Denudation history of the Bocaina Plateau, Serra do Mar, southeastern Brazil: Relationships to Gondwana breakup and passive margin development

The Bocaina Plateau, which is situated on the eastern flank of the continental rift of southeastern Brazil, is the highest part of the Serra do Mar. Topographic relief in this area is suggested to be closely related to its complex tectono-magmatic evolution since the breakup of Western Gondwana and opening of the South Atlantic Ocean. Apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the Bocaina Plateau. The ages range between 303 ± 32 and 46 ± 5 Ma, and are significantly younger than the stratigraphic ages. Mean track lengths vary from 13.44 ± 1.51 to 11.1 ± 1.48 μm, with standard deviations between 1.16 and 1.83 μm. Contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution. The thermal histories inferred from these data imply three periods of accelerated cooling related to the Early Cretaceous continental breakup, Early Cretaceous alkaline magmatism, and the Paleogene evolution of the continental rift of southeastern Brazil. The oldest fission track ages (> 200 Ma) were obtained in the Serra do Mar region, suggesting that these areas were a long-lived source of sediments for the Paraná, Bauru, and Santos basins.     

Structural and vibrational behaviour of fluorapatite with pressure. Part II: in situ micro-Raman spectroscopic investigation

 High-pressure Raman investigations were carried out on a synthetic fluorapatite up to about 7 GPa to analyse the behaviour of the phosphate group's internal modes and of its lattice modes. The Raman frequencies of all modes increased with pressure and a trend toward reduced splitting was observed for the PO₄-stretching modes [(ν_3a(A_g) and ν_3b(A_g); ν_3a(E_2g) and ν_3b(E_2g)] and the PO₄ out-of-plane bending modes [ν_4a(A_g) and ν_4b(A_g)]. The pressure coefficients of phosphate modes ranged from 0.0047 to 0.0052 GPa⁻¹ for ν₃, from 0.0025 to 0.0044 GPa⁻¹ for ν₄, from 0.0056 to 0.0086 GPa⁻¹ for ν₂ and 0.0046 for ν₁ GPa⁻¹, while the pressure coefficients of lattice modes ranged from 0.0106 to 0.0278 GPa⁻¹. The corresponding Grüneisen parameters varied from 0.437 to 0.474, 0.428, 0.232 to 0.409 and 0.521 to 0.800 for phosphate modes ν₃, ν₁, ν₄, ν₂, respectively, and from 0.99 to 2.59 for lattice modes. The vibrational behaviour was interpreted in view of the high-pressure structural refinement performed on the same crystal under the same experimental conditions. The reduced splitting may thus be linked to the reduced distortion of the environment around the phosphate tetrahedron rather than to the decrease of the tetrahedral distortion itself. Moreover, the amount of calcium polyhedral compression, which is about three times the compression of phosphate tetrahedra, may explain the different Grüneisen parameters. Received: 25 April 2000 / Accepted: 20 December 2000     

利用磷灰石裂变径迹法研究鄂尔多斯盆地地热史

本文应用磷灰石裂变径迹法研究了鄂尔多斯盆地的热历史．不同构造单元的磷灰石裂变径迹分析资料表明：在中生代晚期，地温梯度为３．３－４．１℃／１００ｍ，大地热流值为８１－９５ｍＷ／ｍ２，高于盆地现今的平均地温梯度（２．８０℃／１００ｍ）及平均大地热流值（６３ｍＷ／ｍ２）．这次热事件有利于油气的生成、运移及大气田的形成．另外，在２０－２３Ｍａ前，发生了一期显著的冷却事件．     

Counting fission tracks in mica external detectors

The different ways in which minerals and external detectors respond to chemical etching are outlined. While it is prossible that the distribution of angles of inclination of fission tracks in some materials can be determined from the measurement of the aspect ratios of the etch pits this cannot be done using mica detectors. A method of measuring the distribution of angles of inclination in mica detectors based on the frequency distribution of areas of fission track entry holes is proposed. The results of two experiments are presented, and a means of correcting the number of tracks lost during etching is described.     

