Geoscience Reference
In-Depth Information
20
Altaid
Hercynian
Appalachian
18
Superior
Zimbabwe
Grenville
16
Birimian
14
Nagssutoqidian
Svecofenian
12
Pan-African
10
8
6
4
2
0
0
500
1000
1500
2000
2500
3000
3500
207
Pb*/
206
Pb* zircon age (Ma)
Figure 11.17
The distribution of U-Pb (
207
Pb*/
206
Pb*) ages in zircons from four of the major rivers of the
world, the Yangtze, the Mississippi, the Congo, and the Amazon (courtesy Tsuyoshi Iizuka). Only
concordant zircons are plotted. These data clearly show that granitic rocks, the primary carriers of
zircons, and therefore representative of continental crust, formed during very well-defined pulses.
Only a few local names of orogenic events are shown.
volcanoes show isotopic properties indicative that they are largely dominated by melts
from the underlying continental crust rather than creating it. The crustal volume of the
intra-oceanic arcs, such as the Western Aleutians, the Mariannas, or the South Sandwich
Islands is actually quite insignificant. The reason why regular subduction produces so little
crust is that modern oceanic crust is too cold to melt before it disappears into the deep man-
tle. Likewise, avalanches would not allow enough time for the sinking lithosphere to melt.
A popular idea is that Archean subduction was hot enough to allow subducted oceanic
crust to melt, but such a process is very unusual in modern times.
There is, nonetheless, a process of recurrent instabilities that can account for the episodic
character of crustal growth. This is the eruption of plume heads, such as those at the ori-
gin of large igneous provinces (LIP) forming the traps of the Deccan in India or those of
Ethiopia, and whose observable after-effects (plume stems) are the alignments of islands
anchored to the hot spots of Hawaii or Réunion, among others. Other sudden submarine
eruptions of very large size are known, such as the Ontong-Java plateau in the Pacific
and the Caribbean plateau in the Atlantic. These plateaus currently form more than 10%
of the ocean floor. Basalts from juvenile provinces (crustal segments that show a very
short crustal history of the material at the the time they formed), such as the 0.6 Ga old
Pan-African from Egypt and Arabia, the 2.1 Ga old Birimian of Africa, the 2.7 Ga old
Superior of Canada, show remarkable geochemical similarity (
Fig. 11.18
) between conti-
nental protolith, i.e. the material extracted from the mantle and which coalesces to form
continents and the ocean plateaus. Although andesitic volcanic rocks undoubtedly exist in
