Diógenes Custódio de Oliveira

Jaibaras trough: an important element in the early tectonic evolution of the Parnaíba interior sag basin, Northern Brazil

Abstract The Jaibaras Trough (JT) is a late Precambrian to Early Paleozoic basin that is related to the subsequent formation of the Paleozoic–Mesozoic Parnaiba intracratonic basin in northern Brazil. Based on available geological and geophysical data, we propose that the JT is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the NE–SW Transbrasiliano Lineament of the Brazilian–Panafrican Orogeny. The JT evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes. The preserved sedimentary column records, in a broad sense, the sedimentary and tectonic controls on the early subsidence in the Parnaiba Basin. These sequences are constituted by basal fault-scarp-related paraconglomerates followed laterally and vertically by fluvial–lacustrine sandstones interbedded with reddish shale. The JT geological evolution is also constrained by petrology of four magmatic events (Coreau Dike Swarm, Mucambo and Meruoca plutons and mafic Parapui Suite) related to continental rifting. The Late Precambrian–Early Paleozoic age of JT is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western Gondwana at that time. The contemporaneous development of a magmatic-sedimentary system, dated as Cambrian to Ordovician and confined to the rift trend, raised the thermal gradient and produced a mantle softening discontinuity, giving rise to a broad uplifted region. This might explain the close temporal and spatial association between the Transbrasiliano Lineament and the Early Paleozoic depositional axis of the Parnaiba Basin. An integrated interpretation of the JT and the early tectonostratigraphic behaviour of the Parnaiba Basin indicates that the JT might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin.

Single-crystal 40Ar-39Ar dating of pyrite: No fool's clock

Single crystals of dispersed trace pyrite from two igneous rocks were dated by using the 40 Ar- 39 Ar laser probe method, and the resulting ages were compared to step-heating ages of associated amphibole and phlogopite. The isochron ages for 0.1–0.3 mm cubes of pyrite agree well with the respective amphibole ages of these igneous samples. Preliminary step-heating analyses of single pyrite crystals yield internal isochrons with indistinguishable, although less precise, ages. Both pyrite and amphibole are significantly older than associated phlogopite in one sample, which has undergone postcrystallization alteration. These results suggest that pyrite can give reliable and precise 40 Ar- 39 Ar ages even in the presence of subsequent alteration. Given the ubiquity of pyrite in many geologic environments, this technique has great promise for application to the dating of ores, sediments, and hydrocarbon migration, all of which have been very difficult to date directly.

Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview

Atlantic margin in the northeast Brazil (South America) is characterized by a modest magmatic activity compared with the Southern Atlantic margin, where the ParanáEtendeka Province is accounted for eruption of millions of cubic kilometers of mafic and acid magmas (flow basalts, dyke swarms). The most significant magmatic activity in the Equatorial Atlantic margin is the Rio Ceará Mirim Dyke Swarm (CMDS), which marks the early stages of rifting in the Cretaceous. The CMDS consists of an arcuate, nearly 800 km-long mafic dykes that crosscut rocks and structures of the Precambrian basement. The swarm can be divided into two laterally continuous segments, one of E-W strike that is parallel to the southern border of the Mesozoic Potiguar basin, and one barely studied NE-SW segment which, after recent high-resolution aeromagnetic anomalies, can be traced for 300 km along the east border of the Paleozoic Parnaíba basin up to the northern boundary of the São Francisco craton. The E-W-trending segment encompasses essentially tholeiitic basalts including plagioclase, clinopyroxene (±olivine), Fe-Ti oxides and pigeonite in their groundmass. The dykes show oneto 150-meters in width and of up to one kilometer in length; a few dykes exceeding 100 kilometers have been inferred from the aeromagnetic anomalies. The tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2≥1.5 wt.%; Ti/Y >360), and low-Ti tholeiites (TiO2≤1.5 wt%; Ti/Y≤360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The latter exhibit some contamination with crustal (felsic) materials. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic 87Sr/86Sr (~0.7035–0.7039) combined with (in part) radiogenic 143Nd/144Nd and 206Pb/204Pb (>19.1) ratios, which together reveal a contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show quite variable Sr-Nd initial ratios with relatively consistent 206Pb/204Pb ratios clustering toward an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb-Ta anomalies, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Hence, FOZO and EMI components might to be coexisted and contributed in varying extents to the generation of the CMDS primary melts. Plagioclase dating of one evolved high-Ti tholeiite dyke provided two plateau ages of 127.1 ± 0.2 Ma and 128.2 ± 1.3 Ma, with an integrated mean age of 127.7 ± 0.1 Ma. Plagioclase multigrain fractions from one low-Ti tholeiite dyke provided two plateau ages of 131.6 ±0.7 Ma and 131.0 ± 0.4 Ma, with mean age of 131.2 ± 0.1 Ma. These 40Ar/39Ar ages clearly reveal that the low-Ti and high-Ti magmas encompassing the E-W segment of the CMDS were emplaced as two pulses during the Early Cretaceous. AMS investigations of the E-W-trending segment evidenced (at least) two main feeder zones located at the intersection of the dykes with Cenozoic N-trending volcanic centers referred in NE Brazil as the Macau magmatism. Such feeder zones were characterized by vertical magnetic fabrics (via steep-plunging magnetic Maria Helena B. M. HOLLANDA, Carlos J. ARCHANJO, Paul R. RENNE, Donald E. NGONGE, David L. CASTRO, Diógenes C. OLIVEIRA and Antomat A. MACÊDO FILHO, 2016. Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview. Acta Geologica Sinica (English Edition), 90(supp. 1): 109-110.