Geochemistry and U-Pb (SHRIMP) geochronology of Grupelli Granite: New-constraints on the cessation of felsic magmatism in the Pelotas Batholith, Dom Feliciano Belt

摘要

The Pelotas Batholith forms the eastern portion of the Neoprotemzoic Dom Feliciano Belt and records the final stages of the Neopmterozoic Brazilian-Pan-African orogeny. This granite-dominated belt experienced a complex tectonomagmatic evolution, yet constraints on the nature, and especially, the timing of the different magmatic stages are still scarce. This study provides new geochemical and geochmnological constraints on the Grupelli Granite, a post-deformational pluton located in the rural district of Quilombo, Pelotas, RS. This granite crops out as a series of sub-kilometric solid bodies along a NE-oriented fault zone, intruding the Pinheim Machado Complex that dominates the central and southeastern portion of the Pelotas Batholith. The studied granite is widely used as an ornamental rock due to its extremely leucocratic and isotropic appearance. The Grupelli Granite is homogeneous, relatively undeformed, inequigranular and typically medium-to coarse-grained (1-5 mm), with porphyritic and hypidiomorphic textures. Based on modal mineralogy the rock is a monzogranite with 40-50% quartz, 19-25% K-feldspar, and 15% oligoclase, 3-5% biotite, 2-3% magnetite and ilmenite, and accessory chlorite, zircon, apatite, chalcopyrite, and pyrite. Geochemically, the Grupelli Granite is characterized by high SiO2 (74-77 wt%) and K2O (5.08-5.65 wt%) contents combined with low FeOt (1.71-1.93 wt%), MgO and TiO2 (0.2 wt%) concentrations. This highly evolved granite has a high-K calc-alkaline affinity, is weakly peraluminous, with I-type fingerprints. The combined data are consistent with formation in a post-tectonic environment by anatexis of mainly calk-alkaline sources, with a potential minor contribution of a metapelitic source. The low Sr-contents and concomitant high Ba/Sr ratios likely reflect plagioclase fractionation, while ilmenite fractionation may account for the pronounced negative Nb, Ta, and Ti anomalies. Although the rare earth element (REE) and other trace element patterns tend to parallel the patterns in the ca. 600 Ma Dom Feliciano Suite and the Cerro Grande Suite magmatism, the Grupelli Granite presents significantly lower heavy REE (HREE) concentrations and a smaller negative Eu anomaly. The HREE depletion can be attributed to retention of garnet in the source. These geochemical characteristics distinguish it from the earlier magmatic pulses in the Pelotas Batholith. The Grupelli Granite yielded a crystallization age of 578 +/- 3.9 Ma (U-Pb zircon SHRIMP), and is thus younger than the ca. 600 Ma Dom Feliciano and Cerro Grande suites. This crystallization age places new constraints on the felsic post-collisional magmatism during the final stages of the evolution of Pelotas Batholith.