Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates (original) (raw)
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Journal of African Earth Sciences, 2006
This study presents new zircon ages and Sm-Nd whole-rock isotopic compositions for high-grade gneisses from the Udzungwa Mountain area in the central part of the Mozambique belt, Tanzania. The study area comprises a succession of layered granulite-facies paraand orthogneisses, mostly retrograded to amphibolite-facies. The original intrusive contacts became obscured or severely modified during non-coaxial ductile deformation, and extensive shearing occurred during retrogression. Structures reflecting the early deformational history were mostly obscured when the rocks were transported into the lower crust as documented by severe flattening. Only the fragmented gneisses in the eastern part of the area testify to a brittle regime. Structures in narrow low strain zones that predate the currently observed layering are preserved in rootless isoclinal folds and boudins.
Journal of African Earth Sciences, 2014
MS data are presented for 4 granitoid bodies which intrude high grade gneisses of the previously unmapped Ruangwa region in southern Tanzania. The study area forms part of the late Neoproterozoic East African Orogen (EAO). The oldest unit, a coarse-grained migmatitic granitic orthogneiss gave an early Neoproterozoic (Tonian) crystallization age of 899 ± 9/16 Ma, which is similar to, but significantly younger than, Stenian-Tonian basement ages in areas relatively nearby. Crust of this age may extend as far north as the major Phanerozoic Selous Basin, north of which Archaean protolith ages predominate (the "Western Granulites"), except for the juvenile Neoproterozoic "Eastern Granulites", which are not represented in the study area. To the south, the Tonian crust of the study area provides a tentative link with the Marrupa Complex in NE Mozambique. A granite pluton, dated at 650 ± 5/11 Ma is broadly coeval with the main Pan-African tectono-thermal event in the East African Orogen that is recorded across Tanzania north of the Selous Basin. Zircons in this granite contain inherited cores at ca. 770 Ma. This age is within the range of dates obtained from south and west of the study area from juvenile granitoid orthogneisses which might be related to a widespread, but poorly understood, early phase of Gondwana assembly along an Andean-type margin. South of the study area, in NE Mozambique, the latest orogenic events occurred at ca. 550 Ma, and are sometimes attributed to the Ediacaran-aged "Kuunga Orogeny". While metamorphic dates of this age have been recorded from the EAO north of the Selous Basin, magmatic rocks of this event have not been recognized in Tanzania. The two youngest granitoids of the present study are thus the first 500-600 Ma igneous rocks reported from the region. A weakly deformed very coarse-grained granite pluton was dated at 591 ± 4/10 Ma, while a very late, cross-cutting, undeformed granite dyke gave an intrusive age of 549 ± 4/9 Ma. The granitoids ages presented in this study contain elements that are characteristic of the northern, Tanzania-Kenya, segment of the East African Orogen and of the southern, Mozambique, segment. The Tonian orthogneiss sample is typical of (but somewhat younger than) the Marrupa Complex of NE Mozambique. No zircon inheritance was recorded in the sample, typical of the juvenile Marrupa Complex. On the other hand, the ca. 650 Ma granite pluton has an age that is typical of the northern segment of the orogen; this is the first recorded granite of that age intruded into the Tonian-dominated crust of southern Tanzania or NE Mozambique. The two younger granites have provided dates that are typical of the southern segment of the orogen, and that of the Kuunga Orogen. The study area thus appears to represent an area of transitional crust straddling two complex and contrasting segments of the East African Orogen, with elements of both segments present and evidence for a ca. 770 Ma event which appears to be quite widespread and may relate to the early phases of Gondwana amalgamation in southern East Africa.
Precambrian Research, 1997
The Paleoproterozoic basement of northern Malawi shows evidence for granulite-facies metamorphism older than the 1930+30Ma-old Nyika Granite. Low-pressure granulite-facies metamorphism in regionally coherent and extensively exposed cordierite-garnet granulite reached 750-850°C and 5-5.5 kbar. The Chelinda Granite intruded during this event and has been dated by single zircons at 1995 +__0.4 Ma (2°7pb/2°6pb age; 2G-mean errors). Anatectic melt which formed concurrently with cordierite growth in the cordierite-garnet granulite yielded a 2°7Pb/Z°6pb zircon age of 1988 +0.6 Ma. These ages are interpreted to date the peak of regional low-pressure granulite-facies metamorphism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulitefacies metamorphism. Their zircon ages range from 1930 + 30 Ma to 1969 + 0.4 Ma and constrain the end of regional high-temperature conditions. Enderbitic gneiss, which is tectonically intercalated with the cordierite-garnet granulite, records peak-metamorphic conditions of ca 850-880°C and 9-11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a 2°7pb/2°6pb age of 2002+0.3 Ma. This age for high-pressure granulite-facies metamorphism in northern Malawi is similar to the recently reported age for eclogite-facies metamorphism in the Usagaran Belt of central Tanzania and indicates that orogenic activity during the Ubendian-Usagaran Orogeny culminated at ca 2000 Ma.
