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

The southern Brasilia Orogen occurs along the southernmost border of the Sao 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-Guaxupe 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 similar to 625 Ma, when large volumes of high-K diatexitic rocks were formed. Charnockitic and mangeritic magmatic rocks (Sao Jose 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 degrees C). Granitic diatexites (Pinhal-type) were also formed, and zircon thermometry indicates T >= 850 degrees 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 670 Ma cores. The high-luminescence grains show 19 concordant ages at 621 16 Ma. The second typology comprises isometric or soccer-ball-type grains. These grains show sector zoning and a concordia age of 608 +/- 4 Ma. 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 epsilon Hf 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 (similar to 625 Ma), whereas the isometric ones provide evidence of long-lived high-T conditions until similar to 610 Ma. (C) 2014 Elsevier B.V. All rights reserved.