Geochronology of granitic rocks from the Ruangwa region, southern Tanzania: links with NE Mozambique and beyond

New U–Pb zircon LA-ICP-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

Geochronology and structure of the eastern margin of the Tanzania Craton east of Dodoma

The precise position, nature and U-Pb zircon geochronology of the eastern margin of the Tanzania Craton has been studied in the Mpwapwa area, some 60 km east of Dodoma, central Tanzania, in a number of field transects over a ca. 45 km strike length of the craton margin. The rocks to the east of the Tanzania Craton in this area either belong to the Palaeoproterozoic Usagaran belt, or the “Western Granulite” terrane of the Neoproterozoic East African Orogen, according to different authors. The eastern part of the craton is underlain by typical Neoarchaean migmatitic grey granodioritic orthogneisses dated by ICP-MS at 2674 ± 73 Ma. There is a gradual increase in strain eastwards in these rocks, culminating in a 1 to 2 km wide, locally imbricated, ductile thrust/shear zone with mylonites indicating an oblique top-to-the-NW, transpressional sense of movement. East of the craton-edge shear zone, a series of high-grade supracrustal rocks are termed the “Mpwapwa Group”, in view of uncertain age and regional lithostratigraphic correlations. There is an apparent east-west lithological zonation of Mpwapwa Group parallel to the craton margin shear zone. In the west, immediately adjacent to the craton, the group consists of typical “shelf facies” metasediments (marbles, calc-silicates, quartzites etc.). U-Pb dating of detrital zircons from two Mpwapwa Group quartzite samples from this marginal zone contain only Archaean detritus, constraining their maximum depositional age to > ca. 2.6 Ga and suggesting that the group is Neoarchaean in age. The shelf rocks pass eastwards into garnet and kyanite-bearing semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, where the mafic amphibolite gneisses may represent meta-basalts and the felsic rocks may have meta-rhyolite, -granite or –psammite protoliths. Massive garnet-clinopyroxene amphibolite layers in the Mpwapwa Group gneisses may have been intrusive mafic sills and possibly correlate with the Palaeoproterozoic Isimani Suite, which outcrops south of the study area and includes 2 Ga eclogites. Zircons from a quartzo-feldsapthic gneiss sample from the bimodal gneisses were dated and showed it to be a probable Neoarchaean rock which underwent metamorphism during the Palaeoproterozoic Usagaran event at ca. 1950 Ma. This event was broadly coeval with subduction, closure of an ocean basin and eclogite formation further south and led to the initial juxtaposition of the two Archaean blocks. The metamorphism probably dates the tectonic event when the Archaean Mpwapwa Group rocks were juxtaposed against the orthogneissic Tanzania Craton. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1871 ± 35 Ma. Zircons in the granite have metamorphic rims dated between 550 and 650 Ma that grew during the East African orogenic event

Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania

A study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern part of the craton comprises typical Neoarchaean grey granodioritic orthogneisses dated with the LA-ICP-MS method by U–Pb zircon at ca. 2.7 Ga. A gradual eastward increase in strain in these rocks culminates in a 1–2 km wide, locally imbricated, north–south-trending, ductile thrust/shear zone with an oblique top-to-the-NW sense of movement. East of the craton-edge shear zone, high-grade supracrustal rocks are termed the “Mpwapwa Group” in view of uncertain regional correlations. There is an apparent lithological zonation of the Mpwapwa Group parallel to the craton margin shear zone. In the west, the group consists of typical “shelf facies” metasedimentary rocks (marbles, calc-silicates, quartzites, etc.). U–Pb dating of detrital zircons from two quartzites reveal only Archaean detritus, constraining their maximum depositional age to <ca. 2.6 Ga. The shelf rocks pass eastwards into semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, which may have volcanic protoliths and are intruded by multiple mafic sills. Dating of a felsic gneiss showed it to probably be a Neoarchaean rock which underwent Palaeoproterozoic metamorphism during the Usagaran event at ca. 1930 Ma. This date likely represents initial juxtaposition of the (Archaean) Mpwapwa Group against the Tanzania Craton. Consequently the Group are not considered to be stratigraphic correlates of the Usagaran Konse Group or Isimani Suite seen further south, but rather form part of the Neoarchaean crust of the “Western Granulite” terrane of the East African Orogen. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1873 ± 31 Ma. Zircons in the granite have metamorphic rims dated between 550 and 650 Ma that grew during the East African orogenic event. Distinctive para- and orthogneisses, including charnockite dated at 2707 ± 21 Ma, outcrop in the southern part of the area. A sliver of southern tonalitic orthogneiss, entrained in the craton margin shear zone, is an igneous rock of probable early Palaeoproterozoic or Archaean age (>ca. 2300 Ma) with a strong metamorphic overprint at ca. 1960 Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990 Ma, again believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks. The role of the Neoproterozoic East African orogeny in the evolution of the craton margin is unclear, but the geometry of the shear zone, the presence of Neoproterozoic zircon rims in the Palaeoproterozoic granite and published studies from nearby, all suggest that the latest movements on the shear zone may be Neoproterozoic in age and that the structure may represent the local western front of the East African Orogeny

Geochronology of the central Tanzania Craton and its southern and eastern orogenic margins

Geological mapping and zircon U-Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820 Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610 Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcano-sedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725 Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740 Ma, with additional detrital sources 2820 and 2940 Ma. Thus, 200 Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism. In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630 Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran "Pan-African" isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675 Ma, coeval with previous dates of the "Eastern Granulites" of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730 Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa "block" have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920 Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870-1900 Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique. In SW Tanzania, rocks from the Mgazini area gave Ubendian protolith ages of ca. 1980-1800 Ma, but these rocks underwent Late Mesoproterozoic high-grade metamorphism between 1015 and 1040 Ma. One granitoid gave a crystallisation age of ca. 1080 Ma correlating with known Mesoproterozoic crust to the east in SE Tanzania and NE Mozambique. However, while the crust in the Mgazini area was clearly one of original Ubendian age, reworked and intruded by granitoids at ca. 1 Ga, the crust of SE Tanzania is a mixed Mesoproterozoic terrane and a continuation from NE Mozambique. Hence the Mgazini area lies at the edge of the Ubendian belt which was re-worked during the Mesoproterozoic orogen (South Irumide belt), providing a further constraint on the distribution of ca. 1 Ga crust in SE Africa.Hf data from near-concordant analyses of detrital zircons from a sample from the Tanzania Craton lie along a Pb-loss trajectory (Lu/Hf = 0), extending back to ~3.9 Ga. This probably represents the initial depleted mantle extraction event of the cratonic core. Furthermore, the Hf data from all igneous samples, regardless of age, from the entire study area (including the Neoproterozoic granulite nappes) show a shallow evolution trend (Lu/Hf = 0.028) extending back to the same mantle extraction age. This implies the entire Tanzanian crust sampled in this study represents over 3.5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the Tanzanian Craton that was subsequently reworked was composed of a very depleted, mafic source with a very high Lu/Hf ratio. Our study helps to define the architecture of the Tanzanian Craton and its evolution from a single age-source in the early Eoarchaean