Gondwana Research, 2003
The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted MI involved syn-D, high tempcrature/low pressure metamorphism (-4kb and > 8 5 0~) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (-550°C and-5kb) suggests isobaric cooling after MI. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc alkaline magmatic rocks. M, involved syn-D, dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D, tectonism postdated all lithologies in the region, except for synto late-D, granitoid plutons, and is interpreted as a transpressional tectonotherinal reworking of pre-existing (Proterozoic) crust at-1030Ma.
Journal of the Geological Society, 2007
The Taita Hills-Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20-30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630-645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850-960 Ma was derived from zircon cores. Sm-Nd garnet-whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630-645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560-580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.
Journal of the …, 2007
The Taita Hills-Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20-30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630-645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850-960 Ma was derived from zircon cores. Sm-Nd garnet-whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630-645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560-580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.
Journal of Metamorphic Geology, 2003
New data on the metamorphic petrology and zircon geochronology of high-grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan-African event. The metamorphic rocks are characterized by a clockwise P-T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite ⁄ sillimanite-bearing metapelites. The meta-granitoid rocks are of calc-alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet-clinopyroxene-plagioclase-quartz-biotite-amphibole ± K-feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet-biotite-plagioclase ± K-feldspar ± kyanite ⁄ sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12-13 kbar and 750-800°C. Pressures of 5-8 kbar and temperatures of 550-700°C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P-T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970-2500 Ma) and Palaeoproterozoic (c. 2124-1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan-African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan-African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.
U-Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamorphosed during a Pan-African tectonothermal episode. These mineral ages range from 610 to 655 Ma and indicate that peak metamorphic conditions were diachronous in the different granulite domains. U -Pb titanite and rutile ages define integrated cooling rates of 2-5°C/Ma for all investigated granulite areas, and suggest a common process for the post-metamorphic histories of the different granulite areas. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East-and West-Gondwana as the cause of granulite facies metamorphism. Palaeomagnetic data have shown that this collision did not occur until 550 Ma, when the Pan-African granulites in Tanzania had already cooled below 500°C. The high pressure granulites of eastern Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting.
Journal of Petrology, 2022
In collision-type orogens, where high-pressure and ultrahigh-pressure (HP-UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lowerto-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulitefacies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P-T conditions of 2.1-2.6 GPa and 650-860 • C for the M1 stage and 1.4-1.6 GPa and 750-940 • C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7-1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20 • triggers the exhumation of the HP slab sliver with 20-30 Myr eclogite-to granulite transition time of large HP-UHP terranes in major collision zones.
Precambrian Research, 1999
Ion microprobe, evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the Central Zone (CZ) of the Limpopo mobile belt, South Africa define three broad age groups, namely at 3188–3314, 2637–2734 and ca 2500 Ma, respectively. All these granitoids, including the youngest group, display polydeformational features suggesting that much of the tectonic history of the CZ occurred in the late Archaean and early Proterozoic.The oldest group of predominantly tonalitic–trondhjemitic–granodioritic gneisses was found in relatively small domains from the extreme W to the E of the CZ and probably represents remnants of a once more widespread early Archaean terrain. The most abundant granitoid gneisses belong to the 2.6–2.7 Ga age group and were emplaced into already ductilely deformed gneisses of the oldest group. Several of these rocks were derived from intracrustal melting. Rare metamorphic zircons with ages of ∼2560–2575 Ma reflect a high-grade metamorphic event in the CZ that has also been postulated by other workers. A strongly deformed orthogneiss with a protolith emplacement age of ∼2510 Ma proves beyond doubt that at least part of the structural history in the CZ occurred in the early Proterozoic. An anatectic granite in the extreme W of the CZ intruded at ∼2022 Ma and is probably related to the peak of granulite-facies metamorphism previously dated at 2027±6 Ma in the Messina area.The second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite-facies metamorphism in the Northern and Southern Marginal Zones of the Limpopo belt. Broad contemporaneity of extensive granitoid magmatism in all three Zones raises doubts about the exotic nature of the CZ and suggests a revised evolutionary model for the Limpopo belt.