Geochronological data from TTG-type rock associations of the Arroio dos Ratos Complex and implications for crustal evolution of southernmost Brazil in Paleoproterozoic times

摘要

U-Pb isotope analyses by LA-MC-ICPMS (Laser Ablation - Multi Collector - Inductively Coupled Plasma Mass Spectrometry) in zircon crystals from metatonalites, tonalites and granodiorite gneiss from the Arroio dos Ratos Complex (ARC) early magmatism in southernmost Brazil are presented. The ARC is located in the eastern portion of the Sul-rio-grandense Shield, occurring as septa and roof pendants on granitoids emplaced along the Southern Brazilian Shear Belt (SBSB). The SBSB corresponds to a trans-lithospheric structure composed of several anastomosed shear zones of dominantly transcurrent kinematics whose syntectonic magmatism, of Neoproterozoic age, is characteristic of post-collisional environments. The studied rocks comprise TTG-type associations with coeval mafic magmatism, deformed and metamorphosed within a ductile shear zone. Zircon crystals obtained from six samples are interpreted as igneous given that the crystals are subhedral to euhedral, bipyramidal, with concentric zonation, have ratios Th/U between 0.13 and 0.81 and have restricted evidence of overgrowth. The oldest Association 1 (A1) has structures compatible with recrystallization under conditions of high temperature and an igneous age of 2148 ± 33 Ma, obtained in a metatonalite. The rocks of Association 2 (A2) have similar compositions, although with a more significant coeval mafic fraction. They are intrusive into A1 and also show high-temperature recrystallization features. However, they are less deformed and partly preserve their primary, igneous fabric. The igneous ages obtained from two A2 tonalites are 2150 ± 28 Ma and 2136 ± 27 Ma. Association 3 (A3) is represented by tonalitic to granodioritic gneisses whose structure, composition and metamorphic features are similar to those of A1 rocks, except for the absence of coeval mafic magmas in the former. Local features resulting from partial melting are present in A3 rocks. Three samples from A3 were dated. A tonalitic gneiss gives igneous age of 2099 ± 10 Ma and two granodioritic gneisses give igneous ages of 2081 ± 7 Ma and 2077 ± 13 Ma. Restricted to A1, inheritance is represented by one subhedral, zoned, gently rounded zircon crystal interpreted as igneous, of 2732 ± 40 Ma (~(207)Pb/~(206)Pb age), with discordance of 9% and ~(232)Th/~(238)U ratio of 1.17. A single Neo-proteozoic metamorphic date value was obtained from the rim of a zircon crystal of Paleoproterozoic core. The age of 635 ± 6 Ma (~(207)Pb/~(206)Pb age), with Th/U ratio ＜ 0.1 and 1% discordance, is interpreted as compatible with adjacent SBSB magmatism. The three associations are interpreted to represent the record of successive magmatic pulses that mark the evolution of a Paleoproterozoic continental magmatic arc. In the study area, these magmatic arc associations represent relict areas partly reworked and relatively well-preserved from Neoproterozoic tectono-magmatic post-collisional events during the construction of the Southern Brazilian Shear Belt.