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Thomas R.J.,British Geological Survey | Jacobs J.,University of Bergen | Horstwood M.S.A.,British Geological Survey | Ueda K.,University of Bergen | And 2 more authors.
Precambrian Research | Year: 2010

We present new field and geochronological data for two newly identified sequences of Neoproterozoic metasedimentary rocks, the Mecubúri and Alto Benfica Groups, interfolded with the Nampula Complex, NE Mozambique. Both groups comprise clastic sequences of meta-psammites and meta-conglomerates, which were strongly deformed, metamorphosed to sillimanite grade and locally migmatised. The relatively unsheared contacts with the surrounding Mesoproterozoic gneisses suggest that both groups are largely autochthonous. The depositional environment of the sequences is interpreted as a proximal, continental-fluvial environment in small depositional, possibly inter-montane, fault-controlled basins. Evidence of post-Mesoproterozoic exposure to the surface is provided by secondary nodular sillimanite growth in the Mecubúri Group and the underlying high-grade basement, pointing to weathering and/or epigenetic hydrothermal alteration to clays prior to burial, deformation and metamorphism. U-Pb analyses of detrital zircons constrain maximum depositional ages of 530 ± 18 and 938 ± 32 Ma for the Mecubúri and Alto Benfica Groups, respectively. The two groups have different provenances as shown by their detrital zircon age-frequency distributions. The Mecubúri Group was derived from the adjacent eastern Nampula Complex basement (ca. 1100 Ma), with minor early and mid-Neoproterozoic (950-700 Ma) and abundant late Neoproterozoic components (650-550 Ma). The latter were probably derived from Neoproterozoic terranes north of the Lurio belt, such as the Cabo Delgado Nappe Complex (CDNC). Hf isotope data indicate a juvenile origin to the Nampula Complex basement (εHf ca. +4 to 8) which was then re-worked as the source for the late Neoproterozoic melts (εHf ca. 0 to +5). Juvenile magmatic input also continued in the late Neoproterozoic (εHf ca. +11). In addition to the dominant ca. 1100 Ma local source rocks, the Alto Benfica Group, in the western Nampula Complex, records input from older Mesoproterozoic (1200-1400 Ma), Palaeoproterozoic (1600-2000 Ma) and rare Archaean sources, with scant evidence of Neoproterozoic provenance. These sources themselves reflect re-working of older pre-existing crustal components with crustal separation ages up to 3000 Ma. Zircon rims to detrital grains and metamorphic monazite from both groups date high-grade metamorphism at 500 ± 10 Ma, within uncertainty of the age for late to post-tectonic granitoids of the Nampula Complex and the Lurio belt. This suggests an intimate genetic relationship between the metamorphism and migmatisation of the two metasedimentary sequences and granite emplacement at that time. The detrital zircon data from the Alto Benfica Group suggest that the Nampula Complex was juxtaposed with the adjacent terranes of northern Mozambique and SE Africa, which were uplifted, exposed to sub-aerial weathering and locally lateritised at the time of deposition. The terrane juxtaposition event can be equated with the main, NW-SE-directed collision phase of the East African Orogeny at ca. 550 Ma. The resulting mountain belt was rapidly eroding and providing detritus for the two clastic sequences at ca. 530 Ma, probably during the early phases of extensional orogenic collapse. Later in the extensional phase, local SW-NE oriented compression deformed the Mecubúri and Alto Benfica Groups, along with the basement, into tight upright folds, with associated axial planar foliation. Sillimanite-grade metamorphism and migmatisation, associated with the emplacement of late-tectonic granites took place at ca. 500 Ma. It follows that none of the regional fabrics and large-scale structures in the Nampula Complex are Mesoproterozoic in age; the present distribution of lithological units and structures are almost entirely the result of the East African collisional orogeny at ca. 550 Ma and its subsequent extension and collapse. © 2010 Elsevier B.V. All rights reserved. Source


Ueda K.,University of Bergen | Jacobs J.,University of Bergen | Thomas R.J.,British Geological Survey | Kosler J.,University of Bergen | And 2 more authors.
Precambrian Research | Year: 2012

