Kermanshah ophiolites consist of the Paleocene back-arc basin and the lower Eocene arc. These ophiolites extend to the NW (Khoy, NW Iran and Eastern Taurus, Turkey). Slowing Arabia-Eurasia convergence induces the slab retreat with back-arc extension. The lower Eocene arc intrudes the Paleocene back-arc basin close to the Eurasian margin. Obduction of Early Tertiary back-arc, arc assemblage occurred during Oligocene.
U-Pb geochronology on baddeleyite is a powerful technique that can be applied effectively to chronostratigraphy. In southern Africa, the Kaapvaal Craton hosts a wellpreserved Mesoarchean to Paleoproterozoic geological record, including the Neoarchean Ventersdorp Supergroup. It overlies the Witwatersrand Supergroup and its world-class gold deposits. The Ventersdorp Supergroup comprises the Klipriviersberg Group, Platberg Group, and Pniel Group. However, the exact timing of formation of the Ventersdorp Supergroup is controversial. Here we present 2789 ± 4 Ma and 2787 ± 2 Ma U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) baddeleyite ages and geochemistry on mafic sills intruding the Witwatersrand Supergroup, and we interpret these sills as feeders to the overlying Klipriviersberg Group flood basalts. This constrains the age of the Witwatersrand Supergroup and gold mineralization to at least ca. 2.79 Ga. We also report 2729 ± 5 Ma and 2724 ± 7 Ma U-Pb ID-TIMS baddeleyite ages and geochemistry from a mafic sill intruding the Pongola Supergroup and on an east-northeast-trending mafic dike, respectively. These new ages distinguish two of the Ventersdorp Supergroup magmatic events: the Klipriviersberg and Platberg. The Ventersdorp Supergroup can now be shown to initiate and terminate with two large ig-neous provinces (LIPs), the Klipriviersberg and Allanridge, which are separated by Platberg volcanism and sedimentation. The age of the Klipriviersberg LIP is 2791-2779 Ma, and Platberg volcanism occurred at 2754-2709 Ma. The Allanridge LIP occurred between 2709-2683 Ma. Klipriviersberg, Platberg, and Allanridge magmatism may be genetically related to mantle plume(s). Higher heat flow and crustal melting resulted as a mantle plume impinged below the Kaapvaal Craton lithosphere, and this was associated with rifting and the formation of LIPs.
SUMMARY
Many, but not all, current ripples show log‐normal grain‐size distribution and good sorting (P. D. Trask). A range in medians was measured from fifty to several hundred microns. Turbidite ripples are log‐normal, very well‐sorted and are restricted to medians of very fine sand to silt (< 130 μ) because the particles have been supplied in suspension at rippling velocity. Experimental suspension current ripples formed at about 50–60 cm/sec are similar in composition and structure to turbidite ripples. The continuous supply results in climbing ripples. The restricted grain size and the internal structure of turbidite ripples both form strong evidence against the action of normal currents. The number of grain‐size analyses is still rather small.
The Kaapvaal craton (South Africa) was the host of several major magmatic events during the Palaeoproterozoic, including the volcanic Hekpoort and Ongeluk Formations. Their possible comagmatic origin is the subject of a long debate. We performed a palaeomagnetic study of the Hekpoort Formation to be compared with the available palaeopole of the Ongeluk Formation, but also to contribute to the apparent polar wander path of the Kaapvaal craton. Characterization of magnetic mineralogy by three-axis thermal demagnetization of isothermal remanent magnetization and magnetic susceptibility versus temperature points out magnetite as the main remanence carrier in most samples.Five magnetic components were identified in total, of which the least stable (HKE) near parallels the present geomagnetic field. At higher levels of demagnetization (above 400 • C), two components (HKD and HKC) are identified as thermoviscous overprints likely related to the Karoo large igneous province (LIP) and a magmatic event which occurred between the emplacement of the ∼2055 Ma Bushveld Complex and HKD (possibly linked to the Umkondo LIP), respectively. This LIP is known to be associated with extensive remagnetization. The second most stable component HKB was also revealed at higher steps of thermal and alternative-field treatment. The HKB palaeopole (latitude = 28.4 • N and longitude = 54 • E) is similar to those reported from the Bushveld Complex (∼2055 Ma) and the Vredefort impact structure (∼2023 Ma). A potentially primary remanence direction (HKA; declination = 337 • , inclination = 80 • and α = 6.2 • ) was identified in most sites during the highest levels of thermal demagnetization. Note that the HKA pole position (latitude = −44 • N and longitude = 40 • E) is significantly different from the palaeopole for the Ongeluk Formation (latitude = −0.5 • N and longitude = 107 • E). Although, the primary nature of HKA is supported by positive fold and reversal tests, we cannot exclude the possibility that this component represents an overprint. HKA is, however, most likely older than ∼2.0 Ga given its anteriority to HKB components.
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