Browsing by Author "Boniface, Nelson"
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Item Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift(Seismological Research Letters, 2016-09-07) Shillington, Donna J.; Gaherty, James B.; Ebinger, Cynthia J.; Scholz, Christopher A.; Selway, Kate; Nyblade, Andrew A.; Bedrosian, Paul A.; Class, Cornelia; Nooner, Scott L.; Pritchard, Matthew E.; Elliott, Julie; Chindandali, Patrick R. N.; Mbogoni, Gaby; Ferdinand, Richard Wambura; Boniface, Nelson; Manya, Shukrani; Kamihanda, Godson; Saria, Elifuraha; Mulibo, Gabriel; Salima, Jalf; Mruma, Abdul; Kalindekafe, Leonard; Accardo, Natalie J.; Ntambila, Daud; Kachingwe, Marsella; Mesko, Gary T.; McCartney, Tannis; Maquay, Melania; O’Donnell, J. P.; Tepp, Gabrielle; Mtelela, Khalfan; Trinhammer, Per; Wood, Douglas; Aaron, Ernest; Gibaud, Mark; Rapa, Martin; Pfeifer, Cathy; Mphepo, Felix; Gondwe, Duncan; Arroyo, Gabriella; Eddy, Celia; Kamoga, Brian; Moshi, MaryThe Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project acquired a comprehensive suite of geophysical and geochemical datasets across the northern Malawi (Nyasa) rift in the East Africa rift system. Onshore/offshore active and passive seismic data, long‐period and wideband magnetotelluric data, continuous Global Positioning System data, and geochemical samples were acquired between 2012 and 2016. This combination of data is intended to elucidate the sedimentary, crustal, and upper‐mantle architecture of the rift, patterns of active deformation, and the origin and age of rift‐related magmatism. A unique component of our program was the acquisition of seismic data in Lake Malawi, including seismic reflection, onshore/offshore wide‐angle seismic reflection/refraction, and broadband seismic data from lake‐bottom seismometers, a towed streamer, and a large towed air‐gun source.Item Contact Metamorphism in the Supracrustal Rocks of the Sukumaland Greenstone Belt in the North West Tanzania(2011) Boniface, NelsonBiotite-granite intrusions in meta-ironstones at Geita Hills and the Bukoli alkali-granite intrusion in metabasites at Mawemeru area produced heat that baked the respective country rocks through epidote-amphibolite- to amphibolite-facies. Critical and informative mineral assemblages in the metairostones of Geita Hills are garnet-grunerite-epidote-quartz and garnetferrogedrite- biotite-quartz and in the metabasites of Mawemeru is ferrotschermakite-(Naplagioclase)- quartz. Peak temperatures ranging between 438°C and 544°C were calculated from the above mineral assemblages and a pressure not exceeding 3 kbar was inferred to from the composition of magnesium-iron amphiboles (grunerite with XFe ratio of 0.83, i.e. Gru83). Hornfels textures in the metaironstones are suggested by euhedral poikiloblastic garnet and quartz with grain boundaries intersecting at approximately 120° (granoblastic polygonal texture) and biotite aggregates forming an interlocking network of elongate grains aligned in all directions and bounded by rational crystal faces (decussate texture).Item A detailed geochronology of the Rungwe Volcanic Province at the southern end of the East African System.(2013) Maqway, M.; Class, C.; Mesko, G. T.; Boniface, Nelson; Manya, ShukraniItem Eburnian, Kibaran and Pan-African Metamorphic Events in the Ubendian Belt of Tanzania: Petrology, Zircon and Monazite Geochronology(2009) Boniface, NelsonOceanic lithosphere subduction is widely accepted among geoscientists as the major driving force of plate motions on Earth. Ocean floor basalts and gabbros are converted into blueschists and eclogites in the process of oceanic lithosphere subduction. Therefore, outcrops of MORB-like chemistry eclogites in orogenic belts manifest the operation of plate tectonics. The assembly of continental blocks to form supercontinents is derived by plate motions, which result into continental crustal growth by accretions of juvenile volcanicarcs, recycling of oceanic crust into the mantle, volcanism