Syn-collisional lower continental crust anatexis in the Neoproterozoic Socorro-Guaxupé Nappe System, southern Brasília Orogen, Brazil: Constraints from zircon U–Pb dating, Sr–Nd–Hf signatures and whole-rock geochemistry

Abstract

The southern Brasília Orogen occurs along the southernmost border of the São Francisco Craton. This Orogen is structured as a pile of Ediacaran syn-metamorphic thicked-skinned nappes that diachronically migrate toward the cratonic margin. A magmatic arc environment is represented by the Socorro-Guaxupé Nappe (SGN), which is a thick segment of partially molten lower to middle continental crust. The SGN comprises three main units: a lower granulitic, an intermediate diatexitic and an upper metatexitic. High-temperature metamorphic peak was reached at ∼625Ma, when large volumes of high-K diatexitic rocks were formed. Charnockitic and mangeritic magmatic rocks (São José do Rio Pardo suite) and in situ leucosomes of mafic composition are inferred to have formed under hornblende-dehydration melting conditions (i.e. T≥850°C). Granitic diatexites (Pinhal-type) were also formed, and zircon thermometry indicates T≥850°C. Elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust. TIMS zircon U–Pb ages are presented for the lower and middle units. U–Pb dating coupled to Lu–Hf analysis (LA-MC-ICP-MS) was performed on charno-enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende-dehydration melting. Two different zircon typologies were recognized. The first comprises bipyramidal-prismatic grains with oscillatory zoning and high luminescence, sometimes preserving low-luminescence 670Ma cores. The high-luminescence grains show 19 concordant ages at 621±16Ma. The second typology comprises isometric or soccer-ball-type grains. These grains show sector zoning and a concordia age of 608±4Ma. Th/U ratios from soccer-ball grains vary from 1.253 to 2.107, whereas prismatic grains vary from 0.118 to 1.774. Initial ɛHf signatures between −13 and −21 in both typologies provide evidence of a reworked crustal reservoir. The older prismatic grains may be related to the onset of metamorphic peak (∼625Ma), whereas the isometric ones provide evidence of long-lived high-T conditions until ∼610Ma.