Lithospheric thickening during protracted Ediacaran supercontinent assembly was dissipated in various ways along the " Pan-African" East African-Antarctic Orogen. In NE Mozambique, late-tectonic extension and plutonism south of the Lúrio Belt has been modelled in terms of lithosphere delamination, although rigorous testing of the hypothesis by structural analysis has not yet been undertaken.This study presents the first analyses of late-tectonic structures in both the Mesoproterozoic basement and the Cambrian cover sequences in the Nampula Complex, NE Mozambique, supported by, and integrated with, new geochronological data. Both late compressional and extensional fabrics overprint the main Pan-African collisional structures to a variable degree in identified structural domains. The long-lived northern boundary of the Nampula Complex, the Lúrio Belt high-strain zone, initiated in the Ediacaran, was reactivated and segmented during these later phases, with boundary-parallel shortening.U-Pb SIMS ages from selected latest-tectonic units in the Nampula Complex and the Lúrio Belt give consistent ages between 518±2 and 514±5Ma. They are coeval with migmatisation and granitoid plutonism in the Nampula Complex. U-Pb titanite (471±9Ma) and 40Ar/ 39Ar biotite (431±3Ma) data suggest subsequent slow cooling rates in the Nampula Complex, consistent with basal heating by asthenosphere uprise following lithosphere delamination. Consequently, we believe the new data suggest that the observed intense late polyphase deformation has preferentially occurred where the delaminated lithosphere was doubly weakened by loss of the mantle root and the resulting long-lived elevated temperature regime. © 2011 Elsevier B.V.. Source


Ueda K.,University of Bergen | Jacobs J.,University of Bergen | Thomas R.J.,British Geological Survey | Kosler J.,University of Bergen | And 6 more authors.
Journal of Geology | Year: 2012

The postcollisional tectonic development of northeast Mozambique and subsequent cooling from high-temperature metamorphism is delineated with an extensive new set of U-Pb titanite, 40Ar/ 39Ar hornblende, and 40Ar/ 39Ar mica analyses. The complex data suggest a polyphase metamorphic history from the late Neoproterozoic to the Ordovician within the East African-Antarctic Orogen (EAAO), with marked differences between the major constituent blocks. In all the data sets, samples from the basement south of the Lúrio Belt show generally younger ages than those from the north, resulting from a late metamorphic event and slow cooling between ca. 520 and 440 Ma. The ages north and south of the Lúrio Belt are consistently offset by ca. 30-70 Ma, a difference that is maintained and even appears to increase during cooling from very high temperatures to ca. 350°C. Based on the first-order assumption that all the ages are cooling ages, cooling rates in the south are estimated at ca. 7°-8°C/Ma, while those north of the Lúrio Belt are faster at ca. 16°C/Ma. The data are consistent with previous geochronological, petrographic, and field data and suggest a late high-temperature/low-pressure metamorphic event that affected only the basement rocks south of the Lúrio Belt and portions of the latter. This late metamorphism and subsequent delayed, slower cooling agree well with a model of elevated heat flow following lithosphere delamination in the southern part of the orogen, which also explains the observed widespread granitoid magmatism, migmatization, and renewed deformation in the southern basement. © 2012 by The University of Chicago. Source


Emmel B.,University of Bergen | Kumar R.,University of Bergen | Jacobs J.,University of Bergen | Ueda K.,University of Bergen | And 2 more authors.
Gondwana Research | Year: 2014

Fission track (FT) dating of detrital titanites, zircons and apatites combined with sandstone petrography from rocks of the Rovuma Basin was used to constrain the basin's provenance and its post depositional thermal history. A comparison of metamorphic basement and sedimentary titanite and zircon FT data indicates that erosion was localized along a zone which followed the margin of the Rovuma Basin and was the source for the late Jurassic to Cretaceous sandstones. Time-temperature models of apatite FT data show, that after the deposition, the sandstones were heated up to temperatures of ca. 60-110. °C most likely due to a combination of intensified regional heat flux and burial heating caused by fast sedimentation in a transtensional pull-apart setting and intensified by regionally elevated heat flux. The utter western part of the basin was inverted between ca. 60 and 40. Ma, concordant with the drop in the global eustatic sea level which led to a rearrangement of the source-to-sink system. Some reworked zircons were deposited in the Cenozoic strata in the eastern part of the basin. © 2013 International Association for Gondwana Research. Source

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