and degassing at subduction zones. This might result in a long term global climate change and, more of economic importance, the creation of orogenic settings, which are important sites for metal deposits such as gold, porphyry copper and the volcanogenic massive sulfide copper-zinc ores.Item Explanatory Notes for the Minerogenic Map of Tanzania(Geological Survey of Tanzania (GST), 2015) Leger, C.; Barth, A.; Falk, D.; Mruma, A. H.; Magigita, M.; Boniface, Nelson; Manya, Shukrani; Kagya, M.; Stanek, K. P.As one of its efforts to scale up promotional programs for attracting investments in the development and utilization of Tanzania’s mineral resources, the Geological Survey of Tanzania has made major review of the previously existing Mineral Occurrence Map of Tanzania through verification of location of known occurrences of minerals coupled with thorough evaluation and description of geological processes which account for the forͲ mation of these resources as well as their mineral association. The upgrading of this information went hand in hand with the inclusion of the similar data and information for the recently discovered occurrences of some commodities. As a result of these recent reviews and upgrading of information, a new GISͲbased MineroͲ genic Map of Tanzania at a scale of 1:1,500,000 has been developed and published in 2015. The content of this booklet serves as explanatory notes for this newly published map. The aforeͲstated review of information of mineral occurrences and the subsequent publication of the new map was carried out under the implementation of the Sustainable Management of Mineral Resources Project (2009 to 2015), a project that was funded by the World Bank and the Government of Tanzania. Beak Consultants GmbH of Germany was engaged as the consultant for conducting the review and publishing the map and its explanatory notes. Apart from publishing this map using conventional methods (hard and soft copies) the map and all its associated information and explanatory notes are posted in the newly developed web portal of the Geological Survey of Tanzania established in 2015 with address of www.gmisͲtanzania.com, a portal that serves as a platform for online viewing and searching of geoͲdata and information available at the Geological Survey of Tanzania. The Geological Survey of Tanzania is of the opinion that this new map and its explanatory notes, particularly the one placed on the web portal, will facilitate quick and easy disseminaͲ tion of information on the raw materials in the country to potential investors, stakeholders and the general public across the world. This will also allow onͲline quick querying of available geoͲdata related to the extractive industry in Tanzania and hence attracting more investment to the country therefore paving the way to an accelerated economic growth of the country. The Geological Survey of Tanzania encourages all stakeholders to make a good use of the newly developed Minerogenic Map of Tanzania and its explanatory notes and it is committed to providing additional explanations, data and information whenever required in order to ensure thorough understanding of the countryͲwide existing potentials of the minerals to all potential investors, stakeholders and the general public. Let us join hands and efforts to develop the raw materials for the benefit of Tanzania, her people, the investors and the world community at large in line with the “Win – Win” spirit.Item The First Master Program in Petroleum Geology at the University of Dar es Salaam: Lessons and Challenges(2015-11-17) Bertotti, G.; Boniface, Nelson; de Bresser, J. H. P.; Manya, Shukrani; Nkotagu, H.; van Ruitenbeek, F.The UDSM, supported by group of geoscientists from Universities of the Netherlands has been able to establish the first Master program in Petroleum Geology of the country. With the crucial financial support of BG-Group 13 students has enrolled for the program. Courses have been given in the