Protracted deformation during cooling of the Paleoproterozoic arc system as constrained by 40Ar/39Ar ages of muscovite from brittle faults: the Transamazonan Bacajá Terrane, Brazil

ABSTRACT: In the Paleoproterozoic Transamazonas Province, synkinematic granitogenesis has taken place synchronously with compressive tectonic stress. The synkinematic character of the granites is marked by their WNW elongate shape, and by the presence of pervasive and concordant synmagmatic foliation. Ductile shear zones are concordant to the previous regional WNW structures, and tend to be accommodated along contacts between Rhyacian synkinematic granitoids and both Archean orthogneisses and Siderian metabasites. Locally phyllonitic shear zones and brittle-ductile shear zones with cataclasites are oriented subparallel to the preexisting ductile foliation. Late orogenic brittle faults N30E-trending strike-slip faults are either sinistral or destral. 40Ar/39Ar dating of muscovite developed on fault planes gave ages of 1977 ± 8 Ma and 1968 ± 11 Ma. Structural and geochronological data from rocks of the Transamazonas Province permit to conclude that most mylonites and brittle structures were controlled by preexisting structures such as geological contacts and petrographic facies boundaries. Compressive tectonic stress would have initiated at ca. 2100 Ma, since the former magmatic arc (Bacajaí complex), still present at 2070 Ma when syntectonic granites were emplaced and remained until 1975 Ma after granite plutonism and regional cooling.

摘要：在古元古代（Paleoproterozoic）跨亚马孙地块（Transamazonas Province）内，同构造花岗岩作用（synkinematic granitogenesis）与挤压构造应力同步发生。花岗岩的同构造特征以其西西北（WNW）向拉长形态，以及广泛发育且与区域构造协调的同岩浆面理（synmagmatic foliation）为标志。韧性剪切带（ductile shear zones）与早期区域西西北向构造产状一致，且多沿瑞伊克纪（Rhyacian）同构造花岗岩体与太古宙（Archean）正片麻岩、锡德纪（Siderian）变基性岩的接触带发育。局部可见千糜岩剪切带（phyllonitic shear zones）及赋存碎裂岩（cataclasites）的脆韧性剪切带（brittle-ductile shear zones），其走向近平行于先存韧性面理（preexisting ductile foliation）。造山晚期的北东30°（N30E）走向走滑脆性断层兼具左旋（sinistral）与右旋（destral）性质。对断层面上产出的白云母（muscovite）开展⁴⁰Ar/³⁹Ar测年（40Ar/39Ar dating），获得年龄为1977 ± 8 Ma与1968 ± 11 Ma。基于跨亚马孙地块岩石的构造与年代学数据，可得出结论：绝大多数糜棱岩（mylonites）及脆性构造均受先存构造（如地质接触带与岩相边界（petrographic facies boundaries））控制。挤压构造应力大约于2100 Ma开始发育，当时仍存留的古岩浆弧（magmatic arc）——巴卡雅伊杂岩（Bacajaí complex）——在2070 Ma时仍有同构造花岗岩侵位，并在深成岩浆作用（plutonism）与区域冷却过程后持续至1975 Ma。