Pseudosection modeling and U-Pb geochronology on Piranga schists: role of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero, Minas Gerais, Brazil

ABSTRACT In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn- to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.

摘要 在东南部铁四角地区（Quadrilátero Ferrífero），一套原被归属于太古宙里约达斯韦拉斯超群（Archean Rio das Velhas Supergroup）的变泥质岩系在皮兰加产地出露。本研究针对片状石榴子石-十字石云母片岩开展了矿物化学及U-Pb-Hf锆石年代学研究。石榴子石与十字石标型矿物相对于主片理呈同构造至后构造产出。石榴子石变斑晶发育完好的Mg-Fe-Mn-Ca成分环带，其边部铁铝榴石（almandine）与镁铝榴石（pyrope）组分占比升高，锰铝榴石（spessartine）组分占比降低，指示进变质演化模式。变质峰期温压估算结果显示温度介于630~650℃，压力约为7 kbar。P-T伪截面（pseudosections）显示其处于角闪岩相的稳定区间内，变质轨迹呈现温压条件逐步升高的特征。本次研究确定皮兰加云母片岩的最大沉积年龄为1875±51 Ma，表明其沉积历史晚于此前报道的层位。寒武纪变质年龄反映出巴西利亚造山运动（Brasiliano Orogeny）末期构造变形作用对东南部铁四角地区的强烈影响。