first year covering a wide range of relevant disciplines. Students have demonstrated a remarkable dedication to the course and have all performed at high level. The second year of the program will be dedicated to research projects developed in close connection with Industry. Challenges for the future include the development of shared data bases and e-learning facilities, the strengthening of the training the trainers component of the project and the establishment of robust relations with IndustryItem A geochemical constraint on the origin of melts using thermobarometry at Rungwe Volcanic Province, Tanzania(2013) Mesko, G. T.; Class, C.; Manya, Shukrani; Maqway, M.; Boniface, NelsonItem Geochronological constraints of the Rungwe Volcanics and its associated Carbonatite intrusion in the East African Rift System , SW, Tanzania(2014) Maqway, M.; Class, C.; Mesko, G. T.; Boniface, Nelson; Manya, ShukraniItem Geochronology of the Central Tanzania Craton and its Southern and Eastern Orogenic Margins(Elsevier, 2016) Thomas, Robert J.; Spencer, Christopher; Bushi, Alphonce M.; Baglow, Nick; Boniface, Nelson; de Kock, Gerrit; Horstwood, Matthew S. A.; Hollick, Louise; Jacobs, Joachim; Kajara, Sperartus; Kamihanda, Godson; Key, Roger M.; Maganga, Zortosy; Mbawala, Fabian; McCourt, W.; Momburi, Philip; Moses, Fadile; Mruma, Abdulkarim; Myambilwa, Yokbeth; Roberts, Nick M. W.; Saidi, Hamisi; Nyanda, Petro; Nyoka, Khalid; Millar, IanGeological 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 volcanosedimentary 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.Item The Mandawa Basin of Coastal Tanzania and its Reservoir Potential(2015) Dypvik, Henning; Einvik-Heitmann, V.; Hudson, W.; Fossum, K.; Karega, A.; Gundersveen, E.; Nerbaten, K.; Mahmic, O.; Hou, G.; Brink, M. V. D.; Andresen, A.; Rwechungura, R.; Boniface, Nelson; Kaaya, C.; Holtar, E.; Schomacker, E.The Mandawa Basin of Coastal Tanzania is located onshore, adjacent (about 80 km offshore) to giant offshore gas discoveries in Lower Cretaceous to Miocene formations. The Mandawa Basin Project is a research and educational project organised between the Universities of Oslo and Dar Es Salaam, the Tanzania Petroleum Development Corporation (TPDC) and Statoil (Tanzania).Item Mesoproterozoic High-Grade Metamorphism in Pelitic Rocks of the Northwestern Ubendian Belt: Implication for the Extension of the Kibaran Intra-Continental Basins to Tanzania(Elsevier, 2007) Boniface, Nelson; Schenk, Volker; Appel, PeterPaleoproterozoic basement rocks are thought to form the northwestern end of the Ubendian Belt in Tanzania that disappears towards the north below a Mesoproterozoic sedimentary cover. The northwestern end of the Ubendian Belt is known to constitute three litho-tectonic terranes of Katuma, Wakole and Ubende. Through dating of zircon (SHRIMP U–Pb) and monazite (U–Th–total Pb electron microprobe ages) of high-grade metasedimentary rocks of the Wakole Terrane we have detected solely Mesoproterozoic ages, showing no sign of reworking of older Paleoproterozoic basement. These findings signify that the Wakole Terrane hosts younger sediments of Mesoproterozoic times metamorphosed to high-grade P–T conditions (peaked at 670–680 °C/8.5–8.9 kbar). Two distinct phases of Mesoproterozoic metamorphic events separated by 160 Ma have been dated at 1166 ± 14 Ma and 1007 ± 6 Ma (SHRIMP U–Pb zircon). Zircon ages are supported by in-situ dating of monazite with ages at 1170 ± 10 Ma, and 1022 ± 5–1016 ± 10 Ma. The first age is closely related to the period of S-type granitoid emplacement at about 1200 Ma in the Karagwe-Ankolean and Kibaran Belts. The second age cluster overlaps with a period of global Mesoproterozoic orogenic cycle, also recorded in the neighboring Irumide and Kibaran Belts. This age group is associated with the assembly of the hypothetical Mesoproterozoic Rodinia Supercontinent. The recent model for the evolution of the Kibaran Belt suitably explains the spatial and temporal