Possible origin of K-rich volcanic rocks from Virunga, East Africa, by metasomatism of continental crustal material: Pb, Nd and Sr isotopic evidence

1983 ◽  
Vol 64 (3) ◽  
pp. 374-386 ◽  
Author(s):  
R. Vollmer ◽  
M.J. Norry
Keyword(s):  
East Africa ◽  
Volcanic Rocks ◽  
Crustal Material ◽  
Isotopic Evidence ◽  
Continental Crustal Material

Energy in Prehistory

2017 ◽  
Author(s):  
Vaclav Smil
Keyword(s):  
East Africa ◽  
Tool Use ◽  
Foraging Strategies ◽  
Small Populations ◽  
Origins Of Agriculture ◽  
High Quality ◽  
Isotopic Evidence

This chapter discusses the use of energy during prehistoric times. Our direct ancestors spent their lives as simple foragers, and it was only about 10,000 years ago that the first small populations of our species began a sedentary existence based on the domestication of plants and animals. This means that for millions of years, the foraging strategies of hominins resembled those of their primate ancestors, but we now have isotopic evidence from East Africa that by about 3.5 million years ago hominin diets began to diverge from those of extant apes. The chapter first considers how bipedalism started a cascade of enormous evolutionary adjustments such as adaptations underlying tool use and adaptation to high-quality, energy-dense foods (meat, nuts) before providing an overview of foraging societies and the origins of agriculture.

Age and petrogenesis of the Qassiarsuk carbonatite-alkaline silicate volcanic complex in the Gardar rift, South Greenland

1997 ◽  
Vol 61 (407) ◽  
pp. 499-513 ◽  
Author(s):  
Tom Andersen
Keyword(s):  
Volcanic Rocks ◽  
Single Stage ◽  
Volcanic Complex ◽  
Crustal Material ◽  
Crustal Component ◽  
Depleted Mantle ◽  
Common Parent ◽  
Nd Model Age ◽  
Extrusive Carbonatite ◽  
Age Estimates

AbstractThe Qassiarsuk (formerly spelled Qagssiarssuk) complex is located in a roughly E–W trending graben structure between Qassiarsuk village and Tasiusaq settlement in the northern part of the Precambrian Gardar rift, South Greenland. The complex comprises a sequence of alkaline silicate tuffs and extrusive carbonatites interlayered with sandstones, and their subvolcanic equivalents, which represent possible feeders for the extrusive rocks. The Rb-Sr, Sm-Nd and Pb isotopic characteristics of 65 samples of extrusive carbonatite- and silicate tuffs and carbonatite diatremes have been determined by mass spectrometry. The Qassiarsuk complex can be dated to c. 1.2 Ga by Rb-Sr and Pb-Pb isochrons on whole-rocks and mineral separates, agreeing with previous isotopic ages for the volcanic rocks of the Eriksfjord formation in the Eriksfjord area of the Gardar rift, but not with previous, indirect age estimates of >1.31 Ga for assumed Eriksfjord equivalents in the Motzfeldt area further east. Recalculated isotopic compositions at 1.2 Ga indicate that the Qassiarsuk carbonatite- and alkaline-silicate magmas were comagmatic and derived from a depleted mantle source (εNd>4, εSr<−13, time-integrated, single- stage 238U/204Pb ≤ 7.4). The mantle-derived magmas were contaminated with crustal material, equivalent to the local, pre-Gardar granites and gneisses and sediments derived from these. The crustal component has a depleted mantle Nd model age of 2.1-2.6 Ga; at 1.2 Ga it was characterized by εSr = +76, εNd = −8.4, time-integrated, single- stage 238U/204Pb = 8.2−8.3. Strong decoupling of the Pb from the Sr and Nd isotopic systems suggests that the contamination happened only after carbonatitic and alkaline-silicate magmas had evolved from a common parent, by processes such as liquid immisicibility and/or fractional crystallization. Post-magmatic hydrothermal alteration (oxidation, hydration of mafic silicates, carbonatization of melilite) may have contributed further to the contamination of the carbonatite and alkaline silicate rocks of the Qassiarsuk complex.

A Paleozoic Zircon Age of the Westcoast Crystalline Complex of Vancouver Island, British Columbia

1974 ◽  
Vol 11 (12) ◽  
pp. 1717-1722 ◽  
Author(s):  
J. E. Muller ◽  
R. K. Wanless ◽  
W. D. Loveridge
Keyword(s):  
British Columbia ◽  
Volcanic Rocks ◽  
Vancouver Island ◽  
Late Paleozoic ◽  
Basement Complex ◽  
Crystalline Complex ◽  
Zircon Age ◽  
Isotopic Evidence

Zircons from the Westcoast Crystalline Complex near Tofino, Vancouver Island, have yielded the following ages: 206Pb/238U = 265 ± 7 m.y.; 207Pb/235U = 263 ± 7 m.y.; and 206Pb/207Pb = 244 ± 20 m.y. A K–Ar date of an amphibolized dike from the same outcrop yielded 192 ± 9 m.y. This is the first supporting isotopic evidence that the 'basement' complex is derived from late Paleozoic Sicker volcanic rocks and was ultimately migmatized during the major Jurassic plutonic event of Vancouver Island.

