U-Pb geochronology and Lu-Hf zircon isotopy of the Santinho Granitic Association: A remnant of the early magmatic stages of the Florianopolis Batholith, Santa Catarina, Brazil

摘要

One of the main characteristics of the orogenic systems formed during the assembly of southwestern Gondwana in the Late Neoproterozoic is the widespread granitic magmatism. The Florian ' opolis Batholith in South Brazil is an excellent example of this process, as it is part of a ca. 1400 km long intrusive association that is a key unit in the reconstruction of the paleogeography of Gondwana on both sides of the Atlantic. The Santinho Granitic Association in the Island of Santa Catarina is an exceptionally well-exposed section exhibiting a complex lithological variety that includes orthogneisses intruded by multiple generations of leucogranite injections and associated with mafic magmatic enclaves. We present a detailed geochronological study of this unit based on field and petrographic characterizations, which includes U-Pb LA-ICP-MS geochronology of zircon, titanite and monazite, as well as Lu-Hf isotopy in zircon. Single-spot ages in individual hand samples are widespread and determine a magmatic process that mostly extends between ca. 640 and 600 Ma, with average crystallization ages between 625 and 605 Ma that are in accordance with field relationships. Overlapping ages support the magmatic coexistence of different magma batches with contrasting compositions that reflect heterogeneous source rocks with a predominance of old continental crust, as indicated by negative epsilon Hf values (-1.08 to -23.18) and Hf TDM model ages between 2.95 and 1.57 Ga. The strong crustal affinity of the Hf signatures of mafic enclaves suggests a complex petrogenesis associated with the recycling of older magmatic phases, instead of representing a direct mixing of coeval mantellic magmatism. The new geochronological data, together with the complex magmatic and deformational history recorded in the association, suggest that it represents the early stages of granitic magmatism in the Florian ' opolis Batholith in a pre-to syn-collisional setting, preserved in the surrounding main intrusions as a mega-xenolith.