Science and earth science

Acta Geologica Polonica

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Acta Geologica Polonica | 2025 | No 1 (in progress)

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Abstract

This study presents for the first time, the field observations, petrography, mineral chemistry and geochemistry of chloritite hosted in the Al-Barramiya Neoproterozoic ophiolite of the Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). The Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different types of alterations including carbonatization, listvenitization, chloritization and rodingitization. The Al-Barramiya chloritite occurs as thin layers associated with highly serpentinized peridotite. It is a fine-grained rock entirely composed of chlorite (85–95 vol.%) with minor talc and accessory minerals (epidote, rutile, titanite, corundum and opaque minerals). The chlorite minerals in the chloritite are represented mainly by diabantite, while those in the serpentinites include ripidolite. Depending on the chemical composition of the chlorites, the chlorite in chloritite formed at temperatures ranging between 200 and 250°C, which are lower than those of the disseminated chlorite in the serpentinite (310–345oC), indicating their formation in different hydrothermal stages. The chloritite samples are rich in total REE contents (17.9–27.3 ppm) compared with the associated serpentinites (0.69–0.87 ppm). They are characterized by slightly depleted LREE relative to HREE [(La/Lu)n = 0.8–0.9], with a moderately negative Eu-anomaly [(Eu/Eu*)n = 0.4–0.5]. The negative Eu-anomalies are derived from chloritization fluids or reflect the presence of talc in the chloritite. Based on field work, petrography, mineralogical and geochemical data, the studied chloritite has been interpreted as being derived from the associated serpentinized ultramafics by hydrothermal alterations. This is supported by an enrichment of chloritite in compatible trace elements (Cr = 2031–2534 ppm, Ni = 1264–1988 ppm, Co = 76–101 ppm) similar to that which is observed in the associate serpentinite.
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Authors and Affiliations

Bassam A. Abuamarah
1
Amany M.A. Seddik
2
Mokhles K. Azer
3
Omar Bartoli
4
Mahmoud H. Darwish
2

  1. Department of Geology and Geophysics, King Saud University, Riyadh 11451, Saudi Arabia;
  2. Geology Department, Faculty of Science, New Valley University, El-Kharga, 72511 Egypt
  3. Geological Sciences Department, National Research Centre, Cairo, Egypt
  4. Geoscience Department, University of Padova, Padova, Italy
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Abstract

Pelagic microcrinoids from the lower and middle Turonian chalks of the Anglo-Paris Basin (southern England, northern France) have distinctive morphologies that demonstrate rapid evolution among three genera, Dentatocrinus Gale, 2019a, Drepanocrinus Jækel, 1918 and Striacrinus gen. nov., and their constituent species and subspecific formae. These form the basis for a refined biostratigraphic zonation that can be applied across the basin. The genus Striacrinus is described as new (type species Drepanocrinus striatulus Gale, 2019a), with a new species S. ornatus described from the middle Turonian. A succession of stratigraphically restricted formae are recognised within Dentatocrinus dentatus Gale, 2019a, described herein as D. dentatus forma inflatus nov., D. dentatus forma subspinosus nov., D. dentatus forma spinosus nov., and D. dentatus forma conicus nov. These formae had ranges with a mean duration of 100–200 kyr as determined from orbital precession cycles.
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Authors and Affiliations

Andrew Scott Gale
1

  1. School of the Environment and Life Sciences, University of Portsmouth, Burnaby Building, Burnaby Road,Portsmouth PO1 3QL, UK; Department of Earth Sciences, The Natural History Museum, Cromwell Road,London SW7 5BD, UK
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Abstract

Raw sediment samples which are without prior lengthy laboratory preparation can be used to obtain a rough but rapid estimate of the luminescence equivalent dose, and thus of the relative age of the sediment. In this study, we tested this range-finder method on clastic sediments in NE Poland for the first time, with special focus on Pleistocene meltwater sediments generated by highly energetic glacial lake outburst floods, and the post-flooding sediments. Two datasets with known doses from standard measurements were compared to range-finder doses determined from quartz and feldspar in untreated sediments. We found statistically significant correlation between equivalent doses of (1) standard quartz optically stimulated luminescence (OSL) and range-finder feldspar infrared stimulated luminescence (IRSL) at 50 ºC, and (2) standard quartz OSL and range-finder quartz OSL in low-dose samples (<80 Gy). However, these correlations should only be considered as approximate whereby preparing more than three range-finder aliquots has the potential of yielding more accurate results. Correlation between the range-finder quartz OSL and range-finder feldspar IRSL is also significant. The rangefinder measurements can be used for approximate dose determination to preliminary assess the sediment age or as a selection tool to avoid incompletely bleached samples. The sedimentary environment and especially sediment reworking and transportation seem to influence these correlations. We consider the sediments studied here to have undergone between one and four reworking stages, and samples with repeated reworking usually represent well-bleached material for luminescence dating.
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Authors and Affiliations

Edyta Kalińska
1
Helena Alexanderson
2
Piotr Weckwerth
1
Jan A. Piotrowski
1 3
Wojciech Wysota
1

  1. Nicolaus Copernicus University in Toruń, Faculty of Earth Sciences and Spatial Management, Lwowska 1, 87-100 Toruń, Poland
  2. Lund University, Department of Geology, Sölvegatan 12, 223 62 Lund, Sweden;
  3. Aarhus University, Department of Geoscience, Høegh-Guldbergs Gate 2, DK-8000 Aarhus C, Denmark
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Abstract

Karapınar (Konya, Turkey) is located in the central part of the Anatolian micro-plate which is characterized by differently oriented Neogene–Quaternary horst – graben structures. The basement of the region is composed of Mesozoic–Paleogene rocks. In the various Neogene basins, Miocene–Pliocene sedimentary and volcanic rocks overlie the basement rocks and form the secondary rock units. The youngest units are Plio-Quaternary aged terrestrial sediments and volcanics. The main structure of the study area consists of WNW-ESE and NE-SW trending intersecting graben-horst structures and the normal faults forming them. The approximately WNWESE trending basins are filled with Miocene–Pliocene sediments, while the NE-SW trending basins are filled with Pliocene–Quaternary sediments. Kinematic studies show that the faults in the study area are developed in all directions and the distribution of the slicken lines indicates that there is multidirectional crustal extension. The principal stress axes obtained from all the kinematic data show that σ1 is nearly vertical, σ2 and σ3 are horizontal. If all faults are assumed to have been formed due to the same stress system, the study area is predominantly elongated in the NW-SE direction. However, detailed field observations and structural analyses indicate that the NE-SW trending fault systems are younger than the WNW-ESE trending faults. This indicates that firstly NNE-SSW and then NW-SE crustal extension developed in the study area respectively.
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Authors and Affiliations

Yaşar Eren
1
Berkant Coşkuner
1
Şeyda Parlar
1

  1. Department of Geological Engineering, Faculty of Engineering and Natural Sciences, Konya Technical University, Selçuklu, Konya, Turkey

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