Apatite nodules as an indicator of depositional environment and ore genesis for the Mesoproterozoic Broken Hill-type Gamsberg Zn–Pb deposit,Namaqua Province,South Africa

Nodules consisting of fluorapatite are concentrated as a discrete band in the polymetallic sulfide ore of the Gamsberg deposit in central Bushmanland, South Africa. Their shape, zonal features, and geochemistry, in particular rare earth element (REE) abundances, are similar to authigenic francolite concretions that precipitate during diagenesis in organic-rich muds. As a result, the apatite nodules are regarded as primary. The nodules have a strong lithostratigraphic control, occurring at the transition of siliciclastic-hosted pyrite–sphalerite–graphite mineralization to calc-silicate-hosted pyrrhotite–sphalerite mineralization. Mineralogical and chemical evidence indicates that this transition also corresponds to a paleo-redox boundary that separates sediments deposited under oxygen-deficient conditions from those that developed in a more oxygenated environment. The apatite nodules can morphologically and chemically clearly be distinguished from coarse apatite grains that occur in oxide-facies iron formations stratigraphically above the ore horizon. However, similar REE profiles and in particular positive Eu anomalies in both types of apatite are indicative of a close genetic relationship between the two units and favor a common hydrothermal origin of the P. The contrasting morphologies and chemical signatures of the two occurrences can rather be attributed to differences in behavior of the phosphates during precipitation within the contrasting host sediments (chemical versus fine-grained clastic). Taking modern phosphorites as an analogue, these results are used to postulate a genetic model for the deposition of the sulfide ore and associated iron formations. The model envisages differentiation of the depositional basin into oxygen-deficient basin facies hosting the sulfide ore and oxygenated shelf facies, which are composed of manganiferous iron formations. The intimate association of sedimentary apatite with base metal sulfides indicates that basin conditions were conducive to the precipitation of both phases. This relationship might act as a potential exploration guide in similar basins elsewhere.Editorial handling: E. Frimmel     

The compressibility of a natural apatite

In-situ synchrotron X-ray diffraction (XRD) experiments of a natural apatite with the formula of Ca₅(PO₄)₃F_0.94Cl_0.06 were carried out using a diamond anvil cell and angle-dispersive technique at Photon Factory (PF), Japan. Pressure–volume data were collected up to 7.12 GPa at 300 K. The pressures were determined from the ruby fluorescence spectra shift. The unit-cell parameters and volume decreased systematically with increasing pressure, and a reliable isothermal bulk modulus and its pressure derivative were obtained in this study. The third-order Birch–Murnaghan equation of state yielded the isothermal bulk modulus of K_T=91.5(38) GPa, its pressure derivative K_T= 4.0(11), and the zero-pressure volume V₀=524.2(3) ų.     

Long-term landscape evolution of the northeastern margin of the Bohemian Massif: apatite fission-track data from the Erzgebirge (Germany)

To study the relative and absolute timing of post-Variscan cooling and denudation processes in the Erzgebirge of the Mid-European Variscides, eight samples for apatite fission-track (AFT) analysis were collected from a ~1,300 m drill-core. The fission-track data reveal two stages of accelerated cooling through the apatite partial annealing zone (APAZ; i.e., 110±10–60 °C) in the Late Jurassic-Late Cretaceous and in the late Cenozoic, respectively. Late Jurassic-Late Cretaceous cooling corresponding to denudation of 1.5–5.9 km has been related to wrench tectonics along the Elbe Zone during Triassic-Jurassic Pangea breakup. Late Cenozoic exhumation of 2.1–5.6 km, and the increase of the geothermal gradient from 17±5 °C km^–1 (Oligocene/Miocene) to 25–27 °C km^–1 (recent) is likely connected to the formation of the Eger Graben starting from the Oligocene, as a result of the late Alpine orogenic phases.     

The Evolution of the Geothermal Gradient from Lambert Graben and Mahanadi Basin – A Contribution to the Indo-Antarctic Rift Debate

The East Antarctic Lambert Graben and the Indian Mahanadi Basin are considered to represent segments of an intra-Gondwanan rift structure that was active at least since the Paleozoic. Fission track analyses of apatites from a comprehensive data set across the shoulders of both grabens were used to compare their low temperature history, and to estimate the paleo-geothermal gradients before the onset of the last denudation/rifting stage (Late Jurassic). The paleo-geothermal gradients of both juxtaposed Gondwana margins similarly increase from the basement towards the respective rift shoulder from 15–20°Ckm⁻¹ to 25–30°Ckm⁻¹. This trend of increasing paleo-geothermal gradients, together with a denudation episode commencing in the Early Cretaceous and coeval igneous activity, indicates a common rifting stage accompanying the breakup of Gondwana in the India-Antarctica sector.     