Late Silurian – Early Devonian rifting during dextral transpression in the southern Gaspé Peninsula (Quebec): petrogenesis of volcanic rocks

1993 ◽  
Vol 30 (12) ◽  
pp. 2283-2294 ◽  
Author(s):  
J. Dostal ◽  
R. Laurent ◽  
J. D. Keppie
Keyword(s):  
Rift Zone ◽  
Volcanic Rocks ◽  
Significant Proportion ◽  
Spinel Peridotite ◽  
Initial Surface ◽  
Crustal Material ◽  
Early Devonian ◽  
Ree Patterns ◽  
Gaspe Peninsula ◽  
Gaspé Peninsula

The Upper Silurian – Lower Devonian volcanic rocks in the southern Gaspé Peninsula of the Quebec Appalachians crop out at the northeast end of the Connecticut Valley – Gaspé Synclinorium. These shallow marine and subaerial sequences reach a thickness of up to at least 2000 m and comprise two groups: (1) the Late Silurian volcanic rocks, which are mainly transitional alkalic–tholeiitic basalts with steeply sloping REE patterns; (2) the Early Devonian volcanic rocks, which include a significant proportion of intermediate rocks in addition to tholeiitic basalts. Compared with the Silurian rocks, the Devonian basalts have lower abundances of strongly incompatible trace elements such as Ba, Th, Ta, Nb, and light REE and relatively flat heavy REE patterns. Basalts of both groups display negative Nb and Ta anomalies (relative to Th and La).Although the basalts of both sequences were derived from lithospheric mantle, the Silurian basalts were generated from garnet peridotite at ~ 80 km depth while the Devonian basalts appear to have resulted from a larger degree of melting of spinel peridotite at a shallower depth (~ 60 km). Devonian intermediate rocks are probably the result of mixing of the basaltic magma with upper crustal material through assimilation – fractional crystallization processes. The basalts are interpreted to have formed in a northwest-trending rift zone located in the Quebec Reentrant during dextral transpression along the Appalachian Orogen. Rotation during and after the volcanism reoriented the rift zone to a northeast trend. The high density layer at the base of the crust under the Magdalen Basin may be the former magma chamber for the Silurian–Devonian volcanism. The change from transitional to tholeiitic volcanism at the Silurian–Devonian boundary suggests that the stretching value (ratio of final to initial surface area) increased from < 2 to > 2 at that time. This boundary is also coincident with the Salinic disturbance that is inferred to have been produced by erosion of the thermally uplifted block associated with rifting.

Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern Mexico

Lithos ◽  
1993 ◽  
Vol 29 (3-4) ◽  
pp. 295-302 ◽  
Author(s):  
D.J. Lynch ◽  
T.E. Musselman ◽  
J.T. Gutmann ◽  
P.J. Patchett
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Field ◽  
Isotopic Evidence ◽  
Northwestern Mexico

Contributions to the Petrology of British East Africa

1903 ◽  
Vol 13 (61) ◽  
pp. 228-263 ◽  
Author(s):  
G. T. Prior
Keyword(s):  
East Africa ◽  
Rift Valley ◽  
Volcanic Rocks ◽  
Central Africa ◽  
British Museum ◽  
Basaltic Rocks ◽  
Basement Rocks ◽  
Great Rift Valley ◽  
Rock Specimens

The following notes on the petrology of British East Africa are the result of an examination of rock-specimens collected by Professor J. W. Gregory on his well-known expedition from Mombasa to Mt. Kenya and Lake Baringo in 1892-3, and of rock-collections from the Uganda Protectorate which have been recently presented to the British Museum by Sir Harry Johnston.The collections include examples of the Archaean gneisses, schists, and granites which constitute the prevailing basement rocks of Central Africa; of ferruginotls schists, coarse sandstones, and quartzites belonging to the Palaeozoic Karagwe series ; and of an interesting series of Tertiary volcanic rocks comprising phonolites, phonolltic traehytes, riebeckite-rhyolites, kenytes, and basalts from the volcanoes of the Great Rift Valley, as well as of nephelinites and basaltic rocks containing melilite and perofskite from Mr. Elgon.

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