Abstract Detrital zircon grains from three samples of sandstone from the Tswaane Formation of the Okwa Group of Botswana have been dated by U-Pb and analysed for Hf isotopes by multicollector LA-ICPMS. The detrital zircon age distribution pattern of the detrital zircons is dominated by a mid-Palaeoproterozoic age fraction (2 000 to 2 150 Ma) with minor late Archaean – early Palaeoproterozoic fractions. The 2 000 to 2 150 Ma zircon grains show a range of epsilon Hf from -12 to 0. The observed age and Hf isotope distributions overlap closely with those of sandstones of the Palaeoproterozoic Waterberg Group and Keis Supergroup of South Africa, but are very different from Neoproterozoic deposits in the region, and from the Takatswaane siltstone of the Okwa Group, all of which are dominated by detrital zircon grains younger than 1 950 Ma. The detrital zircon data indicate that the sources of Tswaane Formation sandstones were either Palaeoproterozoic rocks in the basement of the Kaapvaal Craton, or recycled Palaeoproterozoic sedimentary rocks similar to the Waterberg, Elim or Olifantshoek groups of South Africa. This implies a significant shift in provenance regime between the deposition of the Takatswaane and Tswaane formations. However, the detrital zircon data are also compatible with a completely different scenario in which the Tswaane Formation consists of Palaeoproterozoic sedimentary rock in tectonic rather than depositional contact with the other units of the Okwa Group.
Abstract A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two‐stage exhumation model of deep‐seated rocks in the eastern sector of the Variscan belt. An early sub‐vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18−20 kbar to depths equivalent to 8−10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub‐horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE‐directed Saxothuringian continental subduction. Fabrics that record the later sub‐horizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north‐directed sub‐horizontal channel. This sub‐horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re‐equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SE‐oriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.
We use structural, petrographic, and geochronological data to examine processes of exhumation of partially molten crust in the Late Devonian–early Carboniferous Chandman dome in the Mongolian tract of the Central Asian Orogenic Belt. The dome is composed of a magmatite-migmatite core and a low-grade metamorphic envelope of early Paleozoic metasediments and Carboniferous clastics. Its tectonic evolution can be divided into three main stages. The oldest fabric is a subhorizontal foliation, S1, in migmatites that is subparallel to the magmatic foliation in granitoids and to the greenschist facies schistosity in the enveloping metasediments. This event is interpreted as a result of horizontal deep crustal flow at depths of 20–25 km. The S1 layering was subsequently transposed into a new foliation, S2, or affected by open to close upright F2 folds that are locally truncated by steep walls of diatexites, suggesting influx of partially molten crust into fold cores. The shallow-dipping magmatic foliation in granitoids is locally reworked by vertical magmatic to gneissic S2 fabrics. Syn-S2 metamorphic assemblages and synkinematic to postkinematic cordierite point to exhumation of the migmatites and granitoids from 20–25 km to ∼10 km, and concomitant isobaric heating of the surrounding upper crust. New 40Ar/39Ar ages of 350–340 Ma from both the high-grade core and the metamorphic mantle overlap with previously published crystallization ages of 360–340 Ma, suggesting that magmatism and cooling in the upper crust are partly synchronous. Late syn-D2, S2-parallel leucogranite sheets crosscutting both the magmatic core and the mantling migmatites either exploit S2 or crosscut horizontal S1 fabrics; they are interpreted as brittle expulsion of magma during ongoing syn-D2 exhumation. We suggest that the partially molten crust and magmas rose vertically into the upper crust, along steep planar fabrics that are parallel to the axial fold plane of a crustal detachment folding, without contribution of buoyancy forces. In order to test that crustal-scale detachment folding can exhume partially molten crust, we apply an analogue model with temperature dependent rheology of the lower crust represented by a partially molten wax layer overlain by an upper crustal sand layer. It is shown that the fold core initially filled by low-viscosity partially molten wax rapidly migrates upward during fold lock-up, enhancing upward extrusion of magma and migmatites along the fold axial plane. The exhumation of the lower crust wax is facilitated by erosional unroofing of the upper crustal sand above the hinge of the antiform. In Chandman, localized siliciclastic lower Carboniferous basins rimming the dome attest to this erosional phenomenon. Using a simple geometrical analysis we show that detachment folding can explain magma collection in an orientation perpendicular to the main shortening direction, and episodic emplacement of magmas during amplification of the antiform. In our view, the detachment folding model provides a new model for the exhumation of a weak zone above a rigid floor (basement from which the fold is detached) and its vertical extrusion related to locking of the fold and post-buckle flattening. This model helps elucidate steep retrograde pressure-temperature-time paths along steep fabrics, overlapping ages from different geochronometers, and emplacement of voluminous syntectonic magmas.
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