U-Pb (LA-ICP-MS) of detrital zircon and whole rock Nd and geochemical constraints on the provenance, depositional age and tectonic setting of the metasedimentary Piriá Basin, northern Brazil: implications for the evolution of the Gurupi Belt

ABSTRACT: The Piriá Basin (Piriá Formation) is a hemi-graben shaped basin that developed over Precambrian rocks of the Gurupi Belt. The lithological content comprises four interfingered lithofacies: (1) arkose and greywacke with pelite layers, (2) laminated siltstones and pelites, (3) arkose with hummocky stratification, and (4) oligomictic conglomerate. This sequence was formed in alluvial fans (conglomerate), and fluvial systems (arkose, greywacke, siltstones and pelites) that were established and evolved during the migration of the subsidence. The sedimentary sequence underwent anquimetamorphism and very weak tectonic deformation. U-Pb analyses of detrital zircon set the maximum depositional age at 591 Ma and indicate several sediment sources, that range in age from the Neoproterozoic to the Archean. The main sources are from the Rhyacian, which is the main period of continental crust formation in the São Luís cratonic fragment and the basement of the Gurupi Belt. Neoproterozoic sources were important in the eastern segment of the basin. Ages from unknown sources in the region so far have also been recorded. Combined, the U-Pb zircon data, the whole-rock geochemical and Sm-Nd results, and the petrographic information revealed proximal felsic to intermediate provenance, in addition to recycled sedimentary sources. As a whole, our data suggest that the Piriá Formation was deposited in a post-orogenic basin related to the final stage of evolution of the Brasiliano cycle of orogenies, which built up the Gurupi Belt.

摘要：皮里亚盆地（Piriá Basin，其沉积序列对应皮里亚组（Piriá Formation））为发育于古鲁皮造山带（Gurupi Belt）前寒武纪岩石之上的半地堑（hemi-graben）型盆地。其岩性组成包含四种交错叠置的岩相：（1）夹有泥质岩层的长石砂岩（arkose）与杂砂岩（greywacke）；（2）纹层状粉砂岩与泥质岩（pelite）；（3）发育丘状层理（hummocky stratification）的长石砂岩；（4）单碎屑砾岩（oligomictic conglomerate）。该沉积序列形成于随沉降作用迁移而形成并演化的冲积扇（alluvial fans，对应砾岩相）与河流系统（fluvial systems，对应长石砂岩、杂砂岩、粉砂岩及泥质岩相）环境。这套沉积序列经历了浅变质作用（anchimetamorphism）与极微弱的构造变形。对碎屑锆石（detrital zircon）开展的U-Pb定年分析显示，其最大沉积年龄为591 Ma，且指示存在多套物源区，物源时代跨度从新元古代（Neoproterozoic）至太古代（Archean）。主要物源来自瑞阿克纪（Rhyacian）地层——该时期是圣路易斯克拉通地块（São Luís cratonic fragment）与古鲁皮造山带基底大陆地壳形成的主要时期。新元古代物源在盆地东部段占据重要地位，同时研究还记录了该区域此前未知来源的年龄数据。综合U-Pb锆石定年数据、全岩地球化学数据、Sm-Nd同位素结果以及岩石学信息，本研究揭示了除再循环沉积物源外，还存在近源长英质至中性物源。整体而言，本研究数据表明皮里亚组沉积于与古鲁皮造山带形成相关的巴西利亚造山旋回（Brasiliano cycle of orogenies）演化末期的造山后盆地（post-orogenic basin）中。