Tjerk Peters

New algae and problematica of algal affinity from the permian of the Aseelah Unit of the batain plain (East Oman)

Permian microfloras occur in limestone boulders of the Aseelah Unit at the lower part of the Batain Group (NE Oman). The boulders are composed of bioclastic limestones deposited during the Permian on a shallow marine warm shelf environment. They yield several rich associations of Dasycladales, tubiphytids and Archaeolithophyllaceae indicating a Yakhtashian to Dzhulfian age (dated by fusulinids). The algal-bearing Aseelah Unit overlies conformably the Permian Qarari Unit deposited on the distal slope of the Arabian carbonate platform. The conglomerate and sandstone sheddings of the Aseelah Unit indicate the end of a major transgressive-regressive cycle at the Permian/Triassic boundary. The oldest age of the depositional event is given by the Dzhulfian age of the youngest reworked boulders. The age of the sandstones and sandy matrix is not well established; it is post-Dzhulfian, probably Triassic on the basis of some palynological indications. Several new taxa are described: Aphralysiaceae nov. fam., Sparaphralysia orientalis nov. sp., Mellporellopsis corpulenta nov. gen. nov. sp., Aseelahella granieri nov. gen. nov. sp., Claracrustinae nov. subfam., Claracrusta ungdarelloidea nov. sp. and Sphairionia galinae nov. sp.

K-Ar ages of the metamorphic sole of the Semail Ophiolite: implications for ophiolite cooling history

K−Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.

The solvus in the system MnCO3-CaCO3

The solvus on the Mn-rich side in the system MnCO3-CaCO3 was studied experimentally at 10 kb and lower pressures. The critical point lies at 540 °C and Mn0.68Ca0.32CO3. Reaction rates of demising are extremely sluggish. X-ray calibration curves for (Mn, Ca)CO3 solid solutions are given. Depicting microprobe analyses of about 300 carbonates from manganese calc silicate marbles affected by different degrees of metamorphism confirm the experimentally derived solvus.Margules parameters were derived from the experimental solvus, giving for spinodal unmixing Wa=20.1+0.032·T and Wb=18.3-0.0033·T, and for binodal unmixing Wa=−22.9+0.035 ·T and Wb=3.36+0.0147·T (All W, in kJ/-mol)A physical interpretation of the Margules parameters based on deformations of the ionic radii in the solid solution is presented. The Margules parameters obtained by this model agree fairly well with the experimental data.

Extremely thin oceanic crust in the Proto-Indian Ocean : Evidence from the Masirah Ophiolite, Sultanate of Oman

The Masirah Ophiolite is a good example of thin oceanic crust. Below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1-1.5 km, the gabbroic lower crust barely exceeds 500 m in thickness. In spite of this reduced thickness, the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence. The crust was formed during the uppermost Jurassic (circa 150 Ma) when the Indian-Madagascar plate separated from the African-Arabian plate and is therefore related to the opening of the coeval Somali basin. Geological relationships indicate that this portion of oceanic crust was formed at a ridge-transform intersect. The peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment, rather than the consequence of a tectonic thinning. The cooling effect due to the vicinity of two large continental lithospheric blocks (Indian-Madagascar and African-Arabian plates) during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness.

Middle permian (midian) foraminiferal assemblages from the batain plain (Eastern Oman): Their significance to neotethyan paleogeography

Exceptionally rich Permian fusulinid assemblages, varying in age from Yakhtashian to Dzhulfian, were found in reworked limestone blocks from conglomerates of the Aseelah Unit, recently defined in the Batain Plain and dated possibly as Upper Triassic. This paper only deals with the Middle Permian, Midian (= Capitanian) microfaunas from the conglomerates. The Aseelah Unit is found associated with the Triassic Sal Formation, tectonically overlying the Qarari Unit, and is dated as Middle to Late Permian based on ammonoid assemblages and conodonts. The boulders of the Aseelah Unit are composed of exclusively Permian skeletal limestones in a sandy matrix; the limestones were deposited on a shallow marine shelf environment from the southern Neotethys. The Qarari Unit is interpreted as a hemipelagic series deposited on the distal slope of the Arabian carbonate platform. The Midian foraminifers from the Batain Plain are similar to some assemblages of Transcaucasia, of the Abadeh area (Iran), of SW Turkey (Taurus Mountains), and some, especially the Neoschwagerinidae, exhibit affinities with microfaunas of the eastern Tethys terranes of Japan. However, similar Neoschwagerinid assemblages were not found in any significant area of the Arabian Peninsula (Oman Mountains, Huqf-Haushi area, and Khuff area).

Fieldbook and geodatabases: tools for field data acquisition and analysis

Abstract We introduce FieldBook and GeoDatabase, 2 new and effective tools for geologic field data acquisition and analysis. FieldBook is an application for Apples Newton MessagePad. Geological data collected at the outcrop, including notes and drawings, can be entered directly and on-site. The formalization of the multiparameter information leads directly to a consistent database. This procedure results in a complete, up-to-date database where all information collected by different researchers in a project is available anytime, and no data are lost. GeoDatabase is an application based on FileMaker™ Pro, representing the FieldBook interface on PC/Macintosh. GeoDatabase provides extensive search possibilities and strong export features that are needed for field-data analysis, either in the field or in the office. It can be used as a central database within a local network with several users on either PC or a Macintosh. FieldBook and GeoDatabase both are simple to use, yet they satisfy the demands of field campaigns involving numerous scientists. Applications of field projects in the crystalline basement of the Salalah area and the Masirah ophiolite are given.

The break-up of East Gondwana along the northeast coast of Oman: evidence from the Batain basin

Recent detailed studies on the Batain nappes (northeast coast of Oman), which represent a special part of the so-called ‘Oman Exotics’, have led to a better understanding of the Neotethyan geodynamic evolution. The Batain Exotics bear witness to volcanic activity, sea-level changes, tectonic instability, rifting and oceanization along the Eastern Oman margin during Late Palaeozoic and Mesozoic times. They allow definition of the Batain basin as an aborted Permian branch of Neotethys. This marine basin was created in Early Permian times extending southward to the East African/ Madagascar region and was linked to the Karoo rift system. The presented revised classification of the Batain nappes considers the Batain basin to be no longer a part of the Hawasina basin and the Neotethyan margin proper. We attribute the Batain basin to a Mozambique–Somali–Masirah rift system (Somoma). This system started in Early Permian, times, creating a marine basin between Arabia and India/Madagascar; rifting in the Late Triassic and oceanization during Late Jurassic times led to the separation of East Gondwana.

Assemblages among tephroite, pyroxmangite, rhodochrosite, quartz: Experimental data and occurrences in the Rhetic Alps

AbstractReactions involving the phases quartz-rhodochrosite-tephroite-pyroxmangite-fluid have been studied experimentally in the system MnO-SiO2-CO2-H2O at a pressure of 2 000 bars and resulted in the following expressions 1

Tephroite-hausmannite-galaxite from a granulite-facies manganese rock of the United Arab Emirates

\begin{gathered} {\text{Rhodochrosite + Quartz = Pyroxmangite + CO}}_2 \hfill \\ {\text{ log}}_{{\text{10}}} K^{{\text{2000 bars}}} = - \frac{{11.765}}{T} + 18.618. \hfill \\ {\text{Rhodochrosite + Pyroxmangite = Tephroite + CO}}_2 \hfill \\ {\text{ log}}_{{\text{10}}} K^{{\text{2000 bars}}} = - \frac{{7.083}}{T} + 11.870. \hfill \\ \end{gathered}