The Ilha Anchieta Quartz Monzonite: the southernmost expression of ca. 500 Ma post-collisional magmatism in the Ribeira Belt

Abstract

The Ilha Anchieta Quartz Monzonite (IAQM) occupies most of the homonymous island in the coast of the state of São Paulo, and is intrusive into foliated rocks of the ∼565 Ma Ubatuba Charnockite. The main petrographic variety is a porphyritic biotite-hornblende quartz monzonite with 2-4 cm tabular microcline megacrysts set in a medium-grained groundmass and magmatic foliation. Outcrop-scale structures indicate cumulative processes (modal and grain-size magmatic banding) and interaction with basic magmas (mafic microgranular enclaves). Lithogeochemical data indicates that the main variety is intermediate to acid (SiO 2 = 63-67%), alkali-calcic, metaluminous and magnesian (mg# ∼30), showing moderate Sr (300-400 ppm) and Ba (∼1500 ppm) contents and relatively high HFSE (Nb = 40 ppm; Zr = 550-700 ppm). The older charnockites are more silicic (SiO 2 = 71-78%), ferroan (mg# = 12-16), and have very low Sr (13-80 ppm) contents, resulting in Ba/Sr ratios remarkably higher than the IAQM (10 versus 4). LA-MC-ICPMS U-Pb zircon dating of the IAQM yielded 499.7 ± 5.9 Ma. This is the youngest magmatic age identified so far in the crystalline basement of the state of São Paulo, and indicates that the pluton is the southernmost expression of the post-collisional "G5" magmatism in the Ribeira Belt.

Geochemistry, Sm-Nd isotopes and SHRIMP U-Pb geochronology of the Morro do Coco Granite (RJ, Brazil): another piece of the post-collisional magmatism of the Ribeira Belt

“…Cambro-Ordovician intrusions also occur at the northern area where there is a transition between the Ribeira and the Araçuaí belts, like the Santa Angélica (513 ± 8 Ma; Söllner et al 2000), Mimoso do Sul and Buarama (495 ± 5 Ma and 480 ± 4 Ma, respectively; Söllner et al 2000) and Várzea Alegre (499 ± 5 Ma; Mendes et al 2005) bimodal intrusive bodies. Post-collisional granitic intrusions have also been identified at the southern Ribeira Belt, such as the Guarujá-Santos granites (497 ± 7 Ma;Janasi et al 2015) and the Anchieta Island (500 ± 6 Ma; Azevedo Sobrinho et al 2011).…”

“…Post-collisional intrusions in Southern Ribeira Belt (e.g., Anchieta Island quartz-monzonite; Azevedo Sobrinho et al 2011) and in Northern Ribeira, and Southern Araçuaí belts transition zone, Espírito Santo State (e.g., Mimoso do Sul, Santa Angélica, Castelo, Pedra Azul and Várzea Alegre) are Mendes et al (2005) SHRIMP: sensitive high resolution ion microprobe; LA-ICPMS: laser ablation inductively coupled plasma mass spectromete; ID-TIMS: thermal ionization mass spectrometry isotopic dilution:…”

“…interpreted to result from the interaction between mantle-derived magmas and partial melting of the lower crust (De , Azevedo Sobrinho et al 2011.…”