Thermal effects of Caledonian foreland basin formation,based on fission track analyses applied on basement rocks in central Sweden

Increasing evidence from fission track studies in Sweden indicate that large parts of the Fennoscandian Shield have been affected by a large-scale thermotectonic event in the Palaeozoic. In this study the results of 17 apatite fission track analyses from central Sweden are presented collected along three NW–SE transects trending from the Bothnian Sea to the Caledonides. On the Bothnian coast samples have been collected directly from the Sub-Cambrian Peneplain. The sedimentary cover protecting this surface until recently is responsible for the thermal increase detected through apatite fission track (FT) thermochronology.The apatite FT ages range between 516 ± 46 Ma (±1σ) on the Bothnian coast around sea level to 191 ± 11 Ma in the Caledonides (～500–1500 m.a.s.l.). The mean track lengths vary from 11.3 ± 2.2 μm (±1σ) in the east to 14.2 ± 2.8 μm in the west, indicating a longer stay in the PAZ in the east, versus a continuous cooling pattern in the west. This pattern in combination with other geological constraints indicates that the crystalline basement rocks near the Caledonian deformation front in the west experienced higher temperatures after the formation of the Sub-Cambrian Peneplain followed by denudation, compared with the basement rocks in the east near the Bothnian coast.The apatite FT data near the Caledonian deformation front indicates prevailing temperatures of more than 110 ± 10 °C prior to the Mid Palaeozoic, causing a resetting of the apatite fission track clock. The temperatures were progressively lower away from the deformation front. Apatite fission track analysis of samples collected from the Sub-Cambrian Peneplain along the Bothnian coast indicate maximum temperatures of 90 ± 15 °C during Late Silurian–Early Devonian time. This heating event is argued to be the result of burial beneath a developing foreland basin in front of the Caledonian orogeny. Assuming a geothermal gradient of 20 °C/km, this temperature increase can be converted to a total burial of the samples. The resulting geometry of this basin can be described as an asymmetrical basin at least 3.5 km deep in the vicinity of the Caledonian deformation front decreasing to about 2.5 km on the Bothnian coast, continuing further onto Finland. The width of this basin was in thus in the order of 600 km. Whether this was formed completely synorogenic or partly synorogenic, broadening after cessation of the orogeny, could not be revealed.The Late Palaeozoic and Mesozoic thermal evolution of this area is related to the extensional tectonics in the North Atlantic Domain.     

Low temperature Phanerozoic history of the Northern Yilgarn Craton, Western Australia

The Phanerozoic cooling history of the Western Australian Shield has been investigated using apatite fission track (AFT) thermochronology. AFT ages from the northern part of the Archaean Yilgarn Craton, Western Australia, primarily range between 200 and 280 Ma, with mean confined horizontal track lengths varying between 11.5 and 14.3 μm. Time–temperature modelling of the AFT data together with geological information suggest the onset of a regional cooling episode in the Late Carboniferous/Early Permian, which continued into Late Jurassic/Early Cretaceous time. Present-day heat flow measurements on the Western Australian Shield fall in the range of 40–50 mW m⁻². If the present day geothermal gradient of  18 ± 2 °C km⁻¹ is representative of average Phanerozoic gradients, then this implies a minimum of  50 °C of Late Palaeozoic to Mesozoic cooling. Assuming that cooling resulted from denudation, the data suggest the removal of at least 3 km of rock section from the northern Yilgarn Craton over this interval. The Perth Basin, located west of the Yilgarn Craton, contains up to 15 km of mostly Permian to Lower Cretaceous clastic sediment. However, published U–Pb data of detrital zircons from Permian and Lower Triassic basin strata show relatively few or no grains of Archaean age. This suggests that the recorded cooling can probably be attributed to the removal of a sedimentary cover rather than by denudation of material from the underlying craton itself. The onset of cooling is linked to tectonism related to either the waning stages of the Alice Springs Orogeny or to the early stages of Gondwana breakup.