Search results for "Proterozoic"

Proterozoic mobile belts compatible with the plate tectonic concept

1983

Tectonic models for accretion of the Central Asian Orogenic Belt

2007

The Central Asian Orogenic Belt ( c . 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with…

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?

2007

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly 91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al2O3 contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peri…

Proterozoic melting in the northern peridotite Massif, Zabargad Island: Os isotopic evidence

1999

The geodynamic history of the three Zabargad peridotite bodies is the key to their significance as samples of the upper mantle. Currently, there are two main hypotheses that differ greatly in their implications for the origin and age of the peridotite complex as a whole. In the first, the Zabargad peridotite bodies all represent young asthenospheric mantle that was juxtaposed with ancient crustal gneisses during the opening of the Red Sea. In the second, the complex may represent a single package of residual mantle and lower crust of Pan-African age that was uplifted during the Miocene. In order to distinguish between these two models, we have analysed five samples of Zabargad peridotite an…

Tectonic evolution in the Archaean and Proterozoic

1991

Kre high heat flow probably favoured global hotspot activity with crustal growth of Iceland type through vertical magmatic accretion. The tectonic environment for the generation of about 3.9-2.5 Ga granite-gneiss-greenstone terrains with volurnlnous production of tonalite-trondhjemite-granodlorite magmas is still uncertain, and both piate ma@ and intrapiate scenarios arc possible. Deformanon atylea in greenstones and adjacent high-grade orthognersses with intercalated shallow-water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking. This probably resulted from collisional and/or rotational motion and produced significant intracrustal melting as early …

Areal extent and first U-Pb age of the Pre-Damaran Abbabis complex in the central Damara belt of South West Africa (Namibia)

1978

The Proterozoic Abbabis Complex emerges from beneath the metasediments of the Late Precambrian Damara Supergroup within mantled gneiss domes and anticlinoria over a wide area in the central granite zone of the Pan African Damara belt. U-Pb dating of composite-, size-, and non-magnetic fractions of metamict zircons from two samples of Abbabis granite-gneiss from the farm Abbabis yielded a concordia intercept age of 1925 −280 +330 Ma. It is speculated that the Abbabis basement rocks are continuous under the Damaran metasedimentary cover and may link up with the Franzfontein basement inlier of similar age. This conclusion supports the ensialic character of the northeastern branch of the Damara…

Crustal age domains in the Kibaran belt of SW-Uganda: Combined zircon geochronology and Sm–Nd isotopic investigation

2008

Abstract Combination of geochemistry, Nd mean crustal residence ages and single zircon ages reveal distinct provinces not recognized previously within the Kibaran belt of SW-Uganda. Geochemical analyses (HFSE and REE) of four granitoids, namely the Ntungamo, Chitwe, Rwentobo and Kamwezi from the Karagwe Tinfield have been interpreted to be mainly composed of melt components from subduction modified parts of the continental lithospheric mantle. In addition we can demonstrate, from these distinctive batholiths crystallization ages that lead to the result of three age groups, one at around 1566 Ma, the second at 1444 Ma to and the third clusters between 1329 Ma and 1363 Ma. The eNd(t) values f…

Precambrian basement around Wadi Halfa, Sudan: a new perspective on the evolution of the East Saharan Craton

1994

This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African (~ 500-900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northern- most Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisse…

La/Nb variation in Proterozoic and Mesozoic mafic dyke swarms in Brazil

1989

Geochronology and palaeomagnetism of a late Proterozoic island arc terrane from the Red Sea Hills, northeast Sudan

1992

Abstract Size fractions and single grains of zircon, hornblende and biotite have been dated to define the formation age and thermal history of the northern Haya Terrane (NHT), a late Proterozoic island arc terrane of the Red Sea Hills (RSH), northeast Sudan. U Pb and 207 Pb/ 206 Pb zircon data for acidic volcanics and granitoid rocks bracket the time of formation of the NHT between 887 ± 20 and 854 ± 9 Ma. 40 Ar/ 39 Ar hornblende and biotite ages of a gabbro near Suakin suggest passage through the argon closing temperature for hornblende at about 841 ± 4 Ma and 782 ± 6 Ma for biotite. The hornblende age is considered to be representative of the age of magnetic blocking and, hence, the age o…