settings of Mesoproterozoic metasedimentary rocks of the Wakole Terrane overlying the Paleoproterozoic Ubendian Belt. Due to its proximity to the Kibaran Belt and by being bound by the Paleoproterozoic Terranes of Ubende and Katuma, it can be interpreted that the Wakole Terrane metasediments initially were deposited in an intra-continental basin that was later squeezed between these old terranes by the regional ca. 1000 Ma compressional event recorded in the Irumide, Kibaran and Karagwe-Ankolean Belts.Item Neoproterozoic Eclogites in the Paleoproterozoic Ubendian Belt of Tanzania: Evidence for A Pan-African Suture Between the Bangweulu Block and the Tanzania Craton.(2012) Boniface, Nelson; Schenk, VolkerGeochronological, petrographic and geochemical data from eclogites ofthe Ufipa Terrane in the Ubendian Belt demonstrate that a Pan-African suture zone dated at 593 ± 20, 548 ± 39 and 524 ± 12 Ma (zircon U–Pb SHRIMP) separates the Tanzania Craton from the Bangweulu Block along the Ubendian Belt. These new and surprising data indicate that during the amalgamation of the Gondwana Supercontinent there was a collision between the Archean Cratons of Tanzania and Bangweulu. A clockwise P–T path that climaxed at pressures of 15–20 kbar and temperatures of 610–790 ◦C were estimated for these eclogites. This indicates a relatively warm subduction with a geothermal gradient of about 10–11 ◦C/km. Magmatic precursor rocks of kyanite-free eclogites crystallized in the back-arc (group I eclogite) and island-arc (group II eclogite) tectonic settings. The light rare earth elements (LREEs) of group I eclogites range between 10 and 30 times chondritic values suggesting a depleted mantle source similar to that of mid oceanic ridge basalts (MORB). Group II eclogites display characteristic depletions of high-fieldstrength elements (Na, Ta, Zr and Hf) relative to LREEs that is typical for island-arc basalts. The U–Pb zircon ages at 593 ± 20 and 524 ± 12 Ma from the kyanite-free eclogites have a difference of about 70 Ma. The time interval of this much long is not likely to represent a single subduction event. Therefore, it is more likely that successive accretions of volcanic-arc rocks occurred at 593 ± 20, 548 ± 39 and 524 ± 12 Ma.Item Paleoproterozoic Eclogites of MORB-Type Chemistry and Three Proterozoic Orogenic Cycles in the Ubendian Belt (Tanzania): Evidence from Monazite and Zircon Geochronology, And Geochemistry(Elsevier, 2012) Boniface, Nelson; Schenk, Volker; Appel, PeterAbstract Eclogites and metapelites from the Ubende Terrane of the Proterozoic Ubendian Belt were studied for the purpose of establishing their metamorphic history. Geochemical, petrological and geochronological data of these rocks indicate that oceanic lithosphere was subducted in the Paleoproterozoic and experienced repeated regional metamorphic cycles following their emplacement into continental lithosphere in the Proterozoic. Eclogite facies metamorphic conditions are recorded by well-preserved porphyroclasts of omphacite (Jd17), matrix plagioclase (Ab72) and garnet core, which give a minimum peak pressure of 15 kbar at 700 °C reflecting a geothermal gradient lower than 13 °C/km. These eclogites have chondrite normalized REE patterns that resemble those of N-MORB and E-MORB. U–Pb SHRIMP dating of zircon in the eclogites reveal metamorphic dates of 1886 ± 16 and 1866 ± 14 Ma. These data indicate that eclogites of the Ubende Terrane in the Ubendian Belt may represent former oceanic crust that was metamorphosed during Paleoproterozoic subduction. The subduction was followed by a regional metamorphic event dated at 1831 ± 11 Ma (monazite in metapelite) and 1817 ± 26 Ma (zircon in metapelite). Mylonitic textures are common in all lithologic units of the Ubende Terrane and reflect regional Mesoproterozoic and Neoproterozoic metamorphic overprints. The Mesoproterozoic event is dated at 1091 ± 9 Ma (U–Pb SHRIMP date on zircon rims in a metapelite), while Neoproterozoic dates