Geology Reference
In-Depth Information
association. They comprise shoshonitic basalts, grading to latites and trachytes.
Barberi et al. considered that rhyolites from Vulcanelo represent the
final stage of
this evolution. In Vulcanelo, leucite tephrite and related rock type were produced by
fractionation of a shoshonitic magma under a f (O
2
) condition of 10
−
2
bar. They
thought that the evolution of Aeolian volcanism might have been related to rapid
deepening of the Benioff zone. The occurrence of shoshonitic rocks and the con-
tinental nature of the crust on both sides of the plate boundaries may suggest that
the Aeolian Arc is in a senile stage of evolution. From their Sr isotopic data, they
considered a mantle source for Aeolian Arc volcanism.
4.13.2 K-Rich Rocks of the Tristan da Cunha Islands
The Island of Tristan da Cunha (Baker et al. 1964; Le Roxe 1990) is located at
37
W in the South Atlantic (Fig.
4.34
a). Tristen Da Cunha is the largest of a
group of three islands. It is nearly circular in shape having a diameter of about
12 km and rises from a depth of
°
S, 12
°
3,700 m to a height of 2,060 m. The outpouring
of lava came from a central vent (Fig.
4.34
b), but there are numerous parallel
channels, through which some eruptions took place. The island has a near-perfect
conical form and is bounded by a sea cliff, which rises nearly 600 m from the sea
level. Of the two islands, one is called Nightingale and the other is named as
Inaccessible. The rock types of Tristan da Cunha vary from ankaramatic basanite
through phonotephrite, tephriphonolite to phonolite (Le Roxe et al. 1990). Earlier,
Baker et al. (1964) thought that these rocks belong to alkali basalt-trachyte series,
but Le Roxe et al. (1990) plotted composition of the rocks in a total alkali-SiO
2
diagram (le Bas et al. 1986) and found the rocks to belong to basanite-phono-
tephrite-phonolite series. The mineralogy of the rocks also support the nomencla-
ture of the rock type (Fig.
4.34
c). Basanite is the dominant rock type (
−
80 %) of
*
the island. Basanites are characterized by the presence of 20
30 vol% euhedral to
-
sudhedral clinopyroxene (Wo 47
20 vol% of olivine
(Fo
79
-
68
) and titano-magnetite phenocrysts. Small plagioclase (An
90
-
34
), pheno-
crysts occur in a
50 En
42
-
35
Fs
11
-
15
), 5
-
-
fine-grained matrix. The plagioclase grans show complex zoning.
The basanites are characterized by the presence of normative nephilines, hence the
name basanite. Some basanite samples contain additional Ti-rich kaersutites as a
phenocrystal phase.
The phonotephrites range from moderately plagioclase-phyric to aphyric in
texture comprising phenocrystal plagioclase (An
88
-
17
), clinopyroxene (Wo
49
-
47
En
41
-
37
Fs
11
-
15
), titano-magnetite (Usp
56
-
76
) and kaersutitic amphibole. They often
contain olivine (Fo
77
-
42
), and alkali feldspar. In some samples leucite occurs as a
late crystalline phase in the groundmass.
Tephriphonolites range in texture from microprophyritic to porphyritic with
micro-phenocrysts of kaersutitic amphibole, titano-magnetite (Usp
38
-
52
), plagio-
clase (An
62
-
46
), clinopyroxene (Wo
49
-
51
,En
36
-
30
,Fs
50
-
19
), sphene and apatite.
Sometimes, clinopyroxene are aegirine-augite with hauine and sodalite.
Search WWH ::
Custom Search