Добірка наукової літератури з теми "Usagaran Belt"

Tanzania is one of the leading gold mining countries in the world and the discovery of new gold resources on its territory is an actual task. Known gold deposits are concentrated mainly in the northwest of the country, in the metallogenic zone of Lake Victoria, where they are associated with the Archean greenstone belts, and to a lesser extent – in the southwest, in the ore regions of Lupa and Mpanda, confined to the Ubendian Paleoproterozoic mobile belt. With regard to the eastern regions of Tanzania, where the Proterozoic structures of the Uzagaran mobile belt are developed, until recently in this region any significant manifestations of gold mineralization were not known. As a result of our research in the northern part of the Morogoro province of the Republic of Tanzania, a new previously unknown gold deposit Mananila was discovered. It is represented by a large volume, up to 400–450 m long, up to 60–80 m thick, mineralized shear zone over intensely leached and schistosed migmatites, gneisses, amphibolites, penetrated by echelon systems of quartz veins and veinlet, steeply dipping bodies of quartz breccia up to 1.0–1.5 m thick. Gold contents range from 0.61 to 8.11 g/t, the average zone content is 2.5–3.0 g/t. Parallel to the main zone, similar structures are developed on the site, although they are of lower thickness. The forecast resources of the deposit are estimated at 20 tons of gold. 2.8 km to the east from the Mananila field, the recently discovered Mazizi gold deposit is located, and a number of small occurrences of gold are also known in the region. All these objects are located within a large shear zone of the northeastern strike, up to 4–5 km width, over 20 km in length. This serves as the basis for the identification of a new gold ore region in the northern part of the Morogoro province of the United Republic of Tanzania, within the Proterozoic mobile belt of Usagaran, the possible gold content of which has never been previously discussed in geological literature.

AbstractGranulite-facies metamorphism recorded in NE Mozambique is attributed to three main tectonothermal events, covering more than 1400 Ma from Palaeoproterozoic – early Palaeozoic time. (1) Usagaran–Ubendian high-grade metamorphism of Palaeoproterozoic age is documented in the Ponta Messuli Complex by Grt-Sil-Crd-bearing metapelites, estimated to pressure (P) 0.75 ± 0.08 GPa and temperature (T) 765 ± 96°C. The post-peak P-T path is characterized by decompression followed by near-isobaric cooling. (2) Irumidian medium- to high-pressure granulite-facies metamorphism is evident in the Unango and Marrupa complexes of late Mesoproterozoic – early Neoproterozoic age. High-pressure granulite-facies is documented by Grt-Cpx-Pl-Rt-bearing mafic granulites in the northwestern part of the Unango Complex, with peak conditions up to P = 1.5 GPa and T = 850°C. Medium-pressure granulite-facies conditions recording P of c. 1.15 GPa and T of 875°C are documented by Grt-Opx-Cpx-Pl assemblage in mafic granulites and charnockitic gneisses of the central part of the Unango Complex. (3) Tectonothermal activity during the Ediacaran–Cambrian Kuunga Orogeny is recorded in the Mesoproterozoic gneiss complexes as amphibolite facies to medium-pressure granulite-facies metamorphism. Granulite facies are documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses, reporting P = 0.99 ± 13 GPa at T = 738 ± 84°C in the Unango Complex and P = 0.92 ± 18 GPa at T = 841 ± 135°C in the Marrupa Complex. This metamorphism is attributed to crustal thickening related to overriding of the Cabo Delgado Nappe Complex, and shorthening along the Lurio Belt during the early Palaeozoic Kuunga Orogeny.

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Eclogites from the Usagaran Orogenic Belt of Tanzania have been reliably dated at 2.0 Ga and are the oldest reported subduction-related eclogites within a well-preserved orogenic belt. Based on limited geochemistry from two samples of eclogite from the Usagaran Belt, Möller et al. (1995) concluded that the protoliths were similar to MORB. This study analyzed a larger number of eclogitic samples and a suite of structurally intercalated mafic and pelitic rocks to establish the tectonic setting of the Usagaran Orogenic Belt rocks. Eclogitic rocks from the Usagaran Orogenic Belt display LILE and LREE enrichment relative to present-day MORB. Variations in εNd values from depleted mantel at 2.0 Ga supported this interpretation. The mantle-derived mafic rocks show strong Nb depletions, indicating that they are subduction-related. Enrichment of mafic rocks in LILE and LREE are likely caused by dehydration of the subducting slab with some contamination from crustally derived materials perhaps via subducted sediment. The intercalated pelites are mainly derived from the Tanzanian Craton, with a significant mafic input evidenced by high Cr & Ni values. Based on the geochemical isotopic compositions and field relationships, the eclogites, mafic rocks and pelites all formed in a subduction setting that operated around 2.0 Ga. Despite the fact that the Earth was hotter in its early history, modern plate tectonics, (i. e., subduction of cold oceanic crust into a warm mantle resulting in high-pressure low-temperature metamorphism), occurred and was recorded in the Usagaran Belt during the Palaeoproterozoic. Thus modern-style plate tectonics have operated since at least 2.0 Ga.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2009