of 596 ± 41 Ma (U–Pb SHRIMP zircon age of an eclogite) and 601 ± 7 Ma (U–Th total Pb age of a monazite rim from a metapelite) reflect later metamorphic events. The Neoproterozoic mylonitization of eclogites occurred under high-pressure amphibolite-facies conditions at 680–750 °C/10–11 kbar. Highlights ► The Ubendian Belt, southwest margin of the Tanzania Craton, has two suture zones marked by eclogites of the Paleoproterozoic and Neoproterozoic Eras. This paper focuses on the Paleoproterozoic suture zone located in the Terrane of Ubende within the Ubendian Belt. ► The Ubende Terrane eclogites have MORB-type geochemistry and experienced subduction-related metamorphism between 1890 Ma and 1870 Ma. We have inferred a geothermal gradient of less than 13 °C/km and a minimum pressure of 15 kbar. ► After the subduction-related metamorphism another metamorphic event of cryptic nature was recorded in metapelite to have occurred between 1830 Ma and 1820 Ma. This event is probably related to an accretion or a crustal thickening metamorphism that followed after the subduction of an oceanic lithosphere. ► During Mesoproterozoic (1180–1090 Ma) and Neoproterozoic (600–570 Ma) rocks of the Ubendian Belt experienced regional metamorphic events attributed to Kibaran and Pan-African orogenic cycles respectively.Item Petrography and Geochemistry of Mafic Granulites of the Ubendian Belt: Contribution to Insights into the Lower Continental Crust of the Paleoproterozoic(2012) Boniface, NelsonMafic granulites of the Ubendian Belt have geochemical signature, trace and REE patterns, similar to that of the lower continental crustal rocks. A SHRIMP U-Pb single zircon date at 1977±40 Ma indicates that these rocks belong to the Paleoproterozoic Ubendian orogenic cycle, which was followed by the Mesoproterozoic tectonic disturbance that contributed to their exhumation. Orthopyroxene, clinopyroxene, garnet, hornblende, plagioclase and quartz is a frequent mineral assemblage for the mafic granulites of the Ubendian Belt, and garnet frequently form corona between plagioclase and clinopyroxene. Geothermobarometric calculations have revealed the equilibration of these rocks at a temperature range between 650 and 740 °C and pressure between 7 and 10.2 kbar equivalent to the geothermogradient of 22-28 °C/km and a depth of 23-33 km. Garnet coronas between plagioclase and clinopyroxene suggest a typical isobaric cooling P-T path for granulite terranes. The absence of preserved earlier (prograde) mineral assemblage suggests deep burial for these granulites, more than 33 km, and long stay for re-equilibration at relatively shallower crustal levels at 23-33 km deep.Item Polymetamorphism in the Paleoproterozoic Ubendian Belt, Tanzania(2007) Boniface, Nelson; Schenk, VolkerThe Paleoproterozoic (ca. 2.0-1.8 Ga) UbendianUsagaran orogens surrounding the SE and SW margins of the Archaean Tanzania craton contain eclogites of MORBlike chemistry which are among the oldest eclogite occurrences exposed in orogenic belts on Earth. Our study of metamorphic events by petrology and U-Pb SHRIMP dating of zircons is aimed to unravel the orogenic history of the Ubendian Belt southwest of the Tanzania CratonItem Structural Analysis, Metamorphism, and Geochemistry of the Archean Granitoids-Greenstones of the Sukumaland Greenstone Belt Around Geita Hills, Northern Tanzania(2012) Boniface, Nelson; Mruma, Abdul H.Greenstone rocks, which include Banded Iron Formations (BIFs), tuffs, volcanic flows (basalt, andesite and rhyolite), and clastic sedimentary rocks (shale-mudstone, greywacke-sandstone and conglomerate), crop out around Geita Hills and are flanked by granites and granodiorites. BIFs and tuffs occupy larger area than other lithological units, which crop out as patches. Structural analysis indicates that layers of greenstone rocks are folded and display a regional fold axis with an attitude of 320˚/40˚. Low-grade metamorphic mineral assemblages (actinoliteepidote-chlorite in basalts