Kodjopa Attoh

Lithostratigraphy and Composition of 2.1 Ga Greenstone Belts of the West African Craton and Their Bearing on Crustal Evolution and the Archean-Proterozoic Boundary

We have determined the lithostratigraphy of three, Early Proterozoic (~2.1 Ga), Birimian greenstone belts in the West African craton and measured the concentrations of major and trace elements in samples of volcanic rocks from those belts. The Nangodi belt largely consists of tholeiitic basalt flows with mg numbers of 37 to 69 and flat to slightly LREE-depleted chondrite-normalized REE patterns, overlain by sodic (

High‐Pressure Granulite Facies Metamorphism in the Pan‐African Dahomeyide Orogen, West Africa

Na_{2}O/K_{2}O = 3.7-29

Metamorphic Temperatures in the Michigamme Formation Compared with the Thermal Effect of an Intrusion, Northern Michigan

), calc-alkaline, dacitic tuffs and lavas that are HREE-depleted (La/Yb = 4.5-8.9 times chondrites, Yb = 3.9-8.7 times chondrites). The volcanic sequence is capped by calc-alkaline andesitic crystal tuffs with high mg numbers (66-75) and a high La/Yb ratio (10.5 times chondrites) similar to Archean and modern sanukitoids. The Dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas (mg numbers = 52-61) that grade upward into high-Mg (mg numbers ~59), calc-alkaline, andesitic tuffs and flows. The Winneba belt largely consists of massive, tholeiitic, high-Mg and siliceous high-Mg basalt flows with mg numbers of 64-72 and La/Sm ratios of 0.8-1.3 times chondrites, overlain by felsic tuffs and flows including a sodic, calc-alkaline, rhyolite with a La/Yb ratio of 16 times chondrites. The association of tholeiitic basalts and sodic, calc-alkaline, felsic volcanic rocks found in Birimian greenstone belts differs from the association of calc-alkaline basalts and potassic felsic rocks which comprises Proterozoic greenstone belts in North America and Scandinavia, but is similar to that present in modern, immature, intraoceanic island arcs and many Archean greenstone belts, which may have formed in ancient island arcs. Like their Archean counterparts, Birimian tholeiites have La/Yb ratios < 4, La/Ta ratios < 35, and Hf/Ta ratios < 10, and most Birimian felsic rocks have chondrite-normalized Yb enrichment factors of < 8. Like modern, wedge-derived arc tholeiites, Birimian tholeiites have higher Ba/La, Rb/La, Th/La and Sr/Nd ratios, and tend to have lower Ta/La ratios, than the corresponding ratios in NMORB. Birimian dacites have high Ba/La, Rb/La and Th/La ratios relative to NMORB, as do modern-arc dacites and, in addition, have high Sr/Nd and Tb/Yb ratios, relative to NMORB, which are particularly distinctive characteristics of modern-arc dacites derived by direct melting of the subducted slab. Taken together, these data suggest that magmas formed in Early Proterozoic, West African island arcs had compositions broadly similar to magmas formed in many Archean island arcs and, hence, the Archean-Proterozoic boundary may not coincide with a worldwide, fundamental change in crust-mantle evolution.

Contrasting Metamorphic Record of Heat Production Anomalies in the Penokean Orogen of Northern Michigan

Abstract The Pan-African Dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the West African Craton was incorporated into Gondwana. Orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in Ghana and Togo. External nappes include the Atacora, composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c. 2.0 Ga foreland basement. The suture zone is represented by high-pressure granulite facies garnet homblende gneiss. We have analyzed muscovite and homblende concentrates from external and suture-zone nappes in incremental heating 40 Ar 39 Ar experiments. Muscovites which define the penetrative foliation in the Atacora nappes and a shear foliation in the basement parautochthon record 40 Ar 39 Ar plateau ages of 579.4 ± 0.8 Ma and 578.1 ± 0.5 Ma respectively. In contrast, c. 600 km to the north, muscovite in quartz schist near the base of the Atacora nappes displays a spectrum which gives a minimum age of 608.1 ± 1.2 Ma, whereas, muscovite in the overthrust Kara nappes yields a plateau age of 633.8 ± 0.5 Ma. The c. 30 m.y. difference between the Atacora nappes in the northern Dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication, however, forward-imbricating thrusting is also suggested by the older age of Kara nappes relative to Atacora nappes in the northern segment. The youngest ages are inferred to date orogen parallel nappe transport at c. 575 Ma. Hornblende from the structurally lower sections of the suture-zone nappes record a 36 Ar 40 Ar versus 39 Ar 40 Ar isotope correlation age of 587 ± 4.3 Ma. Hornblende from a tonalitic vein in the host garnet-mafic gneiss yields an isotope correlation age of 581.9 ± 2.4 Ma. These ages are interpreted to date the exhumation of the suture-zone nappes and thus provide a minimum age of crustal thickening in the Dahomeyide orogen. The age data are consistent with coeval forward- and hindward- propagating thrusting and nappe imbrication, and constrain the amalgamation in northwest Gondwana to have occurred in the latest Proterozoic rather than Cambrian.