Eclogites and other high-P low-T rocks have been used to understand changing tectonic regimes and the processes which have shaped the Earth. Uncertainty exists over the tectonic setting of Palaeoproterozoic high-pressure rock formation with several workers proposing that deep subduction, the most commonly inferred method for modern high-pressure metamorphism, was not functioning at the time these rocks formed. Tanzania hosts two of the oldest in-situ eclogites in the 2.0 Ga Usagaran and 1.85 Ga Ubendian orogenic belts, with km-scale eclogite & retrograde-eclogite sheets in these locations interlayered with felsic gneisses and amphibolites. These orogenic belts form the south-eastern and south-western margins of the Tanzanian Craton. Based on detrital zircon study on stream sediments the Tanzanian Craton formed between 2850 – 2500 Ga with magmas derived from an evolved (TDM crustal = 3.2 Ga) source. Lu-Hf studies on metasedimentary rocks show the Ubende and Usagaran Belt’s crustal material formed between 1850 – 2050 Ma from a more evolved magma source (TDM crustal = 2.6 Ga) that also reworked some Archaean material from the Tanzanian Craton. The Ubende and Usagaran belts have similar but not identical crustal evolution histories through the Archaean and Palaeoproterozoic, but the Mesoproterozoic evolution of the Ubende diverges from the Usagaran. The Songea district, situated at the junction of these orogenic belts, has a separate crustal evolution and does not form the link between the two orogens. The youngest zircon population (1.0 – 1.2 Ga) may be derived from the Irumide Belt, and has a mixed (TDM crustal = 2.1 and 1.3 Ga) magma source. Thus the belts, sometimes referred to as a continuous orogenic system, are in fact distinguishable by their crustal evolution and the timing of sedimentation and metamorphism. Ubendian eclogites have been dated using ⁴⁰Ar/³⁹Ar at 1848±6 Ma (Boven et al., 1999). ⁴⁰Ar/³⁹Ar thermochronology on a suite of five samples coupled with U-Pb geochronology yields a new metamorphic age for the Ubende Belt. An age of ~1070 Ma is preserved in zircon within metasedimentary rocks with cooling below ~500 °C at ~1020 Ma. Structures in the Usagaran Belt are consistent with sinistral transpression associated with collision between the Tanzanian craton and an unknown continent. The metasedimentary rocks of the eastern Usagaran rocks are inconsistent with being derived from the Tanzanian Craton, indicating the existence of an as yet unidentified continental block as part of the collisional event. Rocks from the western Usagaran are more consistent with being derived from the craton, with some input of sediment derived from local igneous activity, possibly a volcanic arc. Two metamorphic events were recognised in the Usagaran belt. Usagaran metamorphism occurred over ~20 My between 2007 and 1991 Ma. The East African orogen affected these rocks during the Neoproterozoic, and is recorded in zircon growth in the east of the orogen at 577 ± 17 Ma. Peak eclogite–facies P-T conditions in the Usagaran Orogeny (17.2 ± 3.6 kbar, 839 ± 173 ºC) were slightly higher temperature than previously published. Lithologies intercalated with eclogite in the eclogite body experienced conditions of at least 13.4 ± 2 kbar, 920 ± 130 ºC, however there is no evidence of pelitic rocks having experienced the 17 kbar which the eclogites saw. Fe-Mg diffusion modeling on garnet-biotite shows that country-rock pelites cooled at ~1-2 °C/My, compared to 25ºC/My for eclogites between 1999 and 1991 Ma. All of these facts are consistent with the eclogite body having formed during subduction of Palaeoproterozoic sea-floor which became intercalated with metasediments and other metaigneous rocks during exhumation. The surrounding blocks of the Isimani suite did not experience eclogite-facies metamorphism but were tectonically juxtaposed during exhumation. All this strongly supports a Palaeoproterozoic subduction-related origin for eclogites in the Usagaran Belt.
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2011

ABSTRACT Records of high-pressure/low-temperature (HP-LT) metamorphic interfaces are not common in Precambrian orogens. It should be noted that the association of HP-LT metamorphic interfaces and strongly deformed ocean plate stratigraphy that form accretionary prisms between trenches and magmatic arcs are recognized as hallmark signatures of modern plate tectonics. In East Africa (Tanzania), the Paleoproterozoic Ubendian-Usagaran Belt records a HP-LT metamorphic interface that we consider as a centerpiece in reviewing the description of tectonic units of the Ubendian-Usagaran Belt and defining a new tectonic model. Our new U-Pb zircon age and the interpretations from existing data reveal an age between 1920 and 1890 Ma from the kyanite bearing eclogites. This establishment adds to the information of already known HP-LT metamorphic events at 2000 Ma, 1890–1860 Ma, and 590–520 Ma from the Ubendian-Usagaran Belt. Arc–back-arc signatures from eclogites imply that their mafic protoliths were probably eroded from arc basalt above a subduction zone and were channeled into a subduction zone as mélanges and got metamorphosed. The Ubendian-Usagaran events also record rifting, arc and back-arc magmatism, collisional, and hydrothermal events that preceded or followed HP-LT tectonic events. Our new tectonic subdivision of the Ubendian Belt is described as: (1) the western Ubendian Corridor, mainly composed of two Proterozoic suture zones (subduction at 2000, 1920–1890, Ma and 590–500 Ma) in the Ufipa and Nyika Terranes; (2) the central Ubendian Corridor, predominated by metamorphosed mafic-ultramafic rocks in the Ubende, Mbozi, and Upangwa Terranes that include the 1890–1860 Ma eclogites with mid-ocean ridge basalt affinity in the Ubende Terrane; and (3) the eastern Ubendian Corridor (the Katuma and Lupa Terranes), characterized by reworked Archean crust.