7 Ma. It is located between two juxtaposed structural metamorphic domains of the Tolombón Complex in the Sierra de Quilmes that are in contact through old shear zones with N–S to NE-SW orientation. The emplacement of the Tres Cerritos pluton was highly conditioned by the high rheological constrast between the intrusive magma and metasediments in low metamorphic grade. Main magmatic structures and the external shape of the pluton domains are roughly parallel to the regional shear zones. On the contrary, the contacts of the intrusive body crosscut the principal fabrics of the metamorphic host, which show negligible deflection close to the granitic body. Therefore, the magma emplacement would have been through stoping mechanism, assisted by hydraulic forces and favored by Famatinian shear zones reactivated. The Tres Cerritos pluton is mostly composed of alkali-calcic, felsic and peraluminous granites, plotted between the magnesian and ferroan fields. Geochemical characteristics point to a clear A-type affinity, with low La/Yb (MBU: La/Yb = 28.72–21.27; MU: La/Yb = 13.58–2.41) and Sr/Y (MBU: Sr/Y = 5.40–3.86; MU: Sr/Y = 1.80–1.32) ratios. εHf isotopic values between −12 and −15, the most negative results obtained from the Carboniferous magmatism in the Eastern Sierras Pampeanas, relate the pluton to an old metasedimentary source. The comparative analysis with other intrusive bodies of the Devonian-Carboniferous silicic and alkali-calcic magmatism scattered along the Eastern Sierras Pampeanas, suggests a homogeneous and long-lasting crustal source for these granitic rocks, with a similar crustal affinity. In this regard, the contribution of juvenile basic magmas at the source decreased during the thermal, likely extensional, event that promoted the Devonian-Carboniferous intracontinental magmatic suite.