and muscoviteepidote-chlorite in granitoids) are common in these rocks; this indicates a regional metamorphism at greenschist facies. However, BIFs and basalts are locally metamorphosed to epidoteamphibolite and amphibolite facies. Basalts belong to the tholeiite series whereas granites, diorites and rhyolites belong to the calc-alkaline series. Chondrite normalized rare earth element pattern of basalt is flat and plot slightly below the average N-MORB values suggesting the enrichment of the light rare earth elements, which means that mantle magma source was an EMORB. Granitoids and rhyolites have strong affinities to the continental arc source magma displaying strong enrichments in the LREEs with (La/Sm)N values ranging between 2.53 and 3.95 in rhyolites and between 4.08 and 5.40 in granitoids. The granitoids are classified as the I-type synorogenic metaluminous granites and granodiorites. Geochemical signatures suggest that the Geita Hills basalts erupted at the enriched mid ocean ridge setting of the back arc setting, and the granites, granodiorite and rhyolite formed in a volcanic arc setting particularly the continental arc.Item Tectonic History of the Mandawa Basin: Implication from Field Structural Observations, Dem And Magnetic Data(2015-11-17) Mtabazi, E.; Boniface, Nelson; Marobhe, I.; Andresen, A.; Hudson, W.; Didas, M.Our new field structural observations, digital elevation modal (DEM), seismic and magnetic data from the Triassic-Jurassic Mandawa Basin of coastal Tanzania demonstrate tectonic results of Gondwana rifting and dextral strike slip movements associated with the rifting and drifting of Madagascar from East Africa in Jurassic time. The results reveal two major deformation events, in the history of Mandawa Basin formation, named D1 and D2 in this study. The D1 event generated the NNW-SSE trending deep-seated normal faults, and T-fractures. The geometry of these structures suggests that, the ENE-WSW extensional movements, probably associated with the rifting of Gondwanaland during Permo-Triassic time, generated them. The D2 event was the most important deformation episode, which is widely distributed on regional scale as well as on outcrop scale. The NNE-SSW, NNW-SSE and ENE-WSW Riedal shears, dextral strike slip faults, sinistral faults, normal faults and T-fractures characterize D2 event. The D2 event is probably related with the NNW dextral shear zone with NW-SE extensional movements, probably generated during the drifting of Madagascar along the Davie transform fault during the Jurassic time. The geometry of Mandawa Basin suggests pull-apart origin, generated by transtensional event, followed by successful reactivations.Item The Timing of Early Magmatism and Extension in the Southern East African Rift: Tracking Geochemical Source Variability with 40Ar/39Ar Geochronology at the Rungwe Volcanic Province, SW Tanzania(2014) Mesko, G. T.; Class, C.; Maqway, M. D.; Boniface, Nelson; Manya, Shukrani; Hemming, S. R.The Rungwe Volcanic Province is the southernmost expression of volcanism in the East African Rift System. Rungwe magmatism is focused in a transfer zone between two weakly extended rift segments, unlike more developed rifts where magmatism occurs along segment axes (e.g. mid-ocean ridges). Rungwe was selected as the site of the multinational SEGMeNT project, an integrated geophysical, geochronological and geochemical study to determine the role of magmatism during early stage continental rifting. Argon geochronology is underway for an extensive collection of Rungwe volcanic rocks to date the eruptive sequence with emphasis on the oldest events. The age and location of the earliest events remains contested, but is critical to evaluating the relationship between magmatism and extension. Dated samples are further analyzed to model the geochemistry and isotopic signature of each melt's source and define it as lithospheric, asthenospheric, or plume. Given the goals, the geochronology focuses on mafic lavas most likely to preserve the geochemical signature of the mantle source. Groundmass was prepared and analyzed at the LDEO AGES lab. Twelve preliminary dates yield ages from 8.5 to 5.7Ma, consistent with prior results, supporting an eruptive episode concurrent with tectonic activity on the Malawi and Rukwa border faults (Ebinger et al., JGR 1989; 1993). Three additional samples yield ages from 18.51 to 17.6 Ma, consistent with the 18.6 ±1.0 Ma age obtained by Rasskazov et al. (Russ. Geology & Geophys. 