Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 221

mineral chemistry and relationships 217
TABLE 34
1 2 3 4 5 6 7 8 9 10
sío2 55.80 53.90 59.92 54.66 59.88 65.29 62.45 60.70 66.22 62.45
tío2 1.87 1.77 1.17 1.60 0.90 0.69 1.90 0.50 0.36 0.38
Al^o^ 18.41 19.00 15.69 19.91 18.21 16.03 16.14 20.50 16.40 17.92
Fe203 3.07 3.37 1.70 3.07 1.68 3.17 1.63 2.30 0.62 2.06
FeO 3.78 3.05 6.04 2.73 3.54 0.70 5.49 0.40 2.95 3.11
MnO 0.09 0.18 0.18 0.18 0.12 0.11 0.18 0.20 0.15 0.16
MgO 2.13 1.68 1.19 1.10 0.97 0.84 0.70 0.20 0.10 0.09
CaO 5.07 6.25 4.17 5.56 2.50 1.45 3.28 1.40 0.56 1.17
NasO 4.57 5.04 5.02 5.85 5.35 5.34 4.65 6.20 5.40 6.47
k2o 4.30 4.53 2.79 5.03 6.19 4.82 3.35 6.70 4.03 6.14
p2o5 0.23 0.74 0.34 0.29 0.29 0.33 0.20 0.03 0.12 0.09
h2o 0.60 0.47 0.52 0.00 0.31 0.79 0.18 1.40 1.41 0.43
Total 99.92 99.98 98.72 99.98 99.94 99.56 100.15 100.16 98.32 100.47
Fe203l 7.27 6.76 7.93 6.10 4.83 3.95 7.72 2.74 3.90 5.52
Liquidus temperatures in °C at 1 atm
Plag 1155 1195 1135 1170 1122 1122
Afsp 1025 1025 1020 995
Compositions of evolved alkaline rocks resembling those of Jan Mayen and the
temperature of liquidus feldspar. 1) G 15 (Le Maitre, 1962), Tristanite, Gough
Island. 2) 230 (Baker et al., 1964), Tristanite, Tristan da Cunha. 3) SNS 8 (Stein-
thorsson, unpubl.), Trachyte, Snaefellsnes, Iceland. 4) 627 (Baker et al., 1964),
Tristanite, Tristan da Cunha. 5) G 24 (Le Maitre, 1962), Trachyte, Gough Island.
6) 60442 (Tilley et al., 1967), Trachyte, Skye. 7) SNS 14 (Steinthorsson, unpubl.),
Trachyte, Snaefellsnes, Iceland. 8) 31 (Baker et. al., 1964), Trachyte, Tristan da
Cunha. 9) SNS 30 (Steinthorsson, unpubl.), Trachyte, Snaefellsnes, Iceland. 10) G
149 (Le Maitre, 1962), Trachyte, Gough Island. The temperature determinations
are from Tilley et al., 1965 and 1967 except for the Snaefellsnes rocks, which are
unpublished (Steinthorsson).
The depths of feldspar crystallization in these evolved rocks are unknown
in detail as is the water content of the magmas, but both factors are of
importance. If the evolution of trachytic liquids takes place in a shallow
magma chamber, as is frequently postulated as the origin of salic magmas
formed by differentiation of basic magmas, then the depth ofthis chamber is
Irequently supposed to be around 3 km.
The feldspars of two syenitic xenoliths (Jan 142 and 324) occurring in the
Jan Mayen rocks have been analysed. Of these Jan 142 is partially remelted
and consists of glass and feldspars and was brought to the surface by a