Compressional reworking of the East African Orogen in the Uluguru Mountains of eastern Tanzania at ~550 Ma: implications for the final assembly of Gondwana

The Uluguru Mountains of eastern Tanzania represent a key area to unravel the tectonic history of Gondwana assembly along the Neoproterozoic East African Orogen. On the basis of combined structural and 40Ar/39Ar geochronological investigations, we have re-interpreted the structural architecture of the region. Texturally-late, ductile to cataclastic low-angle tectonic contacts are recognised, which delimit the contacts between the main lithological units that form a post-metamorphic, NW-verging nappe stack. 40Ar/39Ar geochronology places this nappe-forming event at about 550 Ma, post-dating tonalitic dyke emplacement at about 580 Ma. Results from this study (i) demonstrate that a distinct and later episode of convergence occurred in the region at about 550 Ma, when previous structural elements of the East African Orogen were finally juxtaposed; and (ii) allow to extend models involving the polyphase assembly of Gondwana to East Afric

Età K-Ar delle ignimbriti permiane di Galtellì (Orosei- Sardegna orientale)

Vengono riportati i primi dati petrografici e di geocronologia K-Ar relativi alle vulcaniti permiane affioranti lungo la Valle del Cedrino, nelle immediate vicinanze di Galtellì

Stratigraphy, geochronology and evolution of the Mt. Melbourne volcanic field (North Victoria Land, Antarctica)

Mt. Melbourne (2,732 ma.s.l.) is a large quiescent stratovolcano located in Northern Victoria Land (Antarctica) and is one of a handful of volcanoes on the Antarctic plate with the potential for large-scale explosive eruptions. During the XVIII Italian Expedition in 2002– 2003, the Mt. Melbourne volcanic succession was studied in terms of stratigraphy and sampled for 40Ar/39Ar age determinations and geochemistry. The early, Lower Pleistocene, volcanism was largely alkali basaltic to hawaiitic in composition and monogenetic in style, producing tens of small scoria cones and lava flows scattered over a wide area across the Transantarctic Mountains (Random Hills Period). During the Middle Pleistocene, volcanic activity focused to the area of the Mt. Melbourne stratovolcano, where several monogenetic centres show the transition from early subglacial/ subaqueous conditions to emergent subaerial conditions (Shield Nunatak Period). The oldest exposed deposit associated with the early activity of the Mt. Melbourne stratovolcano (Mt. Melbourne Period) is a trachytic subaerial ignimbrite dated at 123.6±6.0 ka, which reflects the establishment of a crustal magma chamber. Above the ignimbrite a succession of alkali basaltic, hawaiitic, and subordinate benmoreitic lavas and scoria cones is exposed, dated at 90.7±19.0 ka. The Holocene deposits are exposed at the top of Mt. Melbourne, where the crater rim is composed of trachytic to rhyolitic pumice fall deposits, which are also extensively dispersed around the volcano, likely originated from Plinian-scale eruptions. The most recent explosive deposit proved difficult to date accurately because very low quantities of radiogenic 40Ar were released, resulting in imprecise plateau ages of 50±70 and 35±22 ka

Petrogenesis of the Sabongari alkaline complex, cameroon line (central Africa): Preliminary petrological and geochemical constraints

The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene-amphibole (the most abundant), amphibole-biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole-pyroxene and amphibole-biotite quartz syenites; (iii) pyroxene-amphibole-biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite-trachyte-granite (Group 1) bimodal series (SiO2 gap: 44 and 63wt.%) and a basanite-microdiorite-monzodiorite-syenite-granite (Group 2) less pronounced bimodal series (reduced SiO2 gap: 56-67wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO2-undersaturated (modal analcite in the groundmass and 11.04wt.% normative nepheline); its Ni (240ppm) and Cr (450ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO2-saturated (1.56wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line. © 2013 Elsevier Ltd

Stratigraphy, geochronology and evolution of the Mt. Melbourne volcanic field (North Victoria Land, Antarctica)

Mt. Melbourne (2,732 m a.s.l.) is a large quiescent stratovolcano located in Northern Victoria Land (Antarctica) and is one of a handful of volcanoes on the Antarctic plate with the potential for large-scale explosive eruptions. During the XVIII Italian Expedition in 2002-2003, the Mt. Melbourne volcanic succession was studied in terms of stratigraphy and sampled for Ar-40/Ar-39 age determinations and geochemistry. The early, Lower Pleistocene, volcanism was largely alkali basaltic to hawaiitic in composition and monogenetic in style, producing tens of small scoria cones and lava flows scattered over a wide area across the Transantarctic Mountains (Random Hills Period). During the Middle Pleistocene, volcanic activity focused to the area of the Mt. Melbourne stratovolcano, where several monogenetic centres show the transition from early sub-glacial/subaqueous conditions to emergent subaerial conditions (Shield Nunatak Period). The oldest exposed deposit associated with the early activity of the Mt. Melbourne stratovolcano (Mt. Melbourne Period) is a trachytic subaerial ignimbrite dated at 123.6 +/- 6.0 ka, which reflects the establishment of a crustal magma chamber. Above the ignimbrite a succession of alkali basaltic, hawaiitic, and subordinate benmoreitic lavas and scoria cones is exposed, dated at 90.7 +/- 19.0 ka. The Holocene deposits are exposed at the top of Mt. Melbourne, where the crater rim is composed of trachytic to rhyolitic pumice fall deposits, which are also extensively dispersed around the volcano, likely originated from Plinian-scale eruptions. The most recent explosive deposit proved difficult to date accurately because very low quantities of radiogenic Ar-40 were released, resulting in imprecise plateau ages of 50 +/- 70 and 35 +/- 22 ka