Petrological and Geochemical Characteristics of Mafic Granulites Associated with Alkaline Rocks in the Pan-African Dahomeyide Suture Zone, Southeastern Ghana

High‐pressure (HP) granulite facies rocks are associated with eclogites in the Pan‐African Dahomeyide orogen, West Africa. The HP‐granulite and eclogite facies mineral assemblages occur in mafic gneisses within the suture zone exposed in southeastern Ghana and the adjoining part of Togo. Thermobarometric calculations and phase equilibria considerations indicate that the HP metamorphic rocks preserve a record of: (i) prograde garnet crystallization during eclogite facies metamorphism as temperature increased to a maximum of ∼600°C and pressure reached >18 kbar, followed by (ii) HP‐granulite facies metamorphism when temperatures reached 800–900°C at P ∼14 kbar, resulting in partial melting to produce leucocratic veins. Decompression along a retrograde path is inferred from granulites that yield temperatures down to ∼700°C at ∼10 kbar. The inferred P‐T path of the metamorphic record defines a clockwise loop in the kyanite stability field, compatible with orogeny involving subduction during the Pan‐African assembly of Gondwana.

Deep structure of the southeastern margin of the West African craton from seismic reflection data, offshore Ghana

Abstract The association of carbonatite and ultrahigh-pressure (UHP) metamorphic rocks in the Dahomeyide suture zone of southeastern Ghana is unique among the Neoproterozoic orogens that surround the West African craton (WAC). Carbonatite occurs in an alkaline complex that decorates the sole thrust of the suture zone and is characterized by high concentrations of incompatible trace elements such as light rare earth elements (LREE), Sr and Ba. Within the suture zone deformed alkaline rocks, including carbonatite, together with mafic granulites form an imbricate stack of thrust panels that involve 2.1 Ga rocks of the WAC basement. The dominant rock unit of the suture zone is composed of mafic granulites in which garnet megacrysts preserve a diagnostic microstructure of UHP metamorphism; it consists of a crystallographically controlled array of exsolved rutile rods in garnet. Metamorphic pressures estimated from Ti concentrations in the inferred precursor garnet indicate P>3 GPa, which requires subduction (and exhumation) of the suture zone rocks to and from mantle depths during collisional orogeny on the WAC margin. Available age constraints on carbonatite magmatism suggest that continental rifting, leading to the formation of the passive WAC margin c. 700 Ma, occurred c. 100 Ma before intrusion of carbonatite, which was preceded by HP and UHP metamorphism at 610±5 Ma.

Geochemistry of an ultramafic-rodingite rock association in the Paleoproterozoic Dixcove greenstone belt, southwestern Ghana

Temperature and pressure conditions of metamorphism in the Peavy Node area in northern Michigan have been estimated from the garnet-biotite geothermometer and garnet-plagioclase-aluminosilicate-quartz geobarometer. Sillimanite zone rocks yielded an average temperature ~592°C; in the staurolite zone temperatures ranged from 545° to 582°C and in the garnet zone temperature estimates were 485 to 557°C. The pressure estimate of sillimanite-bearing assemblages is 4.4 kb whilst that of andalusite-bearing rocks is 2.9 kb at 534°C. Temperatures due to the Peavy Pond Complex (PVC) intrusion as a heat source were calculated at various positions within the Michigamme Fm assuming that the magma was emplaced at 1000°C and taking into account the latent heat of solidification of magma and the heat consumed by metamorphic dehydration reactions. If cooling proceeds by conduction, a one-dimensional 3.2 km-thick sheet heat source reaches a maximum temperature of 355°C at 2.5 km from contact, within the staurolite zone. During cooling involving convection of the magma, the furthest advance of the 500°C-isotherm is 1.1 km from contact 105 years after intrusion. A two-dimensional 3.2 x 4.4 km magmatic heat source gives a maximum temperature of 200°C at 1.5 km from contact, which is within the sillimanite zone to the south and the staurolite zone to the west. If the upper limit of initial temperature in the Michigamme Fm was 300°C, the calculated temperature due to the rectangular prism heat source was ~504°C at the same distance as the 250°C isotherm if initial temperature was ~0°C. The maximum displacement of 504/250°C and 383/100°C isotherms are respectively 1.1 and 2.7 km from the contact whereas the observed metamorphic temperatures at those positions are 590°C and 560°C respectively. Thus heat transfer by conduction from the intrusion into surrounding rocks appears to have been inadequate to give rise to the observed metamorphic temperatures in the Michigamme Fm.

U-Pb zircon age of deformed carbonatite and alkaline rocks in the Pan-African Dahomeyide suture zone, West Africa

It is proposed that the contrasting metamorphic mineral assemblages of the isolated amphibolite facies metamorphic highs in the Penokean orogen of northern Michigan may be caused by different heat production rates in the Archean basement. This hypothesis is based on concentrations of K, U, and Th in the Archean basement gneisses and Paleoproterozoic metasediments that indicate significant contribution of radiogenic heating during Penokean metamorphism. Heat production was anomalously high (∼10.6 μWm−3) where andalusite‐bearing mineral assemblages indicate that high temperatures were attained at shallow crustal levels (∼550°–600°C at ∼3 kbar). In contrast, where exposed metamorphic rocks indicate peak temperatures of 600°–650°C at 6–7 kbar, heat production in the Archean basement was lower (∼3.7 μWm−3). The effect of heat production rates on the metamorphic pressure–temperature paths was tested with numerical thermal models. The calculations show (1) that if the heat production rate, where andalusite‐bearing assemblages formed, was significantly <6.0 μWm−3, the estimated pressure at peak temperatures ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape