Page 187 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 173

(a) (b) 1000
12
Tephri- Rare earth elements
phonolite 100 OIB
10 Trachyte
Phono- Trachy- Rock/chondrite
tephrite andesite
8 Basaltic 10 MORB
Na 2 O K 2 O (wt %) 6 Tephrite Trachy- andesite Dacite Rhyolite 1 La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu
trachy-
Basanite
basalt
4 Alkaline Andesite Mafic lavas (c) 1000 Spider diagram
Picro- OIB
basalt Felsic lavas 100
2 Flood basalts
Basalt Basaltic Rock/chondrite
andesite
Tholeiitic 10
0
40 45 50 55 60 65 70 75 80 MORB
SiO 2 (wt %)
1
Rb Ba Th U Nb Ta K La Ce Sr NdSm Zr Ti Gd Y
Figure 7.17 (a) Total alkali-silica diagram showing the geochemical characteristics of lavas from Ethiopia (after
Kieffer et al., 2004, by permission of Oxford University Press). Dashed line separates alkaline from tholeiitic basalts. Rare
earth element (b) and spider diagram (c) showing a typical alkaline oceanic island basalt (OIB) and a typical tholeiitic
mid-ocean ridge basalt (MORB) (from Winter, John D., An Introduction to Igneous and Metamorphic Petrology, 1st
edition © 2001, p. 195. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ).

flood basalts also are common and may be associated ambient pressure (Fig. 7.18b). The ascent of hot

with silicic lavas, including rhyolite. Observations in mantle during lithospheric stretching (Section 7.6.2) or
East Africa indicate that a continuum of mafi c rocks the rise of a mantle plume causes a reduction in pres-
generally occurs, including alkaline, ultra-alkaline, sure that leads to decompression melting at a variety
tholeiitic, felsic, and transitional compositions (Fig. of depths, with the degree of melting depending on

7.17a). This diversity reflects both the compositional the rate of ascent, the geotherm, the composition of

heterogeneity of mantle source regions and processes the mantle, and the availability of fluids. A third
that affect the genesis and evolution of mafi c mechanism of melting involves the addition of vola-
magma. tiles, which has the effect of lowering the solidus tem-
There are three ways in which the mantle may perature. All three of these mechanisms probably
melt to produce basaltic liquids beneath rifts. First, contribute to generation of basaltic melts beneath
melting may be accomplished by heating the mantle continental rifts.
above the normal geotherm (Fig. 7.18a). Perturbations Once formed, the composition of mafi c magmas
in the geotherm could be related to the vertical trans- may be affected by partial melting. This process results
fer of heat by deep mantle plumes. It is probable, for in the separation of a liquid from a solid residue, which
example, that the volcanism and topographic uplift can produce a variety of melt compositions from a
associated with the Ethiopian and East African pla- single mantle source. Primary mafic melts also tend to

teaux reflect anomalously hot mantle. Investigations fractionate, whereby crystals are physically removed

of P n wave attenuation beneath the Eastern branch of from melts over a wide range of crustal pressures,
the East African Rift suggest sublithospheric tempera- resulting in suites of compositionally distinctive rocks.

tures that are significantly higher than those in the Current models generally favor fractional crystalliza-
ambient mantle (Venkataraman et al., 2004). A second tion of basaltic melts in shallow magma chambers
mechanism for melting the mantle is to lower the as the dominant process that generates rhyolite.