2003). This eruptive episode is spatially limited to phonolite domes in the Usangu Basin and a mafic lava flow on the uplifted Mbeya Block. These eruptions predate the current tectonic extensional structure, suggesting magmatism predates extension, or that the two are not highly interdependent. No Rungwe samples dated yet can be the source of the of 26Ma carbonatitic tuffs in the nearby Songwe River Basin sequence (Roberts et al., Nature Geoscience 2012). Isochron ages from mica separates of two carbonatite complexes upstream in the drainage basin were dated and yield Jurassic ages of 165.7 ±1.3 Ma for Panda Hill and 154.2 ±0.9 Ma for Mbalizi, older than prior age estimates (Bowden, Nature 1962; Pentel'kov & Voronovskly, Doklady Akad Nauk 1977). These results leave the source of tuffs in the Songwe River Basin unresolved.Item The Timing of Early Magmatism and Extension in the Southern East African Rift: Tracking Geochemical Source Variability with 40Ar/39Ar Geochronology at the Rungwe Volcanic Province, SW Tanzania(2014-12) Mesko, G. T.; Class, C.; Maqway, M. D.; Boniface, Nelson; Hemming, S. R; Manya, ShukraniThe Rungwe Volcanic Province is the southernmost expression of volcanism in the East African Rift System. Rungwe magmatism is focused in a transfer zone between two weakly extended rift segments, unlike more developed rifts where magmatism occurs along segment axes (e.g. mid-ocean ridges). Rungwe was selected as the site of the multinational SEGMeNT project, an integrated geophysical, geochronological and geochemical study to determine the role of magmatism during early stage continental rifting. Argon geochronology is underway for an extensive collection of Rungwe volcanic rocks to date the eruptive sequence with emphasis on the oldest events. The age and location of the earliest events remains contested, but is critical to evaluating the relationship between magmatism and extension. Dated samples are further analyzed to model the geochemistry and isotopic signature of each melt's source and define it as lithospheric, asthenospheric, or plume. Given the goals, the geochronology focuses on mafic lavas most likely to preserve the geochemical signature of the mantle source. Groundmass was prepared and analyzed at the LDEO AGES lab. Twelve preliminary dates yield ages from 8.5 to 5.7Ma, consistent with prior results, supporting an eruptive episode concurrent with tectonic activity on the Malawi and Rukwa border faults (Ebinger et al., JGR 1989; 1993). Three additional samples yield ages from 18.51 to 17.6 Ma, consistent with the 18.6 ±1.0 Ma age obtained by Rasskazov et al. (Russ. Geology & Geophys. 2003). This eruptive episode is spatially limited to phonolite domes in the Usangu Basin and a mafic lava flow on the uplifted Mbeya Block. These eruptions predate the current tectonic extensional structure, suggesting magmatism predates extension, or that the two are not highly interdependent. No Rungwe samples dated yet can be the source of the of 26Ma carbonatitic tuffs in the nearby Songwe River Basin sequence (Roberts et al., Nature Geoscience 2012). Isochron ages from mica separates of two carbonatite complexes upstream in the drainage basin were dated and yield Jurassic ages of 165.7 ±1.3 Ma for Panda Hill and 154.2 ±0.9 Ma for Mbalizi, older than prior age estimates (Bowden, Nature 1962; Pentel'kov & Voronovskly, Doklady Akad Nauk 1977). These results leave the source of tuffs in the Songwe River Basin unresolved.